For safe disposal of the large volume of concrete waste generated during the decommissioning of the Fukushima Daiichi Nuclear Power Station (F1NPS), the state of Cs and Sr contamination in concrete waste must ideally be estimated in advance during waste disposal planning. Cs and Sr are primarily adsorbed by calcium silicate hydrate (C–S–H) in cementitious materials. However, the amount of adsorption varies greatly with the Ca/Si mole ratio (C/S ratio) of C–S–H and with the presence or absence of coexisting elements, both of which vary according to interactions with the environment. This study attempted to model on the results of experiments on Cs and Sr adsorption by synthetic C–S–H with varying C/S ratios. Furthermore, for verifying whether the model can be applied to F1NPS, the results calculated by the model were compared with the experimental results for Cs and Sr adsorption on cement paste subjected to environmental conditions (carbonation, leaching, and presence or absence of coexisting elements (Na, K)). It was confirmed that the present model is overall able to reproduce Cs and Sr adsorption behavior on cement paste, considering changes due to environmental conditions.