AbstractsChemistry

The contact ofthe spent nuclear fuel (SNF) with water dueto a failure in the canister would be a conservative but still plausible hypothesis in the safety assesment of a deep geologic repository (DGR). Concrete and cementitious materia Is will be part ofthe DGR structure. Water in contactwith those materials will have a very alkaline pH. Once the water gets in contact with the SNF the following 4 stages may take place: Radiolysis, Oxidation, Dissolution and Secondary Phase Formation. The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR cede. Two different com plexes were found ata H202/U(VI) ratio lower than 2. A graphical method was u sed in arder to obtain the formation constants ofsuch complexes and the STAR program was used to refine the formation constants values because ofits capacityto treatmultiwavelength absorbance data and refining equilibrium constants. The values obtained far the two equilibrium constants were: lag ¡3°1, 1,4 = 28.1 ± 0.1 and lag ¡3°1,2,6=36.8 ± 0.2. At hydrogen peroxide concentrations higherthan 10-5 mol dm-3, and in the absence of carbonate, the U02(02)2(0H)24-complex is predominant in solution. Time-resolved laser-induced fluorescence spectroscopy(TRLFS) was used to studythe speciation ofuranium(VI) atvery alkaline pH (11-13), at room temperature and in the absence of C02. Fluorescence lifetimes fer (U02)3(0H)7-, U02(0H)3-and U02(0H)42- were determined far pH between 11and13. Measurements at 1 O K were made, obtaining two different lifetimes in the pH range between 12 and 13.5, indicating the presence oftwo different species: U02(0H)3- and U02(0H)42-. The difference between the Iifetimes allowed the calculation ofthe contribution of each species to the total fluorescence signal intensity. lt was observed that hydrogen peroxide produces a quenching effect to the fluorescence of the uranium species. At pH 12 the quenching is static, which points to the formation of a non-fluorescent complex between U(VI) and hydrogen peroxide. Using the Stern-Volmer equation far static quenching, the equilibrium formation constant of the first species, U0202(0H)22-, was calculated to be logKO = 28.7 ± 0.4. A flow-through experimental reactor has been designed in arder to perform studies at both high pressure, high tem perature conditions and high surface salid to volume leachant ratios.Using this new reactor the evolution of uranium concentrations released from an U02 sample was studied atdifferent conditions. The results show that at hydrogen pres sures between 5 and 7 bars, hydrogen peroxide does not seem to significantlyoxidize the uranium (IV) oxide. Uranium concentrations in those experiments remain between 10-8 mol¿l-1 and 10-9 molof-1. The effects of alpha-radiolysis were determined, on one hand, through the generation of radiolytic products: H2, 02, HCIO and H202, and on the other hand from the dissolution ofboth U and Pu. The studies were focused on the effect produced by different dos e rates, different ionic strength as well…