4.02.9.5.1 Dependence of irradiation creep on dpa rate

As mentioned earlier, once swelling begins, irradia­tion creep quickly assumes all the parametric depen­dencies of void swelling. However, for many years it
was assumed that the B0 component of creep was also strongly dependent on dpa rate, increasing as the dpa rate fell, as shown in Figure 81.

The original research that established this percep­tion was performed by Lewthwaite and Mosedale on various cold-worked steels in the Dounreay Fast Reactor at temperatures in the 270-350 °C range.178

The explanation advanced for such a dependence was the decreasing amount of annihilation of point defects by recombination at lower dpa rates, where such an effect is expected to be more pronounced at the lower irradiation temperatures characteristic of this experiment.

An earlier review article was published where this and other data sets were assessed to determine the appropriate rate dependence.1 Some data sets avail­able at that time supported a flux dependence and other data sets supported an independence of dpa rate. On balance it appeared that a strong depen­dence of irradiation creep rate on dpa rate was the more defendable conclusion.

With hindsight and additional published data sup­porting the opposite conclusion, it was later realized that apparent dependence of creep rate on dpa rate was an artifact of the analysis procedure used by

Mosedale and Lewthwaite. The authors had not prop­erly separated the transient and post-transient strains, and all of the lower flux data were in the higher-rate transient regime. When the DFR creep data were reanalyzed by Garner and Toloczko, the creep com­pliance B0 was found to be independent of dpa rate.179