Determination of distribution coefficients of uranium and thorium on bedrock of Anarak Nuclear Waste Repository using Batch sorption experiments

Abstract

One of the key influencing parameters in the safe management of nuclear waste repositories is the distribution coefficient ( 𝐾 𝑑 ) of radionuclides on bedrock. In this study, (𝐾 𝑑 ) of uranium and thorium ions in intact bedrock were determined using batch experiments on crushed bedrock at seven different particle size fractions as well as three different initial concentrations of ions. Sorption experiments have been performed on crushed bedrock prepared from intact drill core samples and an aqueous solution containing desired ions from the local water (LW) near Anarak Nuclear Waste Repository. The results showed that both factors of the initial ions concentration as well as the size of the crushed bedrock particles significantly affect the value of the distribution coefficient so that ( 𝐾 𝑑 ) value increases with decreasing particle size and increasing ion concentration. Also, ( 𝐾 𝑑 ) calculated at three studied concentrations showed that the difference in the ( 𝐾 𝑑 ) values in smaller particles is large but decrease with increasing particle size and ( 𝐾 𝑑 ) curves for two concentrations of (100 and 10) ppm overlap at the end of the chart. Therefore, it can be concluded that for accurate calculation, it is better to use large particles and low concentrations of ions to determine ( 𝐾 𝑑 ) in intact bedrock. In this report, different mechanisms including chemical interactions, physical adsorption, and ion exchange were presented for ion adsorption by bedrock. The proposed mechanisms were related to the type of metal speciation of ions in the solution. Based on the obtained data, the preferred mechanism to describe the adsorption of uranium and thorium ions is composed of chemical interactions and physical adsorption.