Migration Behavior of ¹³⁷Cs, ⁷⁹Se, and ⁹⁹Tc in Clay Rocks: Role of Competitive Adsorption Under Coexistence...

To address the issue of radioactive waste generated by the large-scale promotion and use of nuclear energy, safety evaluations of disposal sites in various surrounding rocks are essential. These evaluations are a prerequisite for ensuring the long-term safe disposal of radioactive waste. This study focuses on the blocking capacity of clay rocks concerning the advection-dispersion behavior of representative radionuclides such as ¹³⁷Cs, ⁷⁹Se, and ⁹⁹Tc. It further examines the effects of competitive adsorption that arise when these three radionuclides coexist. (Since ⁷⁹Se is difficult to obtain, ⁷⁵Se was used as a substitute nuclide. In the mixed-nuclide experiments, the stable isotope Re was used to replace ⁹⁹Tc.) The experimental findings revealed that competitive adsorption can significantly reduce the adsorption capability of clay rocks for ¹³⁷Cs and ⁷⁹Se, altering the adsorption mechanism. During the advection-dispersion process, the weak adsorption sites of ¹³⁷Cs and ⁷⁹Se on clay rocks become active after the strong adsorption sites are preferentially occupied, resulting in a decline in both adsorption quantity and rate. In the case of ⁹⁹Tc, competitive adsorption weakens the effect of anion repulsion, leading to a reduction in the immobile liquid regions (θ_im).