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Based on the importance of having an evaluation index system, a new method that combines PCA with graph distance classification is presented to make up the deficiencies of principal component analysis in the process of index screening, and this method is applied in the construction of an evaluation index system for the environmental quality of decommissioning uranium tailing. The seepage indexes were classified into six classes using graph distance classification, which selects the representative elements, including pH, ∑α, 210Pb, 210Po, F−, and NO3−. All of the representative elements were analyzed by PCA while determining the seepage indexes, including pH, U, Ra, ∑α, NH4-N, and F−, and establishing an index system for environmental quality evaluation that consists of two primary indexes (seepage and radiation environment) and 12 secondary indexes. The results showed that the model had ensured that the sifted indexes had a significant effect on the evaluation result and avoided the deletion of some important indexes and that it had stronger applicability and maneuverability.

In this work, the dense glass matrix of uranium tailings was successfully fabricated via microwave sintering process with Na2CO3 as a sintering aid. The effects of Na2CO3 additive and sintering temperature on the microstructure and mechanical properties of as-prepared solids were systematically investigated. XRD results confirmed the vitrified forms can be achieved at 1200°C within 30 min with 20 wt.% Na2CO3 addition. Importantly, the Na2CO3 additive significantly reduced the firing temperature from 1500°C to 1200°C and promoted densification. FT-IR analysis demonstrated that the main characteristic peaks of the sintered samples were attributed to the vibration of Si-O-Si. Microstructural studies presented the homogeneous distribution of glass phases. The results of mechanical properties of the sintered forms show that bulk density and Vickers hardness increased with increasing Na2CO3 content as well as sintering temperature, and the highest bulk density (2.45 ± 0.01 g/cm3) and Vickers hardness (823 ± 25 HV) were obtained at the temperature of 1300°C with 20 wt.% Na2CO3 addition, the heating rate of 20°C/min, and the soaking time of 30 min. It implied that the combination of microwave sintering with the appropriate addition of Na2CO3 would provide an efficient method for the immobilization of radionuclides in uranium tailings.

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