Engineered Zircaloy Cladding Modifications for Improved Accident Tolerance of LWR Fuel

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.

Although LWR technology has been identified as the key technology for future carbon emissions free electricity generation in the UK, the UK has minimal design and development base for fuel assemblies used for this type of reactors. The current approach relies on purchasing US or Continental European fuel assemblies. However, Westinghouse based in Springfield UK is currently preparing to assemble LWR fuel assemblies in the future. The opportunity to participate in the present programme has the potential for domestically initiating the development of new manufacturing capabilities to produce accident robust fuel assemblies as for instance the coating could be carried out completely separate from cladding manufacturing.

The present project will bring together US and UK researchers from the field of nuclear engineering, physics and materials to work in a seamless team, which will develop a novel new coating system for modern fuel assemblies used in current and future light water reactors. If this development is successful, the benefit will be immense as it will provide accident tolerant fuel assemblies, which will help to avoid accidents such as the one witnessed at the Fukushima plant when the cooling failed. Clearly, this will further help to accept nuclear energy as a safe option.

From a UK plc point of view, the present proposal represents an opportunity to be part of a new manufacturing development right from the beginning. Since Manchester hosts the Dalton Nuclear Institute and one of the two Rolls-Royce Nuclear University Technology Centres, as well as the research arm of the Nuclear Advance Manufacturing Research Centre, knowledge transfer to industry will be easy to achieve. Indeed, we plan to engage UK manufacturing companies as soon as promising results are obtained.

From an academic point of view, it is vitally important that UK academics, working in the field of nuclear engineering, interact closely with colleagues based in a country like the US. The nuclear industry and nuclear research work at a global level. Examples of this are for instance EPRI (funded by many nuclear companies from all around the world) and numerous research projects that are funded by a significant number of countries in a joint fashion (for example SCIP - Studsvik Cladding Interaction Programme). The present project will help UK academics to engage more on a global level and also create better links to US national laboratories, which have unique capabilities currently not found in the UK.