Analysis of venting strategies and hydrogen concentration evolution during a station blackout in a BWR-6 containment using GOTHIC 8.3

Abstract

After the Fukushima accident, the interest on hydrogen combustion hazard management has increased considerably. Many European nuclear plants reinforced their strategies for severe accident management with the installation of Passive Autocatalytic Recombiners (PARs) and Filtered Containment Venting Systems (FCVS), among others. In this study, the hypothesis of considering a preventive venting to try to reduce the hydrogen combustion risk is analyzed for a BWR-6 Mark III containment. The scenario chosen was the in-vessel phase of a Station Blackout accident. Cases with and without PARs have been tested.The venting action has been tested at different conditions to clarify its impact on the hydrogen concentration in the containment. One novelty of this study is to consider the difference between measuring the hydrogen concentration values present in the containment in the venting pipe rather than in the instrumentation position. The purpose is to try to understand the decisions that an operational crew can take with the instrumentation available and analyze the differences by modifying the location of the plant instrumentation.For the numerical analysis, numerous simulations have been carried out using a BWR containment model with GOTHIC 8.3. It has been observed that hydrogen concentration depends to a great extent on the containment venting strategy. The main conclusion is that the venting strategy, if properly performed, can reduce the risk associated with hydrogen for BWR-6 containments. During a SBO scenario, an early venting increases considerably the hydrogen combustion hazard, even in the cases with PARs.