Waste classification of PWR concrete biological shield: Impact of detector-window cavities on neutron streaming

Abstract

Concrete biological shields (CBSs) of large pressurized water reactors (PWRs) yield a significant amount of waste upon decommissioning, and neutron-transport and activation calculations are useful tools for evaluating their radioactivity distributions. However, the impact of the geometric modeling fidelity of the calculation models on decommissioning waste management is not fully understood. This study focused on detector-window (DW) cavities in the CBS and investigated the effects of DW on radioactive waste classification for a representative 4-loop PWR using the three-dimensional continuous-energy Monte Carlo transport simulation code PHITS and its post-processing multi-group activation code DCHAIN-PHITS. The air-filled DW cavity formed a low-attenuation streaming path, producing higher neutron flux near the cavity end and in the deep CBS region. These neutron-flux characteristics were reflected in the spatial distribution of activation products with a bulge around the DW and thus in the waste-class boundaries. Applying Japanese regulatory criteria, the DW cavity produced a bulge in the L3 (VLLW)-clearance boundary in the DW direction, and this effect persisted throughout the cooling time. In contrast, the influence of the DW cavity on the L2 (LLW) region was limited, because the L2 waste diminished as 60 Co decayed. These results demonstrated that the localized deep activation near DW openings is significant enough for considering waste minimization through clearance.