The neutron life cycle in a fast reactor is markedly different than that for a thermal reactor. In a fast reactor, care is taken during the reactor design to minimize thermalization of neutrons. Virtually all fissions taking place in a fast reactor are caused by fast neutrons. Due to this, many factors that are taken into account by the thermal reactor neutron life cycle are irrelevant to the fast reactor neutron life cycle. The resonance escape probability is not significant because very few neutrons exist at energies where resonance absorption is significant. The thermal non-leakage probability does not exist because the reactor is designed to avoid the thermalization of neutrons. A separate term to deal with fast fission is not necessary because all fission is fast fission and is handled by the reproduction factor.

The thermal utilization factor is modified to describe the utilization of fast neutrons instead of thermal neutrons. The reproduction factor is similarly modified to account for fast fission instead of thermal fission.

Summary

The important information in this chapter is summarized on the following pages.

Neutron Life Cycle Summary

The infinite multiplication factor, k_, is the ratio of the neutrons produced by fission in one generation to the number of neutrons lost through absorption in the preceding generation.

The effective multiplication factor, k_eff , is the ratio of the number of neutrons produced by fission in one generation to the number of neutrons lost through absorption and leakage in the preceding generation.

Critical is the condition where the neutron chain reaction is self-sustaining and the neutron population is neither increasing nor decreasing.

Subcritical is the condition in which the neutron population is decreasing each generation.

Supercritical is the condition in which the neutron population is increasing each generation.

The six factor formula is stated as . Each of the six factors is defined below.

The thermal utilization factor can be calculated from the macroscopic cross section for absorption of reactor materials using Equation (3-1).

The reproduction factor can be calculated based on the characteristics of the reactor fuel using Equation (3-2).

The number of neutrons present at any point in the neutron life cycle can be calculated as the product of the number of neutrons present at the start of the generation and all the factors preceding that point in the life cycle.

The thermal utilization factor is effected by the enrichment of uranium-235, the amount of neutron poisons, and the moderator-to-fuel ratio.

The reproduction factor is effected by the enrichment of uranium-235.

The resonance escape probability is effected by the enrichment of uranium-235, the temperature of the fuel, and the temperature of the moderator.

An increase in moderator temperature will have the following effects.

Increase the thermal utilization factor Decrease resonance escape probability Decrease fast non-leakage probability Decrease thermal non-leakage probability