APPROACH AND METHODOLOGY FOR CONDITION ASSESSMENT OF THERMAL REACTOR PRESSURE-VESSELS

In this paper we review the approach employed in the UK to characteris e the embrittlement of thermal reactor pressure vessels (RPVs). The ex perimental techniques developed to characterise the neutron energy spe ctrum and fluence are described; these include the use of multifoil ac tivation packs and a direct damage monitor. The techniques for the ret rospective assessment of the exposure of service irradiated components and the methods for the prediction of end-of -life doses in RPVs are discussed. Displacements per atom (dpa) is the main exposure and corre lation parameter employed and the success of using it to measure damag e dose is evaluated. A variety of techniques is used to characterise t he changes in mechanical properties in both materials test reactors an d power reactors. These include fracture toughness, Charpy impact and tensile testing. Mechanical properties inter-relationships are particu larly helpful when evaluating the condition of irradiated materials; t he Charpy-tensile, Charpy-hardness and Charpy-fracture toughness corre lations are reviewed. An important aspect of research into materials b ehaviour in the UK has been the development of mechanistic models of t he embrittlement phenomena. These models have been reasonably successf ul in the interpretation of embrittlement in copper-containing materia ls, particularly in rationalising data obtained at different dose rate s. The approaches are summarised. Microstructural techniques based on transmission electron microscopy, small angle neutron scattering and a tom probe/field ion microscopy have been developed to characterise the microstructure and microchemistry of unirradiated and irradiated copp er-containing materials. The contributions these techniques have made to the understanding and prediction of embrittlement processes are des cribed.