Experimental determination and chemical modelling of radiolytic processes at the spent fuel/water interface

The spent fuel matrix constitutes a dynamic redox system, due to the time-dependent generation of oxidants and reductants at the spent fuel/water interface by alpha, beta and gamma radiolysis. In this context it is critical to understand the main processes and mechanisms that control the radiolytic production of redox components at this interface and their impact on the stability of the UO2 matrix and in radionuclide release. In order to achieve this, a series of carefully controlled experiments have been carried out in order to determine the time dependence of the radiolytic H2, O2 and H2O2 production and radionuclide release (U, Pu, Np, Tc, Mo, Sr and Cs). These tests have been performed by using PWR Ringhals spent fuel fragments in contact with 10 mM NaHCO3 solutions in an initially anoxic and closed system. The results of these experiments indicate quite a consistent and reproducible behaviour of the radiolytically generated reductants and oxidants. The results indicate that it is possible to define an equilibrium redox potential for the main redox pairs in the bulk system and that the experimentally determined radionuclide release can be rationalised in terms of the system redox potential. (orig.)