Modelling of the Radionuclide Release from an Initially Defective Canister
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become so strong that the pe value could rapidly become larger than 7, and all the fuel surface could be quickly oxidized to higher oxidation states, i.e. U(VI) [4]. With the oxidation of the surface of spent fuel, hydrolysis reactions would proceed swiftly in aqueous solutions inside the canister, because the uranyl ions are very active. Meanwhile, the complexing anions present in the deep groundwater would diffuse through the damage into the canister and accelerate dissolution by stabilizing the dissolved uranyl ions, or limit a further rise in concentration by the formation of secondary phases. The total analytical concentration of aqueous uranium species inside the canister would therefore quickly rise and approach the solubility of uranium under strongly oxidizing conditions, since the transport of all aqueous species outwards from an initially defective canister is very low due to the small hole. As a result, the rate of dissolution of the spent fuel matrix, and subsequently the rate of release of radionuclides trapped in the fuel matrix out of an initially defective canister could be limited by the solubility of uranium under strongly oxidizing conditions. Once significant dissolution has occurred, the dissolution process is expected to be modified by the formation of secondary phases, i.e., secondary phases are likely to be formed rapidly once dissolution commences, but their ability to inhibit further dissolution may take time to develop. The nature of these phases inside the canister present under strongly oxidizing conditions is, in principle, dependent on the chemistry of the dissolution process, and on the concentration and rate of supply of groundwater species [8]. When the interfacial kinetics of dissolution are attenuated by the presence of secondary phases such as schoepite UO3.2H2O, rutherfordite U0 2C0 3, uranyl orthophosphate (U02) 3(PO 4).4H 2 0, soddyite (UOz) 2SiO 4-2H 20, uranophane Ca[(UOJ)(SiOjOH)]2.5H 2 0, autinites Ca(UOJ)PO 4 , becquerelite Ca[(UO,)6 0 4(OH)6l-8H-1 2 0, NaUO 2 PO 4 , Na2UO 4, Na 2 U20 7 and Na4UO 2(CO3) 3 [9, 10, 11], the dissolution of spent fuel and subsequently the release of radionuclides would be controlled predominantly by the properties of the precipitated phases such as composition and morphology, and would be strongly dependent on the geochemical conditions of the repository [2]. It may also be noted that if there were other less soluble compounds for which we do not have the thermodynamic data, the release would decrease further. Our approach will give conservative results in this respect. To investigate the effect of geochemical conditions of the repository on the release of radionuclides, as a first step, a transport model to determine the rate of release of uranium is proposed in the present study. It is combined with a chemical equilibrium model for the solubility of uranium under strongly oxidizing conditions. Applying this model, the sensitivities of the rate of release of uranium to the component concentrations, to the diffusion coefficients and to other
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