Migration of radioactivity in multi-fraction sediments

Abstract

A new 3D radioactivity transport model coupled with multiscale circulation and multi-fractional sediment transport modules is presented. The sediment transport module simulates the transport of a mixture of one cohesive sediment fraction and a number of fractions of non-cohesive sediments of different sizes and densities. The model of radionuclide transport describes the key transport and exchange processes in the system of water-suspended and bottom multi-fraction sediments. Two-step kinetics with two successive reversible fast and slow reactions is used in the model. A noticeable feature of the model is approximation of the sediment and contamination profiles in the bed by multiple well-mixed layers to describe the vertical migration of radioactivity within bottom sediments due to erosion/deposition, molecular diffusion and bioturbation. The model accurately reproduced a laboratory experiment on the uptake of radiocesium by lake sediments. An analytical solution describing mutual adjustment of the concentrations of radioactivity in the pore water and in the multi-fraction sediment showed that activity was redistributed between different fractions of sediments far slower than between water and the total concentration in the sediment. The extended one-layer model of bottom contamination of multi-fraction sediments was derived from a general model and compared with a multilayer model. It was found, however, that the one-layer approximation was not capable of correctly predicting the inventory due to the fact that one-layer averaged concentration can essentially differ from the near-surface value in the multi-layer model. Radionuclide transport in channel with bottom depression was simulated to estimate the effects of erosion/deposition and the multi-fractionality of sediments on the transport process. It was shown that these factors affect the distribution of sediments by forming local maxima and minima of activity at the beginning and end of the depression, respectively, due to the redistribution of contaminated bottom sediments by flow. The developed model can also be used to simulate the transport of a wide class of toxic substances sorbed on sediments.