Study On Injection Cooling During Severe Accidents For The Pressurized Water Reactor Nuclear Power Plant

If there is inadequate cooling during a severe accident in the nuclear power plant, a significant amount of core materials could become molten and relocate to the lower head of the reactor vessel, causing probably severe results such as lower head failure, containment failure and radioactive materials releasing into the environment. Therefore, the research to ensure that the lower head remains intact and the relocated core materials are retained within the vessel becomes more and more importent for severe accident management. As a key severe accident management strategy, In-Vessel Retention (IVR) of core molten debris has been adopted by some operating nuclear power plants and some being designed advanced light water reactors. However, it is not clear whether the proposed reactor vessel cooling without additional enhancements can provide sufficient heat removal to assure IVR for the current operating high power reactors.In this paper, for a 900MWe pressurized water reactor power plant, with the integrative severe accident analysis tool, four kinds of severe accident sequences, such as cold leg large break loss of coolant accident (LLOCA), hot leg middle break loss of coolant accident (MLOCA), loss of feed water (LOFW) and steam generator tunnel rupture (SGTR) are systematically calculated, while in-vessel retention is considered as a severe accident management strategy.The calculation analysis results show that: if the cooling water is adequate, an only External Reactor Vessel Cooling (ERVC) or an Internal Reactor Vessel Cooling (IRVC) measures could provide effective cooling for lower head to maintain the molten core in the vessel and maintain the vessel intact.According to this study, the influence factors of the cooling effect on IVR are summarized as follows:ERVC is implemented under accident conditions,â—‹1 The cavity injection must be of certain flow rate as well as water level in reactor cavity, which ensure that cavity cooling water provides timely adequate and effective cooling for the lower head.â—‹2 The later the injecting time of cooling water into the reactor cavity, the larger the amount of initial injection flow rate of cavity cooling water is. And if the injecting time is too late, the reactor cavity cooling injection effect will not be guaranteed and may ultimately cause the vessel failure.â—‹3 Different molten pool layer configurations and different severe accident sequences affect certainly the IVR results.â—‹4 A gap between the lower head wall and the molten pool shell exists, and the different sizes of the gap affect certainly the results of cavity flooding.IRVC is implemented under accident conditions,â—‹1 A depressurization measure must be done for high pressure core melting accident scenarios, so the cooling water could inject easily into the reactor vessel.â—‹2 A continued injection needs to be taken to ensure the cooling is adequate.â—‹3 Some water must be remained in the reactor vessel when the cooling water is injected into it. If the injection time is too late, the cooling water could not injected into the vessel efficiently, so the molten core could not be cooled timely, eventually causing the lower head failure because of overheated load.