The coupled effect of bentonite colloid and radioactive ingrowth on the transport of actinide decay chains in a single fracture

Abstract

An analytical model is used to illustrate the colloid-facilitated transport of decay chain radionuclides (RNs) in the granitic fractured porous media. The colloid source is assumed from the erosion of the bentonite used as buffer materials in a spent nuclear fuel repository. A conservative assumption that all colloids are mobile and have reached a sufficiently high concentration before RNs start to release is made in the present model. The latest reported distribution coefficient (Kd) values for both local granite and bentonite are used for several calculations. Investigating the simulation results shows that the second peaks of the breakthrough curves and profiles for daughter RNs are found using various initial concentration boundary conditions. The second peak is caused by the radioactive decay of the parent RN. Therefore, the contribution of the parents of a decay chain should not be ignored when performing a safety assessment. Moreover, the existence of colloids promotes a higher influence of the parents on the transport of their daughter RNs. A conservative colloid velocity that could reflect the effect of hydrodynamic chromatography phenomena is also investigated. Simulation results show that the Kd values of RNs on rock and bentonite colloids are the dominant factor that affects the colloid-facilitated-decay chain transport in the fractured rock system.