Engineered Zircaloy Cladding Modifications for Improved Accident Tolerance of LWR Fuel

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This integrated research project aims to evaluate the modified Zircaloy LWR cladding performance under normal BWR/PWR operation and off-normal events. A combination of computational and experimental protocols will be employed to design and evaluate modified Zircaloy cladding with respect to corrosion and accelerated oxide growth, the former associated with normal operation, the latter associated with steam exposure during loss of coolant accidents (LOCAs) and low-pressure core refloods. Urania pellet modifications to improve thermal conductivity will be investigated as well, with the goal of reducing pellet temperature gradients, associated pellet swelling and pellet-cladding interaction (PCI), and fission product release. In addition, Pb-Bi eutectic liquid metal gap fillers will be investigated to promote pellet-cladding heat transfer. Both the cladding and pellet performance evaluations will be incorporated into a reactor system modelling effort of neutronics and thermal hydraulics, thereby providing a holistic approach to accident tolerant nuclear fuel. The proposed project brings together personnel, facilities, and capabilities across a wide range of technical areas relevant to the study of modified nuclear fuel and LWR performance during normal operation and off-normal scenarios. The proposed project leverages existing DOE NEUP support and extends collaborative activities from U.S. academia to U.S. industry, U.S. national laboratories, and to UK academia. Anticipated deliverables will be i) an experimental data base of modified cladding and pellet performance under normal BWR and PWR operational conditions and under off-normal LOCA conditions, ii) improved predictive capability of fuel performance codes, and iii) improved predictive capability of neutronics and thermal hydraulics performance codes. Two pathways toward accident tolerant LWR fuel are envisioned, both based on the modification of existing Zircaloy cladding. The first is the modification of the cladding surface by the application of a coating layer designed to shift the M+O to MO reaction away from oxide growth during steam exposure at elevated temperature. The second is the modification of the bulk cladding composition to promote precipitation of minor phase(s) during fabrication. These precipitates will be stable under normal operation, but dissolve during the temperature excursions; the migration of solute elements to the free surface would then shift the reaction away from oxide formation. Improved pellet thermal conductivity will act to limit cladding hoop stress via reduced fission gas gap pressure and PCI. A synergistic response of the fuel rod is anticipated in which the combined mitigation of brittle exothermic oxide formation, reduced cladding temperature, and reduced cladding stress lead to accident tolerance with respect to cladding failure.