SIMPLIFIED METHOD FOR ASSESSING THE RISK ASSOCIATED WITH CONSEQUENTIAL STEAM GENERATOR TUBE RUPTURE EVENTS

摘要

The U.S. Nuclear Regulatory Commission (NRC) and the nuclear power industry have expended considerable resources over the last two decades to better understand the safety implications and risks associated with consequential steam generator tube rupture (C-SGTR) events; i.e., events in which steam generator (SG) tubes leak or fail as a consequence of the high differential pressures and/or elevated temperatures during accident sequences. The accidents involving SG tube ruptures have shown in various probabilistic risk assessments (PRAs) to be contributors to plant risk, mainly because of their potential to cause a release outside the containment (containment bypass scenarios). This paper summarizes the current approach and the preliminary results of an ongoing NRC study. It documents a methodology for a quantitative risk assessment of C-SGTR during a severe accident after the onset of core damage, and during a Design Basis Accident (DBA) event before the onset of core damage. The study utilized the latest available thermal-hydraulics (TH) for two plants: one plant is a four loop Westinghouse (W) plant, and the other is a Combustion Engineering (CE) design equipped with Power Operated Relief Valves (PORVs). The TH and PRA results for these two plants should not be misconstrued as generic results for W and CE plants. The study developed and utilized the latest SG tube flaw statistics pertinent to current reactors with replaced SGs, based on a limited number of samples and data provided in the periodic inspection reports. The study also developed a software package utilizing the latest available models for estimating the failure probability/timings of other SG tubes, and RCS components (i.e. hot leg and surge line). This software was called "C-SGTR Calculator." It is capable of simulating multiple flaws in SG tubes, and calculating tube leakage probabilities, given Thermal-Hydraulic parameters of an accident sequence, tube flaws, material properties, etc. This study estimated the probability of containment bypass due to C-SGTR, and performed an assessment of the fraction of containment bypass that constitutes LERF (Large Early Release Frequency). The preliminary results of the study showed that the overall contribution of C-SGTR scenarios to LERF may be significantly different between the two selected plants. The study provided an understanding of the specific factors that can significantly contribute to LERF resulting from C-SGTR.