Predicting long-term performance of cement disposal systems for radionuclide-loaded zeolite and titanate ion exchangers

One of the most important processes used to decontaminate nuclear waste streams, such as those resulting from cleanup operations at Fukushima, decommissioning at Sellafield, and other nuclear industry operations, is ion exchange. In this process, the radioactive contamination is removed from water by being bound onto (or into) a solid ion exchange material. Once the capacity of these ion exchange materials (which are used in the form of pellets of zeolites or titanates, in the cases of interest in this project) to take up radioactive contamination is filled ('becoming 'spent'), the pellets must somehow be converted into a solid form to ensure that they are stable for storage and final disposal. The cementation of ion exchangers into solid waste forms has been proposed and trialled in a number of locations, and using a variety of types of cements.

However, there is not yet a good fundamental understanding of how the ion exchangers and the cements will interact in the long term - and this is the core focus of the proposed project. This information is essential to developing a safety case for the use of cementation for the final treatment and disposal of ion exchange resins; we must be able to predict how the materials will behave in the long term, including knowledge of any possible release of radioactivity in the distant future, to enable this to be minimised or avoided. This project will generate the essential fundamental scientific insight related to the stability of ion exchange materials in cementitious environments, using both traditional and newly-developed bespoke cement types. We will characterise the cements and waste forms to an unprecedented level of precision using sorption, solubility and microstructural measurements. This fundamental knowledge will be used to generate a predictive model for the performance of the waste forms over the full timescale required for final disposal of nuclear wastes, tens to hundreds of thousands of years. This will also enable us to provide recommendations for which types of cements should be used, and in which way they should be applied, to give the best outcomes in keeping radioactive contamination away from the environment in the ultra-long term.

The most important impact that will be gained from this project is the availability of a valid predictive model to support the safety case for long-term storage or disposal of conditioned wastes from the Fukushima Daiichi Nuclear Power Plant. The project team has the scope to communicate with leading Japanese government agencies, and report to them the advances made through our research, through the leading role of the Japanese side PI, Professor Sato, who has been involved in the Special Technical Committee at Radioactive Waste Management Funding and Research Center and the Expert Committee at International Research Institute for Nuclear Decommissioning (IRID). We will communicate our research outcomes, and their practical importance in a practical context, through direct interaction with key Japanese organisations such as the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) and the recently-founded International Research Institute for Nuclear Decommissioning (IRID). These groups have, respectively, national and international focus, are supported by the Japanese government and industry, and are intimately involved in the Fukushima clean-up project. A central mission of NDF is to enable appropriate and progressive implementation of decommissioning through the provision of advice, guidance and recommendations to nuclear operators including those involved in decommissioning, and the current technological efforts in Japan for the restoration of Fukushima are being conducted in accordance with the 2015 NDF Road Map. The role of IRID is more technical, and they research and develop technologies for nuclear decommissioning, promote cooperation with international and domestic organizations on nuclear decommissioning, and develop human resources for research and development. These activities also link well to the broader aims of the proposed project, and will bring added impact.

The thermodynamic modelling of cemented wastes, in particular those containing zeolite ion exchangers which are used extensively in the UK, is also a key focus area of UK industry including the National Nuclear Laboratory. Ongoing work in the UK portfolio includes an NDA Bursary project (PI Provis) on thermodynamic modelling of cementitious wasteforms, co-supervised by National Nuclear Laboratory staff, and the proposed project will also make use of the methodologies and collaborations developed through those UK-funded interactions to bring national benefit on both sides of the partnership.

The position of this project within Phase 3 of the UK-Japan Civil Nuclear Research Programme enables direct impact generation through links to projects funded in previous phases. The most direct link will be to the Phase 2 project EP/N017684/1 (Development of solidification techniques with minimised water content for safe storage of secondary radioactive aqueous wastes in Fukushima), 2015-2018, which is led on the UK side by co-I Kinoshita of Sheffield, in partnership with Kyoto University and JAEA. That project is experimentally-focused, and will result in the development of novel phosphate cement matrices for solidification of aqueous radioactive wastes. This proposed project will connect directly to, and build impact from, the work conducted in EP/N017684/1 by providing validated thermodynamic data which will enable the ultra-long-term performance prediction of these novel cementing binders, among the other systems studied here. This connection will bring synergy between the projects, and will enable more extensive leverage of the funding invested in that project as well as the funds requested here.