Insight into immobilization mechanism of nuclear waste management by granite: Impact of waste chemical species

Nuclear waste immobilization is one of the ways to alleviate the harm of radioactive waste produced in the nuclear field to human beings and the environment. In the current study, granite was utilized as a substrate material for the immobilization of simulated radioactive waste. Based on the obtained results, the relationship between the waste chemical species and the physicochemical properties, such as solid-solubility of waste, infrared, density and leaching data, was systematically studied as well. The findings indicate that the granite substrate used for the immobilization of the simulated waste (Nd2O3) exhibits a solid-solubility limit of 27 wt%. When the added content of waste is below 27 wt%, all Nd elements are immobilized into the glassy phase. However, when the waste content exceeds 27 wt%, in addition to being immobilized in the glassy phase, the presence of Nd2O3 reacted with Ca3Si3O9 to form Ca2Nd8(SiO4)6O2. As the Nd2O3 content increased from 20 wt% to 25 wt%, the microstructures related to Si-, Al-, and Fe-are more involved in the immobilization of waste. However, the appearance of Ca2Nd8(SiO4)6O2 and Nd2O3 makes the density and Vickers hardness of the matrix not increase further. The maximum density and hardness values are 3.17 g cm−3 and 7.88 GPa, respectively. The leaching results show that the cured body has good anti-leaching performance (LRNd = ∼10−6 g cm−2 d−1). Finally, the comparison of vacancy formation energy of Nd in Nd2O3 and Ca2Nd8(SiO4)6O2 proved that the formation of Ca2Nd8(SiO4)6O2 is beneficial to the immobilization of simulated waste. This study provides a certain reference for the selection of nuclear waste solidification substrates. This kind of granite material from nature has the potential advantages of low cost and better immobilization effect on waste, and may be more suitable for the treatment of radioactive waste in the future.