A code system with attendant methods has been developed for modeling local fuel rod crud. This tool is used to perform the Crud Induced Localized Corrosion (CILC) risk assessment recommended by the EPRI crud and corrosion guidelines, which were developed in response to industry’s goal of zero cladding failures by 2010. The methodology is in production use. This paper will describe the range of problems to which the methodology has been applied, with special emphasis on low duty fuel applications. Computational Fluid Dynamics (CFD) computations over a fuel assembly grid span provide detailed relative variations in local heat transfer coefficient over the grid span. These very local relative variations are expanded by the HIDUTYDRV code to determine very local thermal hydraulic conditions over the entire axial length of every fuel rod in a reactor core over the life of the rod in reactor. The very local thermal hydraulic conditions are combined with reactor coolant crud concentrations derived from EPRI BOA analysis as input to models for predicting very local fuel rod crud deposition. The advanced crud deposition models also include models for calculating local crud dryout. Local crud deposition and crud dryout are strongly dependent on very local boiling or steaming. The local crud thickness and degree of local crud dryout are key factors in determining the margin or risk for crud induced failure. Based on the observed CILC cladding failures at what would traditionally be interpreted as a “low duty” plant and predictions in a number of real fuel design applications to other plants using very local fuel rod crudding analysis tools developed from the original CILC cladding failure observations, CILC fuel failure risk may be elevated in certain low duty plants. The elevated CILC cladding failure risk appears to arise in those situations in which sufficiently strong fuel rod steaming occurs over relatively small surface areas in the presence of typical or elevated reactor coolant crud concentrations for sufficient time. Assessment of the risk level is expected to require very local crud predictive tools comparable to the tools described herein.
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