In-Reactor Deformation and Fracture of Austenitic Stainless Steels

An experimental technique for determining in-reactor fracture strain was developed and demonstrated. Differential swelling between a specimen holder and a test specimen with a lower swelling rate produced uniaxial deformation in 304 and cold-worked 316 stainless steel specimens. In-reactor deformations of 0.7 to 2.1 percent were achieved in Type 304 stainless steel previously irradiated to fluences up to 8.8 × 10²⁶ neutrons (n)/m² without fracture. These strains are significantly higher than found in postirradiation creep-rupture tests on similar specimens. From the measured strain values and published irradiation creep data and correlations, the stress levels during the irradiation were calculated. On the basis of previous postirradiation creep-rupture results, many of the specimens that did not fail would be predicted to fail. Thus we conclude that the in-reactor rupture life is longer than predicted by postirradiation tests.

Strain in a fractured specimen was estimated to be less than 3.8 percent, and the in-reactor fractures were intergranular—the same fracture mode as found in postirradiation tests. Irradiation creep may relax stresses at crack tips and sliding boundaries, thus retarding the initiation or growth of cracks, or both, and leading to longer rupture lives in-reactor. However, the very high ductility or superplastic behavior predicted by the strain-rate sensitivity of irradiation creep is not achieved because of the eventual interruption of the deformation process by grain boundary fracture.