| Popis: |
Recent observations on experimental low-temperature swelling of irradiated uranium suicide 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 of fission-gas behavior do not predict such a dependence. Grain “subdivision” has been observed in high-burnup uranium oxide. This observation and other indirect evidence from various experiments led the authors to speculate that a dense network of grain boundaries forms in uranium silicide 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 of fission-gas bubbles in the bulk material. A theoretical formulation is presented wherein the stored energy in the material is concentrated on a network of recrystallization sites that diminish with dose due to interaction with radiation-produced defects (vacancy-solute pairs). Recrystallization is induced by statistical fluctuations 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, applied within the context of a mechanistic treatment of gas-bubble behavior, is shown to provide a plausible interpretation of the observed phenomena. |
