Abstract:

Polycrystalline samples of (Gd_1-xCe_x)₂Zr₂O_7+x (0 ≤ x ≤ 0.7) were synthesized by solid-state reaction using NaF as a flux at 1000 ℃ to simulate Pu-immobilization in the Gd₂Zr₂O₇ matrix. Phase transformation and variation of the thermal expansion coefficients (TECs) and thermal conductivities (TCs) of the samples with temperature and composition were investigated. Powder X-ray diffraction (XRD) patterns show that pure Gd₂Zr₂O₇ has a weakly ordered pyrochlore structure, whereas Ce-containing samples (i.e., the Pu-simulated solidified samples) exhibit a defect fluorite structure even if x is as low as 0.1. When x reaches 0.7, the XRD peaks of these samples widen. In the Ce 3d X-ray photoelectron spectrum (XPS) of (Gd_1-xCe_x)₂Zr₂O_7+x there are six peaks located at binding energies of 881.7, 888.1, 897.8, 900.4, 907.1, and 916.1 eV, which are almost the same as the peaks of CeO2. The Ce 3d XPS reveals that the Ce species in (Gd_1-xCe_x)₂Zr₂O_7+x are tetravalent. The TECs of (Gd_1-xCe_x)₂Zr₂O_7+x (0 ≤ x ≤ 0.7) generally increase with increasing temperature. At the same temperature, the TECs and TCs exhibit the same variation trend with the composition of the simulated solidified forms: they decrease from x = 0 to 0.1 and then linearly increase from x = 0.1 to 0.7.

Key words: Immobilization of nuclear waste, Pyrochlore structure, Defect fluorite structure, Thermal expansion coefficient, Thermal conductivity