Fission by thermal neutrons

Uranium A heavy, naturally radioactive, metallic element (atomic number 92). Its two principally occurring isotopes are uranium-235 and uranium-238. Uranium-235 is indispensable to the nuclear industry, because it is the only isotope existing in nature to any appreciable extent that is fissionable by thermal neutrons. Uranium-238 is also important, because it absorbs neutrons to produce a radioactive isotope that subsequently decays to plutonium-239, another isotope that is fissionable by thermal neutrons. 

Table 8.2. Cross sections (Tn,r of nuclear fission by thermal neutrons (energy 0.025 cV) and mean number v of neutrons set free by fission.

Fission by thermal neutrons is induced by the binding energy of the neutron (%6MeV) which leads to excitation of the nucleus, oscillation and finally splitting into two parts, if a critical deformation is exceeded. The stages and the time scale of fission by thermal neutrons arc illustrated in Fig. 8.12. Critical deformation is... 

Fission by thermal neutrons proceeds also via a double-humped barrier as in spontaneous fission (Fig. 5.19). The excitation energy acquired by the uptake of an additional neutron enables easily fissionable nuclei like and Pu to sur-... 

Pu. The isotope Pu is produced by neutron capture in Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons. Pu is converted to a fissionable nuclide by neutron capture. Therefore, like Th and it is a fertile material. It undergoes alpha decay, with a half4ife of 6580 years, to form which then decays to Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes. 

The plutonium produced can contribute to the chain reaction since it is fissionable by thermal neutrons. 

Cm has a half-life of4 730 y. It can be obtained through neutron capture in Cm, which has a half-life of 8 5(K> y the reaction cross-section is 345 b. Both isotopes are also fissioned by thermal neutrons, <>245 b and 0.17 b. Cm also has a n,y cross-section of 1.3 b. Because one does not want to loose too much Cm, Ae irradiation is timed to give a maximum yield of Cm. If the neutron flux is 2 x lO n m s, (a) when does the m concentration teach its maximum (b) What is the ratio between the amount of m produced and amount of Cm consumed at that time ... 

The smallest critical sizes are obtained for homogeneous systems of pure fissile nuclides with maximum neutron reflection. For neutrons with the fission energy spectrum, the critical mass of a metallic sphere of pure is 22.8 kg, that of is 7.5 kg, and that of Pu is 5.6 kg, assuming a 20 cm uranium metal neutron reflector. For fission by thermal neutrons the smallest critical size of a spherical homogeneous aqueous solution of 1102804 without reflector requires 0.82 kg of in 6.3 1 of solution. The corresponding figures for are 0.59 kg in 3.3 1, and of Pu, 0.51 kg in 4.5 1. 

The large majority of nuclear power reactors is based on fission by thermal neutrons and the discussion will begin with this class of reactors. 

LIST the four radioactive materials that fission by thermal neutrons and are used as reactor fuels. 

The reactor core is the heart of any nuclear reactor and consists of fuel elements made of a suitable fissile material. There are presently four radioactive materials that are suitable for fission by thermal neutrons. They are uranium-233 uranium-235 plutonium-239 ( Pu),... 

Radioactive materials suitable for fission by thermal neutrons and used as reactor... 

Fissile material. Material which is capable of undergoing fission by thermal neutrons. Normally used to describe atomic reactor fuel. 

The present invention relates to nuclear physics, and more particularly to an improved means and method of converting an isotope fissionable by thermal neutrons to another or the same thermally fissionable isotope in a neutronic reactor. The term thermally fissionable iso> tope as herein used refers, as is common, to an isotope which is fissionable by thermal neutrons. 

Nuclear fission of actinides is, without doubt, the most important nuclear reaction. Nuclear fission by thermal neutrons may be described by the general equation [Eq. (8)] ... 

FERTILE ISOTOPE. A fertile isotope or fertile material is a substance that is not itself fissionable by thermal neutrons but can be converted into fissfle material. This conversion is typically carried out by irradiation in a nuclear reactor. There are two basic fertile isotopes thorium-232 and uranium-238. When these fertile materials capture neutrons, they are converted into the fissile isotopes uranium-233 and plutonium-239, respectively. 

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