Diffusion of helium in non-stoichiometric uranium dioxide

Diffusion coefficients of helium in solid UO_2+x at high temperature and various stoichiometric compositions are evaluated by using molecular dynamics simulations based on a partly-ionic model in conjunction with a polaron ‘Free Hopping Approximation’ proposed earlier. The results are compared with existing experimental and simulation data. A strong dependence of the apparent diffusion activation energy on stoichiometry is found and the mechanisms of He migration in non-stoichiometric solid UO_2+x are discussed.     

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:

     

Reduction recrystallization of uranium oxides in molybdate melts

The presence of traces of water dissolved in a sodium molybdate melt and the interaction of this water with a metal surface (NP-2 nickel) in contact with the melt are confirmed. It is shown experimentally that U₃O₈ can be reduced to UO₂ in sodium molybdate melt by the hydrogen that is released during such an interaction. A technological process of reprocessing spent oxide nuclear fuel using the phenomenon studied is described. __________ Translated from Atomnaya énergiya, Vol. 102, No. 5, pp. 290–292, May, 2007.     

Diffusion and trapping of ion-implanted helium in nickel

Helium concentration versus depth profiles have been measured and analyzed for 200 keV He implanted into nickel at temperatures between 300 and 1064 K. Below 973 K, the profiles do not exhibit appreciable temperature dependence and are similar to theoretical range distributions. Above 973 K the range peak begins to exhibit broadening with some loss of He. The changes in the peak shape could be accounted for the assuming that the He and its ion-beam generated vacancies migrate simultaneously and become trapped with each other. A helium-vacancy binding enthalpy of 3.2 eV and an effective helium migration energy of approximately 2.6 eV was found to be consistent with the data when analyzed by a limited system of diffusion equations.     

First-principles theory for helium and xenon diffusion in uranium dioxide

The diffusion properties of He and Xe in UO₂ have been investigated, using density-functional calculations employing the projector-augmented-wave (PAW) method and the generalized gradient approximation (GGA). The migration energies corresponding to both interstitial and vacancy-assisted mechanisms have been calculated and the results for the two noble gas atoms are compared with each other. We suggest that He likely diffuses by hopping through a single vacancy with computed low migration energies smaller than 0.79 eV and its diffusivity is much higher than that of Xe. Xe has a quite large migration energy compared to He; the strain energy plays a key role in Xe diffusion in UO₂.     

Predicted mechanisms for radiation enhanced helium resolution in uranium dioxide

Classical molecular dynamics has been performed to predict the behaviour of helium gas bubbles in uranium dioxide, UO₂, when subjected to displacement cascades that mimic the effects of self-irradiation damage. The models presented here examine bubble sizes of 2 and 4 nm with several different gas densities and displacement cascades with energies of up to 50 keV. Of particular interest are the mechanisms by which helium atoms can be returned to solution in the lattice through interaction with displacement cascades. This occurs both via ballistic recoil from high-energy ion fragments traversing the bubble and also a damage assisted resolution whereby the high-pressure gas intermixes into the disordered cascade regions formed adjacent to the surface of the bubble.     

Diffusion in uranium alloys with zirconium and titanium

Moscow Engineering-Physics Institute. Translated from Atomnaya Énergiya, Vol. 72, No. 1, pp. 30–33, January, 1992.     

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.     

Utilization of natural uranium in a homogeneous reactor

One of the main advantages of homogeneous reactors over heterogeneous reactors is the absence of stability problems with heat-emitting elements. Therefore, the permissible degree of uranium utilization, and consequently, the economic characteristics of a homogeneous reactor can be significantly improved.This paper indicates the magnitude of uranium utilization with respect to the initial system parameters and establishes conditions for the probability of capture by U²³⁸ of slowed neutrons, the occurrence of which permits maximum utilization in a reactor using regenerated uranium.     

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.