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 Y 3 Fe 5 O 12 ceramics were prepared by the molten salt method at different sintering temperatures (1000℃, 1100℃, 1200℃) and different m (eutectic salt): m (oxide) ratios (2:1, 3:1, 4:1). The sintered ceramic is Y 3-x Nd x Fe 5 O 12 , 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 1100℃ and 1200℃, 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.
Kexin Lv,Hengcheng Wan,Yi Li,Shengtai Zhang,Qiang He
The garnet structure Y 3 Fe 5 O 12 (YIG) possesses a high immobilization capacity for actinides. However, the related research on the sensitivity of garnet structures to irradiation damage is still in its infancy. Herein, we synthesize Y 3-x Nd x Fe 5 O 12 (x = 0.1, 1.8) ceramics and irradiate them with 2 MeV α-particle irradiation to fluences ranging from 1 × 10 14 to 1 × 10 17 ions/cm 2 at room temperature. We evaluated the effects of irradiation damage on the phase transition and chemical stability. An amorphous layer with a thickness of 84 nm is formed by irradiation, though the grain shape and element distribution remain uniform after irradiation. In addition, Y 3-x Nd x Fe 5 O 12 (x = 0.1, 1.8) ceramics exhibit excellent chemical stability (∼ 10 −5 g m −2 · d −1 ) in the leaching process. Restricted by the topological structure of garnet, the increase in solid solubility does not lead to a change in the radiation tolerance of the garnet structure, which means that garnet of different compositions should have similar and predictable radiation tolerance.
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