Grain growth of NpO 2 and UO 2 nanocrystals.

We report on the crystallite growth of nanometric NpO 2 and UO 2 powders. The AnO 2 nanoparticles (An = U and Np) were synthesized by hydrothermal decomposition of the corresponding actinide(iv) oxalates. NpO 2 powder was isothermally annealed between 950 °C and 1150 °C and UO 2 between 650 °C and 1000 °C. The crystallite growth was then followed by high-temperature X-ray diffraction (HT-XRD). The activation energies for the growth of crystallites of UO 2 and NpO 2 were determined to be 264(26) kJ mol -1 and 442(32) kJ mol -1 , respectively, with a growth exponent n = 4. The value of the exponent n and the low activation energy suggest that the crystalline growth is rate-controlled by the mobility of the pores, which migrate by atomic diffusion along the pore surfaces. We could thus estimate the cation self-diffusion coefficient along the surface in UO 2 , NpO 2 and PuO 2 . While data for surface diffusion coefficients for NpO 2 and PuO 2 are lacking in the literature, the comparison with literature data for UO 2 supports further the hypothesis of a surface diffusion controlled growth mechanism.