Accident sequence precursor analyses for steam generator tube rupture events that actually occurred

Steam generator tube ruptures (SGTRs) at pressurized water reactors are identified as one of the risk significant events in the probabilistic risk assessment studies. In addition, operating experience indicates that SGTRs might result in complex transients, some of which involved additional anomalies such as the delayed recognition of event, resulting in the retarded isolation of the ruptured SG and/or failure to timely equalize the primary and secondary pressures. In order to identify the risk significant anomalies and to obtain generic insights useful for examining alternative mitigation measures for STGR, the present study systematically analyzed ten actual and one potential SGTR events using an accident sequence precursor model consistently. The analysis results show that the SGTR event involving the delayed identification of tube rupture or failure to timely depressurize the reactor could have a relatively high possibility of leading to core damage and would be a significant precursor, though the models and failure probabilities applied might be further examined. This implies the importance of improving the capability to detect SGTR and the operating procedures. It is also shown that some of the other anomalies observed would largely contribute to the possibility of core damage, which points out the need of examining alternative measures for recovering from such conditions.