Small-group model in calculating the critical mass of a VVR-M reactor

Abstract

To obtain intense neutron fluxes, a research reactor must have a compact active zone and a high specific heat intake. This requires a large heat-intake surface per unit volume of the active zone. For VVR-M5 fuel elements, this ratio takes a record value of 6.6 cm squared/cm cubed. The mean neutron path through a volume of metal or water is equal to or less than the neutron path length, so that the reactor may be regarded as homogeneous for almost any neutron energy, except perhaps resonant neutrons. Even in that case, the possibility of multiple neutron passage through a dense lattice homogenizes the resonant capture. Neutron leakage into the internal experimental devices of the reactor and the external reflector is usually large. Its contribution to the breeder factor is considerable and must be calculated with particular care. The optimal number of neutron energy groups must be determined here. The neutrons arriving at the active zone are fast, and those returning from the reflector are thermal, so there must be no fewer than two groups. In metal-water mixtures, the metal facilitates diffusion of the neutrons, and for research reactors, where the proportion of metal is 0.4-0.5, the use of two diffusionalmore » groups of decelerating neutrons is usually more expedient.« less