Mutual shielding of neutron activation foils

Abstract

For decommissioning purposes, stacks of foils are used in order to determine the neutron fluence rates in reactor buildings. Due to the relatively great thickness of these foils, self-shielding, as well as mutual shielding become significant for these stacks. In this paper, the results of Monte Carlo calculations are presented for the self-shielding of single foils, and also for the mutual shielding of foils in a stack. Because the irradiation geometry in a reactor building at a large distance from the reactor core is probably not isotropic, especially not so for neutrons of higher energy, the shielding effects were calculated for both isotropic and mono-directional irradiation geometries. The results obtained show a number of general trends which can be understood with a few rules of thumb.