RELAP5 analysis of the loss-of-RHR accident during the mid-loop operation of Yonggwang nuclear units 3/4

A loss of the residual heat removal (RHR) accident during mid-loop operation of Yonggwang Nuclear Units 3/4 was analyzed using the RELAPS5/MOD3.1.2 code. 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 in wet layup 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. Findings from the RELAP5 calculations are (i) the core boiling begins at -5 min, (ii) the peak RCS pressure is -3.0 bar, which implies a possibility of 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 uncover time is evaluated to be 7.2 hours. Significant mass errors were observed in the calculations