A new concept for steam generator tube integrity assessment

Abstract

Steam generator (SG) tubes are part of the primary circuit of pressurised water reactors and therefore also part of the pressure retaining boundary components. Furthermore, steam generator tubes are the components which guarantee the separation between the primary and secondary circuits. Therefore the knowledge of the loss of tube integrity as soon as possible is an essential. Optimised nondestructive testing methods based on the eddy current array technique are used to find material degradation, especially intergranular stress corrosion cracks during the periodical in-service inspection. These methods are supported by the measurement of the leakage rate during operation based on radioactivity and Tritium measurement regarding a high crack grows. Shutdown criteria depending on leakage rates which are described in the operation procedures but there are no specifications about the conditions for the measured leakage. Is the leakage coming from a single crack or are there multiple cracks or is this the leakage of a critical crack. With the help of FE calculations answer about the uncertainties of leakage rate versus critical crack grows were found. Shutdown criteria must guarantee to avoid critical crack sizes, i.e. between the measured tritium activity and the critical crack size must be a safety gap.