Open issues in the applicability of recombiner experiments and modelling to reactor simulations

Abstract

In the case of a severe accident in light-water reactors, a large amount of hydrogen could be generated from the reaction between steam and zirconium at high fuel clad temperature and also from reactions of molten core debris with concrete. The hydrogen generated will be released into the containment atmosphere, and mixed with air and steam possibly creating local flammable conditions. In order to prevent loads resulting from a possible hydrogen combustion, French and German reactor containments are equipped with passive autocatalytic recombiners (PARs), which recombine hydrogen with oxygen even at concentrations below the lower flammability limit. In common PAR designs, catalytic materials (platinum and palladium on ceramic washcoat) are housed in a metallic structure whose purpose is to optimise the circulation of gases in contact with the catalyst. Numerous tests have been conducted in the past to investigate PAR behaviour in situations representative of severe accidents (Battelle Model Containment in Germany, H2PAR and KALI-H2 in France, AECL Whiteshell Laboratories in Canada, etc.). Furthermore, these tests demonstrated that, provided special care is paid to the design and construction of the catalysts, catalyst poisoning by materials such as carbon monoxide, iodine and aerosols present in the containment atmosphere will not fundamentally reduce the effectiveness of the PARs. Some of the above-mentioned tests also show that PARs could ignite the flammable gas mixture at elevated hydrogen concentrations. These experimental results need however to be corroborated by more detailed experiments and by refined modelling of phenomena occurring in PARs. In order to better characterise the PAR-induced ignition risk, a series of dedicated experiments has started at the REKO-3 facility located in Forschungszentrum Jülich. In parallel, a refined modelling of the recombiners has been developed by IRSN and will be used to gain insights into the phenomena occurring at the PAR catalyst plates. Furthermore, previous tests indicated that the position of the recombiners could have an impact on their overall efficiency. The installation of PARs in the reactor building is influenced by geometric and operational constraints. To this end, numerical models were developed from the experimental data for codes like COCOSYS or ASTEC in order to optimise the PAR location and to assess the efficiency of PAR implementation in different scenarios. However, these models are usually simple (black-box type) and based on the manufacturer's correlation to calculate the hydrogen depletion rate. Recently, enhanced CFD models have been developed at IRSN and Jülich in order to take into account phenomena such as the PAR location effect, gas mixture ignition induced by PARs, and the oxygen starvation effect. A new specifically instrumented facility is also under construction at Jülich to investigate these phenomena in more detail.