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

Poster 3.2: Thermal Diffusivity and Conductivity of Thorium- Uranium Mixed Oxides

Mouna Saoudi1, J. Mouris1, A. Bergeron1, H. Hamilton1, D. Freiss2, M. Cologna2, D. Staicu2
  • 1: Canadian Nuclear Laboratories Limited, Canadian Nuclear Laboratory, Chalk River, Ontario, Canada
  • 2: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Karlsruhe (Germany)

Abstract

Thorium-Uranium oxide pellets with high densities were prepared at CNL by co-milling, pressing, and sintering at 1700°C, with UO2 mass contents of 0, 1.5, 3, 8, 13, 30, 60 and 100%. At ITU, (Th,U)O2 solid solution pellets were fabricated using the spark plasma sintering (SPS) technique with 79 and 93 % wt.% UO2. The thermal diffusivity was measured at CNL and at ITU using the laser flash technique and the thermal conductivity was deduced using the measured density and literature data for the specific heat.

The thermal conductivity for ThO2 is significantly higher than for UO2. The thermal conductivity of (Th,U)O2 decreases rapidly with increasing U content, and for UO2 contents of 60 % and higher, the conductivity of (Th,U)O2 is close to UO2. The new results are compared to the data and models available in the literature.

As the mass difference between the Th and U atoms is small, the thermal conductivity decrease is attributed to the phonon scattering enhanced by lattice strain due to the introduction of uranium in ThO2 lattice. However, this interpretation does not take into account the semi-transparent nature of ThO2, where heat transfer takes place by a coupled conductive and radiative mechanism. The effect of the semitransparency is clearly visible when the thermal diffusivity is measured by the laser pulse technique, with an almost instantaneous temperature rise of the sample surface opposite to the surface receiving the laser pulse. This effect is tempered by coating the front and rear surfaces of the samples with carbon, which ensures the laser energy deposits at the sample surface only, and not in the volume. However, a contribution of the radiative heat transfer remains present on the thermograms, and therefore also on the determined apparent thermal diffusivity. This semitransparency decreases progressively with the addition of UO2 and is almost lost at about 30 % of UO2 in ThO2.