Predicting the Flow Stress of Zircaloy-4 under In-Reactor Accident Conditions

During loss-of-coolant accident (LOCA) and reactivity-initiated accident (RIA), nuclear fuel rods experience high heating rates that change the microstructure and properties of zirconium cladding materials. We sought to determine how different fast heating rates affect the kinetics of transformation and whether their consequences on the material yield stresses in the dual-phase region could be predicted. Phase fraction and texture of cold-rolled Zircaloy-4 were thus measured at fast heating rates similar to those experienced during LOCA and RIA by in situ high-energy synchrotron X-ray diffraction; the yield stress was also measured at different heating rates to the same dual-phase temperature by the electrical resistance method. A crystal plasticity finite-element model was used to simulate the flow stresses with the inputs of the measured phase fraction and texture. The model was calibrated using the flow stress at the heating rate of 10 degrees C s(-1) and used to predict the flow stress at the heating rate of 50 degrees C s(-1) at the same temperature. A significant shift of the beta transus temperature was identified for fast heating rates in the range of 10 to 100 degrees C s(-1). There was great similarity in texture at different heating rates. In the dual-phase temperature region, the alpha texture was almost identical to the starting texture, and the 13 texture was nearly random. Yield stresses of cold-rolled Zircaloy-4 decreased at a faster heating rate at the same temperature of 920 degrees C. A good agreement was found between the experimental and predicted flow stress at different heating rates, indicating this methodology can potentially be used to predict flow behavior in a fast transient regime.