Limiting influence factors for thermal neutrons

The limiting influence factor for thermal neutrons depends on the age of the neutrons and the diffusion length and coefficient of diffusion of thermal neutrons. This paper shows the limiting influence factor versus the neutron age for light and heavy water, beryllium, beryllium oxide, and graphite. It is evident that light water has the highest influence factor μ/sub T//sup lim/ for injector with a fission-neutron spectrum, and in addition a change in the spectrum of the injected neutrons can substantially change its value. For other moderators such changes are less significant. The diffusion-age approximation, however, describes poorly the thermal-neutron distribution near the fast-neutron source. For this reason, multigroup (21 groups) calculations of the influence factor for a point injector with a fission-neutron spectrum in a virtually infinite medium were performed using the Monte-Carlo program