Effect of phase evolution and acidity on the chemical stability of Zr1-xNdxSiO4-x/2 ceramics

Zircon ceramics (ZrSiO₄) are promising candidates for actinide immobilization. Here, a series of Zr_1-xNd_xSiO_4-x/2 (x?=?0, 0.02, 0.20, and 1.0) ceramics are prepared to study the effects of phase evolution and acidity on the chemical durability of ZrSiO₄-based nuclear waste forms. The results show that the chemical stability of the single-phase ZrSiO₄ sample is better than that of the biphasic Zr_0.8Nd_0.2SiO_3.9 sample due to the existence of a secondary highly soluble phase (Nd₂Si₂O₇), which increases the contact area with leachate. The normalized release rates of both zirconium (Zr) and neodymium (Nd) in the Zr_1-xNd_xSiO_4-x/2 ceramic waste forms increase with increasing Nd loading and acid concentration. LR_Zr in ZrSiO₄ ceramics and LR_Nd in Zr_0.98Nd_0.02SiO_3.99 ceramics are the lowest, while LR_Zr and LR_Nd reach the highest values in the Zr_0.8Nd_0.2SiO_3.9 and Nd₂Si₂O₇ ceramics, respectively. The normalized release rates of Zr are lower than those of Nd due to the difference in the energies of their bonds with oxygen atoms. Moreover, the changes in the surfaces of the leached ceramics are consistent with their leaching results.