The science
When compounds in spent nuclear fuel break down, they can release radioactive elements and contaminate soil and water. Scientists know that a spent fuel compound, neptunium dioxide, reacts with water, but they don’t fully understand the process. In this study, advanced electron microscopic techniques were used to study how the microscopic structure of neptunia drives chemical reactions that cause it to dissolve in the environment. The results showed that neptunium tends to dissolve where grains of the material come together, known as grain boundaries. Compared to smaller grains of material, neptunium has less of a tendency to dissolve at the grain boundaries of larger grains of material.
The impact
Nuclear power plants produce highly radioactive waste in the form of spent nuclear fuel. In order to prevent the escape of radiation, plant operators store spent fuel elements in basins and drying casks at nuclear reactor sites. However, this is not a permanent solution. In order to safely store radioactive materials for hundreds of thousands of years, underground final storage at geologically stable locations is required. Planning this storage requires thorough predictions of how the waste will chemically convert to ensure that it is environmentally friendly. This study shows that processing neptunium dioxide in a way that results in larger grains and fewer defects dramatically reduces neptunium’s solubility – its ability to dissolve. This reduces the environmental impact of nuclear waste. These findings will help make policy decisions about how to dispose of nuclear waste.
summary
Neptunium dioxide is found in old nuclear waste, which has a complex structure with nanoscale grains and distinctive grain boundaries. Grain boundaries are places where the crystal order of the solid is disturbed and often lead to increased diffusion and chemical reactivity. Grain boundaries in neptunium dioxide contain a soluble hydroxide phase that is easily oxidized and easily dissolved on contact with water and can lead to increased neptunium concentrations in natural waters. The erosion of grain boundaries causes whole grains to break out of the matrix and ultimately leads to neptunium in both aqueous and colloidal solutions, which can affect environmental behavior and transport assessment. This in-depth study of the microstructure of neptunium dioxide showed that processing the material at high temperature can increase its grain size by an order of magnitude. The high temperature recrystallization induces grain growth which reduces surface defects and surface area and decreases the free energy of the material. Larger neptunia grains provide increased stability and decrease solubility by two orders of magnitude. By investigating dissolution mechanisms at the solid-water interface, this study closes an important gap in understanding the release of radioactive elements into the environment. The results are expected to have wide-ranging environmental impacts for performance evaluation.
financing
This material is based on work supported by the Department of Energy Office of Science, Office of Basic Energy Sciences, and Office of Biological and Environmental Research.
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