A study on the modelling and simulation of the hydrogen behavior

The severe accident hydrogen control regulation of 10CFR50.34 (f) requires that the plant design shall include a hydrogen control system which can safely accommodate the hydrogen resulting from a 100% metal-water reaction, and limit its concentration in the containment to no greater than 10%. This regulation is applied to UCN 3 and 4 design for the first time in Korea. However, the severe accident hydrogen control system such as hydrogen igniters is not designed to be installed at UCN 3 and 4 containment. Further, UCN 3 and 4 do not have the safety-grade containment fan cooler system that will influence on the hydrogen transport and combustion. This study is focussed on the modelling and simulation of the hydrogen behavior by CONTAIN computer code to identify quantitatively how the containment ESFs such as containment fan system, hydrogen igniters and containment spray system have influence on the hydrogen mixing and burning in the severe accident conditions in order to determine whether the hydrogen control system such as igniters is needed for large dry containment and suggest the severe accident mitigation scheme for the hydrogen combustion. However, the direct containment heating (DCH) effects and corium-coolant-concrete interactions including steam explosion in the reactor cavity are not analyzed in this study, the following conclusions are effective only in the severe accident sequences without these phenomena. About 1 hour after RPV breach, high turbulent flow is predicted to prevail in the lower containment compartments. During this period, the irreversible flow loss coefficients govern the hydrogen mixing in the lower compartments. However, after this period, it is estimated that the containment fan system do not have a significant influence on the hydrogen transport in the containment. A more detailed investigation for the estimation of relevant flow loss coefficients for the lumped-parameter code such as CONTAIN should be carried out. After completion of this investigation, the containment fan system effects including the passive mixing concepts should be re-evaluated in detail. The hydrogen igniter system greatly reduced the hydrogen concentrations in the containment. Therefore, it is highly recommended that the hydrogen igniter system should be installed at the future large dry PWR containment in order to meet the severe accident hydrogen regulation of 10CFR 50.34(f) and to reduce the possibility of the hydrogen DDT or detonation. It is suggested that the containment spay system should not be used for the hydrogen mixing purpose in the severe accident conditions, because it suddenly remove a large quantity of steam and rapidly increase the hydrogen concentration to a higher level, which could result in a local detonation