Engineering design elements of a two-phase thermosyphon for the purpose of transferring NGNP thermal energy to a hydrogen plant

Two hydrogen production processes, both powered by Next Generation Nuclear Plant (NGNP), are currently under investigation at the Idaho National Laboratory and University of Idaho. The first is high temperature steam electrolysis utilizing both heat and electricity, and the second is thermo-chemical production through the sulfur iodine process which primarily utilizes heat. Both processes require high temperature (>850 °C) for enhanced efficiency; temperatures indicative of NGNP. Safety and licensing mandates prudently dictate that the NGNP and the hydrogen production facility be physically isolated, perhaps requiring separation of over 100 m. There are several options to transferring multi-megawatt thermal power over such a distance. One of the options is two-phase heat transfer utilizing a high temperature thermosyphon. Heat transport occurs via evaporation and condensation, and the heat transport fluid is re-circulated by gravitational force. A thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. This paper addresses the engineering design elements of an industrial-scale (50 MW), high temperature controllable thermosyphon for NGNP process heat transfer. Although several different working fluids are under consideration, alkali metals are used herein as reference fluids to illustrate elements of design.