1. A CELL STRUCTURE FOR A NUCLEAR REACTOR HEAT EXCHANGER, COMPRISING: AN OUTER LINER DIVIDED INTO TWO PORTIONS OF SUBSTANTIALLY EQUAL LENGTH BY AN INTERMEDAITE PARTITION EXTENDING AT RIGHT ANGLES WITH RESPECT TO THE AXIS OF SAID LINER IN THE INTERMEDIATE PORTION THEREOF, EACH OF SAID PORTIONS COMPRISING AT LEAST ONE INLET DUCT EXTENDING IN AN AXIAL DIRECTION WITHIN SAID LINER FOR THE ADMISSION OF COOLING FLUID IN THE LIQUID PHASE AND A PLURALTIY OF TUBULAR ELEMENTS FOR VAPORIZING SAID FLUID, SAID TUBULAR ELEMENTS BEING DISPOSED IN AN AXIAL DIRECTION AND IN SPACED SYMMETRICAL RELATION WITHIN SAID LINER, AND THROTTLING MEANS DISPOSED IN THE VICINITY OF SAID INTERMEDIATE PARTITION CONNECTING THE INLET DUCT IN EACH LINER PORTION TO EACH OF SAID TUBULAR ELEMENTS THEREIN, SAID THROTTLING MEANS COMPRISING A PLURALTIY OF THROTTLING DUCTS TAPERING INWARDLY IN THE DIRECTION OF FLOW FROM SAID INLET DUCTS TO SAID TUBULAR ELEMENTS, RESPECTIVELY, TO THEREBY THROTTLE SAID LIQUID PHASE, SAID DUCTS COMMUNICATING WITH SAID TUBULAR ELEMENTS IN A TANGENTIAL DIRECTION TO THEREBY FORM A LIQUID FILM ON THE INTERNAL WALLS OF SAID TUBULAR ELEMENTS WHICH FLOWS IN A GENERALLY HELICAL PATTERN PASSING AXIALLY THROUGH SAID TUBULAR ELEMENTS.