Study of a thick CRUD on irradiated fuel rod: correlating thermodynamic calculations and X-ray characterizations

A significantly elevated layer of so-called CRUD (corrosion deposit) was observed on the surface of a boiling water reactor irradiated fuel rod, potentially leading to hot spots and subsequent cladding rupture. To study this phenomenon, comprehensive investigations were conducted employing two complementary approaches: computer-aided chemical thermodynamic calculations and experimental characterizations. Micro-beam X-ray diffraction (XRD) combined with X-ray fluorescence (XRF) at the Swiss Light Source (SLS) facilitated the identification and localization of phases. Notably, the distinctive presence of Si-rich phases (Zn 2 SiO 4 as the main phase and SiO 2 as a minor component) was observed in the thick CRUD sample, while absent in the reference sample. Furthermore, thermodynamic calculations were conducted to simulate CRUD formation at prototypical conditions and scenarios, supplementing the experimental results. These simulations demonstrated that under typical oxidizing conditions, Zn 2 SiO 4 , Fe 2 O 3 , ZnO and Zn-ferrites are the predominant phases formed, which is consistent with experimental observations. The synergy between simulations and experimental data improves our understanding of CRUD formation mechanisms and CRUD composition and highlights the predictive capacity of thermodynamic simulations in studying the intricate corrosion phenomena.