The fn 1 thermal neutrons in the fuel give rise to 17 fn 1 fast neutrons by thermal fission. But, as shown in Fig 1.6 (c) and discussed in Section 3.2.1 of this chapter, fast neutrons (> 1.1 MeV) may induce U-238 to fission. It is not a very likely event because, firstly, the cross-section for fisson is low (~ 0.3 barns) and, secondly, fission is possible only by fast neutrons which have not undergone a scattering event. Scat­tering will reduce the fast neutron energy to below the fission threshold of 1.1 MeV.

Nevertheless, U-238 fission is possible and must be included. The gain in the number of neutrons by fast fission is given by the fast fission factor:

number of fast neutrons arising from _ thermal and fast fission

e = ——————————————————

number of fast neutrons arising from thermal fission only

Because of the condition that fission is possible only for a first collision between fast neutrons and U-238 the value of e may be regarded as being mainly deter­mined by the geometry of the fuel design. The value of 1.03 used here is probably greater than would ap­ply to most practical reactor designs. The total fast neutrons, then, is given by eTjfnj [1.03 x 1160 = 1190 fast neutrons].