Impact of hydrogen on iPWR containment Thermal-Hydraulics in severe accidents

Abstract

Addressing hydrogen issues is an important part of severe accident management for nuclear accidents, and this is doubly important for water-cooled small modular reactors (SMRs), like integral pressurized water reactors (iPWRs). This study uses a combination of containment thermal–hydraulic experiments and modeling to assess the impact of introducing light non-condensable gases under prototypic accident conditions. The experiments evaluated the gas mixing behaviour and steam condensation heat transfer coefficients in the tertiary steam–air-helium system, extending the unified steam condensation correlations by Dehbi (2016) past the density difference transition point. The modeling with the GOTHIC thermal-hydraulics code scaled the experimental observations to reactor-relevant accident scenarios, with steam-hydrogen in a closed containment and steam–air-hydrogen in a loss of containment isolation situation. A strong reduction in the heat transfer capacity caused by the presence of non-condensable gases was observed, which disrupt the steam condensation that drives many of the passive cooling loops that form the basis of accident mitigation strategies. Likewise, the observed gas mixing behaviour inferred that hydrogen combustion may be a further issue that needs to be addressed in severe accident management guidelines.