Phase evolution and chemical stability of Nd-doped Y₃Fe₅O₁₂ waste forms synthesized in molten salt at a low temperature

Molten salt provides a fast mass transfer and nucleation process, so the ceramic solid solution for immobilization of high-level nuclear waste (HLW) can be synthesized at lower temperatures. The chemical stability in the process of interaction with groundwater determines the ability of the matrix phase to prevent radionuclides from entering the biosphere. Nd-doped Y3Fe5O12 ceramics were prepared by the molten salt method at different sintering temperatures (1000celcius, 1100celcius, 1200celcius) and different m (eutectic salt): m (oxide) ratios (2:1, 3:1, 4:1). The sintered ceramic is Y3-xNdxFe5O12, where x is the solubility of Nd in YIG. The results show that the optimum mass ratio of molten salt to oxide is 3:1. The solubility of Nd in garnet is 33.3 mol% (x = 1.1) and 56.7 mol% (x = 1.6) at 1100celcius and 1200celcius, respectively. In the neutral medium, Nd does not transfer to the liquid phase. Acid leaching promotes the strong dissolution of the garnet matrix. In this case, the leaching rate of Nd from ceramics to the liquid phase is two to three orders of magnitude higher than that in a neutral medium. The experimental results suggest that garnet matrices can reliably immobilize actinides in subsurface repositories.