A study of estimating the effective multiplication factor of an individual bare unit for evaluation of criticality safety of units in array

It is necessary to consider the interaction of neutron between individual fuel units for the safety analysis of nuclear fuel facilities. The solid angle method has been developed for these analyses, and it requires the effective multiplication factor in a bare fuel unit. A simple evaluation method was examined to calculate the effective multiplication factor in a bare unit from that with water reflector. It is concluded that the ratio of the effective multiplication factor in a fuel unit without to with water reflector is shown by the function of a dimension of a fuel vessel, and this relation is well explained by the one group diffusion theory with calculating migration area, extrapolation length and reflector savings. (author)