Modelling the morphology of hydrides in fuel rod cladding under the conditions of the long-term bundle test in the framework of the SPIZWURZ project

A long-term bundle tests lasting 250 days were conducted at the QUENCH/LICAS facility in the Karlsruhe Institute of Technology as part of the SPIZWURZ project. The pressurized fuel rod simulators with commercial prehydrided non-irradiated claddings were tested under conditions close to the ones of dry storage of spent nuclear fuel. This paper compares the morphology of hydrides observed after this testing with the results of numerical simulations. The morphology of the hydrides largely determines the fracture mode of hydrated zirconium alloys, as an interconnected network of hydrides can conduct a brittle crack leading to a critical embrittlement. The morphology of hydrides under experimental conditions has been simulated numerically using two approaches. The first approach involves solving a system of differential equations that considers the processes of dissolution, nucleation, and growth of hydrides. This method predicts the average values of the main hydride morphology parameters during heating and cooling, including orientation, average size, and spacing between hydrides. The second approach is the MORPHYD tool, which simulates the nucleation and growth of separated hydrides in a 3D domain. It allows predicting any morphology parameters, including hydride continuity metrics, which are widely used in practice. The MORPHYD tool simulates not only average values of the morphology parameters, but also their statistical distribution based on Monte Carlo method. It makes possible to estimate the probability of a local critical hydride embrittlement. Theoretical approaches showed acceptable agreement with experiments for several hydride morphology metrics. Further model adjustments are planned following the receipt of additional experimental data from SPIZWURZ project.