Since large amounts of radioisotopes of Cs was released after the accidents at Fukushima Daiichi nuclear powerplants, a tremendous number of scientists and engineers have faced several issues in terms of decontamination ofradioactive Cs. In this paper, we present our recent two computational works based on density functional theory, one of which is to clarify mechanism of strong adsorption and retention of Cs in clay minerals to establish an effective Cs removal scheme from large amounts of radioactive wastes left by decontamination activities, and the other of which is to reveal why zeolites can selectively catch Cs even in the presence of other minerals and to make a guideline to create a more improved materials to remove Cs as well as other radioactive ions.