The influence of trivalent dopants (rare-earths, RE) on the structure, composition and electrochemical reactivity of UO 2 has been investigated using scanning electron microscopy (SEM/XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and cyclic voltammetry (CV). This was achieved by comparing the behavior of undoped UO 2.002 , a slightly doped 1.5 at% SIMFUEL, and two rare-earth doped UO 2 (6 wt% Gd-UO 2 and 12.9 wt% Dy-UO 2 ) specimens. The reactivity decreased in the order UO 2.002 > SIMFUEL > Dy-UO 2 > Gd-UO 2 , showing that this decrease is a consequence of RE III doping. Raman spectroscopy showed this could be attributed to the formation of RE III -oxygen vacancy clusters whose formation decreases the availability of the vacancies required to accommodate the injection of oxygen interstitials during anodic oxidation. The behavior of SIMFUEL is complicated by the simultaneous formation of RE III -oxygen vacancy clusters and Zr-O 8 clusters.The safe disposal of spent nuclear fuel (SNF) is one of the key issues facing the modern nuclear power industry, and a major international effort is underway to develop safe management and disposal procedures. One potential management strategy in Canada is permanent disposal in a deep geologic repository. 1 The spent fuel would be sealed in metallic containers, emplaced in a repository and surrounded with compacted clay. The prospects for long term containment using copper containers are very good and corrosion models predict only minimal corrosion damage should be sustained. 2,3 However, if failure were to occur, contact of the fuel wasteform (uranium dioxide (UO 2 )) with groundwater would become possible. Although the solubility of UO 2 is very low under the anticipated anoxic conditions, radiolysis of the groundwater, due to the inherent radioactivity of the spent fuel, could lead to fuel corrosion, the U(IV) in the fuel being oxidized to the significantly more soluble U(VI) state. 4 This would make radionuclide release to the groundwater possible.Spent fuel is mainly UO 2 (> 95%), the remainder being the radioactive fission products and actinides produced during the in-reactor process. The inventory of radionuclides within the fuel depends on in-reactor burn-up and the linear power rating of the fuel. 5 Formation of these products leads to many physical and chemical changes within the fuel, 5 and post irradiation inspection of the fuel shows the presence of both volatile and non-volatile fission products. While volatile products may escape to the fuel-cladding gap, the non volatile products remain fixed within the fuel matrix in three distinct phases: the lanthanides in the fcc-fluorite lattice; the noble metals in metallic precipitates; and radionuclides unstable in the fluorite matrix in mixed metal oxides (perovskites). 6 The key changes likely to influence the chemical reactivity of the UO 2 matrix are the rare earth (RE) doping of the matrix and the development of non-stoichiometry. 7 Micro Raman spectroscopic studies show that non-stoichiometry lead...

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The effect of gadolinium doping on the anodic reactivity of UO 2 in aqueous carbonate solutions has been investigated voltammetrically, potentiostatically, by Raman spectroscopy and X-ray photoelectron spectroscopy. The mechanism of oxidation/dissolution is the same as on undoped UO 2 and lightly doped SIMFUEL (doped UO 2) but the reactivity is significantly reduced. This is attributed to the formation of Gd III-Oxygen vacancy (O V) clusters which limits the availability of these vacancies required to accommodate excess O 2− during matrix oxidation to U IV 1−2x U V 2x O 2+x. The subsequent reaction of this oxidized surface layer to U VI O 3 .yH 2 O/U VI O 2 (CO 3) x (2−2x)+ is also suppressed by Gd-doping. The overall oxidation/alteration (dissolution) reaction appears to be kinetically controlled by the creation of U VI surface species prior to dissolution.

The synergistic influence between H(2)O(2) and H(2) on the corrosion of SIMFUEL (simulated spent nuclear fuel) has been studied in solutions with and without added HCO(3)(-)/CO(3)(2-). The response of the surface to increasing concentrations of added H(2)O(2) was monitored by measuring the corrosion potential in either Ar or Ar/H(2)-purged solutions. Using X-ray photoelectron spectroscopy it was shown that the extent of surface oxidation (U(V) + U(VI) content) was directly related to the corrosion potential. Variations in corrosion potential with time, redox conditions, HCO(3)(-)/CO(3)(2-) concentration, and convective conditions showed that surface oxidation induced by H(2)O(2) could be reversed by reaction with H(2), the latter reaction occurring dominantly on the noble metal particles in the SIMFUEL. For sufficiently large H(2)O(2) concentrations, the influence of H(2) was overwhelmed and irreversible oxidation of the surface to U(VI) occurred. Subsequently, corrosion was controlled by the chemical dissolution rate of this U(VI) layer.