Immobilizing Behavior of Trivalent Actinide Nuclides by YIG Ceramics

Garnet is considered as a potential matrix for immobilizing high-level radioactive waste (HLW) because of its large actinide package capacity and chemical flexibility. Y3-xNdxFe5O12 (0 <= x <= 2) series yttrium iron garnet (YIG) samples was successfully synthesized by solid-state sintering method using Nd3+ to simulate trivalent actinides. The solubility limit of Nd in the YIG and the influence of Nd doping amount on the phase and microstructure of ceramics was studied. Then the chemical durability of Nd-doped yttrium iron garnet solidified body under different pH conditions was evaluated. The results show that the ceramics with x <= 1.7 show homogeneous single YIG, but the ceramics with x >= 1.8 exhibit coexistence of three crystal phases (YIG, NdFeO3 and Fe2O3). The solubility limit of Nd3+ in the pure YIG is about 29.5% (in mass). As the amount of Nd doping increases, the density of the ceramics increases, the volume decreases, and the porosity decreases. The leaching experiment results show that the normalized leaching rate of the elements (LRi) is 10(-6)similar to 10(-5)g.m(-2).d(-1). LRY is smaller than LRNd, and the normalized leaching rate of the elements in acid leachate is slightly higher than those in neutral and alkaline leachates. The results indicate that YIG ceramics are a potential candidate form for trivalent actinides.