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 3.7: Thermal Conductivity of the Uranium-Zirconium Oxide

Alice Seibert, M. Cologna, E. Kassim, J. Somers, V. Tyrpekl, D. Staicu, R.J.M. Konings, T. Wiss
  • European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany

Abstract

A good knowledge of the thermophysical properties of a molten core is required in order to determine or analyse the scenario of a severe accident. For a Light Water Reactor (LWR), in a severe accident the Zircaloy cladding is oxidised by steam and corium forms by melting of the core material, with a composition depending on the reactor type. The typical core inventories range from U/Zr molar ratios of about 40/60 for a PWR to 60/40 for a BWR. A first series of simulated LWR corium was prepared taking into account the requirement of having samples representative of a molten material. Discs of Uranium-Zirconium oxide from a sol-gel assisted preparation method combined with Spark plasma sintering (SPS) were synthesised with U/Zr molar ratios of 0/100, 20/80, 40/60, 50/50, 60/40, 80/20 and 100/0.

The samples were characterised with X-ray Diffraction and SEM. The thermal diffusivity has been measured between 500 and 1600 K by the laser-flash technique. The thermal conductivity was calculated from the measurements of the thermal diffusivity, using literature data for the specific heat and thermal dilatation coefficients. The thermal conductivity of SPS UO2 is in good agreement with the literature values available for sintered samples. The results show a fast degradation of the thermal diffusivity when the fraction of Zr increases. The extrapolation of the observed trend to pure ZrO2 is not straightforward, as ZrO2 is not stable over the whole temperature range, but shows a phase transition in the experimental temperature range and as it is a semi-transparent material requiring coating for the determination of its thermal diffusivity by the laser flash method. The results are compared to literature data and a recommendation is made.