|Fig. 1:Relative redox potential of UO2 and other common oxidants in the environment|
Nuclear waste from reactor power plant is predominantly in the form of uranium dioxide. The long-term stability of uranium dioxide in the waste disposal conditions has attracted significant attention, as leakage of radioactive component due to corrosion can result in serious health and environmental hazard. Typically, a nuclear plant generates around 20 metric tonnes of spent fuel per year. With around 435 nuclear reactors under operation globally, the amount of spent fuel waste generated globally is tremendous. 95% of the spent fuel consists of UO2, with the remainders mainly made up of fission products and transuranium elements. The impact of radionuclide on environment is not only determined by their half-lives and radiotoxicity, but also their mobility under geochemical and hydrologic conditions. It is therefore crucial to understand the corrosion chemistry of the spent nuclear fuel and the various factors that can potentially accelerate or attenuate the corrosion rate.
The redox potential of fuel waste repository environment is critical in determining the dissolution rate of UO2, as oxidation of U(IV) to U(VI) can result is more than three order of magnitude of solubility.  The redox potential of U4+/U6+ is lower than that of most common oxidants found in the environment (Fig. 1), thus corrosion is thermodynamically favorable. Intrinsically, oxidation tends to occur at grain boundary of UO2 where hyperstoichiometry of oxygen (UO2+x) exists. Acidity of the environment will also accelerate corrosion of UO2. Empirical measurements showed a fitted rate equation between dissolution rate and pH for pH between 3 and 6.7. 
where r is the dissolution rate in mol m-2s-1.
The fractional dependence of reaction on [H+] and [O2] indicates that other ionic species may involve in the complexing and dissolution.  It is also possible that both O2 and H+ compete for the active sites that lead to dissolution. Under neutral condition, carbonate ions in the ground water play critical role in accelerating UO2 dissolution due to strong complexation.  The effect of carbonate concentration on UO2 dissolution is generalized by Shoesmith.  At low level of carbonates, UO22+ solubility is increased due to complexation and hence deposition of corrosion product is reduced For intermediate concentration (10-3 to 10-1 mol-1), HCO3-/CO32- will further oxidize the oxygen rich layer (UO2.33) on the UO2 surface, accelerating dissolution. At high carbonate concentration, precipitation of UO2CO3 starts to limit the kinetic of dissolution and dissolution become less dependent on CO32- concentration.
The most common oxidants in the repository environment are O2 in the air and various products of the α, β and γ radiolysis of water contacting the fuel. In the presence of oxygen, oxidized uranium on the surface can transfer electron from the bulk UO2 to the adsorbed O2 atoms, resulting in scission of O-O bond. O2 has very high dissociation energy and normally this process is kinetically very slow. However, the presence of impurity such as noble metal particles (Ru, Mo, Rh, Pd) resulted from in-reactor fission can significantly catalyze O2 reduction. This enhancement can result in O2 reduction activity that is two to four orders of magnitude higher than that of pure UO2 activity.  Oxidation of UO2 caused by radicals (OH-, O2-) produced from radiolysis of H2O is even more severe. It has been shown that for a sufficiently high dose rate, the corrosion rate is proportional to the square root of dose rate. This is because the radical oxidation reaction is diffusion limited and the reaction layer thickness can be described by the radical diffusion coefficient and its effective life time. 
Radiolysis by α particles produces predominantly H2O2 in the vicinity of the spent fuel surface. Experiment results showed that the corrosion effect of alpha radiolysis is similar to the presence of H2O2. Radiolysis of water also produces H2, which can have a counterbalancing effect in suppressing the oxidation of UO2.
Since the lifetime of the radio active spent fuel can last thousands of years, it is practically impossible to have significant experimental data to make long-term prediction of their behavior. One useful approach is to study the depleted U-235 from ore reserve site that has undergone natural nuclear fission. In the uranium rich Gabon, about 15 natural fission reactors have been discovered at Oklo.  The absence of neutron absorbers and presence of neutron reflectors provide conducive conditions for sustained nuclear fission. Water serves as a moderator in the reaction and the reactors were thought to have fissioned more than ten tonnes of U-235 over the past several hundred thousands years. 
Study of the electrochemistry of UO2 corrosion is crucial in accessing the long-term environmental impact of nuclear power. Understanding of factors that affect spent fuel dissolution allow us to select appropriate repository sites for the large amount of nuclear waste that is produced every year.
© Guangyuan Zheng. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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