The Role of Phase-Boundary Fission-Gas Bubbles in the Cavitational Swelling of Irradiated U-Pu-Zr Fuel

Cavitational void swelling, the bias-driven growth of voids, has been identified as a potential swelling mechanism in the alpha-uranium phase of irradiated U-Pu-Zr metal fuels for the Integral Fast Reactor being developed at Argonne National Laboratory. The trends in U-Pu-Zr swelling data prior to fuel cladding contact can be interpreted in terms of unrestrained cavitational-driven void 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 phase-boundary gas bubbles providing the void nuclei, thus avoiding the use of complicated defect-interaction terms required for the calculation of void nucleation. The focus of the examination of the kinetics of fission-gas evolution, as it relates to cavitational void swelling, is on the period prior to the formation of significant interconnected porosity and on the development of small phase-boundary gas bubbles that can act as void nuclei. Calculations for the evolution of phaseboundary fission-gas bubbles show that such bubbles 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.