Self-propagating synthesis and aqueous durability of Nd-bearing zirconolite-rich composites using Ca(NO3)2 as the oxidant

Abstract

Synroc is recognized as the second-generation waste form for safety disposal of high-level radioactive waste (HLW). In this study, zirconolite-rich Synroc waste form was readily synthesized by self-propagating high-temperature plus quick pressing (SHS/QP) using Ca(NO3)2 as the oxidant and Ti as the reductant. As the surrogate of trivalent actinides, Nd2O3 was introduced to equally substitute the Ca and Zr sites of zirconolite with nominal stoichiometry of Ca1−xZr1−xNd2xTi2O7. The results demonstrate that zirconolite, perovskite and pyrochlore (Ca2Ti2O6) coexist as the ceramic components after SHS reaction. The introduction of Nd2O3 promotes the formation of perovskite. Nd is mostly incorporated into the Ca sites of these phases. The normalized elemental leaching rates of Ca and Nd are fairly constant in low values of 1.80 × 10−2 g m−2 d−1 and 6.12 × 10−4 g m−2 d−1 after 42 days.