REDUPP · Reducing Uncertainty in Performance Prediction

At the core of the safety case for long-term geologic disposal for spent nuclear fuel lie the issues regarding spent fuel dissolution. We have today significant knowledge regarding dissolution of uranium oxide in the laboratory. However, uncertainties remain regarding spent fuel dissolution under realistic conditions. Two main questions stand out as necessary to address: First, the synthetic groundwater used in the experiments to date does not contain all of the chemical elements that occur in natural groundwaters. Some of the trace elements may produce radiolysis products that are more aggressive than those produced in the synthetic groundwaters. Second, the fragments used in laboratory experiments contain sharp edges and defects generated by the crushing process. These sites contain atoms that have fewer bonds than the bulk material and constitute high-energy sites. As dissolution occurs, these sites dissolve faster than lower energy sites, such as plane surface sites. As the high-energy sites disappear through dissolution, the dissolution rate decreases. We can approach the long-term dissolution rate in the laboratory, but we cannot at this time estimate how far away we are from it. The two items mentioned above lead to uncertainty concerning the dissolution rate. The objectives of the work proposed here are to reduce the uncertainty in the dissolution rate to be used in the safety case and thereby increase the confidence that can be placed in our ability to demonstrate that the geologic repository will function as designed. A second objective of this work is to provide for the training of young research workers who can continue to support the research needed in the future concerning radioactive waste disposal.