Effect of phase evolution and acidity on the chemical stability of Zr1-Nd SiO4-/2 ceramics

Zircon ceramics (ZrSiO4) are promising candidates for actinide immobilization. Here, a series of Zr1-xNdxSiO4-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 Zr0.8Nd0.2SiO3.9 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 Zr1-xNdxSiO4-x/2 ceramic waste forms increase with increasing Nd loading and acid concentration. LRZr in ZrSiO4 ceramics and LRNd in Zr0.98Nd0.02SiO3.99 ceramics are the lowest, while LRZr and LRNd reach the highest values in the Zr0.8Nd0.2SiO3.9 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.