Chemistry Beyond Containment

QC 20251119

Abstract

Safe disposal of high-level radioactive waste is crucial for protecting human health and the environment over geological timescales. A detailed understanding of the chemical processes that govern fuel corrosion is therefore essential for reliable safety assessments. A scenario of particular concern involves groundwater intrusion following the failure of the engineered barriers designed to contain the spent fuel. In this case, groundwater in contact with the fuel becomes exposed to ionizing radiation, producing radicals and molecules that can shift the conditions from reducing to oxidizing. Among the radiolytic oxidants, hydrogen peroxide (H₂O₂) is expected to have the greatest impact on the rate of fuel oxidation. The presence of carbonate in groundwater enhances the dissolution of oxidized UO₂—the main constituent of spent nuclear fuel—through the formation of soluble uranyl–carbonate complexes. This thesis examines several aspects of the oxidative dissolution of UO₂. One aspect is the effect of uranyl ion (UO₂²⁺) accumulation in the near-surface solution on the overall oxidation rate. It was found that increasing concentrations of UO₂²⁺ suppress H₂O₂-induced oxidation by reducing the concentration of free, reactive H₂O₂ through the formation of inert uranyl–peroxo–carbonate complexes. UO₂²⁺ was also found to affect the stability of H₂O₂ in irradiated solutions. In the presence of O₂, UO₂²⁺ can suppress the concentration of H₂O₂ through selective reduction of uranyl–peroxo–carbonate complexes by the superoxide radical. Under anoxic (N₂) atmosphere, UO₂²⁺ scavenges reducing radicals (e^-_aq and H^●),which increases the H₂O₂ concentration of the γ-irradiated solution. The kinetics of carbonate-facilitated dissolution were also re-evaluated, as previous models were based on systems where oxidation and dissolution occurred simultaneously. In the absence of oxidants, dissolution itself was found to be a multistep process with an apparent activation energy of (34.8 ± 3.2) kJ mol^-1. The reaction order with respect to bicarbonate varied between 0.5 and 1.5 within the temperature range (283–333) K and bicarbonate concentrations of (1.3–15) mM. The dependence on the oxidation state of uranium oxide showed three distinct stages: (1) a rapid initial release of a small fraction, (2) a slower, nearly constant releases rate (independent of the remaining oxidized fraction), and (3) a gradual rate decrease as the oxidized product approached depletion. Finally, comparative studies of Pd-doped and undoped UO₂ thin films in the presence of H₂ confirmed that any uncatalyzed H₂ effect is several orders of magnitude less efficient at inhibiting oxidative dissolution. Palladium was found to reduce the oxide only partially, to a U(V) intermediate, identified as UO₂OH, based on a calculated U:O atom ratio of 1:3 and an average uranium oxidation state of +5. 

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