Tesis doctoral de Rosa María Sureda Pastor
Adequate management of spent nuclear fuel (snf) used in nuclear power plant for electricity generation is, nowadays, one of the most complex challenges which need to be addressed. Storage in deep geological formations is presented as the most viable option for a final disposal of these wastes. To ensure the safety of the repository it is necessary to develop models that predict the behavior of spent nuclear fuel (snf) in the long-term storage, as well as the release and migration of radionuclides to the environment. The alteration of snf under expected representative repository conditions will depend on a number of processes that take place once the groundwater come in contact with the surface of the fuel, such as the formation of radiolitic products, its effect on the matrix oxidation and subsequent dissolution, released of radionuclides contained in the fuel matrix and finally, precipitation of secondary phases. The study of these phases as end products of the oxidative dissolution process is of major interest due to the fact that they may affect the mobilization of actinides and fission products by geochemical processes such as adsorption. In parallel to these processes, the anoxic corrosion of the steel capsule gives rise to the production of large amounts of hydrogen gas which can influence in the dissolution of snf. In order to get deeper into some aspects of radiolytic products and secondary phases in the near-field of a high-level nuclear waste (hlnw) repository, in this thesis we have developed a series of studies which are summarized below. In the first part, the influence of hypochlorite, chlorite and chlorate in the dissolution rate of uo2 as a chemical analogue of the spent fuel matrix, has been studied experimentally. The dissolution rates obtained in this work are found to be higher than the rates determined in the presence of either oxygen or hydrogen peroxide obtained with same experimental methodology. A second part is focused on increasing the knowledge of the most representative secondary phases of u(vi) formed during the oxidative alteration of snf. Specifically, the adsorption of cesium and strontium onto uranyl peroxide, studtite (uo2o2 4h2o), has been studied as a function of time, radionuclide concentration in solution and ph. The results show that this secondary solid phase formation on the snf surface could be an important mechanism involved in the retention of radionuclides. On the other hand, the stability of these mineral phases will be directly affected by two of the most critical variables in the near-field of hlnw are radiation and temperature. Atomic-level effects of ionizing radiation on the structure of u(vi) silicates: soddyite (uo2)2(sio4)(h2o)2 and uranophana ca(uo2)2(sio3oh)2¿5h2o have been investigated using electron beam irradiation in a transmission electron microscope (tem), which allows in situ observation of amorphization/nanocristallization processes of these phases. Thermal studies carried out by thermogravimetry (tg) and differential scanning calorimetry (dsc) combined with x-ray diffraction (drx) have demonstrated the importance of dehydration processes in the structural integrity for each phase. Finally, the reaction between aqueous hydrogen peroxide solutions and hydrogen has been studied at room temperature and in the absence of catalytic products. The reaction evolution has been determined by the decrease of hydrogen peroxide concentration with time. A clear effect has been observed, and the rate of reaction is found to be enhanced as the hydrogen pressure is increased. These results indicate that hydrogen could act as an inhibitor of spent fuel matrix dissolution through the consumption of the main species responsible for the oxidative alteration process
Datos académicos de la tesis doctoral «Disolución del combustible nuclear gastado en un almacenamiento geológico profundo. efecto de los productos radiolíticos y de formación de fases secundarias«
- Título de la tesis: Disolución del combustible nuclear gastado en un almacenamiento geológico profundo. efecto de los productos radiolíticos y de formación de fases secundarias
- Autor: Rosa María Sureda Pastor
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 13/09/2012
Dirección y tribunal
- Director de la tesis
- Francisco Javier Giménez Izquierdo
- Tribunal
- Presidente del tribunal: Jorge Bruno salgot
- mireia Grive sole (vocal)
- daniel Serrano purroy (vocal)
- frederic Clarens blanco (vocal)