Design And Study Of Secondary Passive Residual Heat Removal System For Floating Nuclear Power Plant By RELAP5 Code

The floating nuclear power plant is typically built up and operated in an isolated area in sea.Therefore,once a station black out(SBO)accident happens,the active system which relies on electricity power supply will not be in service effectively.It is great important to incorporate passive residual heat removal system free of external power supply for the safety of floating nuclear power plant.In this paper,of the methodology to design a secondary passive residual heat removal system(SPRHRS)was proposed and three conceptual schemes of passive residual heat removal system were designed by using the methodology.The advantages and disadvantages of the three schemes were compared by utilizing RELAP5 code to simulate the scenario of SBO.The C-type condenser scheme was selected as the most promising one and further analysis of the 72 hours performance was conducted under static condition.The RELAP5 code was in-house modified and developed by adding the ocean condition simulating module into the code package,and the performance of C-type condenser scheme under ocean conditions such as heeling and rolling was studied by this validated code.The the responses of the plant were simulated analyzed for postulated fault cases under SBO accident,which included the activation of SPRHRS by accident,the delayed operation of SPRHRS,the stuck-open case of isolation valve of single residual heat removal loop and the stuck-close case of outlet isolation valve of single steam generator.The simulation results showed that the C-type condenser scheme showed the best performance in terms of heat removal capacity and depressurization of both the pressurizer and steam generator.The similar evolution of natural circulation for either primary loop or SPRHRS loop could be found for three proposed schemes..The water storage in the cooling tank could be utilized most effectively in the scheme with the condenser installed outside the tank as compared with the other two schemes.The simulation result also revealed that the decay heat could be removed in long term of 72 hours.The simulation results and the discussion indicated that heeling would weaken the heat removal capability of the SPRHRS.The heeling of-22.5~o presented greater impact on the system than that of 22.5~o.The leaning of the system will affect the natural circulation of the system.The leaning with 10~o will enhance the natural circulation of the system while the leaning with-10~o will weaken it.However,the impacts for both cases are slight..Even though rolling can cause periodic fluctuations in parameters,little influence can be found for such key parameters as the pressure of pressurizer,the pressure of steam generator,the coolant temperature,etc.If the SPRHRS is activated by accident when the plant is running normally under ocean condition of heeling-22.5~o,the SPRHRS has a limited impact on the operation status of main system.Moreover,the isolation of the SPRHRS in such situation will not affect the operation of the main system dramatically.Under ocean condition of heeling-22.5~o,in case of delayed activation of SPRHRS when SBO occurred,the SPRHRS will cause the peak value of such parameters as the pressure of pressurizer and the steam generator to exhibit similar delay behaviors.The delay of activation of SPRHRS has to be well regulated in order to prevent the possible damage of the core.If one of the two trains of SPRHRSs cannot be in service in SBO accident under ocean condition of heeling-22.5~o,the pressurizer relief valve and steam generator safety valve will be opened timely and the SPRHRS can still remove the residual heat in the core to prevent from meltdown.If the steam isolation valve at the outlet of one of the two steam generators is not closed in case of SBO,and the SPRHRS is in service normally under ocean condition of heeling-22.5~o,the system will still be able to remove the residual heat of the core to ensure the safety of the reactor.