[en] Duplex tubes anchored with both ends in tube plates and featuring two indication gaps are placed in the steam generator suitable for sodium cooled nuclear reactors. Each of the indication gaps is divided into indication sections tightly separated one from the other. The advantage is the speedy location of a leakage and failed tube in the steam generator. Following shut-down, according to the indicated failure location either the water or sodium space of the steam generator will be filled with a mixture of gases and a leak detector will detect the presence of helium from the test mixture in the individual indication sections in turn in both indication gaps. The failure is thereby located and the leakage may be removed. (J.B.)