OBJECTIVES

Many European member states consider spent nuclear fuel as a waste form to be disposed in a deep underground repository. The project tackles the challenge of realistically describing the release of the first batch of radionuclides from disposed spent nuclear fuel upon canister failure (fast/instant release fraction).

The key objectives of the project consider the high burn-up UO₂ fuel and requirements for the Disposal Safety Case independent on the host rock under consideration with respect to:

1. Selection, characterization and preparation of materials and set-up of tools for handling and transportation of the highly radioactive material
2. Experimental determination of fission gas distribution in fuel pellets, the gas release after puncturing of high burn-up spent fuel rods or segments and determination of fission gas release from spent fuel in the case of contact with an aqueous phase.
3. Experimental determination of rim and grain boundaries inventories and the contribution of these inventories to the fast/instant release.
4. Quantification of mechanisms relevant for the release of fission products from fractures, rim and grain boundaries of high burn-up UO₂ fuel by diffusion experiments.
5. Experimental quantification of the fast/instant radionuclide release when high burn-up spent fuel comes into contact with the aquatic phase. Quantification takes into account the presence or absence of cladding, fuel fragments from different radial positions of fuel pellets and the specific fuel and burn-up characteristics.
6. Experimental quantification of the effect of fuel characteristics, burn-up and burn-up history, and of the characteristics of the samples under investigations on fast/instant radionuclide release from high burn-up fuel.
7. Implementation of the fast/instant release mechanisms into models which allow prediction of i) chemical speciation of the relevant elements in the spent fuel, ii) relationship between the fission gas release and non-gaseous fission products, in particular ¹²⁹I, ⁷⁹Se and ¹³⁵Cs , and iii) long-term retention of fission products in the rim and onto grain boundaries.

Strategic objectives

1. In order to make best possible use of existing knowledge, at the beginning of the project emphasis will be given to the evaluation of conceptual models, codes and databases with respect to their use for the project objectives.
2. Communication and dissemination objectives are to ensure that interested parties are informed about the project and its outcome. Within this context, additional groups from different countries will be invited to accompany and contribute to the project.
3. A group of implementation and regulatory oriented organizations are participating as an “End-User Group”. This group of end-users has a number of functions, including:
   - Ensuring that end-user interests are reflected in the project work, reporting, dissemination and communication,
   - Providing for review of the project work and scientific-technical outcome in association with periodic project workshops, including review of scientific- technical contributions to the corresponding workshop proceedings, and
   - Participating in assessment and discussion of the project outcome with respect to the potential impact on the Safety Case.