Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Fission can be induced by several methods, including bombarding the nucleus of a fissile atom with another particle of the correct energy. Usually the other particle is a free neutron moving at the right speed. This free neutron is absorbed by the nucleus, making the nucleus unstable (much like a greengrocer's pyramid of oranges becomes unstable if someone throws another orange at it at the right speed). The unstable nucleus will then split into two or more pieces. These pieces are known as fission products and include two smaller nuclei, two or three other free neutrons, and some photons. The process releases a lot of energy compared to chemical reactions; the energy is released in the form of both photon radiation (like gamma rays) and in the kinetic energy (energy of motion) of the nuclei and neutrons. A typical energy released by a fission event is approximately 200 MeV.

The atomic nuclei released as fission products are of various chemical elements. Which elements are produced is somewhat random, but each nuclei usually ends up with about half the protons and neutrons of the original fissioned atom. Fission products are usually highly radioactive since these other nuclei are not stable isotopes. These isotopes then decay, releasing gamma rays and beta decay radiation. These highly radioactive fission products (or the waste products of their decay, as many of the initial fission products are highly unstable with very short decay times) become the waste of nuclear energy production.

An example of a nuclear fission reaction is as follows:

Neutron + U-235 --> fission products + more neutrons + energy

History

The process was discovered in 1939 by Otto Hahn, Lise Meitner and coworkers at the Kaiser-Wilhelm-Institute for Chemistry in Berlin.

The results of the bombardment of uranium by neutrons had proved interesting and puzzling. First studied by Fermi and his colleagues in 1934, they were not properly interpreted until several years later.

On January 16 1939, Niels Bohr of Copenhagen, Denmark, arrived in the United States to spend several months in Princeton, N. J., and was particularly anxious to discuss some abstract problems with Albert Einstein. (Four years later Bohr was to escape to Sweden from Nazi-occupied Denmark in a small boat, along with thousands of other Danish Jews, in large scale operation.) Just before Bohr left Denmark, two of his colleagues, Otto Robert Frisch and Lise Meitner (both refugees from Germany), had told him their guess that the absorption of a neutron by a uranium nucleus sometimes caused that nucleus to split into approximately equal parts with the release of enormous quantities of energy, a process that they dubbed nuclear "fission."

The occasion for this hypothesis was the important discovery of Otto Hahn and Fritz Strassmann in Germany (published in Naturwissenschaften in early January 1939) which proved that an isotope of barium was produced by neutron bombardment of uranium. Bohr had promised to keep the Meitner/Frisch interpretation secret until their paper was published to preserve priority, but on the boat he discussed it with Leon Rosenfeld, but forgot to tell him to keep it secret. Rosenfeld immediately upon arrival told everyone at Princeton University, and from them the news spread by word of mouth to neighboring physicists including Enrico Fermi at Columbia University. As a result of conversations among Fermi, John R. Dunning, and G. B. Pegram, a search was undertaken at Columbia for the heavy pulses of ionization that would be expected from the flying fragments of the uranium nucleus. On January 26, 1939, there was a conference on theoretical physics at Washington, D. C., sponsored jointly by the George Washington University and the Carnegie Institution of Washington.

Fermi left New York to attend this meeting before the Columbia fission experiments had been tried. At the meeting Bohr and Fermi discussed the problem of fission, and in particular Fermi mentioned the possibility that neutrons might be emitted during the process. Although this was only a guess, its implication of the possibility of a chain reaction was obvious. A number of sensational articles were published in the press on this subject. Before the meeting in Washington was over, several other experiments to confirm fission had been initiated, and positive experimental confirmation was reported from four laboratories (Columbia University, Carnegie Institution of Washington, Johns Hopkins University, University of California) in the February 15 1939, issue of the Physical Review. By this time Bohr had heard that similar experiments had been made in his laboratory in Copenhagen about January 15. (Letter by Frisch to Nature dated January 16 1939, and appearing in the February 18 issue.) Frédéric Joliot in Paris had also published his first results in the Comptes Rendus of January 30 1939. From this time on there was a steady flow of papers on the subject of fission, so that by the time (December 6 1939) L. A. Turner of Princeton wrote a review article on the subject in the Reviews of Modern Physics nearly one hundred papers had appeared. Complete analysis and discussion of these papers have appeared in Turner's article and elsewhere.

Inducing fission

• Though fission is most often and most easily initiated by the absorption of a free neutron, it can also be induced by striking a fissionable nucleus with other particles. These other particles can include protons, other nuclei, or even high intensity high-energy photons (gamma rays).
• Very infrequently, a fissionable nucleus will undergo spontaneous nuclear fission without an incoming neutron.
• Inducing fission is easiest in heavy elements and generally, the more massive the nucleus the more likely it is able to be fissioned. Fission in any element heavier than iron produces energy, and fission in any element lighter than iron requires energy. The opposite is true of nuclear fusion reactions - fusion in elements lighter than iron produces energy, and fusion in elements heavier than iron requires energy. (see: the curve of binding energy)
• The most frequently used elements to produce nuclear fission are uranium and plutonium. Uranium is the heaviest naturally occurring element in any appreciable abundance; plutonium undergoes spontaneous fission reactions and has a relatively short half-life. So, although other elements can be used, these have the best combination of abundance and ease of fission. For further detail, see fissile.

Critical mass

When fission events occur in a mass of uranium (or other fissile material), neutrons are released. Some of these neutrons are captured by other uranium nuclei and lead to fission; some will escape the mass or be absorbed by some other kind of nucleus. If the expected number of neutrons which trigger new fissions is less than one, a nuclear chain reaction may occur but the size will decrease exponentially. If the expected number of neutrons is greater than one, the chain reaction will increase exponentially. This situation (expected number of neutrons causing fission is one or more) is called criticality, and the configuration is called a critical mass (although strictly speaking the shape is as important a factor as the mass; see below).

While any critical mass will in principle lead to exponential growth, the time this will take depends on several factors. The degree to which the mass is supercritical affects the rate of growth. However, as mentioned above, a fraction of the neutrons that cause fission do so only after a brief delay. This delay slows the process of exponential growth and permits the control of nuclear chain reactions. If there are enough neutrons captured so that the ones causing immediate fission are sufficient to lead to exponential growth, then the mass is called prompt critical and it becomes very difficult to control.

A simple nuclear weapon relies on this exponential growth to induce fission in a significant fraction of the fissile nuclei it contains. Such a device must not only be prompt critical, it must be highly prompt critical. Moreover, it must be rapidly converted from a subcritical configuration (for storage) to a highly prompt critical configuration upon detonation. This is a difficult procedure; see nuclear weapon design for an overview.

The relative number of neutrons which escape from a quantity of uranium can be minimized by changing the size and shape. In a sphere any surface effect is proportional to the square of the radius, and any volume effect is proportional to the cube of the radius. Now the escape of neutrons from a quantity of uranium is a surface effect depending on the area of the surface, but fission capture occurs throughout the material and is therefore a volume effect. Consequently the greater the amount of uranium, the less probable it is that neutron escape will predominate over fission capture and prevent a chain reaction. Loss of neutrons by non-fission capture is a volume effect like neutron production by fission capture, so that increase in size makes no change in its relative importance..

Effects of isotopes

Natural uranium contains three isotopes: U-234 (0.006%), U-235 (0.7%), and U-238 (99.3%). The speed required for a fission event vs. non-fission capture event is different for different isotopes.

U-238 tends to capture intermediate speed neutrons (creating U-239, not fission). High speed neutrons tend to have inelastic collisions with U-238, which just slow down the neutrons. Thus, U-238 tends both to reduce the speed of the fast neutrons and then capture them when they get to an intermediate speed.

U-235 fissions with a much wider range of neutron speeds than U-238. Since U-238 affects many neutrons without inducing fission, having it in the mix is bad for promoting fission. So, if we separate the U-235 from the U-238 and discard the U-238 (producing enriched uranium), we promote a chain reaction. In fact, the probability of fission of U-235 by high speed neutrons may be great enough to make the use of a moderator unnecessary once the U-238 has been removed.

U-235 is present in natural uranium only to the extent of about one part in 140. Also, the relatively small difference in mass between the two isotopes makes isotope separation difficult. Nevertheless, the possibility of separating U-235 was recognized early on in the Manhattan Project as being of the greatest importance to their success.

Moderators

Thermal neutrons (that is, slow neutrons) have the highest probability of producing fission of U-235 but the neutrons emitted in the process of fission have high speeds (they are not thermal). It is an oversimplification to say that the chain reaction might maintain itself if more neutrons were created by fission than were absorbed, because the probability both of fission capture and of non-fission capture depends on the speed of the neutrons. Unfortunately, the speed at which non-fission capture is most probable is intermediate between the average speed of neutrons emitted in the fission process and the speed at which fission capture is most probable.

For some years before the discovery of fission, the customary way of slowing down neutrons was to cause them to pass through material of low atomic weight, such as hydrogenous material. The process of slowing down or moderation is simply one of elastic collisions between high speed particles and particles practically at rest. The more nearly identical the masses of neutron and struck particle, the greater the loss of kinetic energy by the neutron. Therefore light elements are most effective as neutron moderators.

It occurred to a number of physicists that it might be possible to mix uranium with a moderator in such a way that the high speed fission neutrons, after being ejected from uranium and before re-encountering uranium nuclei, would have their speeds reduced below the speeds for which non-fission capture is highly probable. The characteristics of a good moderator are that it should be of low atomic weight and that it should have little or no tendency to absorb neutrons. Lithium and boron are excluded on the latter count. Helium is difficult to use because it is a gas and forms no compounds. The choice of moderator therefore lay (and still may lie) among hydrogen, deuterium, beryllium, and carbon. It was Enrico Fermi and Leó Szilárd who first proposed the use of graphite (a form of carbon) as a moderator for a chain reaction.

Reduction of non-fission capture by isotope separation

An additional complication is that natural uranium contains three isotopes: U-234, U-235, and U-238, present to the extent of approximately 0.006, 0.7, and 99.3 per cent, respectively. We have already seen that the probabilities of processes (2) and (4) are different for different isotopes. We have also seen that the probabilities are different for neutrons of different energies.

For neutrons of certain intermediate speeds (corresponding to energies of a few electron volts) U-238 has a large capture cross section for the production of U-239 but not for fission. There is also a considerable probability of inelastic (i.e., non-capture-producing) collisions between high speed neutrons and U-238 nuclei. Thus the presence of the U-238 tends both to reduce the speed of the fast neutrons and to effect the capture of those of moderate speed. Although there may be some non-fission capture by U-235, it is evident that if we can separate the U-235 from the U-238 and discard the U-238, we can reduce non-fission capture and can thus promote the chain reaction. In fact, the probability of fission of U-235 by high speed neutrons may be great enough to make the use of a moderator unnecessary once the U-238 has been removed. Unfortunately, U-235 is present in natural uranium only to the extent of about one part in 140. Also, the relatively small difference in mass between the two isotopes makes separation difficult. Nevertheless, the possibility of separating U-235 was recognized early on in the Manhattan Project as being of the greatest importance.

Production and purification of materials

It has been stated above that the cross section for capture of neutrons varies greatly among different materials. In some it is very high compared to the maximum fission cross section of uranium. If, then, we are to hope to achieve a chain reaction, we must reduce effect (3) - non-fission capture by impurities -to the point where it is not serious. This means very careful purification of the uranium metal and very careful purification of the moderator. Calculations show that the maximum permissible concentrations of many impurity elements are a few parts per million- in either the uranium or the moderator. When it is mentioned that 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, that the total amount of metallic beryllium produced in the USA was not more than a few kilograms, that the total amount of concentrated deuterium produced was not more than a few kilograms, and that carbon had never been produced in quantity with anything like the purity required of a moderator, it is clear that the problem of producing and purifying materials was a major one.

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 uranium that would be necessary for the research to come.

Control - weapons or power?

The problems that have been discussed so far have to do merely with the realization of the chain reaction. If such a reaction is going to be of use, we must be able to control it. The problem of control is different depending on whether we are interested in steady production of power or in an explosion. In general, the steady production of atomic power requires a slow-neutron-induced fission chain reaction occurring in a mixture or lattice of uranium and moderator, while an atomic bomb requires a fast-neutron-induced fission chain reaction in U-235 or Pu-239, although both slow- and fast-neutron fission may contribute in each case. It seemed likely even in 1940, that by using neutron absorbers a power chain reaction could be controlled. It was also considered likely, though not certain, that such a chain reaction would be self-limiting by virtue of the lower probability of fission-producing capture when a higher temperature was reached. Nevertheless, there was a possibility that a chain-reacting system might get out of control, and it therefore seemed necessary to perform the chain-reaction experiment in an uninhabited location.

Up to this point we have been discussing how to produce and control a nuclear chain reaction but not how to make use of it. The technological gap between producing a controlled chain reaction and using it as a large-scale power source or an explosive is comparable to the gap between the discovery of fire and the manufacture of a steam locomotive.

Although production of power has never been the principal object of this project, enough attention has been given to the matter to reveal the major difficulty: the attainment of high-temperature operation. An effective heat engine must not only develop heat but must develop heat at a high temperature. To run a chain-reacting system at a high temperature and to convert the heat generated to useful work is very much more difficult than to run a chain-reacting system at a low temperature.

Of course, the proof that a chain reaction is possible does not itself ensure that nuclear energy can be effective in a bomb. To have an effective explosion it is necessary that the chain reaction build up extremely rapidly; otherwise only a small amount of the nuclear energy will be utilized before the bomb flies apart and the reaction stops. It is also necessary that no premature explosion occur. This entire "detonation" problem was and still remains one of the most difficult problems in designing a high-efficiency atomic bomb.

Three ways of increasing the likelihood of a chain reaction have been mentioned: use of a moderator; attainment of high purity of materials; and use of special material, either U-235 or Pu-239. The three procedures are not mutually exclusive, and many schemes have been proposed for using small amounts of separated U-235 or Pu-239 in a lattice composed primarily of ordinary uranium or uranium oxide and of a moderator or two different moderators. Such proposed arrangements are usually called "enriched piles".

See also

• Nuclear fusion
• Nuclear weapon
• Nuclear reactor
• Nuclear engineering
• Nuclear reaction
• Isotope separation

Links

• SCK.CEN Belgian Nuclear Research Centre (http://www.sckcen.be/) Mol, Belgiumcs:Jaderná reakce

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