Simulation Research On Passive Residual Heat Removal Scheme For NPP

The passive safety design is advantageous because it simplifies the system, reduces the investment and brings higher safety feature. PRHRS(Passive Residual Heat Removal System) is an important part in the passive safety design procedure for NPP(Nuclear Power Plant), whose main function is to remove decay heat when the conventional heat removal system turns invalid. PRHRS is of great significance on NPP safety operation, for which it becomes the research emphasis and focus.This thesis takes the third generation advanced PWR NPP as research object and investigates the steam generator secondary side passive residual heat removal scheme based on RELAP5. Firstly, the PRHRS heat exchanger parameter design is completed with the help of RELAP5. Then the operation characteristic of PRHRS is analyzed under SBO(Station Black Out Accident) and FLB(Feedwater Line Break Accident) to verify its ability on remove residual heat. Moreover, the comparative analyses of PRHRS design capacity and PRHRS transient startup characteristics are carried out. In the last part of the thesis, a modified passive residual heat removal scheme is introduced and calculated under accident condition to verify its ability on accident mitigation.The simulating calculation results can be summarized as follows:(1) When PRHRS designed heat removal capacity is 3.6% of the full power, the PRHRS can remove core residual heat significantly and assure reactor safety under SBO and FLB accident.(2) The comparative analyses of PRHRS designed heat removal capacity is carried out under SBO and FLB accident. The designed heat removal capacities of conditions 1-5 are 2.4%, 3%, 3.6%, 4%, and 4.6% of full power, respectively. Under SBO accident, all designed conditions could assure reactor safety; and the coolant average temperature decreasing rate of condition 1 could meet standard requirements only. Under FLB accident, the designed conditions 2-5 could assure reactor safety; while the condition 1 couldn’t remove core residual heat effectively, which leads to coolant evaporation.(3) At starting moment, the instant-opening valve on PRHRS steam side causes considerable oscillations of pressure and flowrate, which would impact heat exchanger and system pipes. The pressure oscillation amplitude could be decreased through slowing the steam side valve opening speed or adding a nitrogen pressurizer tank on the return water side.(4) The modified passive residual heat removal scheme consists of two parts: the first part is PAFS(Passive Auxiliary Feedwater System), and the other part is PRHRS(Passive Residual Heat Removal System) with 2% of full power designed heat removal capacity. The modified scheme can remove core residual heat significantly and assure reactor safety under SBO and FLB accident. What’s more, the coolant average temperature decreasing rate under SBO could meet standard requirements.