PAR model development exercise in the framework of SAMHYCO-NET

•We assessed the suitability of numerical PAR models for severe accident conditions in the ex-vessel phase.•We compared calculated and experimental reaction/recombination rates of both hydrogen and carbon monoxide.•In the course of three validation steps, hydrogen recombination rates were in general well reproduced under oxygen-rich conditions.•Under oxygen-lean conditions, all models show significant deviations from the experimental data, mostly over-predicting the recombination rates.•The exercise has helped the participating partners to perform first steps in enhancing PAR models towards the interaction with carbon monoxide in the late phase of a severe accident.