Influence of Am on thermodynamic properties of MOX fuels: melting temperature, oxygen potential and heat capacity

Pauline Fouquet-Métivier1

Romain Vauchy1, Philippe M. Martin1, Luka Vlahovic2, Davide Robba2, Dragos Staicu2, Ondrej Benes2, Christine Guéneau3.

1CEA, DEN, DMRC, Université Montpellier, Marcoule, France; 2European Commission, Joint Research Centre, Karlsruhe, Germany; 3CEA, DEN, DPC, Université Paris-Saclay, Gif-sur-Yvette, France. (CEA).


Innovative mixed oxide (MOX) (U,Pu)O2±x fuels for sodium fast neutron reactor (SFRs) systems are currently studied within the framework of the development of advanced nuclear reactors (GEN-IV). With a plutonium content ranging between 20 and 40 mol.%, the fuel will contain few mol.% of 241Am, daughter element generated by 241Pu decay. Furthermore, the oxygen stoichiometry (Oxygen/Metal ratio) of the SFRs fuels needs to meet precise specifications (i.e. to be between 1.95 and 1.99) to minimize cladding corrosion and avoid fuel melting. Then, in order to foresee the fuel behaviour during irradiation, an accurate knowledge of its thermodynamic and structural properties is mandatory. In this work, the thermodynamic properties of the quaternary U-Pu-Am-O system are investigated.

In order to overcome the lack of experimental data, campaigns on five SFR fuels are currently ongoing in the hot lab facilities of the JRC-Karlsruhe (Germany) and the CEA Marcoule (France). U1-y-zPuyAmzO2±x samples with 0.005 < z < 0.03 and 0.23 < y < 0.45 were manufactured by powder metallurgy and sintered in reducing atmosphere to obtain hypo-stoichiometric (O/M<1.98) compositions.

An exhaustive campaign of melting temperature measurements was performed using an updated version of the laser heating set-up implemented at JRC-Karlsruhe. Thanks to the recent addition of oxygen gauges monitoring the atmosphere inside the chamber, the variation of the oxygen partial pressure could be assessed during the experiments, which allowed determining the change in the oxygen stoichiometry of the samples. In addition, the oxygen potential of these fuels was also measured by thermogravimetric analyses (TGA) for temperatures ranging between 1073 and 1673 K. Finally, the enthalpy increments of the materials were measured from 585 to 1790 K by drop calorimetry. The heat capacity functions were then deduced by derivation of the enthalpy data.

In the present work, the measured melting temperatures will be presented and the influence of the Pu, Am and O contents will be discussed. For the first time, the variation of the oxygen stoichiometry of the sample during the shots was estimated. The impact of Am on the oxygen potentials and heat capacity of MOX fuels will also be discussed. Finally, thermodynamic calculations using the CALPHAD method will be compared to the experimental results. Thanks to the inter-comparison between various characterizations performed at CEA Marcoule such as XRD, Raman Spectroscopy, Scanning Electron Microscopy, Electron Probe Micro-Analysis…, new input data for the modelling of the U-Pu-Am-O system, will be selected.

Event Timeslots (1)

Wednesday – 15th September 2021
Pauline Fouquet-Métivier