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(Uranium + americium) mixed oxides are considered as potential targets for americium transmutation in fast neutron reactors. Their thermophysical properties and notably their melting behaviour have not been assessed properly although required in order to evaluate the safety of these compounds under irradiation. In this study, we measured via laser heating, the melting points under inert atmosphere (Ar) of U 1− x Am x O 2± δ samples with x = 0.10, 0.15, 0.20. The obtained melting/solidification temperatures, measured here, indicate that under the current experimental conditions in the investigated AmO 2 content range, the solidus line of the (UO 2 + AmO 2 ) system follows with very good agreement the ideal solution behaviour. Accordingly, the observed liquidus formation temperature decreases from (3130 ± 20) K for pure UO 2 to (3051 ± 28) K for U 0.8 Am 0.2 O 2± δ . The melted and quenched materials have been characterised by combining X-ray diffraction and X-ray absorption spectroscopy.

SSCI-VIDE+CARE+ACV:PVE; International audience; None

Uranium-americium mixed oxides are considered as potential blankets for americium transmutation in fast neutron reactors. Their thermophysical properties and notably their melting behavior have not been properly assessed although required in order to evaluate the safety of these compounds under irradiation. In this study, we measured via laser heating the melting points under inert atmosphere (Ar) of U1 xAmxO2±δ samples with Am/(U+Am) contents equal to 10, 15 and 20 mol.%. The experimental atmosphere was chosen in order to maintain, as much as possible throughout the heating/cooling cycles, the oxygen-to-metal ratio, initially set very close to 2.00. Structural characterizations of the samples carried out before and after melting confirm that this goal was satisfactorily attained. The obtained melting/solidification temperatures, measured here for the first time, indicate that under the current experimental conditions and as far as the investigated AmO2 contents, the solidus line of the UO2-AmO2 system follows with very good approximation the ideal solution behavior. Accordingly, the observed liquidus formation temperature decreases from 3130 ± 20 K for pure UO2 to 3051 ± 30 K for U0.8Am0.2O2±δ.

International audience; This paper is dedicated to the adaptation of the Weak Acid Resin process (WAR) to the elaboration of uranium-americium mixed oxide microspheres as precursors for the fabrication of Americium Bearing Blankets (AmBB). It describes successive developments of WAR process from lanthanide simple oxide synthesis to actinide(s) bearing mixed oxide preparation. Recent studies demonstrated the scientific feasibility of the preparation of a dense uranium-americium mixed oxide pellets with 10 $_{at}$% of Am vs heavy metal and with 95 % of theoretical density (TD) from those innovative microsphere precursors

Over the last decades, XAS has evolved as a very useful tool to investigate nuclear materials with applications for fuel manufacturing of for the behavior for fission products. Nowadays, the possibility of applying HERFD measurements to radioactive compounds offers even more possibilities. Two recent studies dealing with the behavior of fission gases in UO2 nuclear fuel and investigating both the electronic and structural properties of uranium based mixed oxide fuels will be presented to illustrate the use of HERFD. Furthermore, recent experimental developments allowing to perform in-situ XAS measurements on actinide-based materials at high temperature and under controlled atmosphere, will be illustrated by the first results obtained on UO2±x and (U,La)O2±x samples.

Responsable CEA : Delahaye, ThibaudDirecteur universitaire : Ayral, André; National audience; La fermeture du cycle électronucléaire est aujourd’hui une des volontés françaises dans laquelles’inscrit le recyclage des déchets nucléaires en sortie de réacteur. Contributeur majeur à leurradiotoxicité, le plutonium (Pu) est valorisé par la production de combustible MOx où il est mélangéà de l’uranium (U) appauvri, pour être à nouveau irradié. Aujourd’hui coulés dans des verres etstockés dans des sites dédiés, deux autres types de déchets restent à gérer : les produits defission (PF), à demi-vie courte, et les actinides mineurs (AM) qui sont, malgré leur faible teneur(0,7% at. du combustible usé), les principaux contributeurs à la radiotoxicité et à la thermicité desdéchets sur le long terme, après recyclage du Pu. Les AM sont représentés par les troisradioéléments suivants : l’américium (Am), le neptunium (Np) et le curium (Cm).La nécessité d’une gestion à long terme des AM a été définie par la loi Bataille de 1991, complétéeen 2006, qui définit les grands axes de recherche dans ce domaine avec, d’une part l’étude dustockage en grande profondeur, et d’autre part la démonstration de la faisabilité technique de laséparation et de la transmutation des AM en réacteurs à neutrons rapides (RNR). Dans ce derniercas, deux modes de transmutation sont couramment considérés : la transmutation homogène pourlaquelle les AM sont directement associés au combustible en quantités limités (

The incorporation of aliovalent cations into the UO2 structure leads to a change of the U oxidation state and can ultimately affects the physical propertiesy of these U1-yMyO2±x compounds. U1-yLayO2±x, U1-yPuyO2±x and U1-yAmyO2±xhave been investigated in the present work, as these materials are particularly relevant for the nuclear fuel cycle. LIII XANES, LIII HERFD-XANES and MIV HERFD-XANES measurements have been performed to study both electronic and structural environments of these fluorite-type solid solutions.

241Am is one of the most hazardous actinide isotopes present in the spent fuel Transmutation in fast neutrons reactors : conversion of highly radioactive elements into short-lived isotopes Dramatic reduction of the nuclear waste inventory (< 300 years) Two strategies : Homogeneous transmutation : Incorporation of MA in low concentration (

International audience; Mixed uranium-americium oxides are one of the materials envisaged for Americium Bearing Blankets dedicated to americium transmutation in fast neutron reactor. Recently, several processes have been developed in order to improve and validate fabrication in terms of material specifications such as density and homogeneity. The challenge is also to propose simplifications for lowering industrial costs and hazards linked to dust generation of highly contaminating and irradiating compounds such as Am based oxides which presence complicates grinding and milling operations commonly encountered in conventional powder metallurgy processes. Our goal was then to develop an innovative and dust-free process for synthetizing uranium-americium mixed oxide in the form of spherules of the size of hundreds microns in order to facilitate remote handling operations and reduce fine dust dispersion. In this way, the Weak Acid Resin route (WAR), originally dedicated to uranium carbide kernel synthesis, was selected and adapted to mixed oxide fabrication incorporating uranium and americium. This process is based on the fixation of actinide cations in acrylic resin beads and the subsequent mineralization in air of the metal loaded resin into oxide microspheres. A final reducing thermal treatment is then necessary to adjust oxygen stoichiometry and form the mixed oxide phase. This process has been tested and validated for many oxide compounds such as single lanthanide or uranium oxide but also for mixed uranium and lanthanides or americium compounds. Therefore, this contribution aims at describing the development of WAR process for oxide synthesis during recent years at CEA Marcoule and will detail particularly the synthesis and characterization of U0.9Am0.1O2±x mixed oxide precursor. The current study of pelletization and sintering of such precursors, known as Calcined Resin Microsphere Pelletization process (CRMP), will be also presented and will illustrate the application of WAR spherule metallurgy to the fabrication of uranium and americium containing materials.

International audience