A review of the U.K. collaborative programme to test the effects of mechanical and environmental variables on environmentally assisted crack growth of PWR pressure vessel steels

Abstract

Knowledge of the rate of growth of possible defects in the wall of the PWR pressure vessel is necessary for in-service validation of vessel integrity. Large inter-laboratory variations between measured cyclic crack growth rates of pressure vessel steels in a simulated PWR environment have been reported. A collaborative programme is underway in the U.K. in order to resolve the reasons for these discrepancies. Corrosion fatigue testing under this programme is carried out at laboratories at UKAEA Harwell (AERE) and Springfields (SNL), the Central Electricity Generating Board Laboratories (CERL), Babcock Power Ltd (BPL) and Rolls Royce and Associates (RR &A). The current ASME XIA flaw evaluation code is based on data showing the greatest degree of sustained environmental cracking observed to date. The measured rate of crack extension is found to become less reproducible as the cyclic frequency is reduced towards values more representative of operating plant transients. Although instances of crack extension in excess of the ASME code have been recorded, these high cracking rates are not maintained, but decrease towards rates found in an inert environment after a few millimetres of growth. Several interacting parameters are found to influence the rate of crack extension. These include water flow rate, steel sulphur content and water chemistry, including oxygen and sulphate concentration. This paper reviews recent results obtained by the above laboratories under the U.K. collaborative environmentally assisted cracking programme. Current mechanistic interpretations of environmentally assisted cracking are discussed. Knowledge of the processes producing accelerated growth rates is necessary to allow the experimental data base to be extrapolated to plant transient conditions.