Republic of Korea: Methodology Development for Evaluation of Hydrogen Safety in a NPP Containment Using OpenFOAM

During an accident accompanied by a core damage in a water-cooled nuclear reactor, a large amount of hydrogen can be released into a reactor containment. A hydrogen is generated in a nuclear reactor by an oxidation of a heated core during a severe accident. Hydrogen mitigation in a reactor containment is very important because the integrity of the reactor containment can be threatened by an explosive combustion of the hydrogen if the hydrogen concentration is very high globally or locally in a containment. The released hydrogen is mixed with air existing in a containment or steam which is also released from a reactor coolant system. When the hydrogen is well mixed with the other gases, its concentration becomes leaner and finally a probability of hydrogen explosion may be removed. On the contrary if mixing of the released hydrogen is limited and a highly-concentrated hydrogen mixture cloud is developed in a certain region of a containment, it is very probable that hydrogen flame undergoes DDT (deflagration to detonation transition) locally in the containment. A hydrogen mixed with a steam in a containment can be also affected by condensation of the steam and reevaporation of the steam condensates. The concentration of hydrogen in a mixture can be increased by condensation of a steam. If a steam in a hydrogen mixture cloud is condensed and becomes fog, the density of the hydrogen cloud containing the fog becomes high, and the cloud containing the hydrogen may descend to a lower part of a containment. In an analysis of thermal hydraulics in a reactor containment, its multi-dimensional characteristic is very important. In a lumped-parameter (LP) analysis using an integrated accident analysis code such as MELCOR or MAAP, a large flow structure developed in a containment may not be resolved. Flow paths of a released gas in a containment may be partly confined by internal structures such as walls and gates. But because the flow paths are mostly not limited in a large free volume of a containment, a flow-path-based containment modeling of the LP codes may require a sophisticated tuning of the input model. And a large difference in length scales between characteristic lengths of the flow and representative length of the containment makes an analysis of a containment thermal hydraulics very difficult. So, it is believed that a modern computational fluid dynamics (CFD) based detail analysis can be useful for a conservative, realistic and best-estimate evaluation of a hydrogen safety in a containment.