The present invention relates to the use of a mixed carbonate of formula AB(CO.sub.3).sub.2, in which A and B are different and chosen from alkali metals, alkaline-earth metals and rare earths, for the containment of radioactive carbon. This use may for example involve a process comprising: mixing CO.sub.2 having a radioactive carbon to be contained, or a simple carbonate of an alkali, alkaline-earth or rare-earth metal having a radioactive carbon to be contained, with an aqueous solution of a mixture of ACl.sub.n and BCl.sub.m or with an aqueous solution of a mixture of A(OH).sub.n, and B(OH).sub.m in order to obtain a precipitate of AB(CO.sub.3).sub.2, where n and m are integers sufficient to compensate for the charge of A and B respectively; recovery of the AB(CO.sub.3).sub.2 precipitate in powder form; and then pressing and sintering of the powder at a 20 temperature below the decarbonation temperature of the mixed carbonate manufactured in order to obtain sintered pellets of mixed carbonates for the containment of the radioactive carbon.