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 3.6: Valence states and local structure of Gd doped actinide oxides (U1-xGdx)O2 and (Th1-xGdx)O2 measured by X-ray Absorption Spectrometry on the MARS beamline.

René Bès1, J. Pakarinen2, S. Conradson3, I. Makkonen1, A. Baena2, M. Verwerft2, F. Tuomisto1
  • 1: Aalto University, Finland
  • 2: SCK-CEN, Belgium
  • 3: MARS beamline, SOLEIL synchrotron, FRANCE

Abstract

To improve the performance of nuclear fuels, one has to extend the capabilities of UO2 fuels and develop alternatives, such as thoria based fuels [1,2]. To this aim, doped fuels such as (U1-xGdx)O2 and (Th1-xGdx)O2 allow controlled properties along fuel life-time, are of interest.

Gadolinia doped UO2 is widely used to reduce the power peaking and excess reactivity during the first reactor cycle of fresh fuel assemblies as 155Gd and 157Gd isotope act as burnable neutron absorber. According to the fuel assembly design and the core management constraints, the concentration of Gd varies between 2 wt.% and 10 wt.% (weight fraction of Gd2O3). In such concentrations, Gd and UO2 form a solid solution (U1-xGdx)O2 which keeps the fluorite structure of UO2 by substituting U4+ ions by Gd3+. The charge balance is usually considered as only modification of the valence state of U4+ to U5+ in good agreement with the fact that several phases are only observed for x>0.5 [3]. Nevertheless, theoretical studies have recently revealed that it involves not only U4+/U5+ valence state mixing but also oxygen vacancies.

Thorium based nuclear fuels represent potential alternatives to uranium based fuels as the fission of thorium isotopes can reduce the long-lived radioactive nuclides. The influence of gadolinium content on doped thoria is of great interest to evaluate performance of this alternative fuel. The charge compensation is here awaited to be due to the presence of oxygen vacancies only, as recently demonstrated by molecular dynamics calculations [4]. Indeed, for thorium atoms, valence state modification is not allowed and Th4+ remains the only possible valence state in the fluorite structure.

The challenging question of the complex interplay between atomic defects and valence state mixing in (U1-xGdx)O2 and (Th1-xGdx)O2 will be discussed on the basis of X-ray Absorption Spectroscopy and theoretical calculations.

References:
  1. O. Lung et al., Nucl. Eng. Design, 180 (1998) 133.
  2. M. Osaka, et al., J. Nucl. Sci. Technol., 43 (2006) 367.
  3. M. Durazzo, et al., J. Nucl. Mater., 400 (2010) 183.
  4. M. Osaka, et al., J. Nucl. Sci. Technol., 44 (2007) 1543.