Why spent nuclear fuel corrodes at the grain boundaries

What is it about?

Uranium dioxide is widely used as a nuclear fuel. When it corrodes, radioactive material can leak into the environment. Uranium dioxide ceramics are known to be most susceptible to corrosion at the boundaries between the small crystallites (grains). In this work, we show that the preferential corrosion at the grain boundaries can be explained through modified concentrations of chemical species at the grain boundaries. Through a combination of atomistic and continuum simulations, we predict that the grain boundaries become negatively charged and are surrounded by so-called space-charge layers, in which U⁵⁺ ions accumulate. This accumulation of U⁵⁺ can explain the preferential corrosion at the grain boundaries, since it locally increases the solubility of the material in water. On the basis of our electrochemical model, we suggest that corrosion at the grain boundaries can be reduced by acceptor doping, i.e. by introducing lower-valent cation species into the material.

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Photo by Zsolt Palatinus on Unsplash

Photo by Zsolt Palatinus on Unsplash

Why is it important?

Spent nuclear fuel must be safely stored for thousands of years, and uranium dioxide is the main material used in nuclear fuels. By reaching a better understanding of the corrosion of this material, we can better predict how nuclear fuel behaves on long timescales, and potentially find ways to make it more resistant to corrosion, thereby reducing the environmental impact of nuclear waste storage.