REACTIVITY COEFFICIENTSDOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters)ModeratorEffectsAs discussed in the previous module, a moderator possesses specific desirable characteristics.(a) large neutron scattering cross section (b) low neutron absorption cross section (c) large neutron energy loss per collisionWith the exception of the Liquid Metal Fast Breeder Reactor (LMFBR), the remaining majorreactor types that are currently employed use moderating materials to reduce fission neutronenergies to the thermal range. Light moderators (composed of light nuclei) are found to be moreeffective than heavy moderators because the light moderator removes more energy per collisionthan a heavy moderator. Therefore, the neutrons reach thermal energy more rapidly and they areless likely to be lost through resonance absorption. As discussed in a previous module, the ability of a given material to slow down neutrons isreferred to as the macroscopic slowing down power (MSDP) and is defined as the product ofthe logarithmic energy decrement per collision (x) times the macroscopic scattering cross sectionfor neutrons as follows.MS DP x S_{s}Macroscopic slowing down power indicates how rapidly slowing down occurs in the materialin question, but it does not completely define the effectiveness of the material as a moderator.An element such as boron has a high logarithmic energy decrement and a good slowing downpower, but is a poor moderator. It is a poor moderator because of its high probability ofabsorbing neutrons, and may be accounted for by dividing the macroscopic slowing down powerby the macroscopic absorption cross section. This relationship is called the moderating ratio(MR).MR x SsSaThe moderating ratio is merely the ratio of slowing down power to the macroscopic absorptioncross section. The higher the moderating ratio, the more effectively the material performs as amoderator.Another ratio, the moderator-to-fuel ratio (N^{m}/N^{u}), is very important in the discussion ofmoderators. As the reactor designer increases the amount of moderator in the core (that is,N^{m}/N^{u}increases), neutron leakage decreases. Neutron absorption in the moderator (S^{m}_{a}) increasesand causes a decrease in the thermal utilization factor. Having insufficient moderator in the core(that is, N^{m}/N^{u} decreases) causes an increase in slowing down time and results in a greater lossof neutrons by resonance absorption. This also causes an increase in neutron leakage. Theeffects of varying the moderator-to-fuel ratio on the thermal utilization factor and the resonanceprobability are shown in Figure 2.NP-03Rev. 0Page 24