NOVELTY - The reactor comprises a stack of elementary electrolysis cells of which each is formed by a cathode, an anode and an electrolyte that is interposed between the cathode and the anode, an interconnecting plate (8) that is arranged between two adjacent elementary cells and is in contact with an electrode of one of the two elementary cells and an electrode of the other of the two elementary cells, a conduit (10) for the circulation of steam, an opening, and an area to create a buffer volume (14) of the steam around a periphery of an oxygen and hydrogen collection conduit.    USE - The reactor is useful for electrolyzing water at high temperatures to produce hydrogen.    ADVANTAGE - The reactor is capable of simply and efficiently electrolyzing the water at high temperatures thus ensuring safe and reliable production of hydrogen with high yield and purity.    DETAILED DESCRIPTION - The reactor comprises a stack of elementary electrolysis cells of which each is formed by a cathode, an anode and an electrolyte that is interposed between the cathode and the anode, an interconnecting plate (8) that is arranged between two adjacent elementary cells and is in contact with an electrode of one of the two elementary cells and an electrode of the other of the two elementary cells, a conduit (10) for the circulation of steam, an opening, an area to create a buffer volume (14) of the steam around a periphery of an oxygen and hydrogen collection conduit before its arrival at cathode or anode inlet end, ribs arranged around the periphery of the oxygen and hydrogen collection conduit, and additional ribs arranged opposite to the ribs that are arranged around the periphery of oxygen and hydrogen collection conduit. The interconnecting plate defines a cathode compartment (11) and an anode compartment (9) for the gas flow to the cathode and anode. The cathode and anode compartments comprise cathode and anode inlet ends that are connected to a supply adapted to deliver steam. The cathode further comprises a cathode outlet end that is configured for collecting the hydrogen, which is carried through the cell stack and the interconnecting plate. The anode further comprises an anode outlet end that is configured for collecting the oxygen, which is carried through cell stack and the interconnecting plate. The circulation conduit is formed in the interconnecting plate. The first and second ends of the conduit are connected to the inlet to deliver steam. The ribs are configured for discharging the steam at the outlet of the second end of the conduit towards the cathode or anode inlet end. A space between two additional ribs defines cathode or anode inlet end to increase a velocity of repulsed steam. Each cathode and/or anode inlet end is arranged in periphery of the reactor for receiving water vapor around the reactor and arranged near the hydrogen and oxygen collection conduit. The reactor is operated at temperatures of 700-1000 degrees C.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a reactor.    Interconnecting plate (8)    Anode compartment (9)    Steam circulation conduit (10)    Cathode compartment (11)    Buffer volume. (14)