Recent experimental observations on low-temperature swelling of irradiated uranium silicide dispersion fuels have indicated that the growth of fission gas bubbles appears to be affected by fission rate. The swelling curve of the material exhibits a distinct “knee” that shifts to higher fission density with increased fission rate due to higher enrichments. Current state-of-the-art models for fission gas behavior do not predict such a dependence.
Indirect evidence from various experiments leads the authors to speculate that a dense network of subgrain boundaries forms at a dose corresponding to the “knee” in the swelling curve; fission gas bubbles nucleate at the boundaries and then grow at an accelerated rate relative to that in the bulk material. Compositional changes induced by the burnup of uranium could provide a basis for the formation of relatively small-grained crystals. In this paper, an alternative formulation is presented wherein the stored energy in the material is concentrated in a network of “recrystallization“ sites that diminish with dose due to interaction with radiation-produced defects (vacancy-impurity pairs). Recrystallization is induced when the energy per site is high enough that the creation of grain boundary surfaces is offset by the creation of strain-free volumes with a resultant net decrease in the free energy of the material. This formulation is shown to provide a reasonable interpretation of the observed phenomena.