presence of very high concentrations of hydrogen peroxide was evaluated. The results show that in this case, occurring under very intense radiation fields causing radiolysis of most of the water present, studtite is by far the most stable phase within the full range of temperature studied.

A rutherfordine mineral was studied by means of Raman spectroscopy combined with first principle calculations based on the density functional theory (DFT) method. The pseudopotential of a uranium atom was generated and its performance was evaluated for a series of uranium-containing minerals. The structure of rutherfordine was determined for two symmetries (Pmmn and Imm2) and the resulting lattice parameters, bond lengths, bond angles, and X-ray powder diffractogram were found to be in very good agreement with experimental values. The Raman spectrum was experimentally determined in the range 0-1700 cm(-1) and calculated using density functional perturbation theory. The non-scaled theoretical wavenumbers also agreed with the experimental values, and therefore a detailed interpretation of the theoretical spectra allowed us to assign the Raman bands found in the experimental spectrum.

Uranyl silicate mineral soddyite, (UO2)2(SiO4)•2(H2O), is a fundamental component of the paragenetic sequence of secondary phases that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. In this work, soddyite was studied by first principle calculations based on the density functional theory. As far as we know, this is the first time that soddyite structure is determined theoretically. The computed structure of

This work presents a detailed study of hyperstoichiometric UO (0 < x < 0.25) oxides and an assessment of the structural evolution taking place as oxidation proceeds. For this purpose, different UO powder samples with controlled degree of non-stoichiometry have been identified by thermogravimetric analysis and characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD analysis reflects that the commonly assumed Vegard's law is not applicable over the whole hyperstoichiometry range, since a slight increase of the lattice constant is observed for 0.13 < x < 0.20. A quantitative Raman analysis of the UO spectra as a function of the oxidation degree is also shown. A new method to characterize any UO sample (for x < 0.20), based on the shift of the 630 cm band observed in the Raman spectrum, is proposed here for the first time. Moreover, three structure transitions have been detected at x = 0.05, 0.11 and 0.20, giving rise to four distinct regions associated with consecutive structural rearrangements over the hyperstoichiometry range: x < 0.05, 0.05 < x < 0.11, 0.11 < x < 0.20 and 0.20 < x < 0.25.

The Raman spectrum of plutonium dioxide is studied both experimentally and theoretically. Particular attention has been devoted to the identification of high-energy modes at 2120 cm -1 and 2625 cm -1 , whose attribution has so far been controversial. The temperature dependence of both modes suggests an electronic origin. Crystal Field (CF) calculations reported in this work shows that these two modes can be respectively assigned to the Γ 1 → Γ 5 and Γ 1 → Γ 3 CF transitions within the 5 I 4 manifold. These two modes, together with the only vibrational line foreseen by the group theory for the Fm-3m PuO 2 symmetrythe T 2g Pu-O stretching mode observed at 478 cm -1can thus be used as Raman fingerprint of fcc plutonium dioxide.

No abstract

An oxidation and dissolution study has been performed on UO2 pellets containing ∼10 and ∼0.1 wt. % 238Pu, ∼10 wt. % 239Pu and on undoped UO2 to investigate the effects of radiolysis and composition on the corrosion behavior of spent fuel. The so-called alpha-doped UO2 is used to simulate the alpha-radiation field of different types of commercial LWR spent fuel after different storage times. Leaching experiments in demineralized and carbonated water at room temperature under oxidizing conditions showed that relatively high amounts of 238Pu were released. The leached surfaces were examined with X-ray Photoemission Spectroscopy (XPS), and the progressive surface oxidation was monitored. The oxidation of the U(IV) during the leaching experiments, in the materials doped with 238Pu resulted in precipitation of U(VI) phases: enhanced formation of studtite for the strongest radiation field and shoepite at low radiation field was observed on the surface of the pellet. Essentially no precipitation of Pu-rich phases was directly observed. Leaching in carbonated water and characterization of UO2 containing 239Pu under the same experimental conditions were performed and the results compared to those for alpha-doped UO2. The chemistry effects due to the presence of Pu in addition to alpha-radiolysis were investigated.

UO2 containing short-lived α-emitters, the so-called α-doped UO2, can simulate type (i.e. α- decay) and level of activity of spent fuel at the time when it might become exposed to groundwater in a geologic repository during storage. This allows studying α-radiolysis effects on the dissolution of the fuel matrix. Additionally, UO2 with high concentrations of α-emitters accumulate, during experimentally acceptable short times, the amount of decays, hence of property modifications, corresponding to long storage times for spent fuel. UO2 containing ∼10 and ∼0.1 wt% 238Pu was fabricated and tested. Leaching experiments in deionized water under unaerated conditions, with continuous monitoring of the evolution of the redox potential and pH in the leaching solutions, were performed. The Eh measurements showed a fast increase of the redox potential in the case of the material with the highest α-activity, while the UO2 containing ∼0.1 wt% 238Pu increased its potential more slowly. The redox potential for undoped UO2 decreased steadily during the experiment. As previously observed, higher fractions of U were released in the case of α-doped UO2 compared to undoped UO2. The fractions of U and Pu released during leaching from the α-doped materials were very similar, suggesting that congruent dissolution occurred. After leaching times longer than 10 h, only dissolved species were present in the solutions. Under these experimental conditions, characterized by relatively low values of the ratio sample surface/leachant volume, a dependence of the released amounts on the α-activity of the samples was observed. Periodical measurements of parameters like hardness, showed a rapid buildup of radiation damage in the material with the high α-activity. After more than two years, noticeable changes, namely an increase of the hardness, have begun to be observed also for the material with the low concentration of 238Pu.