Toward a Mechanistic Model of Stress Corrosion Cracking in PHWR Fuel Sheaths

Abstract

This work builds on the iodine-induced stress corrosion cracking (ISCC) model of Lewis and Kleczek by integrating the fuel performance model fuel and sheath modeling tool (FAST) to provide thermal and mechanical analysis of the fuel sheath, which was previously required as input parameters. The iodine transport methodology of the Lewis–Kleczek model has been modified to utilize the more mechanistic diffusion model in FAST, and an empirical surface multiplier term has been derived to predict iodine release rates under normal operating conditions based on measured release rates from in-reactor sweep gas tests. A fracture mechanics analysis is implemented using threshold stress intensity values and crack growth rates reported in literature. A correlation to predict crack initiation has been derived by analysis of a database of power histories with known ISCC defects. This correlation is based on the change in sheath hoop strain during a power ramp and is shown to be more accurate at discerning failure versus nonfailure than the correlations used in the previous Lewis–Kleczek model and FUELOGRAMS. Failure time prediction of the model is compared against power ramp test FFO-104 performed at the National Research Experimental (NRX) reactor. Fuel failure is predicted to occur 27% faster than experimentally measured; failure time is explored in a series of sensitivity studies to suggest areas for further development. The model improvements represent a step forward in the mechanistic modeling of stress corrosion cracking (SCC) in pressurized heavy water reactor (PHWR) nuclear fuel.