The Assessment of SG U-tubes Integrity During a Postulated Loss of All Heat Sinks in Wolsong NPPs by the CATHENA Code

A total loss of all sustained heat sinks in Wolsong NPPs is considered as a severe accident with low probability of occurrence. Following a total loss of all sustained heat sinks, the degasser/condenser relief valves (DCRVs), 3332-RV11 and RV21 would be the sole means available for the depressurization of the primary heat transport system (PHTS). A thermalhydraulic analysis code for CANDU6 reactors, CATHENA is used to assess the impact of this kind of accident on fuel cooling and primary circuit integrity. In this study we were especially concerned about the integrity of steam generator (SG) U-tubes. If the SG Utubes fail due to the excessive high temperature of steam flowing into the SG U-tubes, the issue would cause the containment by-pass with the release of fission products to the outside environment through the main steam safety valves (MSSVs)