PAR model development exercise in the framework of SAMHYCO-NET

Abstract

The simulation of passive auto-catalytic recombiner (PAR) operation is a pre-requisite for the assessment of the hydrogen combustion risk and hydrogen mitigation measures in light water reactors (LWRs). However, the ex-vessel boundary conditions of a severe accident pose a challenge for numerical models due to lack of suitable experimental data for validation. Especially the PAR interaction with carbon monoxide (CO) released in significant amounts during molten corium-concrete interaction has not been systematically investigated yet.The international network SAMHYCO-NET has performed several numerical benchmark exercises related to the behavior of combustible gases (hydrogen and carbon monoxide) inside LWR containments during severe accidents. In the field of PAR simulation, an international group of 8 institutions, supported by Framatome and Becker Technologies, have been collaborating towards a realistic and reliable assessment of PAR operation under challenging ex-vessel conditions, including the effect of oxygen starvation and the presence of CO. Focusing on the Areva PAR design, the partners applied various approaches to model the recombination rate, such as the engineering correlation provided by the vendor, a mechanistic model (REKO-DIREKT), and a full chemistry model (SPARK). Based on the experimental data obtained in the THAI and REKO-3 facilities, a step-wise approach towards PAR model assessment has been implemented. While the comparison of model predictions with experimental data shows in general good agreement, significant discrepancies still exist especially for oxygen-lean conditions.