Alternative cooling water flow path for RHR heat exchanger and its effect on containment response during extended station blackout for Chinshan BWR-4 plant

Abstract

The extended Station Blackout (SBO) of 24h has been analyzed with respect to the containment response, in particular the suppression pool temperature response, for the Chinshan BWR-4 plant of MARK-I containment. The Chinshan plant, owned by Taiwan Power Company, has twin units with rated core thermal power of 1840MW each. The analysis is aimed at determining the required alternative cooling water flow capacity for the residual heat removal (RHR) heat exchanger when its tube-side sea water cooling flow path is blocked, due to some reason such as earthquake or tsunami, and is switched to the alternative raw water source. Energy will be dissipated to the suppression pool through safety relief valves (SRVs) of the main steam lines during SBO. The RETRAN model is used to calculate the Nuclear Steam Supply System (NSSS) response and generate the SRV blowdown conditions, including SRV pressure, enthalpy, and mass flow rate. These conditions are then used as the time-dependent boundary conditions for the GOTHIC code to calculate the containment pressure and temperature response. The shaft seals of the two recirculation pumps are conservatively assumed to fail due to loss of seal cooling and a total leakage flow rate of 36gpm to the drywell is included in the GOTHIC model. Based on the given SRV blowdown conditions, the GOTHIC containment calculation is performed several times, through the adjustment of the heat transfer rate of the RHR heat exchanger, until the criterion that the maximum suppression pool temperature is maintained below 200°F is met. This heat transfer rate in conjunction with the RHR heat exchanger UA values (overall heat transfer coefficient times heat transfer area) are used to iteratively determine the heat exchanger effectiveness and the corresponding raw water flow rate. The base case considered in the study assumes that the reactor is first depressurized to ∼800psia at t=1h, and is then further depressurized to ∼300psia at t=5h. The 5th diesel generator and the gas-turbine generator are assumed available at t=5h and are used to power the RHR pump for suppression pool cooling. The analysis results for this base case show that a minimum flow rate of 1346.4gpm is required for the raw water source to meet the criterion that the maximum suppression pool temperature remains below 200°F during the SBO of 24h. Sensitivity studies are performed to investigate the effect of depressurization and the timing for initiating the depressurization. Although it is well recognized that depressurization is beneficial for the later accident mitigation, e.g., injecting of low-pressure fire water to cool down the reactor, the results show that it needs more cooling water flow rate for the RHR heat exchanger to meet the 200°F criterion. Also, the timing to initiate the depressurization may have a significant effect on the required cooling water flow capacity.