Use of proton simulation to study issues of relevance to aging of austenitic internals in LWRs

Full text: s commercial light water cooled power reactors grow older, concerns are arising about various radiation aging processes operating to degrade the structural integrity of austenitic internal components. Some of these processes such as cracking, void swelling and creep relaxation of preloaded components have already begun to be observed. The study of these processes is often limited by insufficient data from the irradiated components and a difficulty to obtain these data without destructive extraction and examination of currently operating structural components. In some cases, however, studies of the processes of interest can be conducted using charged particle simulation. Two examples are presented that utilized proton irradiation. In the first case the relative swelling behavior of annealed 304 and cold-worked 316 was analyzed at PWR-relevant temperatures. These steels comprise the major structural and bolting materials, respectively, of PWRs in the USA, but data on the 304 steel is only available from the EBR-II fast reactor at higher than PWR-relevant temperatures. Also studied was the role of helium and hydrogen gases on swelling, which are found at much higher levels in PWRs. It was shown that the relative swelling behavior of the two steels that was observed in higher-temperature fast reactor irradiation was preserved under PWR-relevant conditions. In the second case, the influence of radiation to relax residual stresses was explored. Welding of BWR shrouds produces surface tensile stresses at the heat affected zone. To reduce irradiation cracking, peening is sometimes used to place the surface in compression. There is concern that compressive stresses are relaxed during irradiation, leading to early loss of compression. It is very difficult to predict the relaxation behavior of such complex, energy-bearing, largely compressive, near-surface stress states. Using proton irradiation, microscopy and X-ray stress analysis, however, it was shown that the stress indeed relaxes but is maintained as compressive throughout the expected maximum damage exposure of the BWR shroud