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Modern spent fuel dissolution and chemistry in failed container conditions

Periodic Reporting for period 2 - DISCO (Modern spent fuel dissolution and chemistry in failed container conditions)

Reporting period: 2018-12-01 to 2020-05-31

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.
Overall, the project is progressing as planned. Three annual meetings have been held where the End User Group (EUG), consisting of representatives of Waste management Organisations and Regulatory Authorities, has had the chance to review and discuss the scientific contributions. The EUG members provide valuable input to the project. The EUG as a group expressed that their comments and guidance were taken into account both regarding the project direction and the organisation of annual meetings. Observations made are that most of the scientific contributions are on track but that some issues and delays are identified within the project. All deliverables defined for the period have been submitted and an important milestone is reached: Deliverable of experimental data to the modellers. This means that during the final year, the model development can be finalised using data generated in the project. The project members actively use the communication tools implemented in the first period. Training and mobility actions, including webinars, mobility grants to attend the annual meetings, and individual training visits at JRC, have been carried out successfully.

The research during the second reporting period has been focused on gathering experimental data and optimizing the experimental systems. This has included improvements in sample preparation, and therefore WP2 has been extended in time compared to what was foreseen in the original work plan. At the end of the second period, all but a few samples have been prepared and characterised. The sample characterisation involves careful analyses of the products during sample preparation and characterisation of the final product: efforts that allow insight regarding the effect of dopants on the UO2 lattice, and effects of long term fuel-water interaction in storage ponds. The results from WP2 are reported in reports D2.1 and D2.2. These will be updated when more information becomes available during the final period.
Dissolution experiments using spent fuel and model materials have produced a significant amount of data. Due to delays, some experiments are however only now initiated. Experiments comparing conventional UOX fuel, Cr-& Al-doped UOX fuel, and MOX fuel are close to finished and some post-mortem analyses are planned. The dissolution experiments of the model materials are designed to complement the spent fuel experiments. The experiments are performed in various conditions, designed to provide a systematic analysis of parameters. The aqueous fluids used are synthetic Young cement water, simplified granitic groundwater, synthetic Callovo-Oxfordian water, and natural granitic groundwater. Results are now available for many of these systems.

Chemical modelling has continued and early versions of models developed in the first period is now more mature; the next stage is to fine-tune these models using the project data gathered in the experiments. The modelling task concerning solid phase thermodynamics, involving a solid solution model for substitution of Cr in uranium dioxide, has allowed some conclusions to be drawn regarding oxygen potential in Cr-doped spent fuel. Models describing matrix dissolution of UOX fuels are developed on two fronts with two different approaches. One approach involves 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 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.
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 ”hydrogen effect”. 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 “Instant Release Fraction” 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.
Overview of dissolution of spent fuel in a repository environment