The long-term safety of a planned spent nuclear fuel repository needs to be demonstrated through a safety assessment. At the heart of this assessment lie the issues regarding spent fuel dissolution. Our present knowledge regarding this now significant; however, a few questions remain. The main objective of this project is to reduce the remaining uncertainty in the dissolution rate of spent uranium oxide fuel. A second objective is to train young scientists for the future needs of research in the field of nuclear waste management.
Nature and scope of the project
Our understanding of interactions between solid surface and fluid during the dissolution process will be significantly improved as a result of this project. The combination of dissolution testing, surface characterisation, and theoretical modelling of the dissolution process will provide a breakthrough in the understanding of the long-term changes in dissolution rate of spent nuclear fuel. The project management and research coordination are handled by SKB and Posiva, two nuclear waste management organisations at the forefront of spent nuclear fuel repository research . Two of the project participants, VTT (Finland) and University of Sheffield (UK) are, along with SKB and Posiva, active in the European Technology Platform for Implementing Geological Disposal. The fifth participant, Uppsala University, has a history of fruitful research collaboration with SKB concerning physics of actinide materials. The consortium has been designed to focus not only on addressing the research topic, but also to develop a training network for young researchers .
The project activities are mainly focused on dissolution experiments and surface characterisation of a suite of materials with the same crystal structure: CaF2, CeO2, ThO2 and UO2. Previous experiments, on which the determined dissolution rate of spent nuclear fuel in repository conditions are based, were performed with UO2 and synthetic ground water of a simplified and controlled composition. These waters do not contain all of the chemical elements that occur in natural ground waters, and the effect of the chemical complexity of natural waters on the dissolution rate is yet unknown. Commonly, in laboratory dissolution experiments, the samples are fragmented and the sample thus contain sharp edges and defects. During dissolution, material is more rapidly dissolved from the sharp edges and defects than from smoother areas, so that the fragments become more rounded as the experiment proceeds. One of the project goals is to determine how this “ageing” of the sample surface affects the measured dissolution rate. 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. Finally, by using first principles calculations and computational modelling, a model of how the surface evolves during dissolution can be produced. Within the project, young scientists will be trained and receive mentoring. The project will interact with PhD-students in associated networks, and provide a lecture series, and thus introduce students to the needs of the nuclear community in Europe. New knowledge which arises from the project will be communicated to a wider audience through a web page, newsletters and a final open meeting.
Through controlled laboratory experiments, the fractional dissolution rate of spent nuclear fuel has been assessed to be in the range of 10-6 to 10-8 per year, meaning complete dissolution of all spent fuel would take about 10 million years. Even though this may seem like a slow rate it is clear that it is faster than rates deduced from observations of uranium ores. There are plenty of examples of uranium oxide ores that have persisted for many hundreds of millions of years, thus implying that the dissolution rate in those ores have been much slower than 10-7 per year. The apparent discrepancy between rates in laboratory and in nature is addressed in this project by investigating processes at the solid-liquid interface. One expected result is a reduced uncertainty regarding the dissolution process, through a deeper understanding of surface changes during dissolution. The studied materials have very low solubility and analysed solutions will therefore contain very low amounts of the elements of interest. Analytical data of high quality is attained through analyses by High Resultion ICP-MS thereby this project will also contribute to method development, personnel training and improvement of data bases. The results will be published in a series of reports and in several scientific journal articles. The project web page and the newsletters will inform the public and interested organisations of the progress of the project. The aim is to present a general model would then be available for application in further research concerning nuclear fuel dissolution.
The project research is both connected to fundamental scientific questions, since it concerns the detailed mechanisms of dissolution, and to applied science in the nuclear industry. Therefore the sectors expected to show an interest in our work is both both nuclear waste management industry and research departments throughout the world. In particular, the project will maintain a close collaboration with the other organisations involved in the European Technology Platform for Implementing Geological Disposal. It is clear that international collaboration is necessary to achieve the goals of our project, and the project helps to strengthen communication between countries in a range of research areas. By ensuring efficient knowledge transfer between the experienced people in the field and young scientists this project contributes to the future European competitiveness in nuclear waste management. The overall aim of the project is to reduce the uncertainty concerning core issues of the safety assessments which will be developed and reviewed by the authorities in the near future. Since questions, comments and requirements are expected as a result of those reviews, we make sure that there are still skilled people available to respond to the comments when that time arrives.
Information about important public events
Planned public events are the publication of the annual reports and newsletters, which will be distributed to interested organisation and available on the project web page. The final project meeting will be an open scientific meeting. This will be announced to the public when more detailed plans are in place.