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

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 as137Cs,79Se, and99Tc. It further examines the effects of competitive adsorption that arise when these three radionuclides coexist. (Since79Se is difficult to obtain,75Se was used as a substitute nuclide. In the mixed-nuclide experiments, the stable isotope Re was used to replace99Tc.) The experimental findings revealed that competitive adsorption can significantly reduce the adsorption capability of clay rocks for137Cs and79Se, altering the adsorption mechanism. During the advection–dispersion process, the weak adsorption sites of137Cs and79Se 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 of99Tc, competitive adsorption weakens the effect of anion repulsion, leading to a reduction in the immobile liquid regions (θim).