Of the fn[ thermal neutrons absorbed in the fuel some will be captured in U-238, others captured in U-235 and the remainder cause fission of U-235. The fission events will be accompanied by release of fast fissions.

The thermal fission factor is a measure of the gain in the number of neutrons by thermal fission:

number of fast neutrons arising from thermal fission

TJ = ——————————————————-

number of thermal neutrons absorbed in the fuel

In Fig 1.13 the fn і thermal neutrons give rise to 7j fn і fast neutrons. [1.33 x 870 = 1160 fast neu­trons. Alternatively, of the 870 thermal neutron ab­sorbed in the fuel, 392 are captured and 478 cause thermal fission of U-235. 478 x v = 478 x 2.43 = 1160 fast neutrons.]

The value of 7] is determined by the enrichment of the fuel and, as calculated in Section 4.4.2 of this chapter, 17 = 1.33 for natural uranium. In Fig 1.14, values of 17 are plotted against the U-235 content in the fuel. The figure demonstrates that if the expense of enrichment is to be undertaken then this may as well be up to 3% or so because of the initial rela­tively large gains in 17; the subsequent modest in­creases will probably not justify higher enrichments.