Fission Product Buildup and Decay
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a nucleus that has a large atomic mass like uranium could fission by splitting into two smaller nuclei, along with a few neutrons. Th
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Fission Product Buildup and Decay
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a nucleus that has a large atomic mass like uranium could fission by splitting into two smaller nuclei, along with a few neutrons. This process results in the release of heat energy, such as kinetic energy of the nuclei, and gamma rays. The fission products themselves are often unstable and radioactive, due to being relatively neutron rich for their high atomic number, and many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta radiation and antineutrinos, even though these particles are not produced directly by the fission event itself.
15.1
Background Introduction
Early in World War II, the scientific community in the United States, including those Europeans now calling the United States their safe home, pursued the idea that uranium fission and the production of excess neutrons could be the source of extraordinary new weapons. They knew that Lise Meitner’s interpretation, in Sweden, of Hahn’s experiments would likely be known in Germany. Clearly, there might now be a race commencing for the development and production of a new, super weapon based on the fission of 235U92 or 239Pu94. By early 1942, it was, known that the two naturally occurring isotopes of uranium reacted with neutrons as follows:
© Springer International Publishing Switzerland 2017 B. Zohuri, Neutronic Analysis For Nuclear Reactor Systems, DOI 10.1007/978-3-319-42964-9_15
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15 Neutron Generation
First
Fission Product Buildup and Decay
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Third
Fourth
U235 fission fragment neutron leading to additional fissions neutron not leading addition fission, available for plutonium production
Fig. 15.1 The first generations of a nuclear chain reaction [1] 235
U92 þ 1 n0 ! fission products þ ð2:5Þ1 n0 þ 200 MeV Energy 238
U92 þ 1 n0 ! 239 U92
239
U92 ! 239 Np93 þ β1 t1=2 ¼ 23:5 min:
239
Np93 ! 239 Pu94 þ β1 t1=2 ¼ 2:33 days
Each U-235 that undergoes fission produces an average of 2.5 neutrons. In contrast, some U-238 nuclei capture neutrons, become U-239, and subsequently emit two beta particles to produce Pu-239. The plutonium is a fissile element also, and it would produce energy by the same mechanism as the uranium. A flow sheet for uranium fission is shown in Fig. 15.1 [1]. The answers to two questions were critical to the production of plutonium for atomic bombs: 1. Is it possible, using natural uranium (99.3 % U-238 and 0.7 % U-235), to achieve a controlled chain reaction on a large scale? If so, some of the excess neutrons produced by the fission of U-235 would be absorbed by U-238 and produce fissionable Pu-239.
15.1
Background Introduction
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2. How can we separate in a reasonable time period the relatively small quantities of Pu-239 from the unreacted uranium and the highly radioactive fission product elements? Although fission had b
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