The C50 concrete is used as a concrete structural materials for engineering disposal units in cavern-type low and intermediate radioactive waste repositories in order to meet the needs of nuclear power plant operation and decommissioning.

This study aims to prepare C50 concrete material and investigate its adsorption performance for radioactive nuclides Ni²⁺ and Cs⁺.

Firstly, according to the design requirements of C50 concrete, C50 concrete specimens were prepared, and then crushed and ground into powder with a particle size less than 75 μm as experimental samples. Then, series of characterization analysis were performed to observe the crystal structure and content of the C50 concrete sample to reveal the main components of the C50 concrete that comprised silicate minerals with high aluminum content and carbonate minerals with high calcium content. Finally, adsorption experiments were carried out to examine the adsorption performance of C50 concrete for radioactive nuclides Ni²⁺ and Cs⁺. Considering the widespread presence of ions in groundwater, the effect of the coexistence of different ions on the adsorption of Cs⁺ and Ni²⁺ in the C50 concrete was tested, and the effect of temperature on adsorption was investigated with concern of the radiation exotherms of radioactive waste.

The analysis results show that the main components of the C50 concrete are SiO₂, CaO, and Al₂O₃ with and specific surface area of 2.786 m²·g^-1, and irregular polyhedral and lamellar structures with strong amorphous characteristics are noted at the microscale. The adsorption experiments reveal that the C50 concrete have a good adsorption capacity for Cs⁺ and Ni²⁺. The K_d of Cs⁺ adsorption on the C50 concrete reaches 19.400 L·mg^-1 with adsorption capacity of 0.316 mg·g^-1 whilst the K_d of Ni²⁺ adsorption on the C50 concrete reaches 465.142 L·mg^-1 with adsorption capacity of 96.375 mg·g^-1. With the increase of initial metal ion concentration, the K_d value of Cs⁺ adsorption on the C50 concrete gradually decreases whilst the adsorption capacity increases. The K_d of Ni²⁺ increases steadily after a gradual decrease, whereas the adsorption capacity gradually increases with the increase of initial metal ion concentration. Experiments results on the influence of environmental factors demonstrate that the effect of pH on the adsorption of Cs⁺ in the C50 concrete is relatively small, and the order of inhibition of ions on adsorption is K⁺ > Ca²⁺ > Mg²⁺ > SO₄^2-> Cl^-> NO₃^-. Increasing the temperature leads to a slow decrease in the absorptivity. Meanwhile, as for the adsorptivity of Ni²⁺ in the C50 concrete increases with pH, and the order of inhibition of ions on adsorption is Mg²⁺ > Ca²⁺ > SO₄^2- > CO₃^2-. The adsorptivity of the C50 concrete for Ni²⁺ increases slowly with an increase in the temperature.

The results of this study provides the basic data for the engineering geological disposal of nuclear waste.