Diffusion Behaviors of Cs and I in Soil Surroundin Long-distance Effluent Pipeline of Proposed Nuclear Power Plant

In the assessment of environmental radiation risk, ¹³⁷Cs and ^129/131I, as high-yield fission products, are crucial indicators in the radioactive effluent of nuclear power plants. Due to the strong mobility and bioavailability of Cs⁺ and I^- in the groundwater-soil system, their diffusion behaviors attracted much attention. In this work, the shallow soil samples under the long-distance effluent pipeline of a proposed nuclear power plant in China were used to study the diffusion behaviors of stable analogue (¹³³Cs and ¹²⁷I) of radioisotopes. The experimental data were fitted by Pycharm, which shows that the shallow soil at different sampling sites retards Cs⁺ through the adsorption process. The effective diffusion coefficient of Cs⁺ ranges from 3.87×10^-11 m²/s to 4.31×10^-11 m²/s, the range of effective diffusion coefficient of I^- is 1.97×10^-11 m²/s to 2.61×10^-11 m²/s, illustrating the diffusion of Cs⁺ and I^- are normal diffusion. The through time of Cs⁺ ranges from 29.24 d to 34.48 d, and the solid-liquid distribution coefficient (Kd) ranges from 3.34×10^-2 L/g to 4.27×10^-2 L/g. On the contrary, the average through time of I- is 0.30 d, which is much less than that of Cs⁺, and the solid-liquid distribution coefficient (Kd) of I^- tends to be zero (1.75×10^-4-2.72×10^-7 L/g), indicating that the weakly adsorbed ^129/131I is difficult to be fixed by soil, and illustrating a faster diffusion of I- than Cs+. The diffusion coefficient of Cs⁺ and I^- in SK02 soil is the largest, which indicating that the migration rate of Cs⁺ and I^- is faster in SK02. The differences of Cs⁺ and I^- diffusion behaviors are affected by soil adsorption. Illite, biotite and montmorillonite in soil can provide stong affinity adsorption sites for Cs+, on the contrary, it is difficult to provide effective adsorption sites for I-. Since the main components of the soil samples are quartz, plagioclase and anorthite, when the concentration of Cs⁺ is higher than the capacity of strong affinity sites, a small amount of clay minerals in the soil will no longer control the diffusion of Cs⁺. In the meantime, the negative charge of the soil surface can attract Cs⁺ and reject I^-, so in the long-term diffusion process, the surface charge is the dominant factor that controlling the diffusion behavior of Cs⁺ and I^- in this area.