Numerical simulation of containment thermal-hydraulics behavior for small modular reactor during Station Black Out severe accident

Hydrogen risk has always been a safety issue for water-cooled reactors. The corresponding phenomenon for small modular reactors (SMR) can differ significantly from the large dry containment because of different design features. Severe accident analysis shows that the oxygen can be depleted, and that can impact the performance of Passive Auto-catalytic Recombiners (PAR). Therefore, the containment thermal hydraulics behavior during a severe accident induced by Station Black Out (SBO) is analyzed with 3D CFD code GASFLOW-MPI focusing on hydrogen risk and PAR performance. Firstly, a numerical simulation on the PANDA PE5 test demonstrates the code performance of the passive containment heat exchanger (PCHR HX) and its influence on hydrogen behavior. Then, the containment thermal hydraulics behavior of the SBO accident is analyzed with mass and energy release obtained from the severe accident analysis. Research shows that the oxygen left in the containment is ∼45 % of the initial dry air before the hydrogen release because of the gas displacement during the pressure suspension system operation. The gas mixture becomes inflammable soon after the hydrogen release. The original correlation of recombination rate consists of an effectiveness coefficient considering oxygen starvation; however, it has a significant uncertainty compared with experimental data. A modified model to describe oxygen starvation is provided in this study based on experimental data. The gas mixture in the containment becomes oxygen-starved soon after the hydrogen release. The modified model can reduce the recombined hydrogen mass to ∼40 %. While analysis shows that the gas mixture does not become oxygen-starved until it becomes inflammable, therefore, its impact on the hydrogen risk analysis is limited from the perspective of hydrogen risk management.