NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Talk 1.5: (U,M)O2±x solid solution: a RT XANES and HT XANES study

Damien Prieur1, Philippe Martin2, Enrica Epihano2, René Bès2,3, Laura Martel1, Jean-francois Vigier1, Daniel Neuville4, Tim Prussmann5, Tonya Vitova5, Kristina Kvahsnina6, Andreas C. Scheinost7, Joseph Somers1
  • 1: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany
  • 2: CEA, DEN, DEC, SPUA, LMPC, Centre de Cadarache, F–13108 Saint Paul Lez Durance Cedex, France
  • 3: Synchrotron SOLEIL, 91190 Saint-Aubin, France,
  • 4: CNRS-IPGP, 75005 Paris, France
  • 5: Karlsruhe Institute of Technology, Institute for Nuclear Waste Management, D–76125 Karlsruhe, Germany
  • 6: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble, France
  • 7: Helmholtz-Zentrum Dresden-Rossendorf , Institute of Resource Ecology, P.O. Box 10119, 01314 Dresden, Germany

Abstract

The introduction of new fast-neutron reactors like ASTRID, MYRRHA, ALLEGRO and AFFRED, is an important step towards a closed nuclear fuel cycle. The first cores will be fueled with (U,Pu)O2 enabling Pu recycling. At a later stage, dedicated (U,MA)O2±x targets for the transmutation of minor actinides (MA = Np, Am, Cm) will be deployed in dedicated assemblies at the reactor core periphery. In these cases, the substitution of tetravalent uranium by aliovalent atoms can lead, under specific conditions, to a modification of the uranium oxidation state leading ultimately to a change of the overall oxygen to metal ratio (O/M). Fresh fuels (e.g. U1-yPuyO2±x and U1-yAmyO2±x) must have an O/M less than 2.00, to ensure that no reaction occurs with the cladding or with the coolant. Nevertheless, an oxidation of the fuel can occur in incidental conditions leading to an increase of the O/M ratio and ultimately affecting phase stability, defect chemistry and oxygen chemical potential. Therefore, an in depth knowledge of the impact of stoichiometry variation is essential for the safety assessment of these materials.

In the present work, the influence of three cations (i.e. La, Pu and Am) has been investigated, their selection being motivated by fundamental interests. The electronic structure and local environments of these materials have been studied combining several short- and long-range order techniques, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. These room temperature methods have shown that ideal fluorite solid solutions are obtained for similar synthesis conditions. However, significant discrepancies have been observed regarding the charge distribution. Indeed, U(V) co exists with La(III) and Am(III), but not with Pu(III). In that latter case, U(V) is observed only when Pu is tetravalent. In addition, in situ XANES measurements have been performed on (U,La)O2±x to study the charge distribution as a function of the temperature and the atmosphere. Thus, the formation mechanism of the solid solution and the accommodation of a aliovalent cation by the UO2 structure can be understood.