Addressing self-irradiation damage and its impact on the long-term behaviour of nuclear waste matrices

Nuclear fission offers a reliable low carbon source of energy, but, the nuclear waste generated as a result of nuclear reactor operation needs proper treatment and confinement in a durable material to ensure that the biosphere is not contaminated with radioactive elements in the near and long-term future. Geological disposal (GD) - which involves confining the host material inside a safety barrier (usually a metal canister) and then permanent deposition of such wastepackages in a pre-selected geological site - is now an internationally accepted methodology including the UK. Nonetheless, after thousands of years, the outer safety barrier will get corroded and the host material will be exposed to the surrounding geological conditions. When in contact with water/moisture, the radioelements may be released from the host matrix into the surrounding geology from where they can be transported into the biosphere. Understanding long-term changes in the wastepackages -starting from the day of their fabrication - is a key element in addressing the eventual release of the radioisotopes. Besides corrosion, one of the reasons why the wastepackages will change under geological disposal conditions is the fact that radioactive decay of the confined radioisotopes will damage the host matrix at atomic level called as self-irradiation damage. This damage accumulation over hundreds of thousands to millions of years can potentially alter the chemical and mechanical durability of the wastepackages. These irradiation induced modifications can have a significant effect on the eventual release of the radioisotopes. Thus, addressing radiation stability of the wastepackages is an essential part of demonstrating long-term safety of the geological disposal.
This research proposal will utilize MIAMI irradiation facility at the University of Huddersfield to study the effects of self-irradiation damage and He accumulation in various types of waste packages ranging from glasses to glass-ceramic composites. Using a transmission electron microscope with in-situ dual-ion-beam irradiation, the irradiation induced modifications will be monitored in real time. The dual-ion-beam irradiation represents the closest analogue to self-irradiation damage in nuclear wasteforms yielding reliable and realistic results. These ion irradiation effects will be compared with actinide doping studies to be undertaken in collaboration with nuclear industry partners, thereby, allowing establishing the irradiation conditions necessary to simulate the self-irradiation damage. The research will be undertaken on leached (gels) and non-leached materials to understand the irradiation induced evolution of the wastepackages and address the effect of radiation damage on the leaching and vice versa. By collaborating with external partners such as ANSTO Australia, CEA Marcoule France, University of Cambridge and, National Nuclear Lab UK, this proposal will bring together the experience and expertise of internationally recognised researchers to develop a better understanding of the wasteform evolution due to self-irradiation damage under geological disposal conditions including leaching.

One of the important outcomes of this research will be to help demonstrate long-term safety of nuclear wasteforms and geological disposal (GD) with regards to radiation damage effects. This is an important aspect in the safety assessment of wasteforms to understand and address any environmental determinants to public health. Through this research, the underpinning role of grain boundaries and grain sizes will directly feed into decisions regarding the future deployment of thermal treatment technology across the NDA estate. In order to apply the results, both Sellafield Ltd. and RWM will need to consider any appropriate changes to their safety cases for storage and disposal and/or waste acceptance criteria. Furthermore, the research will identify and potentially optimize the safe waste loading fraction of different nuclear wasteforms which can help reduce the number of waste packages. This can potentially help bring down the long-term maintenance, transport and disposal costs which are a significant part of the £100 billion to £ 225 billion required over the next 120 years for cleaning and disposal of nuclear waste in the UK. By participation in IAEA's INWARD project, the research will help set up an international standard for studying ion irradiation effects in nuclear wasteforms and help shape research and policy decisions at a global level. The collaborations with; Australian Nuclear Science and Technology organization, Australia; University of Cambridge, UK; National Nuclear Lab, UK and CEA, France and, participation in international projects such as International Atomic Energy Agency's INWARD research project will lead to an effective means of sharing knowledge, practice and expertise among a diverse research base. This will also help strengthen and build further international research collaborations. The workshops, planned to take place at the University of Huddersfield, will also provide a platform to disseminate the results to a wide research base from academia to industry and broaden the research engagement. The research outcomes will have a significant impact on national and global policy decisions and will be critical in aiding modellers and theoreticians in developing models necessary to forecast the behaviour of wasteforms under disposal conditions.