Laboratory experiments and modeling of the transport of 90Sr, 137Cs, 238U, 238Pu in fractures under high flow velocity

Abstract

The presence of fractures in the surrounding rocks of a radioactive waste disposal repository is recognized as a potential pathway for radionuclides to enter the public domain. As is well known, radionuclides transported by groundwater exhibit increased mobility in fractures, with flow velocities significantly faster than those in the pore spaces of the surrounding rock matrix. The principal objective of this study is to investigate the mobility of ⁹⁰Sr, ¹³⁷Cs, ²³⁸U, and ²³⁸Pu in fractures and their fate in the groundwater environment. Concurrently, batch and transport experiments were conducted to determine the sorption coefficients and transport parameters of each radionuclide. The results demonstrated that the adsorption and desorption isotherms of each radionuclide could be quantitatively described using the Freundlich isotherm. The hysteresis area "S" formed by the adsorption and desorption isotherm was utilized to quantify the irreversibility of the four radionuclides on granite particles, with a ranking of ²³⁸Pu > ¹³⁷Cs > ⁹⁰Sr > ²³⁸U. The distribution coefficients (real K_d) of the four radionuclides varied by 2-3 orders of magnitude, but the tendencies and concentrations of ⁹⁰Sr, ¹³⁷Cs, ²³⁸U, and ²³⁸Pu in the sampling holes were not significantly different. These findings suggest that it is essential to introduce the first-order rate constant (α_ch) and consider the kinetic process of adsorption to describe their migration process in granite single fractures. The sorbed concentrations of radionuclides on the surface of the upper and lower granite matrix were positively correlated with "real K_d × α_ch × time," while their concentrations in water flow were opposite. The irreversibility of radionuclides is another critical factor. The greater the irreversibility of radionuclides, the more challenging it is for them to desorb, resulting in a higher sorbed concentration remaining on the surface of the granite matrix and a smaller concentration in the water flow.