The prevention of fracture initiation in reactor structural materials

Abstract

The integrity of pressure retaining circuits in nuclear reactor systems is dependent on their tolerance to defects. When the size of a defect and its stress field reach a critical value, fast unstable fracture will occur. The relationship between defect size and stress field is brought together by the science of fracture mechanics. For materials failing under brittle conditions linear elastic fracture mechanics has been developed and applied, especially in U.S.A. to the fracture of 200–300-mm thick steel pressure vessels for PWR and BWR systems. For materials which fail under more ductile conditions (non-plane strain) general yield fracture mechanics, utilising a critical value of crack opening displacement (C.O.D.), is more applicable. This C.O.D. approach has been developed in terms of testing technique and its applicability to structures. Data are presented which demonstrate the test method as an extension of that used in linear elastic fracture mechanics test methods and, from tests on wide plates and pressure vessels, confirm the validity of the C.O.D. concept. Its direct use in reactor technology is exemplified in considering failure conditions in zirconium-alloy pressure tubes in the Steam Generating Heavy Water Reactor (S.G.H.W.R.). C.O.D. test pieces are used for surveillance of embrittlement due to neutron damage and hydrogen pick-up and the C.O.D. values from such specimens have shown good agreement with the measured failure conditions of experimentally irradiated pressure tubes. It is concluded that general yield fracture mechanics offer a viable concept of quantifying failure conditions in the more ductile materials. Whilst fracture mechanics permits a good prediction of failure conditions of materials, its application is dependent upon a knowledge of stress fields and actual defect sizes in structures. Defects may be found and quantified by non destructive testing before service. During reactor operation growth to the critical size may occur by pressure cycling, thermal cycling, stress corrosion etc. Such growth should be quantified by periodic inspection of the structure during service. The use of surveillance test pieces permits the changes in toughness due to service conditions to be evaluated throughout reactor life; knowledge, by direct measurement, of changing defect sizes in the structure permits the safe remaining life to be calculated.