Book Description

Nuclear fuel cladding undergoes waterside corrosion during normal operating conditions in pressurized water reactors, whereby the zirconium (Zr) in the fuel cladding reacts with the oxygen present in water, creating zirconia (ZrO) and releasing hydrogen. Part of the hydrogen created by the corrosion reaction can be absorbed into the fuel cladding. Once in the cladding, hydrogen redistributes by solid state diffusion in the metal, in response to gradients of concentration, temperature and stress. Once the local hydrogen solubility is exceeded, zirconium hydride precipitates are formed.The precipitation of hydrides may impact the integrity of zirconium-based nuclear fuel cladding, both during normal operation and during extended dry storage. It is important to model hydrogen behavior accurately, so as to assess cladding properties both in reactor and during dry storage. This is because the cladding is the first containment barrier, which prevents fission products to be released into the primary circuit. For this reason, this study aims to first understand hydride precipitation and dissolution and then implement this understanding into a hydride precipitation and dissolution model. To this end, differential scanning calorimetry (DSC) and in-situ synchrotron X-ray diffraction experiments were used to study the precipitation and dissolution of hydrides in Zircaloy-4 under different thermo-mechanical conditions.Results showed that when hydrided samples were cooled at cooling rates above 1C/min the hydrogen content in solid solution decreased, following the Terminal Solid Solubility for Precipitation (TSSP) curve. However, when the samples were held at a fixed temperature for a long anneal, the hydrogen content in solid solution continued to decrease below the TSSP and approached the Terminal Solid Solubility for Dissolution (TSSD). This result suggests that TSSP is a kinetic limit and that a unique solubility limit, i.e. TSSD governs the equilibrium hydrogen concentration in solid solution. DSC was used to perform isothermal precipitation experiments, from which the hydride precipitation rate and the degree of precipitation completion were quantified between 280 and 350C for the first time. The data obtained was used to generate a TTT diagram for hydride precipitation in Zircaloy-4 showing that hydride precipitation is diffusion-controlled at low temperatures and reaction-controlled at high temperatures. The experimental precipitation rate was fitted using the Johnson-Mehl-Avrami-Kolmogorov model to obtain a value of the Avrami parameter of 2.56 (2.5 is the theoretical value for the growth of platelet-shaped precipitates). It was also possible to derive the precipitation activation energy of for each process. Because it was possible to separate hydride nucleation and hydride growth, it was possible to ascertain that if the hydrogen content in solid solution is greater than TSSP, precipitation occurs by hydride nucleation. In contrast, precipitation occurs by hydride growth as long as hydride platelets are present and the hydrogen content in solid solution is above TSSD. Hydride dissolution will take place if hydrides are present and the hydrogen content in solid solution is below TSSP. Using this new understanding of hydrogen precipitation and dissolution mechanisms, experiments were conducted at the Advanced Photon Source (APS) using high temperature change rates to measure hydride nucleation and dissolution kinetics. These observations and measurements were combined to existing theory to a model, entitled Hydride Growth, Nucleation, and Dissolution model (HNGD model) that can accurately simulate hydrogen behavior in Zircaloy fuel cladding and that shows a significant improvement on the model used in BISON.The development of such a model is the first step towards obtaining a model for the impact of the development of hydride microstructure on nuclear fuel cladding mechanical properties during normal operation and to address concerns over fuel handling during dry storage. The use and benchmarking of such a code can be used to justify a safe burnup extension of nuclear fuel, which would reduce the cost of nuclear energy in an increasingly competitive market.