Several months after shut-down,137Cs and134Cs were the main activities in coolant residues from an in-pile sodium loop where a fuel pin had failed at 40000 MWd/t burn-up, FR2-reirradiation of coolant residues increased134Cs and generated136Cs, thus indicating the presence of stable133Cs and of long-lived135Cs in the samples. The amount of135Cs calculated from136Cs activities was of the same quantity as the number of137Cs atoms.