Kinetics of fission-gas-bubble-nucleated void swelling of the alpha-uranium phase of irradiated U-Zr and U-Pu-Zr fuel

Bias-driven growth of voids is identified as a major swelling mechanism in the α-uranium phase of irradiated U-Pu-Zr metal fuels being developed at Argonne National Laboratory for the Integral Fast Reactor (IFR). The trends in U-Pu-Zr swelling data prior to fuel/cladding contact can be interpreted in terms of unrestrained void swelling. It is theorized that the swelling mechanisms at work in the α-uranium phase can be modeled by single-vacancy and single-interstitial kinetics, thus avoiding the use of complicated defect-interaction terms required for the calculation of void nucleation. A mechanistic model is presented wherein gas bubbles that are formed on α/δ phase boundaries during the incubation period act as potential nuclei for cavities. The model has the capability to predict both gas-bubble-driven swelling (overpressurized cavities) and bias-driven void swelling (underpressurized cavities), as well as the transition from one swelling mechanism to the other. The results of this study demonstrate that the relatively long incubation times characteristic of IFR swelling and gas release can be understood in terms of a reduced gas-bubble nucleation rate on the α/δ phase boundaries, i.e., the α/δ phase boundary lamellae are less efficient gas-bubble incubators than normal high angle grain boundaries. Results obtained with the model are compared with U-10Zr and U-19Pu-10Zr deformation data, and with high purity uranium swelling data. (orig.)