To investigate the leaching behavior of radioactive nuclides in leaching samples comparable to core debris (partially molten ZrO 2 /UO 2 between fuel rods) in the Fukushima Daiichi Nuclear Power Station (1 F NPS), the concentration of radionuclides in the leaching solution was measured. The leaching behaviors of actinides (U, Pu, and Np) and Cs from the samples were similar to those from spent fuel. As predicted from the current thermodynamic database, the leaching of U and Pu depends on the pH in the cooling water of the core debris. However, if Mo and Tc are surrounded by zircaloy in the core debris, their leaching amount may increase by one order of magnitude than those from spent fuel.