[en] The protective heat transfer tubes are anchored with both ends in the first and the second protective tubeplates and slipped onto the straight parts of the spacing heat transfer tubes. The space between the protective and the spacing heat transfer tubes is connected to the economizer space filled with an inert gas. The economizer space consists of the intertube space between the second spacing tubeplate and the second protective tubeplate. The spaces between the protective and the spacing heat transfer tubes are filled with a suitable indication material, e.g., liquid metal. The advantage of this design of the steam generator is that the relative dilatation between the protective and the spacing heat transfer tube is accommodated and that no bends are necessary on double-walled tubes. (M.D.)