DYNAMICS OF IRRADIATION-INDUCED GRAIN SUBDIVISION AND SWELLING IN U3SI2 AND UO2 FUELS

Observations on low-temperature swelling of irradiated uranium silicid e dispersion fuels have shown that the growth of fission-gas bubbles i s strongly affected by fission rate. The plot of swelling versus fuel burnup exhibits a distinct ''knee'' that shifts to higher fission dens ity with increased fission rate. State-of-the-art models of fission-ga s behavior do not predict such a dependence. Below the knee, no gas bu bbles can be detected by scanning electron microscopy. Just at the kne e, gas bubbles are seen to form in a heterogeneous fashion. Above the knee, the bubble population rapidly multiplies and the bubble size inc reases with fission density. ''Subdivision'' of the original grains ha s been observed in high-burnup uranium dioxide. In addition, the perip heral region of LWR fuel pellets reveals an increasingly porous micros tructure with burnup. Observations of this ''rim effect'' show that an extremely fine-grained structure formed by subdivision of the origina l fuel grains is associated with the porous microstructure. The singul ar observation that gas-bubble swelling is strongly dependent on fissi on rate has led us to speculate that a dense network of subgrain bound aries forms in UO2 and U3Si2 at high burnup. Fission-gas bubbles nucle ate at the newly formed boundaries and then grow at an accelerated rat e relative to that of fission-gas bubbles in the bulk material. A theo retical formulation is presented wherein the stored energy in the mate rial is concentrated on a network of sink-like nuclei that diminish wi th dose due to interaction with radiation-produced defects. Grain subd ivision is induced when the energy per nucleus is high enough that the creation of grain-boundary surfaces is offset by the creation of stra in-free volumes, with a resultant net decrease in the free energy of t he material. This formulation, applied within the context of a mechani stic treatment of fission-gas-bubble behavior, is shown to provide a p lausible interpretation of the observed phenomenon.