The decommissioning and recovery of the Fukushima Daiichi Nuclear Power Plant will generate a substantial amount of radioactive waste. Among these wastes, fuel debris is expected to be designated as an intermediate-to-high-level waste that needs to be immobilized within a matrix. The immobilization of radioactive waste in an aluminum silicate matrix formed from fly ash or bentonite was investigated in this study. Three matrix compositions were selected: bentonite 100 wt.%, fly ash 90 wt.% - sodium tetraborate decahydrate (borax) 10 wt.%, and fly ash 80 wt.% - bentonite 20 wt.%. The radioactive waste was simulated using uranium dioxide doped with Sr and Eu. The pellets were formed using 70 wt.% matrix material and 30 wt.% radioactive waste, and were heated at 600°C, 800°C, and 1000°C in a nitrogen atmosphere. This study aims to determine the best matrices and conditions for radionuclide immobilization. All pellets exhibited a high compressive strength of several dozen megapascals, which met the criteria of a minimum of 5 MPa. Matrices that showed the highest resistance to the leaching of U were the fly ash-borax matrix heated at 600°C, 800°C, and 1000°C; fly ash-bentonite matrix heated at 800°C; and bentonite matrix heated at 800°C and 1000°C. Matrices that showed good resistance to both U and Sr leaching were the fly ash-borax matrix heated at 1000°C and the bentonite matrix heated at 1000°C. Eu leached out in very small amounts. U was immobilized by incorporating uranium dioxide particles in the aluminum silicate glassy melt formed during heating. The physical absorption of the waste also contributed to reducing Sr leaching. The fixation of Sr on the surface of fly ash also occurred in matrices containing fly ash.