Estimating structural failure frequency of degraded steam generator tubes

We present a methodology to quantify the failure probability of degraded steam generator tubes for design-based, operational and severe accident scenarios. The heart of the tube integrity model is the mathematical correlation relating burst pressure to crack size, tube temperature, and tube dimensions for a given degradation mechanism. We revised and optimized the correlation developed by Pacific Northwest Laboratory (PNL) for outside diameter cracking and used it to extrapolate failure predictions up to severe accident temperatures (>1000 K). PNL developed the original correlation using operating temperature (approximately 600 K) data. The extrapolation has not been validated owing to a lack of high temperature data. Our tube integrity model differentiates between measured physical variability, such as material strength, and estimated uncertainty, such as calculated temperatures. The methodology we describe here can be used to identify the risk-significant accident for a range of tube degradation. Results can be used to assist in determining the reliability of a steam generator tube inspection process for a desired risk profile.