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-d ependent generation of oxidants and reductants at the spent fuel/water inte rface by alpha, beta and gamma radiolysis. In this context it is critical t o understand the main processes and mechanisms that control the radiolytic production of redox components at this interface and their impact on the st ability 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 radiolyt ic H-2, O-2 and H2O2 production and radionuclide release (U, Pu, Np, Tc, Mo , Sr and Cs). These tests have been performed by using PWR Ringhals spent f uel 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 reproducib le behaviour of the radiolytically generated reductants and oxidants. The r esults indicate that it is possible to define an equilibrium redox potentia l for the main redox pairs in the bulk system and that the experimentally d etermined radionuclide release can be rationalised in terms of the system r edox potential.