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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DISCO (Modern spent fuel dissolution and chemistry in failed container conditions)

Teaser

Summary

The DisCo project aims to improve the scientific understanding of the basis of the safety cases for Spent Nuclear Fuel repositories. The specific issue is whether the kinetics of the spent fuel dissolution process is affected by the composition and characteristics of the spent fuel itself. Development and improvement of nuclear power also involves development of the fuel used in the reactors. The behaviour of these modern fuels after being burnt up in the nuclear reactor may be different to the one of the traditional spent nuclear fuel. The overall objectives of the DisCo project are 1) to enhance our understanding of spent fuel matrix dissolution under conditions representative of failed containers in reducing repository environments; and 2) to assess whether novel types of fuel (MOX, doped) behave in a similar manner to conventional fuels. Experimental and modelling tasks are defined to achieve the project objectives.

The expected knowledge gain is essential for Waste Management Organisations and will provide new insights into factors affecting their safety cases as fuel systems have evolved: current repository designs should be appropriate for disposing safely also these new fuel types. The results are also of interest for a wider range of potential users, such as researchers working with fundamental scientific questions concerning, for example, materials science, catalysis, corrosion and environmental geochemistry. Fundamental scientific endeavours lie behind many innovations and improvements in society. Through the planned activities, the project will facilitate knowledge transfer between countries and generations in a research field which will require continuous force of qualified personnel for a long time.

Work performed

Overall, the project is progressing as planned. As a part of the annual meeting, the End User Group (EUG), consisting of representatives of Waste management Organisations and Regulatory Authorities, discussed the scientific contributions. The consensus among the EUG members was that the research appears well aligned with the work plan and scope of the project, the contributions are of high quality and focused on the work ahead.

All deliverables defined for the first reporting period have been submitted. The project employs the communication tools foreseen, such as a functioning web page, a LinkedIn Group, a twitter hashtag and periodical Newsletters informing on events and the advance of the project. The Consortium Agreement is signed by all parties and the foreseen End User Group and Associate Group Agreements have been signed. All groups have started work with no major deviations regarding the initial planned project schedule.
Much of the work during the first reporting period has been focused on sample preparation and characterisation, as expected from the project plan. Most samples have been prepared and characterised for the onset of dissolution experiments. This involves careful analyses of the products both during sample preparation and characterisation of the final product. The work performed has already allowed some insight regarding the effect of dopants on the UO2 lattice. The results are reported in deliverable D2.1. Some model materials have taken longer than foreseen to synthesize, due to various circumstances. This delay is however not expected to have a serious impact of the delivery of dissolution data to the modellers expected later in the project.

Dissolution experiments using real spent fuel and model materials have started. Spent fuels used are conventional UOX fuel, Cr-& Al-doped UOX fuel, and MOX fuel. The dissolution experiments of the model materials are designed to complement the spent fuel experiments. The experiments are mainly performed using hydrogen in the gas phase which is expected to suppress the oxidizing effects of radiolysis. The aqueous fluids used are synthetic Young cement water, simplified granitic groundwater, Callovo-Oxfordian water, as well as natural granitic groundwater. Some results are already available for standard fuel and uranum dioxide in oxidizing conditons as as a reference to compare with the upcoming results from the reducing dissolution experiments. Preliminary data from an experiment using (U, Pu) oxide in Callovo-Oxfordian water is already being collected for use in modelling activities.

The work on chemical modelling has so far resulted in early versions of the models to be developed in the project. The modelling task concerning solid phase thermodynamics is focused on exploring the effect of dopants on the oxygen potential of modern UOX fuels. The development of a solid solution model for substitution of Cr in uranium dioxide is underway. Models describing matrix dissolution of UOX fuels are developed on two fronts and with two different approaches. One approach involves developing a chemical model incorporating redox and electron transfer reactions at the fuel surface and coupling this to reactive transport inside the expected, reducing environment in a failed canister. Another concerns development of an electrochemical model (mixed potential model) to mimic spent fuel corrosion in a storage pond. Finally, modelling of MOX fuel dissolution is developed for Callovo-Oxfordian repository, using reactive transport with a focus on the effect of iron.

Final results

The dissolution data and model developments generated in this project will improve the understanding of processes controlling the oxidative dissolution of uranium dioxide and spent fuel. These processes involve redox reactions and electron transfer at an interface. The results will allow the comparison between the redox response of traditional versus modern (doped and MOX) fuels, including the well known â€hydrogen effectâ€. By systematically investigating the effects of individual dopants in a uranium dioxide matrix, details of the reactions involved in oxidative dissolution will be revealed.

The presence of dopants may also affect the solubility of the solid. Therefore, the chemical, non-oxidative dissolution is also central to the question of how the additives affect fuel dissolution. Changes in spent fuel chemistry invoked by the dopants may also change the fractionation of radionuclides produced during irradiation, implying that the so-called â€œInstant Release Fractionâ€ may be different for doped fuels. The project results will be used to test this hypothesis. Models used to calculate the dissolution rate during the whole of the repository safety assessment period (up to one million years) need to take the hydrogen effect into account and therefore, this project develops chemical models of the dissolution for this purpose.

By resolving issues concerning the dissolution of spent nuclear fuel in a repository environment, the safety case for spent fuel repositories is strengthened. Because the results will reduce uncertainties in critical parameters such as the release rates of radionuclides under repository conditions (source term), the project outcomes are expected to have an impact on both performance assessments and waste acceptance criteria for spent fuel repositories. Both of these issues are closely connected to the licensing process, and thus important for the final solution of how to handle the high-level, long-lived nuclear waste produced in nuclear power plants.