Method of treating sodium contained in the interconnected open pores of a structure placed in a cladding, the pores furthermore containing a radioactive substance. The method comprises the following successive steps: a) at least two slits are made over the entire length of the cladding; b) the sodium is converted to sodium carbonate by a carbonation reaction by bringing the structure into contact, via the slits, with a reactive gas mixture comprising steam, carbon dioxide and a gas inert with respect to sodium, in such a way that the expansion of the carbonate causes the cladding and the structure to open starting from the slits and results in the carbonation reaction propagating into the structure.