Crack stability criteria in steam generator tubes

Steam generator tubes form a substantial proportion of the second fission product barrier in pressurized water reactors (PWRs). These tubes must perform this function with absolute reliability. EDF (Electricite de France) applies the leak-before-break concept to its steam generators to preclude any risk of tube rupture, whatever the operating conditions. Verification of the concept that leaks occur in normal operation, and provide warning of possible ruptures in the case of accidental overpressure, is supplemented and substantiated by periodical in situ examinations (non destructive examinations and taking of tube specimens). These programs are designed to monitor steam generator integrity, determine the type of damage or degradation and check regularly that such damage conforms with stress analysis assumptions (in terms of leakage and hazard to the plant). The 'leak-before-break' concept requires that the mechanical laws governing leakage and rupture of damaged tubes be thoroughly understood. To date, EDF and Framatome have recorded a number of cases of damage actually occurring on operating steam generators, or which could occur in the future based on foreign experience. This has enabled a wide-ranging program of experimental and theoretical investigations to be defined. The main results achieved in the first part of the experimental program are presented in this article. (orig.)