Electrochemical Reduction of (U,Pu)O2 in Molten LiCl and CaCl2 Electrolytes

The electrochemical reduction of UO2-PuO2 mixed oxides (MOX) was performed in molten LiCl at 923K and CaCl2 at 1,123K to evaluate the behavior of the plutonium quantitatively and to define the optimum conditions for the electrochemical reduction of those materials. In LiCl, excess deposition of lithium metal can be avoided and the MOX was smoothly reduced at 0:65V vs. Bi-35 mol per cent Li reference electrode. The reduction ratio calculated from the mass change of the samples taken during the electrochemical reduction and the ratio evaluated by gas-burette method were in good agreement. The cathodic current efficiency remained 30–50 per cent mainly due to the deoxidation of tantalum cathode basket. Although dissolution of plutonium and americium into the electrolyte was found by the chemical analysis, the dissolved amount was negligible and had no immediate influence on the feasibility of the electrochemical reduction process. In CaCl2, reduction of the MOX occurred in whole range of the tested cathode potential (0:15V to 0:40V vs. Ca-Pb reference electrode). The cathodic current efficiency was around 30 per cent. Although the MOX was completely reduced at 0:25 V, the reduction was interrupted by formation of the surface barrier made of the reduced material and the vacancy between the reduced and the non-reduced areas at 0:30 V. Plutonium and americium dissolved also into the CaCl2 electrolyte to slightly higher concentrations than those observed in LiCl electrolyte. The analyses for the reduction products showed that the amount of those actinides lost from the cathode was much larger than that found in the electrolyte, probably due to the formation of mixed oxide precipitate.JRC.E.2-Hot cell

Effects of HiPIMS discharges and annealing on Cr-Al-C thin films

International audienceCr$_2$AlC coatings were synthesized by high power impulse magnetron sputtering (HiPIMS) from Cr$_2$AlC compound target and subsequent thermal annealing in Ar atmosphere. The effect of HiPIMS duty cycle and substrate bias potential (UB) on the thin film composition were investigated. All initial Cr-Al-C coatings exhibit similar compositions close to the target stoichiometry, and dense and amorphous structure independently of the duty cycle. Meanwhile, Al deficiencies up to 15% are observed with UB increasing to −200 V. Based on the measured fraction of ionized metal species flux at the substrate, highly energetic bombardment of the coating with ionized inert gas and metal plasma species causes preferential resputtering of Al. Partially crystallized Cr$_2$AlC thin films were obtained by annealing as-deposited Cr-Al-C coatings at 550 °C for 4 h. The annealed coating is made of an amorphous inner layer and a crystalline Cr$_2$AlC outer layer. A higher annealing temperature of 650 °C led to complete transformation from amorphous phase to crystallized Cr$_2$AlC, and to micro-cracking. These results indicate that the synthesis temperature of MAX phase could be reduced, and the annealing time increased, to obtain protective coatings of Cr$_2$AlC on heat-sensitive components without alteration of the substrate metallurgical properties

Protection of Low-alloy carbon steel in mildly saline environments by dense ZrO2 coatings

International audienceIn the French concept of deep underground nuclear repository, the confinement of high-level nuclear waste is controlled by corrosion of a tight overpack made of low-alloy carbon steel (C-steel). The overpack dimensions are tailored to guarantee confinement during the repository thermal peak, but it may be interesting to increase the overpack lifetime by protective coatings. Due to their physico-chemical properties and corrosion resistance, ceramics such as zirconia (ZrO2) can be used as physical barriers or coatings against interaction between steel surface and corrosive fluids. However, fabrication of oxide coatings has long relied on processes using hazardous or polluting solvents. This limitation can be overcome by techniques such as physical vapor deposition (PVD), which have been spurred by the recent development of High-power impulse magnetron sputtering (HiPIMS). Therefore, the goal of this study was to assess the protective efficiency of C-steel protection against corrosion by PVD-deposited zirconia coatings. A ferrito-perlitic C-steel (P285GH) was were polished, degreased, and used as a substrate in the PVD setup. The steel surfaces were first etched for 10 minutes, and then ZrO2 ceramic coatings were fabricated by reactive PVD-HiPIMS using a zirconium target. The total deposition time, the duration of HiPIMS impulsion, the power density at the target surface and the target-to-substrate distance were varied. Characterization by X-ray diffraction, scanning electron microscopy and Raman microscopy showed that the coatings were homogeneous, relatively dense and composed of monoclinic zirconia forming columns. They were adherent over the entire surface, and displayed only few defects. Variations in the deposition parameters resulted in differences in crystalline properties and coatings thicknesses (between 5 and 10 µm).The corrosion behavior of these coatings in reference (borate buffer) and mildly saline (40 mM NaCl) solutions were assessed in a three-electrode setup (Hg/HgSO4 reference electrode, Pt mesh counter electrode) by monitoring of the open circuit potential (EOCP) and by anodic polarization. Potentials are quoted against the Normal Hydrogen electrode (NHE). The EOCP values measured were generally within ± 0.1 V of the value for bare steel (-0.56 V /NHE), but the corrosion currents were significantly lower, by one to three decades ( ≤ 0.2-5.8×10-8 versus 2.7×10-6 A/cm2, respectively, in 40 mM NaCl). Immersion experiments over three weeks showed that significant variations in the corrosion current occurred, possibly due to corrosion and scaling within coating defects. Overall, however, the corrosion current remained lower than the value measured after one day. These results indicate that dense zirconia coatings may offer interesting protective properties against corrosion in mildly saline environments. Additional experiments over longer timescales, at higher temperatures, and in environmentally representative solutions would aim to further assess the relevancy of such protective coatings

High-temperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy

International audienceProtective coatings of Zr-based claddings have been proposed for the development of Accident Tolerant nuclear Fuel (ATF). Coatings forming stable oxides at high temperature such as MAX phases are attractive candidates for these applications. In this study Cr-Al-C coatings were deposited on coupons of Zr-based alloy (Zr702) by High Power Impulse Magnetron Sputtering (HiPIMS). Cr2AlC coatings were then obtained by annealing of the asdeposited films at a temperature below metallurgical degradation of Zr alloys. The behavior of as-deposited Cr-Al-C and annealed Cr2AlC coatings with respect to high-temperature oxidation slightly differ for short oxidation times but converge for longer durations. Oxidized coatings are made of (i) an external dense, covering, adherent and thin scale of aluminum and chromium oxides, (ii) an intermediate thicker, porous layer of chromium carbide and (iii) an interdiffusion layer. Both coatings are protective in dry and wet air (up to 1473 K for 2 h in air-28 % H2O for an initial thickness of 7 µm), and are thermal shock-resistant. Self-healing capability is observed for submicronic defects. The top oxide scale acts as a barrier against oxygen diffusion, thus efficiently protecting the Zr702 substrate from extended oxidation except near coupon edges. The results indicate that Cr-Al-C thin films grown by HiPIMS process and annealed Cr2AlC coatings are both promising candidates for ATF cladding coatings

Development and Validation of a Simple, Rapid and Robust Method for the Chemical Separation of Uranium and Plutonium.

A simple and robust method was developed for the chemical separation of uranium and plutonium from mixed U/Pu/fission product solutions. The method is based on a commercially available resin, UTEVA-spec. It yields pure U and Pu fractions with recoveries superior to 95% respectively, using a single column. The method is simple and robust, allowing a rapid separation with a minimum of secondary waste being created. The method was validated for application in routine analysis of nuclear materials covering a wide range of concentrations and U/Pu ratios.JRC.E-Institute for Transuranium Elements (Karlsruhe

Equilibrium Distribution of Actinides Including Cm Between Molten LiCl-KCl Eutectic and Liquid Cadmium

Equilibrium distribution of actinides both in molten LiCl-KCl eutectic and liquid cadmium were measured from the concentration data obtained in electrorefining tests and reductive extraction tests. Separation factors for U, Np, Am, Cm against Pu were derived in the practical temperature range of 700 K to 783 K. The derived separation factors are consistent with the reported values measured at 773 K and 723 K. The temperature dependence for Cm is different compared to the other actinides (U, Np and Am). This behavior remains unclear and additional experimental measurements of distribution coefficient of Cm are required before ruling on the real behavior.JRC.E.5-Nuclear chemistr

Study of Molten Salt Electrorefining of U-Pu-Zr Alloy Fuel.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Development of Pyrochemical Separation Processes for Recovery of Actinides from Spent Nuclear Fuel in Molten LiCl-KCl

Contribute to the ERA efforts in Actinide recovery by pyrochemical separation methods in molten Chloride and molten salt.JRC.E.5-Nuclear chemistr

Recent HKED Instrumentations for Analytical Measurements in Conventional and Advanced Nuclear Fuel Reprocessing

Two Hybrid K-edge Densitometers (HKED¿s) recently designed at ITU for different applications are presented and described. One of them, designated for installation in the RT-1 reprocessing plant of Mayak, Russia, follows the well-known route of standard HKED applications, i.e. the analysis of process samples from Purex type reprocessing. The second HKED is embedded into a more enlarged non-destructive assay (NDA) station including an additional neutron coincidence counter and a highresolution gamma spectrometer for the analysis of minor actinides in process samples originating from pyro-reprocessing test facilities at ITU. In addition, the paper also provides an evaluation of new HKED analysis software recently developed at the Los Alamos National Laboratory.JRC.E.8-Nuclear safeguards and Securit

Synthesis of UF4 and ThF4 by HF gas fluorination and re-determination of the UF4 melting point

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF4 and ThF4. The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF4 was revised using a very pure sample and a new value is suggested.JRC.G.I.3-Nuclear Fuel Safet