Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons (in the form of gamma rays). Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). For fission to produce energy, the total binding energy of the resulting elements has to be higher than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom.

Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are made possible because certain substances called nuclear fuels undergo fission when struck by free neutrons and in turn generate neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.

The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very tempting source of energy; however, the products of nuclear fission are radioactive and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power.




[edit] Physical overview

[edit] Mechanics

A 3D representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons. Also shown is the capture of a neutron by uranium-238 to become uranium-239.Occasionally nuclear fission occurs without neutron bombardment, as a type of radioactive decay, but this type of fission (called spontaneous fission) is rare except in a few heavy isotopes. Most nuclear fission occurs as a "nuclear reaction"-- a bombardment-driven process that results from the collision of two subatomic particles. In nuclear reactions, a subatomic particle collides with an atomic nucleus and causes changes to it. Nuclear reactions are thus driven by the mechanics of bombardment, not by the relatively constant exponential decay and half-life characteristic of spontaneous radioactive processes.

A great many nuclear reactions are known. Nuclear fission differs importantly from other types of nuclear reactions in that it can be amplified and sometimes controlled via a nuclear chain reaction. In such a reaction, free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions.

The chemical element isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be fissile. The most common nuclear fuels are 235U (the isotope of uranium with an atomic mass of 235 and of use in nuclear reactors) and 239Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 u (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha/beta decay chain over periods of millennia to eons. In a nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events.


[edit] Energetics
Typical fission events release about two hundred million eV of energy for each fission event. By contrast, most chemical oxidation reactions (such as burning coal or TNT) release at most a few eV per event, so nuclear fuel contains at least ten million times more usable energy than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic radiation in the form of gamma rays; in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid, usually water or occasionally heavy water.

When a uranium nucleus fissions into two daughter nuclei fragments, an energy of ~200 MeV is released. For uranium-235, typically ~169 MeV is the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 neutrons are emitted with a kinetic energy of ~2 MeV each (total of 4.8 MeV). Finally, the fission reaction releases ~7 MeV in prompt gamma ray photons.

This energy amounts to about 181 MeV, or 89% of the total energy. The remaining 9% is released in beta decays and delayed gamma emissions. For example, in uranium-235 this amounts to about 6.5 MeV in betas, 8.8 MeV in anti-neutrinos, and an additional 6.3 MeV in delated gamma emission from the excited beta-decay products, for a total of ~ 10 gamma ray emissions per fission, in all.

The 8.8 MeV/202.5 MeV = 4.3% of the energy which is released as antineutrios is not captured by the reactor material as heat, and escapes directly through all materials (including the Earth) at nearly the speed of light, and into interplanetary space. Almost all of the remaining radiation is converted to heat, in the reactor core or shielding.

Some processes involving neutrons are notable for absorbing or finally yeilding energy-- for example neutron kinetic energy does not yield heat immediately if the neutron is captured by a uranium-238 atom to breed plutonium-239, but this energy is emitted if the plutonium-239 is later fissioned. On the other hand, so called "delayed neutrons" emitted as radiactive decay products with half-lives up to a minute, from fission-daughters, are very important to reactor control because they give a characteristic "reaction" time for the total nuclear reaction to double in size, if it run in a neutron-production zone which relies on these neutrons for a "run-away" chain-reaction (one in which each fission cycle yields more neutrons than it absorbs). Without their existance, the nuclear chain-reaction would increase in size faster than it could be controlled by human intervention, and the first experimental atomic reactors would have run-away to a dangerous and messy "prompt critical reaction" before their operators could have shut them down. If these neutrons are captured without producing fissions, they produce heat as well. [1]


[edit] Product nuclei and binding energy
In fission there is a preference to yield fragments with even proton numbers, which is called the odd-even effect on the fragments charge distribution. However, no odd-even effect is observed on fragment mass number distribution. This result is attributed to nucleon pair breaking.

In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120; the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u.[2] Unequal fissions are energetically more favorable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei (another statement of this, is that the atomic binding energy curve is slightly steeper to the left of mass 120 u than to the right of it).


[edit] Origin of the active energy and the curve of binding energy
Nuclear fission of heavy elements produces energy because the specific binding energy (binding energy per mass) of intermediate-mass nuclei with atomic numbers and atomic masses close to 62Ni and 56Fe is greater than the nucleon-specific binding energy of very heavy nuclei, so that energy is released when heavy nuclei are broken apart.

The total rest masses of the fission products (Mp) from a single reaction is less than the mass of the original fuel nucleus (M). The excess mass Δm = M – Mp is the invariant mass of the energy that is released as photons (gamma rays) and kinetic energy of the fission fragments, according to the mass-energy equivalence formula E = mc². The variation in specific binding energy with atomic number is due to the interplay of the two fundamental forces acting on the component nucleons (protons and neutrons) that make up the nucleus. Nuclei are bound by an attractive strong interaction between nucleons, which overcomes the electrostatic repulsion between protons. However, the strong nuclear force acts only over extremely short ranges, since it follows a Yukawa potential. For this reason large nuclei are less tightly bound per unit mass than small nuclei, and breaking a very large nucleus into two or more intermediate-sized nuclei releases energy.

Because of the short range of the strong binding force, large nuclei must contain proportionally more neutrons than do light elements, which are most stable with a 1–1 ratio of protons and neutrons. Extra neutrons stabilize heavy elements because they add to strong-force binding without adding to proton-proton repulsion. Fission products have, on average, about the same ratio of neutrons and protons as their parent nucleus, and are therefore usually unstable because they have proportionally too many neutrons compared to stable isotopes of similar mass. This is the fundamental cause of the problem of radioactive high level waste from nuclear reactors. Fission products tend to be beta emitters, emitting fast-moving electrons to conserve electric charge as excess neutrons convert to protons inside the nucleus of the fission product atoms.


[edit] Nuclear or "fissile" fuels
The most common nuclear fuels, 235U and 239Pu, are not major radiological hazards by themselves: 235U has a half-life of approximately 700 million years, and although 239Pu has a half-life of only about 24,000 years, it is a pure alpha particle emitter and hence not particularly dangerous unless ingested. Once a fuel element has been used, the remaining fuel material is intimately mixed with highly radioactive fission products that emit energetic beta particles and gamma rays. Some fission products have half-lives as short as seconds; others have half-lives of tens of thousands of years, requiring long-term storage in facilities such as Yucca Mountain nuclear waste repository until the fission products decay into non-radioactive stable isotopes.


[edit] Chain reactions

A schematic nuclear fission chain reaction. 1. A uranium-235 atom absorbs a neutron and fissions into two new atoms (fission fragments), releasing three new neutrons and some binding energy. 2. One of those neutrons is absorbed by an atom of uranium-238 and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. 3. Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases between one and three neutrons, which can then continue the reaction.Main article: Nuclear chain reaction
Several heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission, a form of radioactive decay and induced fission, a form of nuclear reaction. Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable; isotopes that undergo fission when struck by a thermal, slow moving neutron are also called fissile. A few particularly fissile and readily obtainable isotopes (notably 235U and 239Pu) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be useful.

All fissionable and fissile isotopes undergo a small amount of spontaneous fission which releases a few free neutrons into any sample of nuclear fuel. Such neutrons would escape rapidly from the fuel and become a free neutron, with a half-life of about 15 minutes before they decayed to protons and beta particles. However, neutrons almost invariably impact and are absorbed by other nuclei in the vicinity long before this happens (newly-created fission neutrons are moving at about 7% of the speed of light, and even moderated neutrons are moving at about 8 times the speed of sound). Some neutrons will impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough nuclear fuel is assembled into one place, or if the escaping neutrons are sufficiently contained, then these freshly generated neutrons outnumber the neutrons that escape from the assembly, and a sustained nuclear chain reaction will take place.

An assembly that supports a sustained nuclear chain reaction is called a critical assembly or, if the assembly is almost entirely made of a nuclear fuel, a critical mass. The word "critical" refers to a cusp in the behavior of the differential equation that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by radioactive decay, but if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual mass of a critical mass of nuclear fuel depends strongly on the geometry and surrounding materials.

Not all fissionable isotopes can sustain a chain reaction. For example, 238U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But too few of the neutrons produced by 238U fission are energetic enough to induce further fissions in 238U, so no chain reaction is possible with this isotope. Instead, bombarding 238U with slow neutrons causes it to absorb them (becoming 239U) and decay by beta emission to 239Np which then decays again by the same process to 239Pu; that process is used to manufacture 239Pu in breeder reactors. In-situ plutonium producion also contributes to the neutron chain reaction in other types of reactors after sufficient plutonium-239 has been produced, since plutonium-239 is also a fissile element which serves as fuel. It is estimated that up to half of the power produced by a standard "non-breeder" reactor is produced by the fission of plutonium-239 produced in place, over the total life-cycle of a fuel load.

Fissionable, non-fissile isotopes can be used as fission energy source even without a chain reaction. Bombarding 238U with fast neutrons induces fissions, releasing energy as long as the external neutron source is present. This is an important effect in all reactors where fast neutrons from the fissile isotope can cause the fission of nearby 238U nuclei, which means that some small part of the 238U is "burned-up" in all nuclear fuels, especially in fast breeder reactors that operate with higher-energy neutrons. That same fast-fission effect is used to augment the energy released by modern thermonuclear weapons, by jacketing the weapon with 238U to react with neutrons released by nuclear fusion at the center of the device.


[edit] Fission reactors
Critical fission reactors are the most common type of nuclear reactor. In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors. Such devices use radioactive decay or particle accelerators to trigger fissions.

Critical fission reactors are built for three primary purposes, which typically involve different engineering trade-offs to take advantage of either the heat or the neutrons produced by the fission chain reaction:

power reactors are intended to produce heat for nuclear power, either as part of a generating station or a local power system such as a nuclear submarine.
research reactors are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.
breeder reactors are intended to produce nuclear fuels in bulk from more abundant isotopes. The better known fast breeder reactor makes 239Pu (a nuclear fuel) from the naturally very abundant 238U (not a nuclear fuel). Thermal breeder reactors previously tested using 232Th to breed the fissile isotope 233U continue to be studied and developed.
While, in principle, all fission reactors can act in all three capacities, in practice the tasks lead to conflicting engineering goals and most reactors have been built with only one of the above tasks in mind. (There are several early counter-examples, such as the Hanford N reactor, now decommissioned). Power reactors generally convert the kinetic energy of fission products into heat, which is used to heat a working fluid and drive a heat engine that generates mechanical or electrical power. The working fluid is usually water with a steam turbine, but some designs use other materials such as gaseous helium. Research reactors produce neutrons that are used in various ways, with the heat of fission being treated as an unavoidable waste product. Breeder reactors are a specialized form of research reactor, with the caveat that the sample being irradiated is usually the fuel itself, a mixture of 238U and 235U. For a more detailed description of the physics and operating principles of critical fission reactors, see nuclear reactor physics. For a description of their social, political, and environmental aspects, see nuclear reactor.


[edit] Fission bombs

The mushroom cloud of the atom bomb dropped on Nagasaki, Japan in 1945 rose some 180 kilometers (110 miles) above the bomb's hypocenter.One class of nuclear weapon, a fission bomb (not to be confused with the fusion bomb), otherwise known as an atomic bomb or atom bomb, is a fission reactor designed to liberate as much energy as possible as rapidly as possible, before the released energy causes the reactor to explode (and the chain reaction to stop). Development of nuclear weapons was the motivation behind early research into nuclear fission: the Manhattan Project of the U.S. military during World War II carried out most of the early scientific work on fission chain reactions, culminating in the Little Boy and Fat Man and Trinity bombs that were exploded over test sites, the cities Hiroshima, and Nagasaki, Japan in August 1945.

Even the first fission bombs were thousands of times more explosive than a comparable mass of chemical explosive. For example, Little Boy weighed a total of about four tons (of which 60 kg was nuclear fuel) and was 11 feet (3.4 m) long; it also yielded an explosion equivalent to about 15 kilotons of TNT, destroying a large part of the city of Hiroshima. Modern nuclear weapons (which include a thermonuclear fusion as well as one or more fission stages) are literally hundreds of times more energetic for their weight than the first pure fission atomic bombs, so that a modern single missile warhead bomb weighing less than 1/8th as much as Little Boy (see for example W88) has a yield of 475,000 tons of TNT, and could bring destruction to 10 times the city area.

While the fundamental physics of the fission chain reaction in a nuclear weapon is similar to the physics of a controlled nuclear reactor, the two types of device must be engineered quite differently (see nuclear reactor physics). It is impossible to convert a nuclear reactor to cause a true nuclear explosion[citation needed], or for a nuclear reactor to explode the way a nuclear explosive does, (though partial fuel meltdowns and steam explosions have occurred), and difficult to extract useful power from a nuclear explosive (though at least one rocket propulsion system, Project Orion, was intended to work by exploding fission bombs behind a massively padded vehicle).

The strategic importance of nuclear weapons is a major reason why the technology of nuclear fission is politically sensitive. Viable fission bomb designs are, arguably, within the capabilities of bright undergraduates (see John Aristotle Phillips) being incredibly simple, but nuclear fuel to realize the designs is thought to be difficult to obtain being rare (see uranium enrichment and nuclear fuel cycle).


[edit] History
This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (July 2008)

Criticality in nature is uncommon. At three ore deposits at Oklo in Gabon, sixteen sites (the so-called Oklo Fossil Reactors) have been discovered at which self-sustaining nuclear fission took place approximately 2 billion years ago.

Ernest Rutherford is credited with splitting the atom in 1917. His team bombarded nitrogen with naturally occurring alpha particles from radioactive material and observed a proton emitted with energy higher than the alpha particle. In 1932 his students John Cockcroft and Ernest Walton, working under Rutherford's direction, attempted to split the nucleus by entirely artificial means, using a particle accelerator to bombard lithium with protons, thereby producing two helium nuclei.

Enrico Fermi and his colleagues studied the results of bombarding uranium with neutrons in 1934. The first person who mentioned the idea of nuclear fission in 1934 was Ida Noddack.[3]


The experimental apparatus with which the team of Lise Meitner, Otto Hahn and Fritz Strassmann discovered Nuclear Fission in 1938After the Fermi publication, Lise Meitner, Otto Hahn and Fritz Strassmann began performing similar experiments in Germany. Meitner, an Austrian Jew, lost her citizenship with the Anschluss in 1938. She fled and wound up in Sweden, but continued to collaborate by mail and through meetings with Hahn in Sweden. By coincidence her nephew Otto Robert Frisch, also a refugee, was also in Sweden when Meitner received a letter from Hahn describing his chemical proof that some of the product of the bombardment of uranium with neutrons, was barium and not barium's much heavier chemical sister element radium (barium's atomic weight is about 60% that of uranium). Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and the techniques were well-known. According to Frisch:

Was it a mistake? No, said Lise Meitner; Hahn was too good a chemist for that. But how could barium be formed from uranium? No larger fragments than protons or helium nuclei (alpha particles) had ever been chipped away from nuclei, and to chip off a large number not nearly enough energy was available. Nor was it possible that the uranium nucleus could have been cleaved right across. A nucleus was not like a brittle solid that can be cleaved or broken; George Gamow had suggested early on, and Bohr had given good arguments that a nucleus was much more like a liquid drop. Perhaps a drop could divide itself into two smaller drops in a more gradual manner, by first becoming elongated, then constricted, and finally being torn rather than broken in two? We knew that there were strong forces that would resist such a process, just as the surface tension of an ordinary liquid drop tends to resist its division into two smaller ones. But nuclei differed from ordinary drops in one important way: they were electrically charged, and that was known to counteract the surface tension.

The charge of a uranium nucleus, we found, was indeed large enough to overcome the effect of the surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly unstable drop, ready to divide itself at the slightest provocation, such as the impact of a single neutron. But there was another problem. After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and hence a very large energy, about 200 MeV in all; where could that energy come from? ...Lise Meitner... worked out that the two nuclei formed by the division of a uranium nucleus together would be lighter than the original uranium nucleus by about one-fifth the mass of a proton. Now whenever mass disappears energy is created, according to Einstein's formula E=mc2, and one-fifth of a proton mass was just equivalent to 200MeV. So here was the source for that energy; it all fitted!

[4]

In December 1938, the German chemists Otto Hahn and Fritz Strassmann sent a manuscript to Naturwissenschaften reporting they had detected the element barium after bombarding uranium with neutrons;[5] simultaneously, they communicated these results to Lise Meitner. Meitner, and her nephew Otto Robert Frisch, correctly interpreted these results as being nuclear fission.[6] Frisch confirmed this experimentally on 13 January 1939.[7] In 1944, Hahn received the Nobel Prize for Chemistry for the discovery of nuclear fission. Some historians have documented the history of the discovery of nuclear fission and believe Meitner should have been awarded the Nobel Prize with Hahn.[8][9][10]

Meitner’s and Frisch’s interpretation of the work of Hahn and Strassmann crossed the Atlantic Ocean with Niels Bohr, who was to lecture at Princeton University. Isidor Isaac Rabi and Willis Lamb, two Columbia University physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said: “Young man, let me explain to you about something new and exciting in physics.”[11] It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States,[12] which was done in the basement of Pupin Hall; the members of the team were Herbert L. Anderson, Eugene T. Booth, John R. Dunning, Enrico Fermi, G. Norris Glasoe, and Francis G. Slack. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C. under the joint auspices of the George Washington University and the Carnegie Institution of Washington. There, the news on nuclear fusion was spread even further, which fostered many more experimental demonstrations.[13]

Frédéric Joliot-Curie's team in Paris discovered that secondary neutrons are released during uranium fission, thus making a nuclear chain-reaction feasible. The figure of about two neutrons being emitted with nuclear fission of uranium was verified independently by Leo Szilard and Walter Henry Zinn. The number of neutrons emitted with nuclear fission of 235U was then reported at 3.5/fission, and later corrected to 2.6/fission by Frédéric Joliot-Curie, Hans von Halban and Lew Kowarski.

"Chain reactions" at that time were a known phenomenon in chemistry, but the analogous process in nuclear physics, using neutrons, had been foreseen as early as 1933 by Leo Szilard, although Szilard at that time had no idea with what materials the process might be initiated (Szilard thought it might be done with light neutron-rich elements). Szilard, a Hungarian born Jew, also fled mainland Europe after Hitler's rise, eventually landing in the US.

With the news of fission neutrons from uranium fission, Szilard immediately understood the possibility of a nuclear chain reaction using uranium. In the summer, Fermi and Szilard proposed the idea of a nuclear reactor (pile) to mediate this process. The pile would use natural uranium as fuel, and graphite as the moderator of neutron energy (it had previously been shown by Fermi that neutrons were far more effectively captured by atoms if they were moving slowly, a process called moderation when the neutrons were slowed after being released from a fission event in a nuclear reactor).

In August Hungarian-Jewish refugees Szilard, Teller and Wigner thought that the Germans might make use of the fission chain reaction, and persuaded German-Jewish refugee Einstein to warn President Roosevelt of this possible German menace. The letter suggested the possibility of a uranium bomb deliverable by ship, which would destroy "an entire harbor and much of the surrounding countryside." The President received the Einstein-Szilárd letter on 11 October 1939 — shortly after WWII began in Europe, but two years before U.S. entry into it.

In England, James Chadwick proposed an atomic bomb utilizing natural uranium, based on a paper by Rudolf Peierls with the mass needed for critical state being 30–40 tons. In America, J. Robert Oppenheimer thought that a cube of uranium deuteride 10 cm on a side (about 11 kg of uranium) might "blow itself to hell." In this design it was still thought that a moderator would need to be used for nuclear bomb fission (this turned out not to be the case if the fissile isotope was separated).

In December, Heisenberg delivered a report to the Germany Department of War on the possibility of a uranium bomb.

In Birmingham, England Otto Robert Frisch teamed up with Rudolf Peierls who had also fled German anti-Jewish laws. They conceived the idea of utilizing a purified isotope of uranium, 235U, and worked out that an enriched uranium bomb could have a critical mass of only 600 grams, instead of tons, and that the resulting explosion would be tremendous. (The amount actually turned out to be 15 kg, although several times this amount was used in the actual uranium (Little Boy) bomb). In February 1940 they delivered the Frisch-Peierls memorandum. Ironically, they were still officially considered "enemy aliens" at the time.

Glenn Seaborg, Joe Kennedy, Art Wahl and Italian-Jewish refugee Emilio Segrè shortly discovered 239Pu in the decay products of 239U produced by bombarding 238U with neutrons, and determined it to be fissionable like 235U.

On June 28, 1941, the Office of Scientific Research and Development was formed in the U.S. to mobilize scientific resources and apply the results of research to national defense. In September, Fermi assembled his first nuclear "pile" or reactor, in an attempt to create a slow neutron induced chain reaction in uranium, but the experiment failed for lack of proper materials, or not enough of the materials which were available.

Producing a fission chain reaction in natural uranium fuel was found to be far from trivial. Early nuclear reactors did not use isotopically enriched uranium, and in consequence they were required to use large quantities of highly purified graphite as neutron moderation materials. Use of ordinary water (as opposed to heavy water) in nuclear reactors requires enriched fuel — the partial separation and relative enrichment of the rare 235U isotope from the far more common 238U isotope. Typically, reactors also require inclusion of extremely chemically pure neutron moderator materials such as deuterium (in heavy water), helium, beryllium, or carbon, the latter usually as graphite. (The high purity for carbon is required because many chemical impurities such as the boron-10 component of natural boron, are very strong neutron absorbers and thus poison the chain reaction.)

Production of such materials at industrial scale had to be solved for nuclear power generation and weapons production to be accomplished. Up to 1940, the total amount of uranium metal produced in the USA was not more than a few grams, and even this was of doubtful purity; of metallic beryllium not more than a few kilograms; concentrated deuterium oxide (heavy water) not more than a few kilograms. Finally, carbon had never been produced in quantity with anything like the purity required of a moderator.

The problem of producing large amounts of high purity uranium was solved by Frank Spedding using the thermite process. Ames Laboratory was established in 1942 to produce the large amounts of natural (unenriched) uranium metal that would be necessary for the research to come. The success of the Chicago Pile-1 which used unenriched (natural) uranium, like all of the atomic "piles" which produced the plutonium for the atomic bomb, was also due specifically to Szilard's realization that very pure graphite could be used for the moderator of even natural uranium "piles". In wartime Germany, failure to appreciate the qualities of very pure graphite led to reactor designs dependent on heavy water, which in turn was denied the Germans by Allied attacks in Norway, where heavy water was produced. These difficulties prevented the Nazis from building a nuclear reactor capable of criticality during the war.

Unknown until 1972 (but postulated by Paul Kuroda in 1956), when French physicist Francis Perrin discovered the Oklo Fossil Reactors, it was realized that nature had beaten humans to the punch. Large-scale natural uranium fission chain reactions, moderated by normal water, had occurred some 2 billion years in the past. This ancient process was able to use normal water as a moderator only because 2 billion years in the past, natural uranium was richer in the shorter-lived fissile isotope 235U (about 3%), than natural uranium available today (which is only 0.7%, and must be enriched to 3% to be usable in light-water reactors).

For more detail on the early development of the first nuclear reactors and nuclear weapons, see Manhattan Project.

Nuclear fusion

In nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus. It is accompanied by the release or absorption of energy, which allows matter to enter a plasma state.

The fusion of two nuclei with lower mass than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy while the fusion of nuclei heavier than iron absorbs energy; vice-versa for the reverse process, nuclear fission. In the simplest case of hydrogen fusion, two protons have to be brought close enough for their mutual electric repulsion to be overcome by the nuclear force and the subsequent release of energy.

Nuclear fusion occurs naturally in stars. Artificial fusion in human enterprises has also been achieved, although has not yet been completely controlled. Building upon the nuclear transmutation experiments of Ernest Rutherford done a few years earlier, fusion of light nuclei (hydrogen isotopes) was first observed by Mark Oliphant in 1932; the steps of the main cycle of nuclear fusion in stars were subsequently worked out by Hans Bethe throughout the remainder of that decade. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project, but was not successful until 1952. Research into controlled fusion for civilian purposes began in the 1950s, and continues to this day.

Overview
Nuclear physics

Radioactive decay
Nuclear fission
Nuclear fusion [show]Classical decays
Alpha decay · Beta decay · Gamma radiation
[show]Advanced decays
Double beta decay · Double electron capture · Internal conversion · Isomeric transition · Cluster decay · Spontaneous fission
[show]Emission processes
Neutron emission · Positron emission · Proton emission
[show]Capturing
Electron capture · Neutron capture
R · S · P · Rp
[show]High energy processes
Spallation · Cosmic ray spallation · Photodisintegration
[show]Nucleosynthesis
Stellar Nucleosynthesis
Big Bang nucleosynthesis
Supernova nucleosynthesis
[show]Scientists
Becquerel · Bethe · Marie Curie · Pierre Curie · Fermi

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Fusion reactions power the stars and produce all but the lightest elements in a process called nucleosynthesis. Although the fusion of lighter elements in stars releases energy, production of the heavier elements absorbs energy.

When the fusion reaction is a sustained uncontrolled chain, it can result in a thermonuclear explosion, such as that generated by a hydrogen bomb. Reactions which are not self-sustaining can still release considerable energy, as well as large numbers of neutrons.

Research into controlled fusion, with the aim of producing fusion power for the production of electricity, has been conducted for over 50 years. It has been accompanied by extreme scientific and technological difficulties, but has resulted in steady progress. At present, break-even (self-sustaining) controlled fusion reactions have been demonstrated in a few tokamak-type reactors around the world. These have enabled the creation of workable designs for a reactor which will deliver ten times more fusion energy than the amount needed to heat up plasma to required temperatures (see ITER which is scheduled to be operational in 2018).

It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. This is because all nuclei have a positive charge (due to their protons), and as like charges repel, nuclei strongly resist being put too close together. Accelerated to high speeds (that is, heated to thermonuclear temperatures), they can overcome this electromagnetic repulsion and get close enough for the attractive nuclear force to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, generally releases more energy than it takes to force the nuclei together; this is an exothermic process that can produce self-sustaining reactions.

The energy released in most nuclear reactions is much larger than that in chemical reactions, because the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 electron volts—less than one-millionth of the 17 MeV released in the D-T (deuterium-tritium) reaction shown in the diagram to the right. Fusion reactions have an energy density many times greater than nuclear fission; i.e., the reactions produce far greater energies per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones, which are themselves millions of times more energetic than chemical reactions. Only direct conversion of mass into energy, such as that caused by the collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion.


[edit] Requirements
A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances two naked nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the attractive nuclear force which is stronger at close distances.

When a nucleon such as a proton or neutron is added to a nucleus, the nuclear force attracts it to other nucleons, but primarily to its immediate neighbours due to the short range of the force. The nucleons in the interior of a nucleus have more neighboring nucleons than those on the surface. Since smaller nuclei have a larger surface area-to-volume ratio, the binding energy per nucleon due to the strong force generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons.

The electrostatic force, on the other hand, is an inverse-square force, so a proton added to a nucleus will feel an electrostatic repulsion from all the other protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei get larger.


At short distances the attractive nuclear force is stronger than the repulsive electrostatic force. As such, the main technical difficulty for fusion is getting the nuclei close enough to fuse. Distances not to scale.The net result of these opposing forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei. Eventually, the binding energy becomes negative and very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable. The four most tightly bound nuclei, in decreasing order of binding energy, are 62Ni, 58Fe, 56Fe, and 60Ni.[2] Even though the nickel isotope ,62Ni, is more stable, the iron isotope 56Fe is an order of magnitude more common. This is due to a greater disintegration rate for 62Ni in the interior of stars driven by photon absorption.

A notable exception to this general trend is the helium-4 nucleus, whose binding energy is higher than that of lithium, the next heaviest element. The Pauli exclusion principle provides an explanation for this exceptional behavior—it says that because protons and neutrons are fermions, they cannot exist in exactly the same state. Each proton or neutron energy state in a nucleus can accommodate both a spin up particle and a spin down particle. Helium-4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons; so all four of its nucleons can be in the ground state. Any additional nucleons would have to go into higher energy states.

The situation is similar if two nuclei are brought together. As they approach each other, all the protons in one nucleus repel all the protons in the other. Not until the two nuclei actually come in contact can the strong nuclear force take over. Consequently, even when the final energy state is lower, there is a large energy barrier that must first be overcome. It is called the Coulomb barrier.

The Coulomb barrier is smallest for isotopes of hydrogen—they contain only a single positive charge in the nucleus. A bi-proton is not stable, so neutrons must also be involved, ideally in such a way that a helium nucleus, with its extremely tight binding, is one of the products.

Using deuterium-tritium fuel, the resulting energy barrier is about 0.01 MeV.[citation needed] In comparison, the energy needed to remove an electron from hydrogen is 13.6 eV, about 750 times less energy. The (intermediate) result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV.[citation needed] The recoil energy of the remaining 4He nucleus is 3.5 MeV,[citation needed] so the total energy liberated is 17.6 MeV.[citation needed] This is many times more than what was needed to overcome the energy barrier.

If the energy to initiate the reaction comes from accelerating one of the nuclei, the process is called beam-target fusion; if both nuclei are accelerated, it is beam-beam fusion. If the nuclei are part of a plasma near thermal equilibrium, one speaks of thermonuclear fusion. Temperature is a measure of the average kinetic energy of particles, so by heating the nuclei they will gain energy and eventually have enough to overcome this 0.01 MeV. Converting the units between electronvolts and kelvins shows that the barrier would be overcome at a temperature in excess of 120 million kelvins, obviously a very high temperature.

There are two effects that lower the actual temperature needed. One is the fact that temperature is the average kinetic energy, implying that some nuclei at this temperature would actually have much higher energy than 0.01 MeV, while others would be much lower. It is the nuclei in the high-energy tail of the velocity distribution that account for most of the fusion reactions. The other effect is quantum tunneling. The nuclei do not actually have to have enough energy to overcome the Coulomb barrier completely. If they have nearly enough energy, they can tunnel through the remaining barrier. For this reason fuel at lower temperatures will still undergo fusion events, at a lower rate.


The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.The reaction cross section σ is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution with thermonuclear fusion, then it is useful to perform an average over the distributions of the product of cross section and velocity. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:


If a species of nuclei is reacting with itself, such as the DD reaction, then the product n1n2 must be replaced by (1 / 2)n2.

increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of 10 – 100 keV. At these temperatures, well above typical ionization energies (13.6 eV in the hydrogen case), the fusion reactants exist in a plasma state.

The significance of as a function of temperature in a device with a particular energy confinement time is found by considering the Lawson criterion.


[edit] Gravitational confinement
One force capable of confining the fuel well enough to satisfy the Lawson criterion is gravity. The mass needed, however, is so great that gravitational confinement is only found in stars (the smallest of which are brown dwarfs). Even if the more reactive fuel deuterium were used, a mass greater than that of the planet Jupiter would be needed. In stars heavy enough, after the supply of hydrogen is exhausted in their cores, their cores (or a shell around the core) start fusing helium to carbon. In the most massive stars (at least 8-11 solar masses), the process is continued until some of their energy is produced by fusing lighter elements to iron. As iron has one of the highest binding energies, reactions producing heavier elements are generally endothermic. Therefore significant amounts of heavier elements are not formed during stable periods of massive star evolution, but are formed in supernova explosions and some lighter stars. Some of these heavier elements can in turn produce energy in nuclear fission.


[edit] Magnetic confinement
See Magnetic confinement fusion for more information.
Electrically charged particles (such as fuel ions) will follow magnetic field lines (see Guiding center#Gyration). The fusion fuel can therefore be trapped using a strong magnetic field. A variety of magnetic configurations exist, including the toroidal geometries of tokamaks and stellarators and open-ended mirror confinement systems.


[edit] Inertial confinement
See Inertial fusion energy for more information.
A third confinement principle is to apply a rapid pulse of energy to a large part of the surface of a pellet of fusion fuel, causing it to simultaneously "implode" and heat to very high pressure and temperature. If the fuel is dense enough and hot enough, the fusion reaction rate will be high enough to burn a significant fraction of the fuel before it has dissipated. To achieve these extreme conditions, the initially cold fuel must be explosively compressed. Inertial confinement is used in the hydrogen bomb, where the driver is x-rays created by a fission bomb. Inertial confinement is also attempted in "controlled" nuclear fusion, where the driver is a laser, ion, or electron beam, or a Z-pinch. Another method is to use conventional high explosive material to compress a fuel to fusion conditions.[3][4] The UTIAS explosive-driven-implosion facility was used to produce stable, centered and focused hemispherical implosions[5] to generate neutrons from D-D reactions. The simplest and most direct method proved to be in a predetonated stoichiometric mixture of deuterium-oxygen. The other successful method was using a miniature Voitenko compressor,[6] where a plane diaphragm was driven by the implosion wave into a secondary small spherical cavity that contained pure deuterium gas at one atmosphere.

Some confinement principles have been investigated, such as muon-catalyzed fusion, the Farnsworth-Hirsch fusor and Polywell (inertial electrostatic confinement), and bubble fusion.


[edit] Production methods
A variety of methods are known to effect nuclear fusion. Some are "cold" in the strict sense that no part of the material is hot (except for the reaction products), some are "cold" in the limited sense that the bulk of the material is at a relatively low temperature and pressure but the reactants are not, and some are "hot" fusion methods that create macroscopic regions of very high temperature and pressure.


[edit] Locally cold fusion
Muon-catalyzed fusion is a well-established and reproducible fusion process that occurs at ordinary temperatures. It was studied in detail by Steven Jones in the early 1980s. It has not been reported to produce net energy. Net energy production from this reaction is not believed to be possible[citation needed] because of the energy required to create muons, their 2.2 µs half-life, and the chance that a muon will bind to the new alpha particle and thus stop catalyzing fusion.


[edit] Generally cold, locally hot fusion
Accelerator based light-ion fusion. Using particle accelerators it is possible to achieve particle kinetic energies sufficient to induce many light ion fusion reactions. Accelerating light ions is relatively easy, cheap, and can be done in an efficient manner – all it takes is a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kilovolt between electrodes. The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb interaction cross sections. Therefore vast majority of ions ends up expending their energy on bremsstrahlung and ionization of atoms of the target. Devices referred to as sealed-tube neutron generators are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement which allows ions of these nuclei to be accelerated against hydride targets, also containing deuterium and tritium, where fusion takes place. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves. Despite periodic reports in the popular press by scientists claiming to have invented "table-top" fusion machines, neutron generators have been around for half a century. The sizes of these devices vary but the smallest instruments are often packaged in sizes smaller than a loaf of bread. These devices do not produce a net power output.

In sonoluminescence, acoustic shock waves create temporary bubbles that collapse shortly after creation, producing very high temperatures and pressures. In 2002, Rusi P. Taleyarkhan reported the possibility that bubble fusion occurs in those collapsing bubbles (aka sonofusion). As of 2005, experiments to determine whether fusion is occurring give conflicting results. If fusion is occurring, it is because the local temperature and pressure are sufficiently high to produce hot fusion.[7] In an episode of Horizon, on BBC television, results were presented showing that, although temperatures were reached which could initiate fusion on a large scale, no fusion was occurring, and inaccuracies in the measuring system were the cause of anomalous results.

The Farnsworth-Hirsch Fusor is a tabletop device in which fusion occurs. This fusion comes from high effective temperatures produced by electrostatic acceleration of ions. The device can be built inexpensively, but it too is unable to produce a net power output.

The Polywell is a concept for a tabletop device in which fusion occurs. The device is a non-thermodynamic equilibrium machine which uses electrostatic confinement to accelerate ions into a center where they fuse together.

Antimatter-initialized fusion uses small amounts of antimatter to trigger a tiny fusion explosion. This has been studied primarily in the context of making nuclear pulse propulsion feasible. This is not near becoming a practical power source, due to the cost of manufacturing antimatter alone.

Pyroelectric fusion was reported in April 2005 by a team at UCLA. The scientists used a pyroelectric crystal heated from −34 to 7°C (−30 to 45°F), combined with a tungsten needle to produce an electric field of about 25 gigavolts per meter to ionize and accelerate deuterium nuclei into an erbium deuteride target. Though the energy of the deuterium ions generated by the crystal has not been directly measured, the authors used 100 keV (a temperature of about 109 K) as an estimate in their modeling.[8] At these energy levels, two deuterium nuclei can fuse together to produce a helium-3 nucleus, a 2.45 MeV neutron and bremsstrahlung. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces.[9][10][11][12]


[edit] Hot fusion
In "standard" "hot" fusion, the fuel reaches tremendous temperature and pressure inside a fusion reactor or nuclear weapon.

The methods in the second group are examples of non-equilibrium systems, in which very high temperatures and pressures are produced in a relatively small region adjacent to material of much lower temperature. In his doctoral thesis for MIT, Todd Rider did a theoretical study of all quasineutral, isotropic, non-equilibrium fusion systems. He demonstrated that all such systems will leak energy at a rapid rate due to bremsstrahlung produced when electrons in the plasma hit other electrons or ions at a cooler temperature and suddenly decelerate. The problem is not as pronounced in a hot plasma because the range of temperatures, and thus the magnitude of the deceleration, is much lower. Note that Rider's work does not apply to non-neutral and/or anisotropic non-equilibrium plasmas.


[edit] Important reactions

[edit] Astrophysical reaction chains

The proton-proton chain dominates in stars the size of the Sun or smaller.
The CNO cycle dominates in stars heavier than the Sun.The most important fusion process in nature is that which powers the stars. The net result is the fusion of four protons into one alpha particle, with the release of two positrons, two neutrinos (which changes two of the protons into neutrons), and energy, but several individual reactions are involved, depending on the mass of the star. For stars the size of the sun or smaller, the proton-proton chain dominates. In heavier stars, the CNO cycle is more important. Both types of processes are responsible for the creation of new elements as part of stellar nucleosynthesis.

At the temperatures and densities in stellar cores the rates of fusion reactions are notoriously slow. For example, at solar core temperature (T ≈ 15 MK) and density (160 g/cm³), the energy release rate is only 276 μW/cm³—about a quarter of the volumetric rate at which a resting human body generates heat.[13] Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates strongly depend on temperature (exp(−E/kT)), then in order to achieve reasonable rates of energy production in terrestrial fusion reactors 10–100 times higher temperatures (compared to stellar interiors) are required T ≈ 0.1–1.0 GK.


[edit] Criteria and candidates for terrestrial reactions
In man-made fusion, the primary fuel is not constrained to be protons and higher temperatures can be used, so reactions with larger cross-sections are chosen. This implies a lower Lawson criterion, and therefore less startup effort. Another concern is the production of neutrons, which activate the reactor structure radiologically, but also have the advantages of allowing volumetric extraction of the fusion energy and tritium breeding. Reactions that release no neutrons are referred to as aneutronic.

In order to be useful as a source of energy, a fusion reaction must satisfy several criteria. It must

be exothermic: This may be obvious, but it limits the reactants to the low Z (number of protons) side of the curve of binding energy. It also makes helium 4He the most common product because of its extraordinarily tight binding, although 3He and 3H also show up;
involve low Z nuclei: This is because the electrostatic repulsion must be overcome before the nuclei are close enough to fuse;
have two reactants: At anything less than stellar densities, three body collisions are too improbable. It should be noted that in inertial confinement, both stellar densities and temperatures are exceeded to compensate for the shortcomings of the third parameter of the Lawson criterion, ICF's very short confinement time;
have two or more products: This allows simultaneous conservation of energy and momentum without relying on the electromagnetic force;
conserve both protons and neutrons: The cross sections for the weak interaction are too small.
Few reactions meet these criteria. The following are those with the largest cross sections[citation needed]:

(1) 2
1D + 3
1T → 4
2He ( 3.5 MeV ) + n0 ( 14.1 MeV )
(2i) 2
1D + 2
1D → 3
1T ( 1.01 MeV ) + p+ ( 3.02 MeV ) 50%
(2ii) → 3
2He ( 0.82 MeV ) + n0 ( 2.45 MeV ) 50%
(3) 2
1D + 3
2He → 4
2He ( 3.6 MeV ) + p+ ( 14.7 MeV )
(4) 3
1T + 3
1T → 4
2He + 2 n0 + 11.3 MeV
(5) 3
2He + 3
2He → 4
2He + 2 p+ + 12.9 MeV
(6i) 3
2He + 3
1T → 4
2He + p+ + n0 + 12.1 MeV 51%
(6ii) → 4
2He ( 4.8 MeV ) + 2
1D ( 9.5 MeV ) 43%
(6iii) → 4
2He ( 0.5 MeV ) + n0 ( 1.9 MeV ) + p+ ( 11.9 MeV ) 6%
(7i) 2
1D + 6
3Li → 2 4
2He + 22.4 MeV
(7ii) → 3
2He + 4
2He + n0 + 2.56 MeV
(7iii) → 7
3Li + p+ + 5.0 MeV
(7iv) → 7
4Be + n0 + 3.4 MeV
(8) p+ + 6
3Li → 4
2He ( 1.7 MeV ) + 3
2He ( 2.3 MeV )
(9) 3
2He + 6
3Li → 2 4
2He + p+ + 16.9 MeV
(10) p+ + 11
5B → 3 4
2He + 8.7 MeV

Nucleosynthesis
Stellar nucleosynthesis
Big Bang nucleosynthesis
Supernova nucleosynthesis
Cosmic ray spallation

Related topics
Astrophysics
Nuclear fusion
R-process
S-process
Nuclear fission

edit

For reactions with two products, the energy is divided between them in inverse proportion to their masses, as shown. In most reactions with three products, the distribution of energy varies. For reactions that can result in more than one set of products, the branching ratios are given.

Some reaction candidates can be eliminated at once.[14] The D-6Li reaction has no advantage compared to p+-11
5B because it is roughly as difficult to burn but produces substantially more neutrons through 2
1D-2
1D side reactions. There is also a p+-7
3Li reaction, but the cross section is far too low, except possibly when Ti > 1 MeV, but at such high temperatures an endothermic, direct neutron-producing reaction also becomes very significant. Finally there is also a p+-9
4Be reaction, which is not only difficult to burn, but 9
4Be can be easily induced to split into two alpha particles and a neutron.

In addition to the fusion reactions, the following reactions with neutrons are important in order to "breed" tritium in "dry" fusion bombs and some proposed fusion reactors:

n0 + 6
3Li → 3
1T + 4
2He
n0 + 7
3Li → 3
1T + 4
2He + n0

To evaluate the usefulness of these reactions, in addition to the reactants, the products, and the energy released, one needs to know something about the cross section. Any given fusion device will have a maximum plasma pressure that it can sustain, and an economical device will always operate near this maximum. Given this pressure, the largest fusion output is obtained when the temperature is chosen so that <σv>/T² is a maximum. This is also the temperature at which the value of the triple product nTτ required for ignition is a minimum, since that required value is inversely proportional to <σv>/T² (see Lawson criterion). (A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating.) This optimum temperature and the value of <σv>/T² at that temperature is given for a few of these reactions in the following table.

fuel T [keV] <σv>/T² [m³/s/keV²]
2
1D-3
1T 13.6 1.24×10-24
2
1D-2
1D 15 1.28×10-26
2
1D-3
2He 58 2.24×10-26
p+-6
3Li 66 1.46×10-27
p+-11
5B 123 3.01×10-27

Note that many of the reactions form chains. For instance, a reactor fueled with 3
1T and 3
2He will create some 2
1D, which is then possible to use in the 2
1D-3
2He reaction if the energies are "right". An elegant idea is to combine the reactions (8) and (9). The 3
2He from reaction (8) can react with 6
3Li in reaction (9) before completely thermalizing. This produces an energetic proton which in turn undergoes reaction (8) before thermalizing. A detailed analysis shows that this idea will not really work well, but it is a good example of a case where the usual assumption of a Maxwellian plasma is not appropriate.


[edit] Neutronicity, confinement requirement, and power density

The only fusion reactions thus far produced by humans to achieve ignition are those which have been created in hydrogen bombs, the first of which, Ivy Mike, is shown here.Any of the reactions above can in principle be the basis of fusion power production. In addition to the temperature and cross section discussed above, we must consider the total energy of the fusion products Efus, the energy of the charged fusion products Ech, and the atomic number Z of the non-hydrogenic reactant.

Specification of the 2
1D-2
1D reaction entails some difficulties, though. To begin with, one must average over the two branches (2) and (3). More difficult is to decide how to treat the 3
1T and 3
2He products. 3
1T burns so well in a deuterium plasma that it is almost impossible to extract from the plasma. The 2
1D-3
2He reaction is optimized at a much higher temperature, so the burnup at the optimum 2
1D-2
1D temperature may be low, so it seems reasonable to assume the 3
1T but not the 3
2He gets burned up and adds its energy to the net reaction. Thus we will count the 2
1D-2
1D fusion energy as Efus = (4.03+17.6+3.27)/2 = 12.5 MeV and the energy in charged particles as Ech = (4.03+3.5+0.82)/2 = 4.2 MeV.

Another unique aspect of the 2
1D-2
1D reaction is that there is only one reactant, which must be taken into account when calculating the reaction rate.

With this choice, we tabulate parameters for four of the most important reactions.

fuel Z Efus [MeV] Ech [MeV] neutronicity
2
1D-3
1T 1 17.6 3.5 0.80
2
1D-2
1D 1 12.5 4.2 0.66
2
1D-3
2He 2 18.3 18.3 ~0.05
p+-11
5B 5 8.7 8.7 ~0.001

The last column is the neutronicity of the reaction, the fraction of the fusion energy released as neutrons. This is an important indicator of the magnitude of the problems associated with neutrons like radiation damage, biological shielding, remote handling, and safety. For the first two reactions it is calculated as (Efus-Ech)/Efus. For the last two reactions, where this calculation would give zero, the values quoted are rough estimates based on side reactions that produce neutrons in a plasma in thermal equilibrium.

Of course, the reactants should also be mixed in the optimal proportions. This is the case when each reactant ion plus its associated electrons accounts for half the pressure. Assuming that the total pressure is fixed, this means that density of the non-hydrogenic ion is smaller than that of the hydrogenic ion by a factor 2/(Z+1). Therefore the rate for these reactions is reduced by the same factor, on top of any differences in the values of <σv>/T². On the other hand, because the 2
1D-2
1D reaction has only one reactant, the rate is twice as high as if the fuel were divided between two hydrogenic species.

Thus there is a "penalty" of (2/(Z+1)) for non-hydrogenic fuels arising from the fact that they require more electrons, which take up pressure without participating in the fusion reaction. (It is usually a good assumption that the electron temperature will be nearly equal to the ion temperature. Some authors, however discuss the possibility that the electrons could be maintained substantially colder than the ions. In such a case, known as a "hot ion mode", the "penalty" would not apply.) There is at the same time a "bonus" of a factor 2 for 2
1D-2
1D because each ion can react with any of the other ions, not just a fraction of them.

We can now compare these reactions in the following table.

fuel <σv>/T² penalty/bonus reactivity Lawson criterion power density (W/m3/kPa2) relation of power density
2
1D-3
1T 1.24×10-24 1 1 1 34 1
2
1D-2
1D 1.28×10-26 2 48 30 0.5 68
2
1D-3
2He 2.24×10-26 2/3 83 16 0.43 80
p+-6
3Li 1.46×10-27 1/2 1700 0.005 6800
p+-11
5B 3.01×10-27 1/3 1240 500 0.014 2500

The maximum value of <σv>/T² is taken from a previous table. The "penalty/bonus" factor is that related to a non-hydrogenic reactant or a single-species reaction. The values in the column "reactivity" are found by dividing 1.24 × 10-24 by the product of the second and third columns. It indicates the factor by which the other reactions occur more slowly than the 2
1D-3
1T reaction under comparable conditions. The column "Lawson criterion" weights these results with Ech and gives an indication of how much more difficult it is to achieve ignition with these reactions, relative to the difficulty for the 2
1D-3
1T reaction. The last column is labeled "power density" and weights the practical reactivity with Efus. It indicates how much lower the fusion power density of the other reactions is compared to the 2
1D-3
1T reaction and can be considered a measure of the economic potential.


[edit] Bremsstrahlung losses in quasineutral, isotropic plasmas
The ions undergoing fusion in many systems will essentially never occur alone but will be mixed with electrons that in aggregate neutralize the ions' bulk electrical charge and form a plasma. The electrons will generally have a temperature comparable to or greater than that of the ions, so they will collide with the ions and emit x-ray radiation of 10-30 keV energy (Bremsstrahlung). The Sun and stars are opaque to x-rays, but essentially any terrestrial fusion reactor will be optically thin for x-rays of this energy range. X-rays are difficult to reflect but they are effectively absorbed (and converted into heat) in less than mm thickness of stainless steel (which is part of a reactor's shield). The ratio of fusion power produced to x-ray radiation lost to walls is an important figure of merit. This ratio is generally maximized at a much higher temperature than that which maximizes the power density (see the previous subsection). The following table shows the rough optimum temperature and the power ratio at that temperature for several reactions.[15]

fuel Ti (keV) Pfusion/PBremsstrahlung
2
1D-3
1T 50 140
2
1D-2
1D 500 2.9
2
1D-3
2He 100 5.3
3
2He-3
2He 1000 0.72
p+-6
3Li 800 0.21
p+-11
5B 300 0.57

The actual ratios of fusion to Bremsstrahlung power will likely be significantly lower for several reasons. For one, the calculation assumes that the energy of the fusion products is transmitted completely to the fuel ions, which then lose energy to the electrons by collisions, which in turn lose energy by Bremsstrahlung. However because the fusion products move much faster than the fuel ions, they will give up a significant fraction of their energy directly to the electrons. Secondly, the plasma is assumed to be composed purely of fuel ions. In practice, there will be a significant proportion of impurity ions, which will lower the ratio. In particular, the fusion products themselves must remain in the plasma until they have given up their energy, and will remain some time after that in any proposed confinement scheme. Finally, all channels of energy loss other than Bremsstrahlung have been neglected. The last two factors are related. On theoretical and experimental grounds, particle and energy confinement seem to be closely related. In a confinement scheme that does a good job of retaining energy, fusion products will build up. If the fusion products are efficiently ejected, then energy confinement will be poor, too.

The temperatures maximizing the fusion power compared to the Bremsstrahlung are in every case higher than the temperature that maximizes the power density and minimizes the required value of the fusion triple product. This will not change the optimum operating point for 2
1D-3
1T very much because the Bremsstrahlung fraction is low, but it will push the other fuels into regimes where the power density relative to 2
1D-3
1T is even lower and the required confinement even more difficult to achieve. For 2
1D-2
1D and 2
1D-3
2He, Bremsstrahlung losses will be a serious, possibly prohibitive problem. For 3
2He-3
2He, p+-6
3Li and p+-11
5B the Bremsstrahlung losses appear to make a fusion reactor using these fuels with a quasineutral, anisotropic plasma impossible. Some ways out of this dilemma are considered—and rejected—in Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium by Todd Rider.[16] This limitation does not apply to non-neutral and anisotropic plasmas; however, these have their own challenges to contend with.

Atomic bombings of Hiroshima and Nagasaki

The atomic bombings of Hiroshima and Nagasaki were nuclear attacks near the end of World War II against the Empire of Japan by the United States at the executive order of U.S. President Harry S. Truman on August 6 and 9, 1945, respectively. After six months of intense fire-bombing of 67 other Japanese cities, followed by an ultimatum which was ignored by the Shōwa regime, the nuclear weapon "Little Boy" was dropped on the city of Hiroshima on Monday,[1] August 6, 1945, [2] followed on August 9 by the detonation of the "Fat Man" nuclear bomb over Nagasaki. These are to date the only attacks with nuclear weapons in the history of warfare.[3]

The bombs killed as many as 140,000 people in Hiroshima and 80,000 in Nagasaki by the end of 1945,[4] roughly half on the days of the bombings. Amongst these, 15–20% died from injuries or the combined effects of flash burns, trauma, and radiation burns, compounded by illness and radiation sickness.[5] Since then, more have died from leukemia (231 observed) and solid cancers (334 observed) attributed to exposure to radiation released by the bombs.[6] In both cities, the overwhelming majority of the dead were civilians.[7][8][9]

Six days after the detonation over Nagasaki, on August 15, Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2, officially ending the Pacific War and therefore World War II. (Germany had signed its unavoidable[2] Instrument of Surrender on May 7, ending the war in Europe.) The bombings led, in part, to post-war Japan adopting Three Non-Nuclear Principles, forbidding that nation from nuclear armament.[10]

The Manhattan Project
Main article: Manhattan Project
The United States, with assistance from the United Kingdom and Canada in their respective secret projects Tube Alloys and Chalk River Laboratories,[11] designed and built the first atomic bombs under what was called the Manhattan Project. The scientific research was directed by American physicist J. Robert Oppenheimer. The Hiroshima bomb, a gun-type bomb called "Little Boy", was made with uranium-235, a rare isotope of uranium. The atomic bomb was first tested at Trinity Site, on July 16, 1945, near Alamogordo, New Mexico. The test weapon, "the gadget," and the Nagasaki bomb, "Fat Man", were both implosion-type devices made primarily of plutonium-239, a synthetic element.[12]


Choice of targets

Map showing the locations of Hiroshima and Nagasaki, Japan where the two atomic weapons were employedOn May 10–11, 1945 The Target Committee at Los Alamos, led by J. Robert Oppenheimer, recommended Kyoto, Hiroshima, Yokohama, and the arsenal at Kokura as possible targets. The target selection was subject to the following criteria:

They are larger than three miles in diameter and are important targets in a large urban area.
The blast would create effective damage.
They are unlikely to be attacked by August 1945. "Any small and strictly military objective should be located in a much larger area subject to blast damage in order to avoid undue risks of the weapon being lost due to bad placing of the bomb."
These cities were largely untouched during the nightly bombing raids and the Army Air Force agreed to leave them off the target list so accurate assessment of the weapon could be made. Hiroshima was described as "an important army depot and port of embarkation in the middle of an urban industrial area. It is a good radar target and it is such a size that a large part of the city could be extensively damaged. There are adjacent hills which are likely to produce a focussing effect which would considerably increase the blast damage. Due to rivers it is not a good incendiary target." The goal of the weapon was to convince Japan to surrender unconditionally in accordance with the terms of the Potsdam Declaration. The Target Committee stated that "It was agreed that psychological factors in the target selection were of great importance. Two aspects of this are (1) obtaining the greatest psychological effect against Japan and (2) making the initial use sufficiently spectacular for the importance of the weapon to be internationally recognized when publicity on it is released. In this respect Kyoto has the advantage of the people being more highly intelligent and hence better able to appreciate the significance of the weapon. Hiroshima has the advantage of being such a size and with possible focussing from nearby mountains that a large fraction of the city may be destroyed. The Emperor's palace in Tokyo has a greater fame than any other target but is of least strategic value."[13]

During World War II, Edwin O. Reischauer was the Japan expert for the US Army Intelligence Service, in which role he is incorrectly said to have prevented the bombing of Kyoto.[14] In his autobiography, Reischauer specifically refuted the validity of this broadly-accepted claim:

"...the only person deserving credit for saving Kyoto from destruction is Henry L. Stimson, the Secretary of War at the time, who had known and admired Kyoto ever since his honeymoon there several decades earlier."[15]

The Potsdam ultimatum
On July 26, Truman and other allied leaders issued The Potsdam Declaration outlining terms of surrender for Japan. It was presented as an ultimatum and stated that without a surrender, the Allies would attack Japan, resulting in "the inevitable and complete destruction of the Japanese armed forces and just as inevitably the utter devastation of the Japanese homeland" but the atomic bomb was not mentioned. On July 28, Japanese papers reported that the declaration had been rejected by the Japanese government. That afternoon, Prime Minister Kantaro Suzuki declared at a press conference that the Potsdam Declaration was no more than a rehash (yakinaoshi) of the Cairo Declaration and that the government intended to ignore it (mokusatsu lit. "kill by silence").[16] The statement was taken by both Japanese and foreign papers as a clear rejection of the declaration. Emperor Hirohito, who was waiting for a Soviet reply to noncommittal Japanese peace feelers made no move to change the government position.[17] On July 31, he made clear to his advisor Kōichi Kido that the Imperial Regalia of Japan had to be defended at all costs.[18]

In early July, on his way to Potsdam, Truman had re-examined the decision to use the bomb. In the end, Truman made the decision to drop the atomic bombs on Japan. His stated intention in ordering the bombings was to bring about a quick resolution of the war by inflicting destruction and instilling fear of further destruction in sufficient strength to cause Japan to surrender.[19]


Hiroshima

Hiroshima during World War II
At the time of its bombing, Hiroshima was a city of some industrial and military significance. A number of military camps were located nearby, including the headquarters of the Fifth Division and Field Marshal Shunroku Hata's 2nd General Army Headquarters, which commanded the defense of all of southern Japan. Hiroshima was a minor supply and logistics base for the Japanese military. The city was a communications center, a storage point, and an assembly area for troops. It was one of several Japanese cities left deliberately untouched by American bombing, allowing a pristine environment to measure the damage caused by the atomic bomb.[citation needed]


A postwar "Little Boy" casing mockupThe center of the city contained several reinforced concrete buildings and lighter structures. Outside the center, the area was congested by a dense collection of small wooden workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were of wooden construction with tile roofs, and many of the industrial buildings also were of wood frame construction. The city as a whole was highly susceptible to fire damage.

The population of Hiroshima had reached a peak of over 381,000 earlier in the war, but prior to the atomic bombing the population had steadily decreased because of a systematic evacuation ordered by the Japanese government. At the time of the attack the population was approximately 255,000.[citation needed] This figure is based on the registered population used by the Japanese in computing ration quantities, and the estimates of additional workers and troops who were brought into the city may be inaccurate.


The bombing
For the composition of the USAAF mission see 509th Composite Group.

Seizo Yamada's ground level photo taken from approximately 7 km northeast of Hiroshima.Hiroshima was the primary target of the first nuclear bombing mission on August 6, with Kokura and Nagasaki being alternative targets. August 6 was chosen because there had previously been cloud cover over the target. The 393d Bombardment Squadron B-29 Enola Gay, piloted and commanded by 509th Composite Group commander Colonel Paul Tibbets, was launched from North Field airbase on Tinian in the West Pacific, about six hours flight time from Japan. The Enola Gay (named after Colonel Tibbets' mother) was accompanied by two other B29s, The Great Artiste which carried instrumentation, commanded by Major Charles W. Sweeney, and a then-nameless aircraft later called Necessary Evil (the photography aircraft) commanded by Captain George Marquardt.[20]

After leaving Tinian the aircraft made their way separately to Iwo Jima where they rendezvoused at 2,440 metres (8,000 ft) and set course for Japan. The aircraft arrived over the target in clear visibility at 9,855 metres (32,330 ft). On the journey, Navy Captain William Parsons had armed the bomb, which had been left unarmed to minimize the risks during takeoff. His assistant, 2nd Lt. Morris Jeppson, removed the safety devices 30 minutes before reaching the target area.[21]


Hiroshima, in the aftermath of the bombingThe release at 08:15 (Hiroshima time) was uneventful, and the gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 57 seconds to fall from the aircraft to the predetermined detonation height about 600 metres (2,000 ft) above the city. Due to crosswind, it missed the aiming point, the Aioi Bridge, by almost 800 feet (240 m) and detonated directly over Shima Surgical Clinic.[22] It created a blast equivalent to about 13 kilotons of TNT. (The U-235 weapon was considered very inefficient, with only 1.38% of its material fissioning.)[23] The radius of total destruction was about one mile (1.6 km), with resulting fires across 4.4 square miles (11 km2).[24] Americans estimated that 4.7 square miles (12 km2) of the city were destroyed. Japanese officials determined that 69% of Hiroshima's buildings were destroyed and another 6–7% damaged.[5]

70,000–80,000 people, or some 30%[25] of the population of Hiroshima were killed immediately, and another 70,000 injured.[26] Over 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured; most had been in the downtown area subject to the greatest damage.[27]


The energy released by the bomb was powerful enough to burn through clothing. The dark portions of the garments this victim wore at the time of the blast were emblazoned on to the flesh as scars, while skin underneath the lighter parts (which absorb less energy) was not damaged as badly.[28]Although the United States had previously dropped leaflets warning civilians of air raids on twelve other Japanese cities,[29] the residents of Hiroshima were given no notice of the atomic bomb.[30][31][32]

About an hour before the bombing, Japanese early warning radar detected the approach of some American aircraft headed for the southern part of Japan. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small—probably not more than three—and the air raid alert was lifted. To conserve fuel and aircraft, the Japanese had decided not to intercept small formations. The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29s were actually sighted, but no raid was expected beyond some sort of reconnaissance.


Japanese realization of the bombing
The Tokyo control operator of the Japanese Broadcasting Corporation noticed that the Hiroshima station had gone off the air. He tried to re-establish his program by using another telephone line, but it too had failed.[33] About twenty minutes later the Tokyo railroad telegraph center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within 16 kilometers (10 mi) of the city came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the headquarters of the Japanese General Staff.

Military bases repeatedly tried to call the Army Control Station in Hiroshima. The complete silence from that city puzzled the men at headquarters; they knew that no large enemy raid had occurred and that no sizeable store of explosives was in Hiroshima at that time. A young officer of the Japanese General Staff was instructed to fly immediately to Hiroshima, to land, survey the damage, and return to Tokyo with reliable information for the staff. It was generally felt at headquarters that nothing serious had taken place and that it was all a rumor.

The staff officer went to the airport and took off for the southwest. After flying for about three hours, while still nearly one hundred miles (160 km) from Hiroshima, he and his pilot saw a great cloud of smoke from the bomb. In the bright afternoon, the remains of Hiroshima were burning. Their plane soon reached the city, around which they circled in disbelief. A great scar on the land still burning and covered by a heavy cloud of smoke was all that was left. They landed south of the city, and the staff officer, after reporting to Tokyo, immediately began to organize relief measures.

By August 8, 1945, newspapers in the US were reporting that broadcasts from Radio Tokyo had described the destruction observed in Hiroshima. "Practically all living things, human and animal, were literally seared to death," Japanese radio announcers said in a broadcast captured by Allied sources.[34]


Post-attack casualties
According to most estimates, the immediate effects of the blast killed approximately 70,000 people in Hiroshima. Estimates of total deaths by the end of 1945 from burns, radiation and related disease, the effects of which were aggravated by lack of medical resources, range from 90,000 to 140,000.[4][35] Some estimates state up to 200,000 had died by 1950, due to cancer and other long-term effects.[1][7][36] Actually, from 1950 to 1990, roughly 9% of the cancer and leukemia deaths among bomb survivors was due to radiation from the bombs, the statistical excess being estimated to 89 leukemia and 339 solid cancers.[37] At least eleven known prisoners of war died from the bombing.[38]





Survival of some structures

Above shows a small-scale recreation of the Nakajima area around ground zero. There remains modern "Rest House" (right) and a few structures.Some of the reinforced concrete buildings in Hiroshima were very strongly constructed because of the earthquake danger in Japan, and their framework did not collapse even though they were fairly close to the center of damage in the city. Eizo Nomura (野村 英三, Nomura Eizō?) was the closest known survivor, who was in the basement of a modern "Rest House" only 100 m (330 ft) from ground-zero at the time of the attack.[39] Akiko Takakura (高蔵 信子, Takakura Akiko?) was among the closest survivors to the hypocenter of the blast. She had been in the solidly built Bank of Hiroshima only 300 metres (980 ft) from ground-zero at the time of the attack.[40] Since the bomb detonated in the air, the blast was more downward than sideways, which was largely responsible for the survival of the Prefectural Industrial Promotional Hall, now commonly known as the Genbaku, or A-bomb Dome designed and built by the Czech architect Jan Letzel, which was only 150 m (490 ft) from ground zero (the hypocenter). The ruin was named Hiroshima Peace Memorial and was made a UNESCO World Heritage site in 1996 over the objections of the U.S. and China.[41]


Events of August 7-9
Truman announcing the bombing of Hiroshima

President Truman announces the bombing of Hiroshima.

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After the Hiroshima bombing, President Truman announced, "If they do not now accept our terms, they may expect a rain of ruin from the air the likes of which has never been seen on this earth."

The Japanese government still did not react to the Potsdam Declaration. Emperor Hirohito, the government and the War council were considering four conditions for surrender: the preservation of the kokutai (Imperial institution and national polity), assumption by the Imperial Headquarters of responsibility for disarmament and demobilization, no occupation, and delegation to the Japanese government of the punishment of war criminals.

The Soviet Foreign Minister Molotov had informed Tokyo of the Soviet Union's unilateral abrogation of the Soviet-Japanese Neutrality Pact on April 5. At two minutes past midnight on August 9, Tokyo time, Soviet infantry, armor, and air forces had launched Manchurian Strategic Offensive Operation. Four hours later, word reached Tokyo that the Soviet Union had declared war on Japan. The senior leadership of the Japanese Army began preparations to impose martial law on the nation, with the support of Minister of War Korechika Anami, in order to stop anyone attempting to make peace.

Responsibility for the timing of the second bombing was delegated to Colonel Tibbets as commander of the 509th Composite Group on Tinian. Scheduled for August 11 against Kokura, the raid was moved forward to avoid a five-day period of bad weather forecast to begin on August 10.[42] Three bomb pre-assemblies had been transported to Tinian, labeled F-31, F-32, and F-33 on their exteriors. On August 8 a dress rehearsal was conducted off Tinian by Maj. Charles Sweeney using Bockscar as the drop airplane. Assembly F-33 was expended testing the components and F-31 was designated for the mission August 9.[43]


Nagasaki

Nagasaki during World War II

Urakami Tenshudo (Catholic Church in Nagasaki) in January 1946, destroyed by the atomic bomb, the dome of the church having toppled off.The city of Nagasaki had been one of the largest sea ports in southern Japan and was of great wartime importance because of its wide-ranging industrial activity, including the production of ordnance, ships, military equipment, and other war materials.

In contrast to many modern aspects of Hiroshima, the bulk of the residences were of old-fashioned Japanese construction, consisting of wood or wood-frame buildings, with wood walls (with or without plaster), and tile roofs. Many of the smaller industries and business establishments were also housed in buildings of wood or other materials not designed to withstand explosions. Nagasaki had been permitted to grow for many years without conforming to any definite city zoning plan; residences were erected adjacent to factory buildings and to each other almost as closely as possible throughout the entire industrial valley.

Nagasaki had never been subjected to large-scale bombing prior to the explosion of a nuclear weapon there. On August 1, 1945, however, a number of conventional high-explosive bombs were dropped on the city. A few hit in the shipyards and dock areas in the southwest portion of the city, several hit the Mitsubishi Steel and Arms Works and six bombs landed at the Nagasaki Medical School and Hospital, with three direct hits on buildings there. While the damage from these bombs was relatively small, it created considerable concern in Nagasaki and many people—principally school children—were evacuated to rural areas for safety, thus reducing the population in the city at the time of the nuclear attack.

To the north of Nagasaki there was a camp holding British Commonwealth prisoners of war, some of whom were working in the coal mines and only found out about the bombing when they came to the surface. At least eight known POWs died from the bombing and as many as thirteen POWs may have died:

One British Commonwealth.[44][45][46][47][48]
Seven Dutch (two names known)[49] died in the bombing.
At least two POWs reportedly died postwar from cancer thought to have been caused by Atomic bomb.[50][51]

The bombing
For the composition of the USAAF mission see 509th Composite Group.

A post-war "Fat Man" modelOn the morning of August 9, 1945, the U.S. B-29 Superfortress Bockscar, flown by the crew of 393rd Squadron commander Major Charles W. Sweeney, carried the nuclear bomb code-named "Fat Man", with Kokura as the primary target and Nagasaki the secondary target. The mission plan for the second attack was nearly identical to that of the Hiroshima mission, with two B-29s flying an hour ahead as weather scouts and two additional B-29s in Sweeney's flight for instrumentation and photographic support of the mission. Sweeney took off with his weapon already armed but with the electrical safety plugs still engaged.[52]


Illustration of the implosion concept employed in "Fat Man".Observers aboard the weather planes reported both targets clear. When Sweeney's aircraft arrived at the assembly point for his flight off the coast of Japan, the third plane, Big Stink, flown by the group's Operations Officer, Lt. Col. James I. Hopkins, Jr. failed to make the rendezvous. Bockscar and the instrumentation plane circled for forty minutes without locating Hopkins. Already 30 minutes behind schedule, Sweeney decided to fly on without Hopkins.[52]

By the time they reached Kokura a half hour later, a 70% cloud cover had obscured the city, prohibiting the visual attack required by orders. After three runs over the city, and with fuel running low because a transfer pump on a reserve tank had failed before take-off, they headed for their secondary target, Nagasaki.[52] Fuel consumption calculations made en route indicated that Bockscar had insufficient fuel to reach Iwo Jima and they would be forced to divert to Okinawa. After initially deciding that if Nagasaki were obscured on their arrival they would carry the bomb to Okinawa and dispose of it in the ocean if necessary, the weaponeer Navy Commander Frederick Ashworth decided that a radar approach would be used if the target was obscured.[53]


Nagasaki before and after bombingAt about 07:50 Japanese time, an air raid alert was sounded in Nagasaki, but the "all clear" signal was given at 08:30. When only two B-29 Superfortresses were sighted at 10:53, the Japanese apparently assumed that the planes were only on reconnaissance and no further alarm was given.

A few minutes later, at 11:00, The Great Artiste, the support B-29 flown by Captain Frederick C. Bock dropped instruments attached to three parachutes. These instruments also contained an unsigned letter to Professor Ryokichi Sagane, a nuclear physicist at the University of Tokyo who studied with three of the scientists responsible for the atomic bomb at the University of California, Berkeley, urging him to tell the public about the danger involved with these weapons of mass destruction. The messages were found by military authorities but not turned over to Sagane until a month later.[54] In 1949 one of the authors of the letter, Luis Alvarez, met with Sagane and signed the document.[55]

At 11:01, a last minute break in the clouds over Nagasaki allowed Bockscar's bombardier, Captain Kermit Beahan, to visually sight the target as ordered. The "Fat Man" weapon, containing a core of ~6.4 kg (14.1 lbs.) of plutonium-239, was dropped over the city's industrial valley. Forty-three seconds later it exploded 469 meters (1,540 ft) above the ground exactly halfway between the Mitsubishi Steel and Arms Works in the south and the Mitsubishi-Urakami Ordnance Works (Torpedo Works) in the north. This was nearly 3 kilometers (2 mi) northwest of the planned hypocenter; the blast was confined to the Urakami Valley and a major portion of the city was protected by the intervening hills.[56] The resulting explosion had a blast yield equivalent to 21 kilotons of TNT. The explosion generated heat estimated at 3,900 degrees Celsius (7,000 degrees Fahrenheit) and winds that were estimated at 1005 km/h (624 mph).


A Japanese report on the bombing characterized Nagasaki as "like a graveyard with not a tombstone standing".Casualty estimates for immediate deaths range from 40,000 to 75,000.[57][58][59] Total deaths by the end of 1945 may have reached 80,000.[4] The radius of total destruction was about a mile (1.6 km), followed by fires across the northern portion of the city to two miles (3.2 km) south of the bomb.[60][61]

An unknown number of survivors from the Hiroshima bombing had made their way to Nagasaki, where they were bombed again.[62][63]


Plans for more atomic attacks on Japan
The United States expected to have another atomic bomb ready for use in the third week of August, with three more in September and a further three in October.[64] On August 10, Major General Leslie Groves, military director of the Manhattan Project, sent a memorandum to General of the Army George Marshall, Chief of Staff of the United States Army, in which he wrote that "the next bomb . . should be ready for delivery on the first suitable weather after 17 or August 18." On the same day, Marshall endorsed the memo with the comment, "It is not to be released over Japan without express authority from the President."[64] There was already discussion in the War Department about conserving the bombs in production until Operation Downfall, the projected invasion of Japan, had begun. "The problem now [August 13] is whether or not, assuming the Japanese do not capitulate, to continue dropping them every time one is made and shipped out there or whether to hold them . . . and then pour them all on in a reasonably short time. Not all in one day, but over a short period. And that also takes into consideration the target that we are after. In other words, should we not concentrate on targets that will be of the greatest assistance to an invasion rather than industry, morale, psychology, and the like? Nearer the tactical use rather than other use."[64]


The surrender of Japan and subsequent occupation
Main articles: Surrender of Japan and Occupation of Japan
Up to August 9, the war council was still insisting on its four conditions for surrender. On that day Hirohito ordered Kido to "quickly control the situation" "because Soviet Union has declared war against us". He then held an Imperial conference during which he authorized minister Tōgō to notify the Allies that Japan would accept their terms on one condition, that the declaration "does not compromise any demand which prejudices the prerogatives of His Majesty as a Sovereign ruler".[65]

On August 12, the Emperor informed the imperial family of his decision to surrender. One of his uncles, Prince Asaka, then asked whether the war would be continued if the kokutai could not be preserved. Hirohito simply replied "of course".[66] As the Allied terms seemed to leave intact the principle of the preservation of the Throne, Hirohito recorded on August 14 his capitulation announcement which was broadcast to the Japanese nation the next day despite a short rebellion by militarists opposed to the surrender.

In his declaration, Hirohito referred to the atomic bombings :

“ Moreover, the enemy now possesses a new and terrible weapon with the power to destroy many innocent lives and do incalculable damage. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.
Such being the case, how are We to save the millions of Our subjects, or to atone Ourselves before the hallowed spirits of Our Imperial Ancestors? This is the reason why We have ordered the acceptance of the provisions of the Joint Declaration of the Powers.
”

In his "Rescript to the soldiers and sailors" delivered on August 17, he stressed the impact of the Soviet invasion and his decision to surrender, omitting any mention of the bombs.

During the year after the bombing, approximately 40,000 U.S. occupation troops were in Hiroshima. Nagasaki was occupied by 27,000 troops.


Atomic Bomb Casualty Commission
In the spring of 1948, the Atomic Bomb Casualty Commission (ABCC) was established in accordance with a presidential directive from Harry S. Truman to the National Academy of Sciences-National Research Council to conduct investigations of the late effects of radiation among the survivors in Hiroshima and Nagasaki. Among the casualties were found many unintended victims including:

Allied POWs.
Korean and Chinese labourers.
Students from Malaya on scholarships.
Some 3,200 Japanese American citizens.[67]
One of the early studies conducted by the ABCC was on the outcome of pregnancies occurring in Hiroshima and Nagasaki, and in a control city, Kure located 18 miles (29 km) south from Hiroshima, to discern the conditions and outcomes related to radiation exposure. Some would say ABCC was not in a position to offer medical treatment to the survivors except in a research capacity. One author has claimed that the ABCC refused to provide medical treatment to the survivors for better research results.[68] In 1975, the Radiation Effects Research Foundation was created to assume the responsibilities of ABCC. [69]


The hibakusha
Main article: hibakusha

Panoramic view of the monument marking the hypocentre, or ground zero, of the atomic bomb explosion over Nagasaki.
Citizens of Hiroshima walk by the Hiroshima Peace Memorial, the closest building to have survived the city's atomic bombing.The surviving victims of the bombings are called hibakusha (被爆者?), a Japanese word that literally translates to "explosion-affected people." The suffering of the bombing has led Japan to seek the abolition of nuclear weapons from the world ever since, exhibiting one of the world's firmest non-nuclear policies. As of March 31, 2008[update], 243,692 hibakusha were recognized by the Japanese government, most living in Japan.[70] The government of Japan recognizes about 1% of these as having illnesses caused by radiation.[71] The memorials in Hiroshima and Nagasaki contain lists of the names of the hibakusha who are known to have died since the bombings. Updated annually on the anniversaries of the bombings, as of August 2008[update] the memorials record the names of more than 400,000 hibakusha—258,310[72] in Hiroshima and 145,984[73] in Nagasaki.


Korean survivors
During the war Japan brought many Korean conscripts to both Hiroshima and Nagasaki to work as forced labor. According to recent estimates, about 20,000 Koreans were killed in Hiroshima and about 2,000 died in Nagasaki. It is estimated that one in seven of the Hiroshima victims was of Korean ancestry.[8] For many years, Koreans had a difficult time fighting for recognition as atomic bomb victims and were denied health benefits. However, most issues have been addressed in recent years through lawsuits.[74]


Double survivor
Main article: Tsutomu Yamaguchi
On March 24, 2009, the Japanese government recognized Tsutomu Yamaguchi as a double hibakusha. Tsutomu Yamaguchi was confirmed to be 3 kilometers from ground zero in Hiroshima on a business trip when the bomb was detonated. He was seriously burnt on his left side and spent the night in Hiroshima. He got back to his home city of Nagasaki on August 8, a day before the bomb in Nagasaki was dropped and was exposed to residual radiation while searching for his relatives. He is the first confirmed survivor of both bombings.[75]


Debate over bombings
Main article: Debate over the atomic bombings of Hiroshima and Nagasaki
Further information: Operation Downfall
“ The atomic bomb was more than a weapon of terrible destruction; it was a psychological weapon. ”
— Former U.S. Secretary of War Henry L. Stimson, 1947[76]

The role of the bombings in Japan's surrender and the United States' ethical justification for them has been the subject of scholarly and popular debate for decades. J. Samuel Walker wrote in an April 2005 overview of recent historiography on the issue, "the controversy over the use of the bomb seems certain to continue." Walker noted that "The fundamental issue that has divided scholars over a period of nearly four decades is whether the use of the bomb was necessary to achieve victory in the war in the Pacific on terms satisfactory to the United States."[77]

Supporters of the bombings generally assert that they caused the Japanese surrender, preventing massive casualties (some estimates put Allied casualties at 1 million, Japanese casualties would be in the millions[78]) on both sides in the planned invasion of Japan: Kyūshū was to be invaded in October 1945 and Honshū five months later; and those who oppose the bombings argue that it was inherently immoral, a war crime, a form of state terrorism,[79] or militarily unnecessary.[80]