One of the key parameters used in the safety management of nuclear Waste Repository is determination of distribution coefficient (K_d) of radionuclides on bedrock. In this study, we determined the K_dof uranium and thorium ions in intact bedrock 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 ground water of Anarak Nuclear Waste Repository. The results showed that as the particle size increases, K_(d )decreases and remains almost constant at higher particle size. Also, K_(d )calculated at three studied concentrations showed that the differences in K_(d ) values in smaller particles are noticeable, but decreases with increasing particle size so that 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 lower concentrations to determine K_(d )in intact bedrock. In this report, in general, two different mechanisms including weak chemical or physical adsorption and ion-exchange for ion adsorption by bedrock were presented. The proposed mechanisms were related to the type of metal speciation of ions in solution. The preferred mechanism for uranium and thorium ions due to the absence of exchangeable ions in the bedrock structure was identified through weak chemical or physical adsorption.