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 4.4: Thermodynamic assessment of the neptunium-oxygen system: mass spectrometric studies and thermodynamic modelling

Anna L. Smith1, C. Guéneau2, J.-Y. Colle3, O. Benes3, B. Sundman4, R.J.M. Konings3
  • 1: Delft University of Technology, Faculty of Applied Sciences, Department of Radiation Science and Technology, Mekelweg 15, 2629 JB Delft, Netherlands
  • 2: DEN/DANS/DPC/SCCME/LM2T, CEA Saclay, 91191Gif-sur-Yvette cedex, France
  • 3: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany
  • 4: INSTN, CEA Saclay, 91191 Gif sur Yvette, France


A thorough knowledge of the inherent characteristics and behavior under normal and accidental conditions of advanced nuclear fuels to which minor actinides have been incorporated, i.e. (U,Pu,Np,Am,Cm)O2 fuel, is essential for the safe use of future Generation IV nuclear reactors. Temperatures can reach up to 2273 K in normal operating conditions at the centre of the fuel pin of Sodium-cooled Fast Reactors, and about 893–923 K on the pellet edge [1]. The prediction of the nature of the oxide phases formed and their compositions under specific temperature and oxygen potential conditions is crucial. Moreover, the determination of their liquidus temperatures and evaporation processes is also needed in the scenario of an accident, with uncontrolled temperature increase.

The binary U-O, Pu-O and ternary U-Pu-O systems have been investigated extensively already, and thermodynamic models have been developed for these systems using the CALPHAD method [2,3]. The data available on the Np-O system are much more limited [4], however, and there is no satisfactory overall description using CALPHAD. In the context of heterogeneous in-pile recycling where a high concentration of minor actinides is added to UO2 fuel assemblies, the knowledge of this system is essential, and a sound description via models is needed.

The experimental data available on the neptunium-oxygen system, encompassing phase equilibria, thermodynamic data, oxygen chemical potential, and vaporization studies of NpO2, have been critically reviewed in the present work. Knudsen effusion cell mass spectrometry measurements have also been performed on neptunium dioxide to improve the understanding of its vaporization behavior. A thermodynamic model for the neptunium-oxygen system has furthermore been developed using the CALPHAD method. The non stoichiometric NpO2-x phase is described herein using the compound energy formalism with ionic constituents, while the liquid phase is represented with the ionic two-sublattice model. The reliability and consistency of all optimized parameters has been verified by comparison of the calculated and experimental data.

  1. Y. Guerin, Chapter 2.21 in Comprehensive Nuclear Materials (2012) 547–578
  2. C. Guéneau et al., J. Nucl. Mater. 419, 145 (2011)
  3. C. Guéneau et al., J. Nucl. Mater. 304, 161 (2002)
  4. C. Guéneau et al., Chapter 2.02 in Comprehensive Nuclear Materials (2012) 22–59