Release of Radiotoxic Elements from High Burn-Up UO 2 and MOX Fuel in a Repository
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RODUCTION In the European Union the discharged spent fuel is kept in intermediate dry or wet storage for a time span between 50 and a few hundred years before it is reprocessed or, after conditioning disposed in geological formations. Due to a limited reprocessing capacity installed, disposal is the dominant of the two options. In order to obtain licensing and public acceptance for this fuel management option, the behavior of the fuel and its interaction with the environment has to be well known. It is generally accepted that the main processes by which radionuclides originated from a spent fuel waste repository will be released are dissolution and transport as a result of the groundwater flow [1]. The cladding will be the last barrier before the water comes into contact with the fuel, namely with the outer rim of the pellet. The release of the actinide elements including the matrix elements uranium and plutonium will be limited by the low solubility product of the actinide-bearing solids and the flow rate of the water through the waste package [1,2]. On the other hand, the release of highly soluble radionuclides like cesium and iodine depends mainly on: • the heterogeneous distribution of these radionuclides within the fuel as a consequence of
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migration to the grain boundaries (together with other fission products) and to the pellet periphery (together with fission gases). the durability of the fuel matrix in a potential repository. It is generally agreed [1] that the dissolution of soluble radionuclides from spent fuel can be divided into components that come from three different areas in the fuel: The fuel/cladding gap, including the free spaces between fuel pellets and the open porosity or cracks inside of the fuel pellets. The grain boundaries of the fuel pellets. The UO2 matrix.
If groundwater penetrates inside the fuel rod through a defect, S/V ratios of porous UO2 are extremely high and the relative importance of α-radiolysis is increased due to an increased inventory of α-emitters as a consequence of increased epithermal neutron capture in the outer pellet zone. The radiolysis creates locally oxidizing conditions, under which UO2 is known to be thermodynamically unstable. Surface oxidation of the UO2 is followed by dissolution; both processes are influenced by a number of parameters such as S/V ratios, pH or the carbonate concentration. Furthermore, depending on the redox conditions precipitation of secondary uranium phases can be relevant [2,3]. In an anaerobic environment mild steel from the canister material will corrode to Fe3O4 with hydrogen evolution. The equilibrium hydrogen pressure for this reaction is very high and some time after water intrusion, there will be a hydrogen pressure in the near environment of the fuel equivalent at least to the hydrostatic pressure at the repository level. This corresponds to a partial pressure of hydrogen of about 5 MPa. Excess hydrogen will influence the yield of radiolytic species in the system. At ITU a series of experiments (integral and single effe
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