Analysis of loss of residual heat removal(RHR) during mid-loop operation of Kori 3 and 4 nuclear power plants

This study aims to understand the thermal-hydraulic behavior, to predict allowable time until core uncovery and core damage, and to evaluate the appropriateness of the abnormal operation procedures during loss of RHR at mid-loop operation for Kori 3 and 4. RELAP5/mod3.2.2beta thermal-hydraulic transient code was used to analyze the thermal-hydraulic behavior for the following cases identified according to the combination of RCS configurations, venting path and steam generation status: 1) Case 1 : normal venting path (pressurizer and reactor vessel venting path are 3/4 inch and 1 inch venting valves, respectively) with water-filled steam generators (SG). 2), Case 2 : normal venting path with emptied SG, 3) Case 3 : the pressurizer manway opening ( the size of manway is 16 inches) with emptied SG, 4) Case 4 : the SG manway opening (the size of manway is 16 inches) located in hot leg with emptied SGs. As a result, the RELAP5 code appropriately predicted the RCS boil-off in core region and the system pressurization during the transient period. In the Case 1, it was taken long time to reach the core uncovery and heatup because the decay heat was enough transferred from RCS to secondary side. For all cases, the core boil-off was reached about 10 ∼ 12.4 minutes after the event. Also, it was estimated that the core uncovery time was strongly dependent on the conditions of RCS venting path and the status of secondary side. This result indicated that the Case 3 is the worst one among the sequences investigated. In addition, the Case 3 was estimated that the containment isolation should be achieved within 133 minutes to prevent fission products from releasing to the environment