INVESTIGATION OF STEAM GENERATOR COOLING CAPABILITY UNDER THE LOSS-OF-RHR ACCIDENT DURING THE MIDLOOP OPERATION OF YONGGWANG NUCLEAR UNITS 3/4

摘要

A loss of the residual heat removal (RHR) accident during mid-loop operation of Yong-gwang Nuclear Units 3/4 was analyzed using the best estimate code RELAP5/ MOD3.1.2 and CATHARE2 V1.3u. In this work the following assumptions are used; (i) initially the reactor coolant system (RCS) above the hot leg center line is filled with nitrogen gas, (ii) two 3/4-inch diameter vent valves on the reactor vessel head and the top of pressurizer in the reactor coolant system are always open, and a level indicator is connected to the RHR suction line, (iii) the two steam generators are operated in wet lay-up status and the steam generator atmospheric dump valve assemblies are removed so that the secondary side pressure remains at nearly atmospheric condition throughout the accident, and (iv) the loss of RHR is presumed to occur at 48 hours after reactor shutdown. Slightly different input models and initial conditions were used for the RELAP5 and CATHARE2 code, which turned out to entail large difference in the results. Overall findings are (i) the core boiling begins at ～5 min, (ii) the peak RCS pressure is ～3.0 bar, which may lead to temporary seal break, (iii) ～94% of the decay heat is removed by reflux condensation in the steam generator U-tubes in spite of the presence of noncondensable gas, (iv) the core uncovery time is estimated to be 7.2 hours with a conservative approach. It was also found that, from the CATHARE sensitivity calculation, one steam generator is enough to remove the decay heat with slight increase of the RCS pressure. In the RELAP5 calculations, significant mass errors were observed.