Correlation of irradiation creep data obtained in fast and thermal neutron spectra with displacement cross-sections

Irradiation creep data are available for 20% cold-worked M316 stainless steel, 20% cold-worked FV548 stainless steel, solution-annealed 304 stainless steel and aged Nimonic PE16 for the same heats of material and state of stress tested in thermal and fast neutron reactors. The transient and steady state irradiation creep coefficients were calculated using spectrum-averaged displacement cross-section dose units (i.e. displacements per atom). The transient irradiation creep coefficients appear to be subject to considerable scatter. The steady state irradiation creep coefficients were found to be a consistent factor of 2.5 larger in thermal than fast reactors. These results are in excellent agreement with the fast reactor dose rate dependence of the steady state irradiation creep rate observed by Lewthwaite and Mosedale. However, the dose rate effect directly depends on the displacement cross-section, which is a calculated dependent parameter. The available information suggests that the displacement cross-section function uncertainty is the major effect responsible for the 2.5 factor in irradiation creep coefficients. (orig.)