The effect of hydrogen on the mechanical behavior of zircaloy-4 alloy

Zirconium alloys are used as fuel cladding material in Light Water Reactors (LWR) and act as the first barrier to fission product release by physically separating uranium oxide fuel pellets and primary coolant. During the operation of the reactor, the surface of the cladding reacts with water to form an oxide film layer on the surface. This reaction also produces hydrogen, which has important consequences for steady state and accident situations. Hydrogen can spread from the metal-oxide interface to the matrix due to its small atomic size. In cladding, hydride precipitation occurs above the solubility limit. Hydrides are fragile and can reduce the ductility of the cladding. Exothermic character of the reaction at higher temperatures leads to faster reaction speed. Although the mechanical behavior of zirconium alloys as a function of temperature and the strong effect of hydrogen on the fracture behavior of zirconium alloys are relatively well established, relatively little is known about the effects of hydrogen (and the resulting hydrides) on mechanical behavior. Recent findings have made it clear that hydrogen plays an important role in the embrittlement of zirconium-based alloys and therefore must be correctly estimated with fuel performance codes to assess the condition of the fuel rods after various design-based accidents. The potential presence of such effects is particularly important for high combustion fuel where hydrogen uptake within the fuel cladding can be significant. The mechanical behavior is one of the critical inputs for fuel modeling codes, and therefore a mechanical understanding of the deformation behavior is critical for reactor safety. In this study, Zircaloy-4 type zirconium-based fuel cladding material, which is frequently used in pressurized water reactors (PWR), is modeled as a one-dimensional cylinder with the HUNEM-1.0 fuel performance code which is developed in this study. In the model, the time-dependent diffusion coefficient and mass gain of oxygen were first calculated and then the oxide displacement model was developed with oxide metal interface temperatures. An oxide thickness was determined by including the effect of heat flux, temperature gradient, rapid neutron flux and coolant chemistry. Finally, the mechanical and also the deformation behavior of the Zircaloy-4 alloy in terms of the hydrogen concentration was modeled with the help of empirical correlations by HUNEM-1.0 fuel performance code through the correlations obtained from the literature with the thickness of the metal-oxide interface and the hydrogen collected in the alloy. In order to verify some of the HUNEM-1.0 fuel performance code outputs, a similar cylindrical fuel rod in one dimension was modeled with the FRAPCON-3.4 fuel performance code and the results were evaluated. As a result of the study, it has been shown that the effect of the hydrogen concentration on the mechanical and therefore deformation behavior of the surface of the zirconium-based Zircaloy-4 alloy in contact with the coolant is reasonably low under normal operating conditions.