Recovery of actinides from actinide-aluminium alloys by chlorination: Part I

Pyrochemical processes in molten LiCl–KCl are being developed in ITU for recovery of actinides from spent nuclear fuel. The fuel is anodically dissolved to the molten salt electrolyte and actinides are electrochemically reduced on solid aluminium cathodes forming solid actinide–aluminium alloys. A chlorination route is being investigated for recovery of actinides from the alloys. This route consists in three steps: Vacuum distillation for removal of the salt adhered on the electrode, chlorination of the actinide– aluminium alloys by chlorine gas and sublimation of the formed AlCl3. A thermochemical study showed thermodynamic feasibility of all three steps. On the basis of the conditions identified by the calculations, experiments using pure UAl3 alloy were carried out to evaluate and optimise the chlorination step. The work was focused on determination of the optimal temperature and Cl2/UAl3 molar ratio, providing complete chlorination of the alloy without formation of volatile UCl5 and UCl6. The results showed high efficient chlorination at a temperature of 150 °C

Investigation of ammonium diuranate calcination with high temperature X-ray diffraction

The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in-situ high-temperature x-ray diffraction (HT-XRD), thermogravimetry, and differential thermal analysis (TG/DTA). Data have been collected in the temperature range from 30 to 1000 °C, allowing to observe the sequence of phase transformations and to assess the energy changes involved in the calcination of ADU. The starting material 2UO3•NH3•3H2O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO3 is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO3 phase appears, as displayed by ex-situ XRD. The self-reduction of UO3 into orthorhombic U3O8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilize the structure of U3O8 at room temperature. U3O8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U3O8-x, as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.JRC.E.6-Actinide researc

Recovery of actinides from actinide–aluminium alloys by chlorination: Part II

A chlorination route is being investigated for recovery of actinides from actinide–aluminium alloys, which originate from pyrochemical recovery of actinides from spent metallic nuclear fuel by electrochemical methods in molten LiCl–KCl. In the present work, the most important steps of this route were experimentally tested using U–Pu–Al alloy prepared by electrodeposition of U and Pu on solid aluminium plate electrodes. The investigated processes were vacuum distillation for removal of the salt adhered on the electrode, chlorination of the alloy by chlorine gas and sublimation of the AlCl3 formed. The processes parameters were set on the base of a previous thermochemical study and an experimental work using pure UAl3 alloy. The present experimental results indicated high efficiency of salt distillation and chlorination steps, while the sublimation step should be further optimised

Probing magnetism in the vortex phase of PuCoGa5 by X-ray magnetic circular dichroism

We have measured X-ray magnetic circular dichroism (XMCD) spectra at the Pu M4;5 absorption edges from a newly-prepared high-quality single crystal of the heavy fermion superconductor 242PuCoGa5, exhibiting a critical temperature Tc = 18.7 K. The experiment probes the vortex phase below Tc and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a SQUID device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa5 have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.JRC.G.I.5-Advanced Nuclear Knowledg

Synthesis of nc-UO 2 by controlled precipitation in aqueous phase

A new strain of actinobacteria, designated ACD1, was isolated from a Saharan soil sample in the Hoggar region (Algeria). Morphological study led to this strain being classified as a member of the Actinomadura genus. Phylogenetic analysis based on the 16S rRNA gene showed that the strain is closely related to Actinomadura sediminis DSM 45500T (98.5% sequence similarity). Furthermore, strain ACD1 presented a strong activity against mycotoxigenic and phytopathogenic fungi, including Aspergillus and Fusarium strains, and other pathogenic microorganisms. The kinetics of antimicrobial activity were investigated on ISP-2, Bennett and TSB media. Four solvents (n-hexane, dichloromethane, ethyl acetate and n-butanol) were used for the extraction of the produced antibiotic. The highest antimicrobial activity was obtained using the butanolic extract from the ISP-2 medium after seven days of fermentation culture. The active antibiotic was purified by reverse-phase HPLC using a C18 column. The UV-visible and mass spectra were determined. The minimum inhibitory concentrations (MIC) of this antibiotic were determined against pathogenic microorganisms

An Atomic-Scale Understanding of UO2 Surface Evolution During Anoxic Dissolution

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of non-local characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic–scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionised water. No amorphisation of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the centre of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces

Electronic Structure of Actinide Hydrides Studied by Photoelectron Spectroscopy

Actinide hydrides constitute an interesting class of materials. From Np on, cubic dihydrides and hexagonal trihydrides exist, very similarly to the rare earths. Uranium only forms the trihydride UH3 (the dihydride is missing), while Th forms a tetragonal dihydride and a complex cubic higher hydride Th4H15. These differences are related to the 5f electrons which, from Np on, are localized in the hydrides, resulting in the rare-earth like behavior. The localization-delocalization threshold is thus shifted to the lighter actinides, and lies between U and Np. It is instructive to compare the evolution of the cohesive energy of metals and hydrides (Fig. 1). While for the metals, 5f localization takes place between Pu and Am, as seen by the sudden drop in density, the hydrides always have an even lower density and do not show any density change between Pu and Am. It is thus concluded that AmH3, PuH3 and NpH3 have localized 5f states. Only UH3 has an increased density, and this may now be taken as indication for participation of the f-states in bonding. And indeed, UH3 is an itinerant ferromagnet and it has an enhanced low temperature specific heat compared to the a-metal. All thisis consistent with formation of a narrow U-5f-band.JRC.E.6-Actinides researc

Préparation, caractérisation et évaluation de catalyseurs de décomposition de nouveaux monergols azotés pour la propulsion spatiale (comparaison avec l'hydrazine et l'eau oxygénée)

L'hydrazine est le monergol le plus couramment utilisé dans la propulsion orbite. En raison de ses propriétés toxiques, l'intérêt pour de nouveaux monergols s'est développé. Parmi les plus intéressants, les monergols à base de nitrate d'hydroxyle amine (HAN) ont été suggérés. Le but de cette recherche a été de préparer, caractériser et tester l'activité de différents catalyseurs pour la décomposition de mélange à base de HAN. Les supports les stables à haute température ont servi alors à la préparation de catalyseurs à base de métaux nobles puis caractérisés avant d'être testés pour leur activité dans un réacteur batch. L'hydrazine a été testée comme référence. Une étude comparative a été aussi effectuée sur le peroxide d'hydrogène (90% et 30%; Ag et Ag/Al2O3) en collaboration avec le centre spatiale du Surrey (SSC).Hydrazine is the most used monopropellant in orbit propulsion and its decomposition is ensured by an alumina supported iridium catalyst. Because of hydrazine toxicity, the interest for new non toxic propellants has been developped, and among potential substitute, the hydroxyl ammonium nitrate (HAN) based liquid propellants has been suggested. The purpose of our research was to prepare, to characterize and to test the activity of catalysts for the decomposition of HAN blended propellant. Due to the high temperature of gas generated by this propellant, a screening test was developped to determine and to prepare the most stable supports after a thermal treatment at 1200 ʿC during 5 h. Noble metals based catalysts have been prepared on the selected supports, characterized and then tested for the catalytic activity in a batch reactor. An other possible propellant, the hydrogen peroxide, has seen its interest renewed as a substitute propellant too. An association with the Surrey Space Centre led to a training work on the catalytic decomposition of H2O2 (90 %) permitted to compare supported and unsupported silver activity.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.JRC.G.I.5-Advanced Nuclear Knowledg

U2O5 film preparation via UO2 deposition by direct current sputtering and successive oxidation and reduction with atomic oxygen and atomic hydrogen

We describe a method to produce U2O5 film in-situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, essential part of the PAMEC (Properties of Actinides under Extreme Condition) laboratory allows preparing films and study samples surfaces using surface analytical techniques such as X-ray and Ultra-violet photoemission spectroscopy (XPS, UPS). All studies are made in-situ; the films, transferred under ultra-high vacuum from preparation to analyses chamber, are never in contact with the atmosphere. Initially a film of UO2 is prepared by direct current (DC) sputter deposition on an Au foil and then oxidised by atomic oxygen to produce UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are done after each step of oxidation and reduction, using high resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction time and the corresponding temperature of the substrate during the process have a severe effect on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging. First uranium-oxygen systems exist in numerous intermediate phases. Second the differentiation of the oxidation states of uranium is mainly based on satellites peaks whose intensity peak is weak. Also, one should be aware that the technique of x-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1 to 5%. Thus, it is important to obtain the complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s and the valence band (VB). The programmes used on the Labstation include the linear transfer programme developed by Specs Company, and the data acquisition programme and the sputter source programme, both developed in-house.JRC.G.I.5-Advanced Nuclear Knowledg