Dan Yin

Self-propagating high-temperature synthesis of ZrO 2 incorporated Gd 2 Ti 2 O 7 pyrochlore

In this research, Zr-doped Gd2Ti2O7 pyrochlores, with the composition of Gd2(Ti1-xZrx)2O7, were firstly synthesized by self-propagating high-temperature synthesis plus quick pressing (SHS/QP) using CuO as the oxidant and Ti as the reductant. To improve the radiation resistance of titanate–pyrochlore, up to 35 at% Zr was incorporated to substitute the Ti site of Gd2Ti2O7 pyrochlore (Gd2(Ti0.75Zr0.35)2O7). XRD and SEM microstructural characterizations showed the formation of a composite ceramic with the major pyrochlore phase and the minor Cu phase. The generated temperature of samples decreased from 1702 to 1011°C with increasing Zr content. The effects of sintering temperature and pressure time on phase composition and microstructure were systematically studied. Besides, the influence of thermal transmission on the whole combustion process was also explored. The pyrochlore-based waste form possessed high bulk density of 6.25 g/cm3 and Vickers hardness of 10.81 GPa. The MCC-1 leaching test showed the normalized elemental leaching rates (42 d) of Cu, Gd, and Zr are 1.27×10-2, 1.33×10-3, and 8.44×10-7 g·m-2·d-1, respectively.

Self-propagating high-temperature synthesis, phase composition and aqueous durability of Nd–Al bearing zirconolite-rich composites as nuclear waste form

ABSTRACT In this study, the zirconolite-rich composite was rapidly synthesised from self-propagating high-temperature synthesis plus quick pressing using MoO3 as the oxidant and Ti as the reductant. As a surrogate of trivalent actinides, Nd was introduced to substitute the Ca site of zirconolite and Al was employed as the charge compensator to replace the Ti site (nominally Ca1−xNdxZrTi2−xAlxO7). The phase composition and Nd occupancy were analysed after Nd2O3 addition. Nd-bearing zirconolite was produced as the major ceramic phase. Nd mostly substitutes the Ca sites of zirconolite, which results in waste loading higher than 8.6 wt-% Nd. The aqueous durability of Nd–Al codoped sample (with 15 at.-% Nd substitutes the Ca site of zirconolite) was evaluated at 90°C as well. The 42 days normalised leaching rates of Mo, Ca and Nd were measured to be 3.70, 1.90 × 10−2 and 3.46 × 10−4 g m−2 d−1.

Preparation and aqueous durability of Sr incorporation into rutile TiO2

TiO2 was employed as the waste form for disposal of simulated nuclide Sr. Preparation of Sr bearing rutile was explored under different sintering temperatures and Sr contents. The optimal treatment temperature was confirmed as 1 300 °C for the incorporation of SrO in rutile TiO2. Perovskite type SrTiO3 was prepared as the resultant product. The limited containment capacity of SrO in rutile was speculated to be 56.5wt%. As the SrO content increases, the as-synthesized sample exhibits more porosity because SrTiO3 phase demonstrates higher density than rutile and SrO. The 28 day normalized leaching rate (LRi) of Sr and Ti will decrease congruously as the SrO incorporation increases. The LRSr value is lower than 0.1 g·m-2·d-1, which is about 3 orders of magnitude higher than the LRTi value.