When compounds in spent nuclear fuel break down, they can release radioactive elements and contaminate the ground and water. Scientists know that one spent fuel compound, neptunium dioxide, reacts with water, but they do not fully understand the process. A study has used advanced electron microscopy techniques to investigate how the microscopic structure of neptunium dioxide drives chemical reactions that lead it to dissolve into the environment. The results revealed that neptunium tends to dissolve where grains of the material come together, known as grain boundaries. Neptunium is less prone to dissolve at the grain boundaries of larger grains of material as compared to smaller grains of materials.