Development and Optimization of Multiphase Alloys and Oxide Composites for Corrosive Environments

Nuclear energy is considered to become one of the main sources to meet the clean electricity demand. However, the storage and disposal of spent nuclear fuel is of environmental concern. Pyroprocessing is being developed to recover and recycle the spent nuclear fuel to be reused in new fast nuclear reactors. This process generates highly radioactive waste streams, some of which are processed separately producing metallic and ceramic waste forms. Combining different waste streams will benefit the waste management operations and reduce its total volume. However, the metallurgical processing of these characteristically different waste streams can be challenging, to mitigate any possible radionuclide release in a geological repository environment. In this study, representative nuclear metallic waste forms have been fabricated that incorporate metal and ceramic wastes generated by the processing of used stainless steel-clad fuels into a single composite metal/oxide waste form. Four metallic alloys were made first with the same base formulation to represent waste streams dominated by 316L stainless steel cladding with small amounts of residual zirconia. The formulations were modified slightly by adding minor amounts of passivating elements, Mo and Zr, to improve the durabilities (corrosion performance) of the primary host phases. The metallurgical effects of Mo and Zr additions to the 316L stainless steel and the impact on the corrosion performance in multiple environmental conditions, such as pH, oxidation power (redox), and temperature were investigated. A series of well-established electrochemical tests were conducted and correlated with microstructural changes to determine the optimum multiphase alloy composition. The second part of the investigation focuses on determining the maximum oxide loading capacity of the optimal multiphase alloy for fabrication of a robust multiphase alloy/oxide composite and corrosion resistance. Electrochemical tests under the same wide range of Eh-pH-T conditions were conducted to demonstrate that durable multiphase alloy/ceramic waste forms can be formulated for the range of waste streams compositions that could occur during spent fuel reprocessing. This investigation provides a range of possible compositions to produce durable multiphase alloy and robust multiphase alloy-oxide composite waste forms to reduce the high-level nuclear waste to be stored in deep geological repositories.