Actinide incorporation in a zirconia based pyrochlore (Nd1.8An0.2)Zr2O7+x (An=Th, U, Np, Pu, Am)

Actinides (thorium, uranium, neptunium, plutonium, and americium) were infiltrated into a porous Nd1.8Zr2O6.7 matrix, prepared by gel-supported precipitation. (Nd1.8An0.2)Zr2O7+x pyrochlores were formed after sintering in Ar/H2 and the pyrochlore structure remains during oxidation at 800 °C in air. X-ray diffraction reveals a linear relationship between the pyrochlore lattice parameter and the ionic radii of the actinides. EXAFS measurements on actinide L3-edge show a split shell of nearest neighbour oxygen atoms similar to that surrounding of Nd. The actinide–oxygen bond distances decrease with the actinide ionic radii, which verifies that these actinides adopt the Nd site in the (Nd1.8An0.2)Zr2O7+x pyrochlore. The oxidation susceptibility of Np is related to the availability of oxygen vacancies and in contrast to stabilised zirconia Np(V) can be obtained in zirconia based pyrochlore.