Fission gas bubble nucleated cavitational swelling of the alpha-uranium phase of irradiated U-Pu-Zr fuel

Cavitational swelling has been identified as a potential swelling mechanism for the alpha uranium phase of irradiated U-Pu-Zr metal fuels for the Integral Fast Reactor being developed at Argonne National Laboratory. The trends of U-Pu-Zr swelling data prior to fuel cladding contact can be interpreted in terms of unrestrained cavitational driven swelling. It is theorized that the swelling mechanisms at work in the alpha uranium phase can be modeled by single vacancy and single interstitial kinetics with intergranular gas bubbles providing the void nuclei, avoiding the use of complicated defect interaction terms required for the calculation of void nucleation. The focus of the kinetics of fission gas evolution as it relates to cavitational swelling is prior to the formation of a significant amount of interconnected porosity and is on the development of small intergranular gas bubbles which can act as void nuclei. Calculations for the evolution of intergranular fission gas bubbles show that they provide critical cavity sizes (i.e., the size above which the cavity will grow by bias-driven vacancy flux) consistent with the observed incubation dose for the onset of rapid swelling and gas release.