2021 Volume 70 Issue 7 Pages 217-227
This study aims to estimate the stress relaxation of the shot-peened austenitic stainless steel storage container （called as the canister） for the used nuclear fuel, at temperatures lower than 200℃ during the long term service period （approximately 80 years）. We first show the relaxation data of residual compressive stresses of the peened Type-304 and -316 steels which are aged at 340 C in air for 18,080 hours, and then the relaxation of residual stresses at temperatures lower than 500 C. We estimate that the activation energy for the stress relaxation is 84～89 kJ/mol by the Arrhenius plot of the times for the initial residual stresses to come down to 80％ that of the initial stress values （called as the 80％ time）. We, next, compute the Larson-Müller Parameter （LMP） using the constant of 10, and determine two boundary LMP lines which separate the three relaxation levels, i.e., higher than 60％ of the initial stresses, lower than 60％ but higher than 30％, and lower than 30％, on the temperature vs. time diagram. We estimate that the compressive residual stresses of the shot-peened austenitic stainless steel canister will be relaxed to the stress levels higher than 30％ of the initial stresses.
Though the relaxation of residual stresses is closely related to the change of structure and mechanical properties of the aged stainless steels, these will be discussed in the next technical report.
