Past exam of the MPhil in nuclear energy course of the University of Cambridge / 2020 / Module NE3 / 1 / d / Solution

Void swelling is generally favored by an intermediate irradiation temperature window. The balance involves defect mobility, stable nucleation, absorption at defect sinks and thermal emission; a larger vacancy population alone does not guarantee a larger void.

At low temperature, vacancy migration is slow. Self-interstitials may migrate and form dislocation loops, and small clusters can cause considerable irradiation hardening, while vacancies cannot readily feed large growing cavities. Defects retained in the matrix can evolve during later heating.

At intermediate temperature, vacancies can migrate far enough to reach stable cavities. Interstitial bias and the competition among defect sinks sustain a net vacancy flux into these cavities. Once the incubation period has produced suitable nuclei and sink populations, substantial void swelling can develop.

At high temperature, the thermal equilibrium vacancy population and vacancy emission from small cavities rise. A small void's curvature further favors emission, so maintaining stable nuclei becomes harder. Recovery changes the dislocation population, and recombination, gas transport and evolving precipitates also change the flux balance. The irradiation-driven vacancy supersaturation can fall, suppressing void nucleation and net swelling. Existing cavities may still coarsen, and gas bubbles, material creep and other damage can remain important.

Thus low-temperature defect retention, intermediate-temperature cavity growth and high-temperature recovery/emission are different regimes. The boundaries depend on alloy composition, initial microstructure, gas production, accumulated displacements per atom and displacement dose rate. No alloy-independent peak temperature can be inferred from the paper's data.