Accident Tolerant Fuels (ATFs) are the next generation of nuclear fuels, designed to maintain their integrity when exposed to the extreme conditions present during an incident, thereby preventing release of radioactive material to the environment. Uranium nitride is a leading ATF candidate due to its high thermal conductivity high specific heat capacity and low thermal expansion that suppress overheating and swelling in the event of a loss of coolant accident (LOCA). However, a significant obstacle to adoption of UN fuels in light water reactors (LWRs) is its extremely low oxidation resistance. Corrosion of fuel exposed to high-temperature pressurised water or steam, following a breach of the fuel cladding, would lead to release of fuel debris and fission products to the coolant system. Therefore, a strategy for increasing the oxidation resistance of UN, either through coating or doping is essential.
The aim of this project is to develop a mechanistic understanding of the corrosion of UN under reactor conditions, enabling the establishment of doping strategies to reduce the reactivity of the fuel’s surface making it safe for operation in LWRs. The project will employ a combination of state-of-the-art quantum mechanical simulations, supported by electrochemical and Raman spectroscopic studies of UN thin films, to understand how water interacts with the surface and how the introduction of defects and dopants, such as Cr and Si, will modify the surface chemistry and whether this can inhibit oxidation.
Originally discounted for use in light water reactors (LWRs) due to its unacceptably high oxidation rate when exposed to high temperature water and steam, uranium nitride is currently a leading candidate for use as an accident tolerant fuel (ATF). However, before UN can be employed in LWRs it is essential to develop a strategy to reduce the oxidation rate to acceptable levels and this forms the focus of this project.
An attractive method of passivating the UN surface is to use an additive that is homogeneously dispersed in a solid solution with the fissile material. The additive must react preferentially with the water molecules at the surface and form a stable oxide that is insoluble in water, which can then act as an oxidation barrier. The choice of additive is further constrained as it must be able to substitutionally solve into the nitride, leaving Zr, Al, Si and Cr as the leading candidates.
An important goal in the study of corrosion processes is to determine what is the rate determining step (i.e. the slowest process). This is often the transfer of oxide and hydroxyl ions through the protective oxide or the chemical reactions occurring at the surface. The objective of this work, therefore, is to determine the rate determining step in the oxidation process of UN and how this changes due to the incorporation of additives, thereby allowing the development of a doping strategy to improve the accident tolerance of UN fuel.
Contact: Dr Samuel Murphy