Fatigue loading can be very detrimental for situations involving cyclic loading, especially when associated with thermal cycling such as might occur in the first wall of a fusion device. As shown in preceding sections, radiation changes the microstructure and affects the phase stability ofsteels as well as generating deleterious gases such as helium and hydrogen.

Therefore it is not unexpected that fatigue life will be adversely affected by irradiation as shown in Figure 90.192

Fatigue tests are by necessity conducted out-of­reactor and therefore are not fully representative of in-reactor conditions, especially not being subject to the mitigating influence of radiation creep to reduce local stress concentrations. In this sense out-of­reactor results may be conservative. The tests can be conducted in a variety of ways, however, generally using either strain-controlled or load-controlled methods, with the former being more relevant to low cycle fatigue arising from thermal cycling. Guidance on the application of fatigue data is provided by Tavassoli.195

Figure 90 presents the usual engineering curves of total strain versus the number of cycles to failure. In this representation the lifetimes of irradiated and unirradiated materials are not really so dissimilar. The observed difference is the result of competing influences, degradation due to irradiation, and improvement due to hardening. As pointed out by Boutard,196 it is better to isolate the irradiation effect on the lifetime in which the controlling parameter is the plastic strain range.

As shown in Figure 91, there is a significant effect of radiation on the lifetime at a given plastic strain.196,197 The lower the plastic strain, the greater the decrease in lifetime. Under conditions where the crack initiation phase controls the lifetime of the unirradiated material, irradiation will result in much earlier crack formation

and much earlier failure. Other researchers have reached the same conclusion.198

In general it appears that most researchers agree that helium is a contributing but not primary cause of the radiation-induced degradation in lifetime.195-199

4.02.10 Conclusions

In general there are no beneficial aspects of radia­tion on austenitic steels when exposed to neutron

irradiation. Structural components used in various nuclear reactors may have been constructed from alloys with carefully tailored and optimized proper­ties, but there is an inevitable degradation of almost all engineering properties of interest as irradiation proceeds. Even more importantly, having labored to build a device with well-defined dimensions, separations, and tolerances, it must be recognized that these dimensional attributes can also change dramatically, requiring that the design anticipate such changes in order to maximize safe and efficient operation for the longest possible lifetime.

This evolution of properties and dimensions frequently determines the lifetime of any given structural component, a lifetime that will be very specific to each nuclear environment. It is important to recognize that all potential degradation processes may not yet have been identified and that others may lie over the current exposure horizon, espe­cially as light water reactors are being considered for life extension to 60 or 80 years, and as fast reactors are being designed for doses well beyond 200 dpa.