CFD simulation of hydrogen mitigation by a passive autocatalytic recombiner

Highlights: • CFD simulations of H₂ mitigation by a Passive Autocatalytic Recombiner are presented. • Simulated tests are OECD THAI-HR2 (ambient pressure) and THAI-HR5 (high pressure) • CFD Best Practice Guidelines are followed (mesh and time-step sensitivity) • Hydrogen recombination rate is specified by vendor-supplied empirical correlation. • All important features of tests are well predicted by the CFD approach. - Abstract: In the case of a nuclear severe accident, hydrogen produced by the metal-water reaction and core-concrete interaction is released in the containment in which large amounts of oxygen are present. The flammable mixture can deflagrate or even lead to detonation, increasing the pressure and potentially endangering the integrity of the containment. To mitigate the hydrogen risk, Passive Autocatalytic Recombiners (PARs) are used in many power plants to passively remove the hydrogen by recombination with the oxygen in air. The PAR presence in the containment volume induces secondary flows that are three dimensional in nature and hence Computational Fluid Dynamics (CFD) tools are most appropriate to tackle PAR issues in containments. This paper aims to validate a CFD model for hydrogen consumption by a PAR using experimental data in reduced-scale geometries. The CFD PAR model is implemented in the ANSYS Fluent code and makes use of a correlation given by the PAR manufacturer to compute the reaction rate of hydrogen. The PAR unit is considered as a channel where the reaction takes place without any chemistry details. The CFD model is validated against two experimental tests (HR2 and HR5), which were performed in the THAI facility in the frame of the OECD/NEA THAI project. The tests, conducted with no initial steam content, were designed to investigate PAR response to different starting pressures (1 and 3 bar). The simulations show that the predictions of the main variables (hydrogen recombination rates, hydrogen concentration, temperature, pressure) agree well with the experimental data. This demonstrates the suitability of CFD treatments of PAR behavior in a single compartment. Future validations should address the presence of steam in the initial mixture as well as multi-compartment geometries in order to increase confidence in CFD methods before upscaling to full containment scales.