Validatiom of the theoretical approach for Modelling of the Hydride Morphology with experimental Data obtained from Spizwurz Bundle Test

This paper presents the calibration and assessment of two complementary simulation tools using new experimental data on hydride morphology in zirconium alloy fuel cladding. The first tool is based on the numerical solution of a system of ordinary differential equations, predicting averaged microstructural quantities including the radial hydride fraction, hydride length and hydride volume density. The second, MORPHYD explicitly simulates the nucleation and diffusion-controlled growth of disc-shaped hydrides within a virtual 3D domain. In the present work, MORPHYD is applied only to 100 wppm cases because its current version is limited to precipitation-only simulations of small, non-overlapping hydrides. MORPHYD also enables the evaluation of hydride network connectivity metrics and their statistical distributions based on the Monte Carlo method, repeating the simulations a statistically significant number of times. In both approaches, hydride nucleation is described within a classical heterogeneous framework in which the defect energy for nucleation sites is defined by the user-specified distribution. Both modelling approaches were calibrated using new experimental data obtained from the SPIZWURZ bundle test on cladding tubes manufactured from Zircaloy-4 and opt. ZIRLO. The SPIZWURZ included long-term hydride reorientation experiments conducted at the LICAS facility of the Karlsruhe Institute of Technology. The test bundle consisted of 21 hydrogen-charged cladding tubes, each 2.5 meters long, which were subjected to slow, stepwise cooling over 250 days to reproduce as close as possible dry storage conditions for spent nuclear fuel. The following morphological parameters were measured experimentally across a spectrum of hydrogen concentrations and applied stresses: radial hydride fraction, hydride length and surface density, hydride continuity coefficient (HCC) and radial hydride continuity path (RHCP). The results show that, after calibration, both simulation tools reproduce the main experimental trends in hydride morphology and connectivity, supporting their further development and application for assessing hydride embrittlement of fuel rod cladding under dry storage conditions.