Stability of UO2 in systems containing radiolytic oxidants : The role of uranyl peroxide species

Sammanfattning: Nuclear power constitutes a major pillar on the global energy market, and it will most probably increase in importance as fossil fuels are gradually phased out. The main problem connected to nuclear power is the generation of highly radiotoxic spent nuclear fuel (95% UO2 and 5% radioactive fission products and heavier actinides). Many countries plan to place the spent nuclear fuel in geological repositories where the hazardous material will be protected by engineered and natural barriers for periods exceeding 100 000 years. Given the extremely long time periods, groundwater intrusion is a potential scenario that must be considered in safety assessments of geological repositories. In this thesis, the stability of UO2in either solutions containing HCO3- or saline waters under exposure to radiolytic oxidants has been studied. In addition, the stability of uranyl peroxide minerals, which are common secondary phases on UO2 surfaces, in either solutions containing HCO3- or saline waters has been studied. The solution chemistry was also investigated in the two systems mentioned above with the focus on uranyl speciation. It was shown that oxidants such as O2, H2O2, and other radiolytic oxidants can oxidize UO2 to UO2.33 in either HCO3- solution or saline solutions. The hyper stoichiometric UO2 surface was not expected to form in HCO3- solution, since UO2was expected to oxidize to U(VI) which is highly soluble in HCO3- solution. XPS results illustrate that the oxidized surface was dominated by U(V) and no U(VI) was observed. The accumulation of U(V) on the surface resulted in the decrease ofUO2 reactivity towards oxidants. The dissolved U(VI), in the form of UO22+, is a good electron acceptor in solution, which will interact with the anions in aqueous solutions forming highly soluble complexes. In addition to complexes, UO22+ can also interact with H2O2 and precipitate uranyl peroxides, i.e., studtite UO2O2.4H2O and meta-studtite UO2O2.2H2O. We have found that studtite formation in solutions containing UO22+ and H2O2 was affected by anion concentrations (other than O22-), pH, and ionic strength. Formation of (meta)-studtite was observed on the surface of UO2 after exposure to γ-radiation in pure water. The stability of studtite in pure water and in aqueous solutions containing either 1-10 mM HCO3- or 1-5 M salts was studied. It was found that studtite dissolved to UO22+ and H2O2 at [HCO3-] ≥ 5 mM, and exposure to γ-radiation can accelerate the dissolution. While there was no measurable UO22+ at [HCO3-] ≤ 2 mM. Several 2 uranyl-(peroxo)-carbonanto complexes was characterized by 13C NMR during studtite and meta-studtite dissolution in 10 mM HCO3- solution. Ternary uranylperoxo-halo complexes were identified in saline solutions containing UO22+ and H2O2 using vibrational spectroscopies and 17O NMR. Interestingly, H2O2 was found to be catalytically decomposed in solutions containing UO22+ and either HCO3-/CO32- or Br-.