1.4.2.1 Spectrum

Changes in core configuration or core materials all have an effect on the neutron spectrum (Fig. 1.14). It is instructive to note the differences between a light water thermal system and the current LMFBR designs.

(a) Sodium is substituted for the water coolant. This decreases the mo­deration and hardens the spectrum, thus leading to less parasitic and non­parasitic absorption effectiveness (Table 1.4).

(b) Stainless steel cladding is used instead of Zircaloy. This can be done, since parasitic absorptions are less important in the harder spectrum and absorption in steel is not significant. The steel also allows the use of higher cladding temperatures with consequent improved efficiencies.

(c)

The fissile enrichment is increased from about 3 to 15%. More enrich­ment is required to cope with increased inelastic scatter due to the harder spectrum.

(d) The volumetric proportions of fuel to coolant and structure are re­tained at the same levels (about 35:45:20) approximately, although, if anything, the LMFBR is a tighter lattice.

Thus the LMFBR has a harder spectrum, almost entirely above 100 eV, whereas the light water thermal system has a considerable thermal peak (Fig. 1.14).

The breeding ratio calculated from a static neutron balance in a clean core is the ratio of fertile captures to fissile absorption:

where rj239 is the number of neutrons produced per neutron absorbed. In the hardest possible spectrum, rj23i is highest and thus the harder the spectrum the better the breeding ratio. Table 1.5 shows that breeding is impossible with plutonium in a thermal environment, whereas it is very good in a fast spectrum.