Efecto de la radiólisis y de los productos radiolíticos en la disolución del UO2Aplicación al modelo de alteración de la matriz del combustible nuclear gastado

Zusammenfassung

The management of the high level radioactive waste supposes one of the greatest technological challenges for our society, considering the disposal in a Deep Geological Repository as the only viable alternative for the definitive management of these wastes. During the repository performance assessment one studies, among others, how it will be the release and the transport of the RN contained in the spent fuel matrix inside and between each one of the barriers that form the repository. The Matrix Alteration Model (MAM), as its name indicates, is the one in charge to mainly study the alteration of the spent fuel matrix mainly UO2, that constitutes the most internal barrier of the repository. In the case of Spanish repository, the considered MAM is based on an alfa-radiolytic model, where the radiolysis of the groundwater caused by the alpha radiation emitted from the spent fuel (SF) itself generates oxidizing conditions in the layers of water more closed to SF surface that cause an increase in the rate of spent fuel matrix dissolution, and the consequent increase in the release of actinides and fission products that are embedded in it. The main deficiencies in the MAM are related to the uncertainties in data or in fundamental processes such as the SF surface site densities, the kinetic constants rates for secondary phases precipitation, to separate the uranium dioxide oxidation from the dissolution process in oxygen media and the SF alteration by hydrogen peroxide. In order to decrease some of these limitations in the MAM, in the present thesis, using non-irradiated UO2 as a chemical analogue of the spent fuel matrix, a series of experimental work has been undertaken. Thus, in chapter 4 the surface site densities of UO2 and U3O8 has been determined by means of a potentiometric acid-base titrations of solid suspensions. In chapter 5, the UO2 oxidation and dissolution kinetics rates have been determined in oxygen and bicarbonate media by means of batch tests, in a specifically designed reactor to prevent the presence of any gas phase. The main conclusion is that the dissolution rates obtained at low bicarbonate concentrations are not representative for the real alteration of the UO2. In chapter 6, the effect of the hydrogen peroxide in the oxidative alteration of the UO2 has been studied in function of different parameters as the hydrogen peroxide concentration, with and without bicarbonate media, the concentration of bicarbonate, the pH, the temperature and the pressure, determining the respective empirical equations for UO2 dissolution kinetics rates.