Free occupying mechanism of simulated tetravalent waste ions in Gd2Zr2O7 and performance evaluation of the waste forms

Abstract

To extend the practicability of ceramics in immobilizing nuclear waste with fluctuant composition, structural design should be abandoned. The simulated tetravalent actinide waste An4+ (Ce4+) was directly doped into prepared Gd2Zr2O7, and the waste forms were synthesized by high-temperature solid-state reaction. It has been shown that the maximum loading of CeO2 in Gd2Zr2O7 lies between 20 and 30 wt.%, and Ce elements are uniformly distributed in the matrix. Existing in Ce3+ and Ce4+, cerium ions automatically occupied both the Gd and Zr sites in Gd2Zr2O7 according to valence equilibrium. This occupation causes the change of r(A3+)/r(B4+) and eventually leads to the structure transition from pyrochlore to fluorite. In addition, the normalized leaching rate of the sample with 60 wt.% of dopant was about 2.5 × 10−7 g m−2 d−1 on the 35th day. In this study, a free occupation of simulated waste ions in ceramics was proposed.