A Probabilistic Approach to the Estimation of Lifetime Distribution of Alloy 800 Sg Tubing

Abstract

Alloy 800 has been used for steam generator (SG) tubing for more than 30 years, primarily in CANDU reactors and in reactors in Germany. Extensive laboratory testing and in-service experience suggest that the Alloy 800 tubing has excellent resistance to corrosion-related degradation under specified and appropriate operating conditions. In planning refurbishment of CANDU stations, a key concern is the longevity of existing SGs up to the 60 year lifetime of the refurbished plant. For SGs with Alloy 800 tubing, the lack of corrosion-related degradation to date implies that assumptions need to be made about the probability that a corrosive environment could occur (in the domain of electrochemical corrosion potential and pH) that would cause degradation of the tubing. The objective of this paper is to develop a probabilistic approach to estimate the degradation free lifetime (DFL) distribution of A800 tubing and to quantify the probability of the lifetime exceeding 60 years. The paper reviews an existing methodology based on the concept of the improvement factor, and estimates it based on experimental data specific to CANDU operating conditions. The paper modifies the improvement factors method and determines the Weibull lifetime distribution using the Bayesian method in which the in-service experience of CANDU SGs is integrated.