Dissolution of unirradiated UO2 fuel in synthetic groundwater. Final report (1996-1998)

This study was a part of the EU R and D programme 1994-1998: Nuclear Fission Safety, entitled 'Source term for performance assessment of spent fuel as a waste form'. The research carried out at VTT Chemical Technology was focused on the effects of granitic groundwater composition and redox conditions on UO2 solubility and dissolution mechanisms. The synthetic groundwater compositions simulated deep granitic fresh and saline groundwaters, and the effects of the near-field material, bentonite, on very saline groundwater. Additionally, the Spanish granite/bentonite water was used. The redox conditions (Eh), which are obviously the most important factors that influence on UO2 solubility under the disposal conditions of spent fuel, varied from strongly oxidising (air-saturated), anaerobic (N2, O2 < l ppm) to reducing (N2, low Eh). The objective of the air-saturated dissolution experiments was to yield the maximum solution concentrations of U, and information on the formation of secondary phases that control the concentrations, with different groundwater compositions. The static batch solubility experiments of long duration (up to 1-2 years) were performed using unirradiated UO2 pellets and powder. Under anaerobic and reducing conditions, the solubilities were also approached from oversaturation. The results of the oxic, air-saturated dissolution experiments with UO2 powder showed that the increase in the salinity (< 1.7 M) had a minor effect on the measured steady-state concentrations of U. The concentrations, (1.2 ...2.5) x 10-5 M, were at the level of the theoretical solubility of schoepite or another uranyl oxide hydrate, e.g. becquerelite (possibly Na-polyuranate). The higher alkalinity of the fresh (Allard) composition increased the aqueous U concentration. Only some kind of oxidised U-phase (U3O8-UO3) was identified with XRD when studying possible secondary phases after the contact time of one year with all groundwater compositions. Longer contact times are needed to identify secondary phases predicted by modelling (EQ3/6). In the anoxic dissolution experiments with UO2 pellets, the solubilities of U in synthetic groundwaters ranged from 9.6 x 10-10 ... 3.5 10-8 M. The lowest concentrations (10-9 M) were measured in synthetic groundwaters with redox control (low Eh: -0.2 ... 0.3 V). The composition of the groundwater had a minor effect. The solubilities measured in the oversaturation experiments were generally in good agreement with the pellet experiments. A trend towards lower solubility was observed with fresh (Allard) composition. According to the analyses with XRD, a weakly crystalline UO2-U3O7, was precipitated with all compositions. In saline groundwaters, an impure U(IV)-oxide, including possibly Sr or Ca, seemed to precipitate with the UO2-U3O7. The measured Eh values in the anoxic experiments suggest that the presence of the U solid phase had a buffering effect on Eh. (orig.)