The present invention relates to a process for packaging radioactive wastes, in which the following successive steps are carried out: a/ radioactive wastes, the solids content of which comprises at least 90% of compounds selected from CaCO2, Fe2O3, SiO2, Al2O3 and B2O3, are supplemented, so as to achieve a target composition of said supplemented wastes after calcination, and b/ said supplemented radioactive wastes are melted and c/ said melt is poured into a container, so as to obtain, after cooling, a product comprising a vitreous or vitro-crystalline synthetic rock, having said target composition, characterized in that said target composition corresponds to the following definition, in a CaO, SiO2 and X2O3 ternary system, in which X2O3 is a trivalent oxide or a mixture of trivalent oxides selected from Al2O3, Fe2O3 or B2O3, PC and PS being the mass percentages of CaO and SiO2: PC is from 35 to 60%, and PS is from 10 to 45%.