The significance of residual stresses in relation to the integrity of LWR pressure vessels

Abstract

This review is concerned with assessing the significance of residual stresses in relation to the integrity of nuclear reactor pressure vessels. The discussion is largely centred on light water reactors (LWRs), with particular emphasis on pressurised water reactors (PWRs); moreover, residual stresses are only explicitly considered in connection with PWR primary pressure vessels. However, much that is discussed is of general relevance to other reactor systems and pressure boundary components. The level and distribution of residual stresses in heavy section weldments and the factors affecting their relaxation with post-weld heat treatment (PWHT) are discussed; residual stresses are also considered in relation to the deposition of austenitic strip cladding and repair welding. A brief survey is made of currently available methods of measuring surface and sub-surface residual stresses in heavy section weldments; the effects of compressive residual stresses on the detection and sizing of planar defects are similarly considered. Available fracture mechanics methodologies with the capability of evaluating defect significance in the presence of residual and other secondary stresses are reviewed in some detail. On-going experimental investigations of the effects of residual stresses on structural integrity are also described. Following a general discussion, the desirability of internationally agreed Codes and Methods for assessing defects in the presence of residual and other self-limiting stresses is pointed out. It is argued that before such agreement could be achieved, however, further work is necessary in order to: (i) establish the level and distribution of surface and sub-surface residual stresses due to fabrication and repair welding: (ii) relate material properties, especially those influencing relaxation behaviour, to the level and distribution of residual stresses following PWHT; (iii) develop (or improve), and suitably validate, numerical and analytical methods for quantifying residual stresses and their effects on fracture behaviour. Within this context, a number of recommendations are given in Appendix 1 concerning areas requiring further research or engineering investigation.