Analyses of Beyond Design Basis Accident Homogeneous Boron Dilution Scenarios

Abstract

Abstract Homogeneous boron dilution scenarios in a VVER-440 reactor were analyzed using the coupled KIKO3D-ATHLET code. The scenarios are named “homogeneous” because of the very slow dilution caused by a rupture in the heat exchanger of the makeup system. In the case of the presented Beyond Design Basis Accident (BDBA) scenarios, it was supposed that after 30 min the operator doesn't stop the dilution, violating the prescriptions of the Limits and Conditions document. Without the presented analyses, a significant contribution of the homogeneous boron dilution to the Core Damage Frequency (CDF) had to be assumed in the Probabilistic Safety Analyses (PSA). According to the combined results of the presented deterministic and probabilistic analyses, the final conclusion is that boron dilution transients don't give significant contribution to the CDF for the investigated VVER-440 NPP.