Grain growth of NpO2 and UO2 nanocrystals.

We report on the crystallite growth of nanometric NpO2 and UO2 powders. The AnO2 nanoparticles (An = U and Np) were synthesized by hydrothermal decomposition of the corresponding actinide(iv) oxalates. NpO2 powder was isothermally annealed between 950 °C and 1150 °C and UO2 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 UO2 and NpO2 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 UO2, NpO2 and PuO2. While data for surface diffusion coefficients for NpO2 and PuO2 are lacking in the literature, the comparison with literature data for UO2 supports further the hypothesis of a surface diffusion controlled growth mechanism.