Hydrogenation of Zircaloy-4 in a Defective Fuel Pellet for Pressurized Water Reactors

Abstract

The integrity of structural components is a major concern in reactor safety. Cladding materials can fail when hydrogen uptake is in excess of the terminal solid solubility (TSS). Previous studies on hydrogenation or secondary hydriding (SH) focused mainly on loss of coolant accident (LOCA) and fuel ballooning. Most of these studies are carried out in autoclave conditions and they preclude the contribution of ionizing radiations (neutrons and gamma) to hydrogen generation during normal reactor condition. This study is devoted to the hydrogenation of Zircaloy-4 (Zry-4) cladding material during normal reactor operation and during the early phase of core degradation in a LOCA event. Radiolysis and corrosion are considered as hydrogen sources during normal reactor operation while corrosion is considered in the early phase of core degradation. Using appropriate initial and boundary conditions peculiar to pressurized water reactors (PWRs), diffusion equation is solved in a defective Zry-4 fuel system using COMSOL Multiphysics 5.2. Results show that the average corrosive source term (1.279E−4 mol m−3 s−1) is higher than the radiolytic source term (3.6594E−7 mol m−3 s−1) by a factor of 350 during normal reactor operation. By integrating in two kinetics regimes and setting 6 years as the maximum lifetime of Zry-4, the amount of H dissolved in the cladding material at a temperature of 633 K are 6.54E−03 and 2.02 wt. ppm due to radiolysis and corrosion respectively. These values are safe compared to the TSS of dissolution (CTSSD) of Zry-4 which is 117 wt. ppm at the same referenced temperature. However, in the early phase of core degradation where the source term is 9.405E−01 mol m−3 s−1, SH is observed in 13 days. A comparison is made between hydrogenation by integration in two regimes and hydrogenation using Sievert’s law. The results obtained in this study are useful for the prediction of the onset of hydrogen embrittlement, and to determine the service time of Zry-4 cladding material.