Axial deposition mechanism of corrosion-related unidentified deposits on fuel cladding

The formation of corrosion-related unidentified deposits (CRUD) on fuel cladding remains a critical technical bottleneck for the operational safety of pressurized water reactors (PWRs). Current researches generally assume that CRUD morphology parameters are axially uniform. This reliance on CRUD axial uniform distribution stems from the inherent limitations of traditional axial thermodynamic condition-restricted experiments, which fail to capture the axial evolutionary continuum of CRUD. To address these issues and fill this gap, a CRUD advanced loop tester has been developed to disclose successive flow-boiling regimes with thermodynamic development, enabling high-resolution characterization of axial spatiotemporal evolution along the full length of simulated fuel rods. The results reveal that the CRUD layer thickness, micro-chimney, and porosity follow a non-monotonic V-shaped distribution along the flow path. The substantial axial variations are observed in both the local critical heat flux (CHF) and the degree of neutron poison hideout, with maximum differences reaching 300% and 400%, respectively. A dual-exponential axial distribution mechanism with two competing processes is identified, whereby CRUD deposition is driven by particle inertial deposition in the upstream region and governed by boiling-induced crystallization growth in the downstream region due to enhanced subcooled nucleate boiling (SNB). A CRUD axial deposition dual-exponential model (CADDEM) is established to quantitatively characterize this spatial interaction. Furthermore, the CADDEM provides various potential application.