The Physics and Technology of Fast Reactors

The physics aspects of the fast reactor have been touched on at various points in the earlier chapters. On account of the considerable differences of emphasis which are necessitated by the differences between fast and thermal reactors, however, it is useful to summarize the physics of the former separately at this stage. 

The principal distinction between the thermal and fast reactor is that whereas, in the former, moderating material is deliberately introduced in order to slow the neutrons down as rapidly as possible through the region of resonance capture, in the fast reactor the amount of moderating material is 

Because of the effect of inelastic scattering in the spectrum softens as the proportion of fertile to fissile material is increased. Further softening is produced by the introduction of light elements, for example by the use of oxide or carbide fuel. As a result, the spectrum in a full-scale oxide-fueled fast reactor is considerably softer than that in the early compact cores of the EBR type. It may in fact be desirable deliberately to introduce a certain amount of additional moderation in order to improve the response of the reactor to power transients (see Section 11.3). 

As was illustrated earlier (Fig. 4.1), the value of vj = v/ I + a) for both and Pu is appreciably higher in a fast spectrum. The fission and capture cross sections themselves are of course much smaller at the higher energies (see, for example. Fig. 2.8 for U ). Table 11.1 gives values of the important parameters of the fissile isotopes averaged over a typical fast reactor spectrum. 

The superior performance of the plutonium isotopes, as evidenced by the value of (rj — 1), and in particular the advantage of the buildup of the higher isotope Pu as a result of neutron capture in Pu , are clearly seen. In 

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