CHEMISTRY OF ACTINIDOS

Author: CLARENS BLANCO FREDERIC.
Year: 2003.
University: POLITÉCNICA DE CATALUÑA.
Place of defense: SALA AUDIOVISUAL DEL DEPT. D'ENG. QUÍMICA..
Place of preparation: ETSEIB, EDIFICI H PLANTA 10 Campus SUD.

Summary: The management of high-level radioactive waste is one of the greatest technological challenges of today's society, considering the provision in a Deep Geological Warehouse (AGP) as the only viable alternative for the management of these wastes final. To assess the safety of the repository is studied, among other things, how will the release and transport of RN content in the matrix of spent fuel (CG) both within each of the barriers that make up the PGA between them. The Model of the Alteration of the Matrix (MAM), as its name suggests, is to study the alteration of the parent of CG composed mainly of UO2, which is the barrier inside the repository. In the case of Spanish repository, the MAM considered is based on a model á-radiolítico where radiólisis groundwater caused by the radiation emitted by the GC á generates oxidizing conditions in the water surrounding the surface of the same cause an increase in the rate of dissolution of the matrix of CG, and consequently, increased the release of actinides and fission products it contains. The main shortcomings of MAM refer to the uncertainties in the data or fundamental processes such as: the density focal points of CG, the kinetics of precipitation of secondary phases, determining the oxidation process by oxygen, and the alteration of CG by hydrogen peroxide. To try to answer these limitations, this thesis has developed a series of works using UO2 unirradiated as a chemical analogue of the matrix CG. Thus, in chapter 4 has been determined density focal points of UO2 and U3O8 by valuations potenciométricas acid. In chapter 5 has been studied the alteration of UO2 the effect of oxygen through static tests amid carbonate in a reactor specifically designed to avoid the presence of gas phase, determining the rate of oxidation and dissolution separately. The main conclusion is that the speed of dissolution obtained at low concentrations of bicarbonate not represent the actual state of alteration of UO2. In Chapter 6, has studied the effect of hydrogen peroxide in the alteration of UO2 depending on various parameters such as the concentration of hydrogen peroxide, both in the presence and in absence of bicarbonates, the concentration of bicarbonates, pH, temperature and pressure, determining the respective empirical equations speed of dissolution. Based on the results of the kinetics of dissolution earlier, alongside a series of experiments developed to study the involvement of hydroxyl radical in the oxidation of UO2 by hydrogen peroxide, has proposed a conceptual model based on the reaction of Fenton, for the alteration of UO2 by oxidative hydrogen peroxide that can explain both behavioral changes observed in the various segments of this thesis as the comments published in the literature. In chapters 5 and 6 has studied the process of alteration of UO2 at the nano-scale by force microscopy (SFM), with the technique that has been able to determine the kinetics of precipitation of studtita. Finally, in Chapter 7, has been made the first studies publi 8 ments at 452 scientific literature, which has studied the effect of radiation à ¢ oxidative disruption of UO2. These studies are necessary in order to understand the experiments conducted by CG fresh, to justify the non-inclusion of this radiation within the MAM. In these experiments has been shown that hydrogen peroxide generated is the main species responsible for the alteration of UO2 the effect of radiation à ¢.