Study on Heat Removal Capacity of PCCS of Advanced Passive PWR During Severe Accidents

In the advanced passive pressurized water reactor, a passive containment cooling system (PCCS) has been adopted to cool the containment—comprising a cylindrical steel vessel—during postulated accidents, whereby the decay heat is removed through water film evaporating enhanced by air cooling outside the containment. In this study, an integrated safety analytical code is used to study the heat removal capacity of PCCS during severe accidents and its influence on severe accident management measures. The coupled analytical model includes the reactor cooling system, engineered safety features, containment system, and PCCS. Containment responses during typical design-basis accidents and integrated severe accident scenarios are calculated and validated using a design control document and probabilistic risk assessment, respectively. Four typical severe accident sequences that contribute to core damage frequency or containment high pressure are selected to evaluate the containment response. The results show that the containment pressure can be controlled at a relatively low level within 72 h with the heat removal by PCCS. Analysis of the effects of PCCS water cooling recovery during the late period of the accident sequence in severe accident management guidelines alerts as to the risk of hydrogen combustion after breaking the steam-inert atmosphere inside containment. Moreover, sensitivity analysis has been performed to study the influence of the water film coverage rate and environmental air temperature, and it shows that a decrease of the water film coverage rate and an increase of the environmental air temperature reduce the PCCS cooling capacity.