Local Atomic Structure in (Zr,U)N Inert Matrix Fuel (Contribution to ANKA Annual Report)

The transmutation of minor actinides (MA: Np, Am, Cm) in fast flux reactors or accelerator driven systems is a recent approach to reduce their long¿term radiotoxicity. To obtain a high MA burn up, inert fuels are necessary as host matrix for the minor actinides. Due to its advantageous physical properties (high thermal conductivity, high melting point), ZrN has been proposed as such an inert matrix1. ZrN and the actinide mononitrides such as UN crystallize in a cubic face centered structure (Fm3m ). Consequently, ZrN and UN form solid solutions, in which the lattice parameter nearly follows the Vegard¿s rule2. However, the lattice parameter of solid solutions as obtained by X-ray diffraction is averaged over several unit cells and does not reflect the local structure and bonding behaviour of the different metals. This information (bond length, local disorder, cooperative effects) can be provided by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. EXAFS was used to determinate the local structure of several dopant cations in stabilized zirconia3 and in (La1-xUx)S 4. It had been shown that the local structure and disorder of the dopant cation can be quite different from that of the host cation. In the case of (Zr,U)N ¿ which represents the experimentally most simple (Zr,MA)N system ¿ not such information are available yet. These investigations form part of an extensive project to determine structural and thermodynamic properties (Cp, ¿G, ¿S) of these materials, and will eventually culminate in irradiation testing of selected candidate materials.

WALTER Marcus;  SOMERS Joseph; 

2008-01-16

ANKA Angstroemquelle Karlsruhe

JRC42528