The recrystallization of cold-rolled uranium

This paper gives the results of investigations into the effect of rolling and annealing in the region on the structure and mechanical properties of uranium. The initial material was uranium which had been cast, rolled in the region and quench-hardened from the ß region. Smallgrain recrystallized uranium has much higher strength than the initial coarse-grained uranium. Approximate recrystallization diagrams are plotted for 5–40% degrees of reduction and annealing at temperatures of 350–650 °C with soaking for 10 hr. The recrystallization is almost independent of the investigated initial states of the material. Data are given on the kinetics of the recrystallization processes. The addition of 0.1 wt.% of molybdenum slows down considerably the recrystallization of uranium and increases the strength of the small-grained uranium by 20%.     

Inverse photoemission of uranium oxides

Understanding the itinerant-localised bonding role of the 5f electrons in the light actinides will afford an insight into their unusual physical and chemical properties. In recent years, the combination of core and valance band electron spectroscopies with theoretic modelling have already made significant progress in this area. However, information of the unoccupied density of states is still scarce. When compared to the forward photoemission techniques, measurements of the unoccupied states suffer from significantly less sensitivity and lower resolution. In this paper, we report on our experimental apparatus, which is designed to measure the inverse photoemission spectra of the light actinides. Inverse photoemission spectra of UO₂ and UO_2.2 along with the corresponding core and valance electron spectra are presented in this paper. UO₂ has been reported previously, although through its inclusion here it allows us to compare and contrast results from our experimental apparatus to the previous Bremsstrahlung Isochromat Spectroscopy and Inverse Photoemission Spectroscopy investigations.     

Helium diffusion in uranium and plutonium oxides

Samples of UO₂, (U,Pu)O₂ and PuO₂ containing up to several 100 at. ppm helium were submitted to thermal annealing in a Knudsen-cell provided with a mass spectrometer. Gas release was measured on line with a great accuracy. In the examined materials helium was created by α-decay of plutonium or laboratory infused at high temperature and high pressure. The selected samples exhibited different types of lattice damage, including reactor burn-up and high α-radiation doses. Analysis of helium release as a function of temperature enabled the elementary diffusion processes to be investigated and the atomic diffusion coefficient to be deduced for a defined state of helium-in-solid. The helium diffusion coefficient has the expression:

     

Diffusion of radiogenic helium in natural uranium oxides

The issue of nuclear waste management – and especially spent fuel disposal – demands further research on the long-term behavior of helium and its impact on physical changes in UO₂ and (U,Pu)O₂ matrices subjected to self-irradiation. Helium produced by radioactive decay of the actinides concentrates in the grains or is trapped at the grain boundaries. Various scenarios can be considered, and can have a significant effect on the radionuclide source terms that will be accessible to water after the canisters have been breached. Helium production and matrix damage is generally simulated by external irradiation or with actinide-doped materials. A natural uranium oxide sample was studied to acquire data on the behavior of radiogenic helium and its diffusion under self-irradiation in spent fuel. The sample from the Pen Ar Ran deposit in the Vendée region of France dated at 320 ± 9 million of years was selected for its simple geological history, making it a suitable natural analog of spent fuel under repository conditions during the initial period in a closed system not subject to mass transfer with the surrounding environment. Helium outgassing measured by mass spectrometry to determine the He diffusion coefficients through the ore shows that: (i) a maximum of 5% (2.1% on average) of the helium produced during the last 320 Ma in this natural analog was conserved, (ii) about 33% of the residual helium is occluded in the matrix and vacancy defects (about 10⁻⁵ mol g⁻¹) and 67% in bubbles that were analyzed by HRTEM. A similar distribution has been observed in spent fuel and in (U_0.9,Pu_0.1)O₂. The results obtained for the natural Pen Ar Ran sample can be applied by analogy to spent fuel, especially in terms of the apparent solubility limit and the formation, characteristics and behavior of the helium bubbles.     

Amorphization and recrystallization of the ABO3 oxides

Single crystals of the ABO₃ phases CaTiO₃, SrTiO₃, BaTiO₃, LiNbO₃, KNbO₃, LiTaO₃, and KTaO₃ were irradiated by 800 keV Kr⁺, Xe⁺, or Ne⁺ ions over the temperature range from 20 to 1100 K. The critical amorphization temperature, T_c, above which radiation-induced amorphization does not occur varied from approximately 450 K for the titanate compositions to more than 850 K for the tantalates. While the absolute ranking of increasing critical amorphization temperatures could not be explained by any simple physical parameter associated with the ABO₃ oxides, within each chemical group defined by the B-site cation (i.e., within the titanates, niobates, and tantalates), T_c tends to increase with increasing mass of the A-site cation. T_c was lower for the Ne⁺ irradiations as compared to Kr⁺, but it was approximately the same for the irradiations with Kr⁺ or Xe⁺. Thermal recrystallization experiments were performed on the ion-beam-amorphized thin sections in situ in the transmission electron microscope (TEM). In the high vacuum environment of the microscope, the titanates recrystallized epitaxially from the thick areas of the TEM specimens at temperatures of 800–850 K. The niobates and tantalates did not recrystallize epitaxially, but instead, new crystals nucleated and grew in the amorphous region in the temperature range 825–925 K. These new crystallites apparently retain some ‘memory' of the original crystal orientation prior to ion-beam amorphization.     

Calorimetric investigations on stoichiometric barium and uranium oxides

Heat capacities and enthalpy increments of barium uranates: BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) were measured using a differential scanning calorimeter and a high-temperature Calvet calorimeter. The heat capacities and enthalpy increments were measured in the temperature range 126-304 K and 299-1011 K, respectively. A set of self consistent thermodynamic functions such as entropy, Gibbs energy function, heat capacity and Gibbs energy and enthalpy of formation values for BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) have been computed for the first time using the data obtained in the present study and other available experimental data.     

Role of oxidation-reduction processes in the solution of uranium oxides in acid media

The degree of oxidation of the components of the ore material is of decisive importance in the extraction of uranium. Since the solution of uranium is composed of various chemical processes and complicated by the presence of impurities in the ores, it is necessary to study the rules of uranium oxidation and the effect of various compounds which dissolve during the leaching of uranium ores.In the present work we present the results of an investigation of the oxidation and solution of uranouranic oxide in acid media and the effect of ferrous compounds on it. A comparison of the oxidizing power of various oxidants with respect to uranium is given for the first time. It was shown that the oxidation of tetravelent uranium is not determined unequivocally by the normal oxidation-reduction potential. The results obtained show that the reasons for incomplete extraction of uranium by a mixture of nitric and sulfuric acids at low concentrations when considerable amounts of iron ions are present in the solutions are associated with the formation of complexes between Fe²⁺, SO ₄ ^2– , and NO ₃ ^– ions.     

Determination of uranium content in Th, U mixed oxides using EDXRF

An energy dispersive X-ray fluorescence (EDXRF) technique has been developed for the determination of U content in the range 2-80% in mixed oxides of thorium and uranium. The experimental parameters for XRF measurements were optimized using 1 cm diameter pellets made from 1:1 mixture of thorium-uranium mixed oxide standards and cellulose. Calibration plots were established using both L_α as well as L_β peaks of uranium. The reproducibility of the determination was evaluated to be better than 3% for L_α peak, while for L_β peak, it was better than 4%. The measured values of the uranium concentration in the synthetic samples were found to be in good agreement with the added values. Simultaneously Th was also determined using its L_α peak. The present method will be useful for fast routine measurements of mixed oxide samples of Th and U, without the need for dissolution.     

Reduction of uranium hexafluoride adsorbed on sodium fluoride

The problem of the formation of uranium residue in spent sodium fluoride sorbent is studied. The mechanisms of the formation of un-desorbed uranium residue with thermal desorption of uranium hexafluoride from the surface sodium fluoride are examined. It is proved that hydrolysis of sorbed and gaseous uranium hexafluoride under the conditions of sublimate-separation production is impossible. The most likely mechanism of residue formation is partial reduction of hexavalent uranium, which is due to the chemical properties of the system UF₆–NaF. A possible process of uranium reduction as a result of nuclear-chemical transformations requires study and experimental confirmation.