In situ characterization of uranium and americium oxide solid solution formation for CRMP process: first combination of in situ XRD and XANES measurements

Abstract

Transmutation of americium in heterogeneous mode through the use of U1-xAmxO2±δ ceramic pellets, also known as Americium Bearing Blankets (AmBB), has become a major research axis. Nevertheless, in order to consider future large-scale deployment, the processes involved in AmBB fabrication have to minimize fine particle dissemination, due to the presence of americium, which considerably increases the risk of contamination. New synthesis routes avoiding the use of pulverulent precursors are thus currently under development, such as the Calcined Resin Microsphere Pelletization (CRMP) process. It is based on the use of weak-acid resin (WAR) microspheres as precursors, loaded with actinide cations. After two specific calcinations under controlled atmospheres, resin microspheres are converted into oxide microspheres composed of a monophasic U1-xAmxO2±δ phase. Understanding the different mechanisms during thermal conversion, that lead to the release of organic matter and the formation of a solid solution, appear essential. By combining in situ techniques such as XRD and XAS, it has become possible to identify the key temperatures for oxide formation, and the corresponding oxidation states taken by uranium and americium during mineralization. This paper thus presents the first results on the mineralization of (U,Am) loaded resin microspheres into a solid solution, through in situ XAS analysis correlated with HT-XRD.

Melting behaviour of americium-doped uranium dioxide

“…The radiotoxicity and heat load of ultimate nuclear waste would thus be decreased as well as the ecological footprint of deep geological repositories [2,3]. In this context, several studies have been dedicated to U 1Àx Am x O 2±d compounds, not only to investigate synthesis methods [4][5][6][7][8][9][10][11] and assess their behaviour under irradiation in reactors [4,7,12,13], but also to determine their structural and thermophysical properties, the latter remaining scarcely known [14][15][16][17][18][19][20]. Among them, no data regarding the U 1Àx Am x O 2±d melting behaviour has been reported, despite the importance of such information with respect to safety margins during irradiation, notably in accidental conditions.…”

How to Measure the Oxidation State of Multivalent Elements in Minerals, Glasses, and Melts?

“…Hence, its use for either standard experiments, sample mapping, or for in-situ experiments at extreme conditions of temperature, pressure, or oxygen fugacity has increased in the last decade. XANES spectroscopy works for all transition elements (like Cu, Cr, Ti, or Eu; for example, see Berry et al, 2006a,b;Doyle et al, 2016;Burham et al, 2015) up to U for stable element (Caisso et al, 2014) and also for transuranian elements like Americium (Caisso et al, 2015;Prieur et al, 2018). Moreover, for some elements, it is also possible to acquire data at different edges.…”

Fifteen Years of Radionuclide Research at the KIT Synchrotron Source in the Context of the Nuclear Waste Disposal Safety Case

“…A group from CEA Marcoule and CEA Cadarache, France, has investigated the formation of (U,Am) mixed oxide microspheres (as precursors for a later pelletizing step) obtained by wet impregnation of porous exchange resin microspheres. For the first time, the two-step high-T calcination process (i.e., (i) removal of the organic skeleton in synthetic air and (ii) formation of the final solid solution at a reduced atmosphere) has been investigated at INE-Beamline using-situ XAS measurements at the U L3-and Am L3-absorption edges at temperatures up to 1400 K [14]. The temperature of the sample was controlled using a specially designed micro-furnace.…”

“…High-T in situ experiment at INE-Beamline with micro-furnace cell inside a plexiglass box acting as second containment during Am L3 fluorescence XAFS detection in March 2014 (cooperation with CEA Cadarache, France[14]). …”