Performance Evaluation for Overpressure Transient at Reactor Coolant System of SHINKORI 3/4 NPP by Loss of Turbine Load

Commercial NPP include depressurization-devices such as pressurizer safety valve and main steam safety valve with reactor protection system. In SINKORI 3/4 NPP (SK34), first commercial APR1400-type plant, the depressurization devices have distinguishing features. Fig. 1 shows schematic diagram for pilot operated safety relief valve (POSRV) as pressurizer safety valve of SK34. The valve includes two spring loaded safety valve (pilot valve) to set up the opening/closing pressure of main valve. And main valve operates with delay time after opening/closing of pilot valve. Main steam safety valve (MSSV) of SK34 is so-called IAD-type valve, for which opening set pressure was defined by manufacturer as initial audible discharge whereas previous definition was popping point. In this study, we evaluated the performance of SK34 for overpressure transient with conservative conditions. From calculation results, RCS and SG peak pressure is maintained lower than 110% of design pressure under conservative conditions. Therefore, performance for overpressure protection of RCS in SK34 will be satisfactory. And we also identify significant parameters or factors in overpressure transient analysis with sensitivity study. It was confirmed that loss of offsite power could be effective to overpressure significantly and MSSV and POSRV should also be modelled appropriately in overpressure transient analysis