J. Quiñones

Effect of secondary phases formation in the leaching of UO{sub 2} under simulated radiolytic products

The leaching of the spent fuel matrix (UO{sub 2}) is function of the radiolytic products formation. The effect of each radiolytic product on the leaching process is not totally understood. In the literature, the influence of H{sub 2}O{sub 2} on the dissolution process is described from the qualitative point of view, and most of the studies were performed for pH values from 8 to 12. In this paper the authors report on the effect of the H{sub 2}O{sub 2} in the leaching process of UO{sub 2} by dissolution experiments at various H{sub 2}O{sub 2} concentrations. Also, the influence of S/V ratio (surface area exposed to the leaching media) on the UO{sub 2} leaching and secondary phases formation was tested. The formation of secondary phases on the UO{sub 2} surface was identified. Solid phases characterization was carried out by X-ray Photoelectron Spectrometry (XPS), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. By XPS studies the secondary phase formed corresponded to a U(VI) phase. By XRD analyses the solid was identified as studtite, UO{sub 4}{center_dot}4H{sub 2}O. A comparison of the U(VI) phases formed in spent fuel and UO{sub 2} leaching experiments in various media has been carried out.

Diffusion bonding of alumina to steel using soft copper interlayer

The diffusion bonding of a low steel to alumina has been studied in the present work. Thin foils of a soft metal (copper) were used to reduce the effects of the residual stresses produced in the joint by thermal expansion mismatch. The strength of the joint was found to be influenced by the bonding parameters, but principally by the oxygen content both on the surface and in the copper matrix. The diffusion bonds have been mechanically tested using a three-point bending test. Maximum bending strengths of 100 MPa were achieved by using a 0.1 mm copper foil, and bonding in a oxidizing atmosphere (PO2=104Pa). SEM and EDS investigations have shown the presence of reaction products in the copper-alumina interface which controls the mechanical properties of the joint.

Diffusion bonding of an aluminium-lithium alloy (AA8090) using aluminium-copper alloy interlayers

Diffusion bonds have been produced between sheets of an Al-Li-Cu-Mg-Zr alloy using aluminium-4% copper vapour deposited metallic interlayers. Microstructural changes occurred both in the parent alloy and in the bond interface after diffusion bonding cycles and post-bonding heat treatments were analysed. Different metallographic techniques (light microscopy, scanning and transmission electron microscopy) have been used. Diffusion bonding trials were carried out using the same alloy (AA8090), both in non-superplastic (T6) and superplastic conditions. Differences in their behaviours in relation to diffusion bonding were observed.

COMPARISON OF THE LEACHING BEHAVIOUR OF IRRADIATED FUEL, SIMFUEL, AND NON-IRRADIATED UO2 UNDER OXIC CONDITIONS

The aim of this study was to compare the leaching behaviour of irradiated U02 and MOX and non-irradiated U02 and SIMFUEL. Direct comparisons using the very large amount of experimental data available in the literature are often difficult, not only because of the differences existing between SIMFUEL or non-irradiated U02 and spent fuel (i.e. the presence of intense radiation fields surrounding irradiated fuel, hence radiolysis effects, or the formation of cracks and pores due to the volatile fission products, hence larger surface area), but also because of different experimental procedures used by different authors. In the case of MOX, the possible effects due to the different initial composition and microstructure also affect the leaching behaviour. This work presents results of sequential leaching experiments in deionized water at room temperature under oxic conditions using irradiated and non-irradiated fuel. The experimental conditions and procedure for all types of fuel were kept similar as much as possible. The SIMFUEL and U02 samples underwent a more extensive characterisation due to easier handling. The specimens used were U02 (54 GWd/tU) and mixed oxide (MOX, 30 GWd/tU) LWR spent fuels, and U02 and SIMFUEL (30 and 50 GWd/tU) as non-irradiated chemical analogues. The highest fractional release of uranium was measured for U02 spent fuel. The uranium release data for MOX fuel showed behaviour closer to non-irradiated fuels. Comparison of fission products release between SIMFUEL and irradiated fuel was also performed.

Mechanisms governing the release of radionuclides from spent nuclear fuel in geological repository: major outcomes of the European Project SFS

European Commission supported a wide research project entitled “Spent Fuel Stability under repository conditions” (SFS) within the 5 th FWP, the aim of which was to develop a common understanding of the radionuclides release from spent nuclear fuel in geological disposal and build a RN release model in order to assess the fuel performance. This project achieved by the end of 2004 focuses both on the Instant Release Fraction (IRF) model and the Matrix Alteration Model (MAM). A new IRF model was developed based on the anticipated performances of the various fuel microstructures (gap, rim, grains boundaries) and the potential diffusion of RN before the canister breaching. However, this model lets the choice to the end-user about the degree of conservativeness to consider. In addition, fuel alteration has been demonstrated to be linked to the production of radiolytic oxidants by water radiolysis at the fuel interface, the oxidation of the fuel interface by radiolytic oxidants and the subsequent release of uranium under the influence of aqueous ligands. A large set of experimental data was therefore acquired in order (i) to upgrade the current radiolytic kinetic scheme, (ii) to experimentally correlate the fuel alteration rate and the fuel specific alpha activity by performing experiments on alpha doped samples, (iii) to experimentally test the potential inhibitor effect of hydrogen on fuel dissolution. Based on these results, a new MAM was developed, which was also calibrated using the experiments on inactive UO 2 samples. This model was finally applied to representative granitic, salt and clayey environment to predict spent fuel long-term fuel performance.

Latest developments for microstructural and chemical characterization of diffusion bonding in superplastic 8090 Al–Li alloys

This paper describes a new application of two complementary surface characterization techniques to study solid-state bonding in an Al–Li alloy. Through the two mentioned techniques, Atomic Force Microscopy (AFM) and Secondary Ion Mass Spectrometry (SIMS), important findings about what takes place in the bond interface have been determined. These findings enclose both the formation of discontinuous mixed oxides and the evolution of Li through the bond line and into theadjacent diffusion affected zones. Homogenization of Li and Cu alloyelements has been detected even in those cases where a metallic interlayer was used to favor the union.

Interface reactions and bonding strength in aluminium alloy (AA-7075)-alumina diffusion bonds

Diffusion bonds between alumina and high-strength aluminium alloy (AA-7075) have been produced and studied in the present work. Direct diffusion bonding in the solid state was tested as a possible joining method for both materials. The nature of the AA7075-Al2O3 interface was investigated paying special attention to the chemical interaction processes between the alloying elements and the ceramic material, as well as their influence on the joint strength. SEM images and energy-dispersive microanalysis were used to determine the formation of reaction layer between both parent materials. Shear strength was used as an optimum method to evaluate the bond strength and the influence of the bonding parameters on it. A maximum shear strength of 60 MPa was achieved using bonding temperatures and pressures of 360 °C and 6 MPa, respectively, during very prolonged bonding times (100h). Fractographic studies of the failure surface gave additional information on those aspects.

Determination of a stability potential-pO 2- diagram of samarium in the eutectic LiCl-KCl

Abstract:Reaction between samarium trichloride and oxide ions was studied in the fused LiCl – KCl eutectic mixture at 723 K, by potentiometry with an yttria-stabilised zirconia membrane electrode. Titration curve demonstrated the existence of a precipitated samarium oxychloride (SmOCl), under this experimental conditions, with a solubility product value of 7.59 ţ 0.3 (molality scale). The combination of this value with the standard potentials allowed setting up the potential–pO 2- stability diagram which summarises the properties of Sm species in the eutectic LiCl – KCl melt.

Mathematical models of diffusion processes

Abstract The capabilities of a PC program that can be used for predicting different metallurgical processes controlled by solid state bonding are described in this paper. Program considers different solutions to the Ficks second law, such as: thin films, pair of semi-infinite solids, finite solid and infinite solid. Theoretical results are compared with experimental data obtained in two different situations: surface oxidation in Al-Li alloys and diffusion bonding of ferreous materials.

Simfuel Leaching Experiments in Presence of Gamma External Source ( 60 CO)

One of the factors considered within the studies of performance assessment on spent fuel under final repository conditions is the effect of the radiation on its leaching behavior. Radiation from spent fuel can modify some properties of both solid phase and leachant and therefore it would alter the chemical behavior of the near field. Particularizing in the effect of the radiation on the leachant, it will cause generation of radiolytic species that could change the redox potential of the environment and therefore may bring on variations in the leaching process. In this work, the chemical analogue utilized was SIMFUEL (natural UO 2 doped with non-radioactive elements simulating fission products) and the leachants selected were saline and granite bentonite waters both under initial anoxic conditions. To emulate γ radiation field of a spent fuel, leaching experiments with external 60 Co sources in a irradiation facility (Nayade) were performed. Initial dose rate used was 0.014 Gy/s. Preliminary results indicate that radiation produces an increase of the uranium dissolution rate, being the concentrations measured close to those obtained in oxic atmosphere without radiation field. In addition the solubility solid phases from experimental conditions were calculated, for both granite bentonite water and 5 m NaCl media. On the other hand, a tentative approach to model the role of γ radiolysis in these SIMFUEL tests has been carried out as well.