The safety case of geological disposal
Much research has been conducted and significant knowledge has been amassed on the complex solid-fluid interaction of radioactive material and natural groundwater entering the spent fuel canister. However, there are still open questions. Among these is how results of laboratory experiments can be translated to the real repository environment. Scientists initiated the EU-funded project 'Reducing uncertainty in performance prediction' (REDUPP) to answer two different aspects of this question. The team investigated how results of relatively short-term dissolution experiments can be extrapolated to the very long times of the process in the repository. In addition, they explored the effect of trace elements found in natural groundwater. Spent nuclear fuel is mainly composed of uranium oxide (UO2), which has a fluorite structure. REDUPP research focused on a series of materials with similar fluorite structure, including cerium oxide and calcium fluoride. Extensive laboratory dissolution experiments were performed using fragmented or powdered samples that contained sharp edges and defects. During dissolution, the fragments became more rounded. Among the project goals was to determine how this gradual change of the sample surface affects the dissolution rate. By integrating their experimental results with computational modelling, REDUPP scientists developed a theoretical model of how the solid surface structure interacts with water during dissolution of spent nuclear fuel in groundwater. The effect of the presence of trace elements was studied using real groundwater in dissolution experiments with UO2. Previous experiments used synthetic groundwater having a different chemical composition than natural groundwater. The lack of all the chemical elements that appear in natural waters introduced a non-trivial uncertainty in past modelling efforts needing to be reduced. Extremely low solubility of some of the materials used required careful and precise analyses of the solutions. Highly specific analytical data was obtained by means of high-resolution inductively coupled plasma mass spectroscopy. The improvement of databases for spent fuel dissolution was another outcome of the REDUPP project. The results of REDUPP research are of interest to both research departments throughout the world and stakeholders in the nuclear waste management industry. Collaborations such as this are expected to have major impact not only on the fundamental issue of how solid surface structure affects dissolution, but also in reducing uncertainties in disposal safety assessments.
Keywords
Geological disposal, spent nuclear fuel, dissolution processes, underground repositories, groundwater