Estimation of a boundary to distinguish between radioactive materials and non-radioactive materials around a Main Steam line and a Feed Water line in a Biological Shielding Wall of a BWR for decommissioning

Abstract

In order to estimate amount of radioactive concrete waste from decommissioning activities of a Boiling Water Reactor (BWR), we performed an estimation of a boundary to distinguish between radioactive materials and non-radioactive (NR) materials, which we call NR boundary in our work. So as to estimate the boundary reliably, we measured radioactivities induced by neutrons with activation foils around a Main Steam (MS) line in a MS tunnel room and also calculated three dimensional (3D) distribution of neutron-flux around both the MS line and a Feed Water (FW) line, which penetrate a Biological Shielding Wall (BSW) of the BWR, by using 3D discrete-ordinate method (Sn) calculation code TORT. We estimated the NR boundary by using TORT and verified a validity with comparing the calculation with the measurements. In the calculation, combinations of various numbers of directional mesh sets and orders of scattering cross section were examined by comparing the calculations with the measurements to survey the best set that simulates neutron transport phenomena in the MS tunnel room. The calculation that we judged reliable showed that the NR boundary around these two lines was located in the BSW concrete and that all materials in the MS tunnel room are non-radioactive.