Resumen de Study of near field processes involved in the oxidative dissolution of the spent nuclear fuel and radionuclides release
Nowadays nuclear power plants (NPP) and dedicated facilities are storing the spent nuclear fuel (SNF). In the case of Spain, the NPP have enough space in their pools to keep storing the SNF produced from their activity; therefore, it is necessary to provide a final solution for the SNF management. It has been internationally accepted that the safest and most economically viable option to finally dispose the SNF is the Deep Geological Repository (DGR) which is based on the protection and confinement of the SNF by using a multibarrier system. This system considers the SNF as the first barrier where all the radionuclides (RN) are contained due to the physical and chemical properties of the fuel waste.
With the goal of assessing the safety of the DGR and determining the SNF behavior under relevant conditions for the repository, in this work, four different scenarios have been selected that might alter the nuclear waste. These scenarios were studied by using non irradiated analogues of the SNF.
During the alteration of the barriers of the DGR, the anoxic corrosion of the metallic container will produce high partial pressures of hydrogen that might get in contact with the SNF. At the same time, it is possible that vapor will be produced due to the water entry and the temperature of the SNF. In this work, the effect of water vapor and hydrogen in contact with UO2 has been studied. It was observed that uranium is oxidized in contact with water vapor in anoxic and reducing environment and as a function of the temperature. On the other hand, when palladium nanoparticles were used as analogues of the epsilon and metallic particles of the SNF, no oxidation was observed under H2 atmosphere.
At the time when the water gets in contact with the SNF, the RN that are segregated and are more volatile and soluble than UO2 will be released to the water faster than the SNF matrix and will contribute to the Instant Release Fraction (IRF). With the aim of predicting the RN release, a mathematical model and a specific algorithm were designed, in order to identify and quantify the segregation of the RN that contributes to the IRF. Real leaching data was used to discuss the model and the algorithm by using the results obtained that allowed to classify the RN depending on their release source (grain boundary, oxidized phases and matrix). The dissolution rates obtained from the model agree with reported values in the bibliography.
Since the alteration mechanisms of the SNF and the UO2 matrix depend on the ground water composition, and given the presence of concrete components in the DGR, this work studied the SNF corrosion in contact with cement waters. A SIMFUEL electrode was used to obtain cyclic voltammograms, potentiostatic and corrosion potential measurements under hyper alkaline conditions in presence of calcium and silicate as a analogue of cementitious water. The results obtained showed that the oxidation of the SIMFUEL was partially avoided in the presence of calcium and silicate. This might be caused by the precipitation of solid phases on the electrode surface or by the stabilization of reduced phases.
Finally, saturation conditions might occur in the SNF near field and secondary solid phases might precipitate in the SNF vicinity. In this context, this work studied the influence of two uranyl silicate solid phases (soddyite and uranophane) on the Cs and Sr mobility. It was determined that both solid phases can retain the two RN on their surface and the sorption process proceeds through sorption on active sites. For the sorption of Cs, soddyite has more sorption capacity while in the case of Sr, uranophane has more sorption capacity. Therefore, the formation of these secondary phases might decrease the concentration of Cs and Sr released to the geosphere.
