Validation of the PARUPM and GOTHIC 8.3 code coupling using THAI hydrogen recombination tests

• PARUPM was coupled with GOTHIC 8.3, enabling dynamic PAR simulation in containment scenarios. • The model matched THAI HR-2 and HR-12 tests in recombination, pressure, and flow, even with low oxygen. • Despite detailed modeling, computation rose by <2% vs. empirical models, allowing full-containment use. • The PARUPM-GOTHIC framework is robust and scalable for advanced hydrogen risk assessment in nuclear safety. In case of a severe accident in a nuclear power plant (NPP), large amounts of H 2 and CO could be generated, potentially leading to uncontrolled combustion if concentrations within the flammability thresholds are reached. To mitigate this hazard, many NPPs equipped their containment buildings with passive autocatalytic recombiners (PARs). Thus, there is an interest in the developing of mechanistic models capable of predicting the behaviour of these devices. PARUPM is a code that simulates the behaviour of PARs using a physicochemical model approach. In the framework of the AMHYCO project (EU-funded Horizon 2020 project), the code has been validated as a standalone tool using experimental data. Nevertheless, the containment thermal hydraulics have a significant impact on the PAR behaviour, thus, in a next phase, PARUPM has been integrated as an add-on program within the thermohydraulic simulation code, GOTHIC. The present paper provides an overview of the capabilities of the joint simulation with PARUPM – GOTHIC 8.3. This coupling enables a detailed simulation of the recombination process under dynamically evolving conditions, allowing to represent the feedback between the PAR and the containment atmosphere. The validation was conducted based on a sequence of experiments on H 2 recombination by PARs performed at the THAI experimental facility. The results of these simulations confirm that the coupled PARUPM-GOTHIC model can predict the behaviour of PARs in full containment scenarios while maintaining low computational efforts, making the tool suitable for detailed safety analysis and scalable for full-containment simulations involving multiple recombiners.