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 3.8: Assessment of the Phase Equilibrium in the ThO2-ZrO2 System

Emtethal Kassim, Jean-Fran├žois Vigier, Mohamed Naji, Dario Manara and Joseph Somers
  • European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany


Nuclear Fuel oxides such as UO2, PuO2 and ThO2 are known to form solid solutions with ZrO2 with monoclinic, tetragonal and cubic (fluorite) structures [1–3]. Studies of the phase equilibrium of these oxides with the ZrO2 witnessed a vast amount of research due to its importance for nuclear fuel disposition. Thermodynamic modelling has been demonstrated for the majority of the oxide systems with experimental data widely existing for the UO2 system [4–7]. However, very limited experimental data is present for the ThO2 oxide. This may be related to the lack of interest in thorium oxide as a nuclear fuel due to it being not fissile. However, Thorium can still be used as a fuel in a nuclear reactor as a fertile material for the production of U–233, or as an inert matrix in the presence of a fissile material (e.g. U–233, U–235 or Pu–239) which acts as a driver to initiate a chain reaction. The development of such thorium based fuels is currently under investigation e.g. in Norway and India.

In this work, the phase equilibrium of ThO2-ZrO2 is investigated. A series of different molar ratio (80:20, 60:40, 40:60, and 20:80 ThO2:ZrO2) compositions of ThO2 and ZrO2 were synthesised using external sol-gel method utilised at JRC-ITU. The XRD data of samples sintered at 1650ºC shows bi-phasic behaviour throughout the range compositions with small solubility (< 5%) of ZrO2 into ThO2 and limited solubility of ThO2 into ZrO2. These results were supported with RAMAN spectroscopy and SEM microscopy to have a better understanding of the distribution of the components within each other. Furthermore, solid/liquid equilibria in the ThO2-ZrO2 system are investigated here for the first time by laser heating coupled with fast optical thermometry. These results contribute to assessing the thermal stability of the system in a ThO2 fuel based reactor.

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