Research On The Strategy Of Operator's Action In Core Emergency Cooldown Process For PWR

The maintenance of cooling ability for reactor core is quite important in PWR accidents,the core may melt due to the insufficient of cooling, especially when BDBA occurs. Thus, the strategies for the core emergency cooling at BDBA is the Primary Feed-and-Bleed operation and Secondary Feed-and-Bleed operation. In this thesis, a typical CNP600 PWR plant is modeled by using the best-estimate thermal hydraulic RELAP5/MOD3.4 code to evaluate the operator's strategy for core emergency cooldown process. A typical small break loss of coolant accident (SBLOCA) without High pressure safety injection (HPSI) is selected for the calculation of Primary Feed-and-Bleed process, and SBLOCA without HPSI and auxiliary feedwater (AFW) accident and total loss of feedwater (TLOFW) accident are selected for the calculation of Secondary Feed-and-Bleed process.This thesis drew the conclusions as follows: The flash of coolant process after the clearance of water seal in the broken loop has a great influence on the operators' Secondary Feed-and-Bleed operation during the SBLOCA without HPSI mitigation. The operators should implement the Secondary Feed-and-Bleed before the clearance of water seal with a higher cooling rate than the 56?/h within the design limit of RCS.After the failure of Secondary Feed-and-Bleed operation, the operators should turn to the Primary Feed-and-Bleed operation for mitigation. The results show that the entry for SAMG is suitable for the timing for initiation of depressurization by opening the relief valves normally. Also, Opening only one relief valve is not enough for the depressurization of RCS,and opening three relief valves is a more favorable strategy for mitigation.When implementing Primary Feed-and-Bleed operation for TLOFW mitigation with EOPs, there is a risk that the relief valve failure due to the long terms of discharge with water and repeated open/reseat. The timing for initiating safety injection should delay for a moment after the entry of H2 procedure. While implementing Primary Feed-and-Bleed operation for TLOFW mitigation with SOPs, the risk of relief valve failure can be reduced significantly,and there is no pressurizer overflow occurred during the mitigation.