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4-4
Table 4.0-1 gives some of the pertinent neutronic properties of the different fis-
sile nuclides for a specific thermal-neutron energy.
In discussing these properties,* it
is necessary to distinguish between the two functions of a fissile material: the production
of energy (i.e., power) and the production of ezcess neutrons which when absorbed by fertile
material will produce additional fissile fuel.
Nuclide
Table 4.0-1. Nuclear Parameters of the Principal Fissile Nuclides
233U, 235U, 239Pu, and 241P~a9b at Thermal Energy
(Neutron Energy = 0.0252 eV, velocity = 2200 m/sec)
Cross Section (barns)
‘a uf uc
a V n
23311 578 2 2 531 2 2 47 1 0.089 2 0.002 2.487 2 0.007 2.284 2 0.006
2351) 678 2 2 580 2 2 98 2 1 0.169 2 0.002 2.423 2 0.007 2.072 2 0.006
239Pu 1013 2 4 742 2 3 271 3 0.366 2 0.004 2.880 5 0.009 2.109 2 0.007
241Pu 1375 2 9 1007 7 368 2 8 0.365 2 0.009 2.934 2 0.012 2.149 2 0.014
~~
a
G. C. Hanna et al., Atomic Energ. Rev. 7, 3-92 (1969); figures in the referenced article
were all given to one additional significant figure,
b~a = uf + uc; a = uc/uf; v = neutrons produced per fission; 11 = neutrons produced per atom
destroyed = v/(l + a).
The energy-production efficiency of a fissile material is directly related to its
neutron capture-to-fission ratio (a), the smaller the ratio the greater the fraction of
neutron-nuclide interactions that are energy-producing fissions. As indicated by Table
4.0-1, at thermal energy the value of a is significantly smaller for 233U than for the
other isotopes, and thus 233U has a greater energy-production efficiency than the other
isotopes. (lhe energy released per fission differs only slightly for the above isotopes.)
The neutron-production efficiency of a fissile material is determined by the number
of neutrons produced per atom of fissile material destroyed (n), the higher the number the
more the neutrons that will be available for absorption in fertile material. Table 4.0-1
shows that the n value for 233U is higher than that for any of the other nuclides, although
plutonium would at first appear to be superior since it produces more neutrons per fission
(v). The superiority of 233U results from the fact that a is lower for 233U and 0 = v/(l + a).
Thus at thermal energies 233U both yields more energy and produces more neutrons per atom
destroyed than any of the other fissile nuclides.
In the energy range of interest for fast reactors (~0.05 - 4.0 MeV), the situation
is not quite so straightforward. Here again, the a value for 233U is significantly lower
than the values for the other fissile nuclides, and, moreover, the microscopic cross section
for fission is higher (see Fig. 4.0-1). The energy release per fission of 233U is
somewhat less than that of the plutonium nuclides, but the energy release per atom of 233U
destroyed is significantly higher than for th, othcr nuclides. Thus, from ths standpoint
*Much of this discussion on the neutronic properties of nuclides is based on refs. 1 - 3.