Technical Report NTB 04-09

A Report of the Spent Fuel Stability (SFS) Project of the 5th Euratom Framework Program:Spent Fuel Evolution under Disposal Conditions – Synthesis of Results from the EU Spent Fuel Stability (SFS) Project

Over the period 2002-2004, a large number of European organisations cooperated on the EU Project SFS – Spent Fuel Stability under disposal conditions. The objective of the SFS Project was to develop a reliable and robust model for the spent fuel source term which can be used in performance assessment exercises by the waste management agencies responsible for assessing the feasibility and safety of potential geological disposal systems for spent fuel, whatever the countries and disposal system designs may be.

A new model for short-term release of fission products (Instant Release Fraction or IRF) was developed based on the anticipated fission product release from various fuel microstructures (gap, rim, grain boundaries) and the potential solid-state diffusion of fission products prior to canister breaching. For the oxide matrix of the spent fuel, a Matrix Alteration Model (MAM) was developed, which is linked to the production of oxidants by water radiolysis at the fuel interface, the oxidation of the fuel interface by radiolytic oxidants and the subsequent release of uranium under the influence of aqueous ligands. A large set of experimental data was therefore acquired in order to (i) upgrade the current radiolytic kinetic scheme, (ii) determine the relationship between fuel alteration rate and alpha activity by performing experiments on alpha-doped samples of UO2 and (iii) experimentally test the potential inhibition effect of hydrogen on fuel dissolution. Based on these results, a new MAM was developed that was calibrated using experiments on inactive UO2 samples, although the hydrogen effects remain to be incorporated completely into the model. The integrated model combining the IRF and MAM was used to illustrate long-term performance of representative spent fuel disposed of in granite, salt and clay host rock environments.

The findings of the SFS Project have significantly enhanced the understanding of phenomena that may affect radionuclide release from spent fuel under disposal conditions and have helped to more clearly identify areas in which uncertainties should be reduced through future research.