| Abstract Scope |
The formation of Chalk River Unidentified Deposits (CRUD) on zirconium-based fuel cladding in Light Water Reactors (LWRs) poses a significant challenge to reactor performance and safety. CRUD can impair heat exchange efficiency, potentially causing localised overheating and cladding failures. This study focuses on characterising CRUD formed on a fuel rod segment after two cycles in a Pressurised Water Reactor (PWR) to improve the understanding of CRUD formation mechanisms and support the development of thermodynamic models for heat transfer in CRUD under operating conditions. Microanalytical techniques included Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA), and 3D tomography using a shielded Focused Ion Beam (FIB) with Energy Dispersive X-ray Spectroscopy (EDXS). The CRUD was observed to be tightly bonded to the underlying ZrO2 layer formed on the cladding surface, but exhibited an inhomogeneous thickness ranging from 2 to 7 µm along the azimuthal direction. Elemental mapping produced by Wavelength Dispersive Spectroscopy (WDS)-EPMA revealed a multiphase composition rich in Fe, Ni, and Zn, with minor amounts of Cr, Mn, and Co. Quantitative WDS-EPMA point analyses identified phases predominantly consistent with α-Fe2O3, Fe3O4, (Ni,Zn)Fe2O4. Additionally, localised Zn-enriched and sporadically Cr-enriched oxide phases were detected. 3D FIB tomography revealed that the CRUD comprises aggregates of oxide crystallites interspersed with a few large, isolated pores. This microstructural morphology is expected to affect heat transfer through the CRUD layer, an effect currently under investigation through thermodynamic modelling. |