Hydrogen risk analysis in small containment with a pressure suppression tank

Abstract

The total volume of the small containment of a small modular reactor (SMR) is significantly reduced, and the pressure in the containment may rise rapidly when the non-condensable gas and water steam jet into the small volume containment space during a loss of coolant accident (LOCA). SMR has adopted the pressure suppression tank (PST) as a rapid and efficient measure to mitigate the containment pressure and temperature increasing after a relevant accident. It significantly affects the transfer and distribution of gas mixture consisting of air, steam or hydrogen, and the hydrogen concentration and combustion probability may be infinitely increased. The three-dimensional parallel Computational Fluid Dynamics (CFD) code GASFLOW-MPI, which provides 3D gas flow field and hydrogen concentration in detailed to analyze the hydrogen safety, is appropriate to analyze the effects of PST to the mitigation of hydrogen risk. The PST model, which is coupled with the external gas by solving the three-dimensional Navier-Stokes equation to realize the coupling of mass, energy and momentum, is specially developed based on the GASFLOW-MPI. The PST model is validated against the special pressure suppression experiment implemented in a small containment suppression characteristic test device. The comparisons indicate good agreements between the predictions and the experimental data. At last, the validated code is used to simulate hydrogen risk of the small containment with a PST in a pressurized water SMR during severe accident. The computation results show that the oxygen concentration in the small containment space is relatively low but high in the gas space of PST. Hydrogen in the small containment flows into the PST through the connection pipe between the containment and PST in the later stage of the accident, causing a continuing increase of hydrogen concentration in the gas space of the PST. PARs mitigate the hydrogen risk well in the small containment, but more attention is required for the potential of flame acceleration in the gas space of PST and the discharge pipe in the later stage of the accident.