6.2.1 High-temperature gas-cooled reactor (HTGR)
Hydrogen production using thermochemical cycles has been studied in the last four decades. The thermal energy source for these processes has been high-temperature gas-cooled reactors (HTGR), which have been and/or are being developed. The maximum operating temperature of these cycles has been limited to about 800°C, which is the available temperature level of gas-cooled nuclear reactors. The high-temperature gas-cooled reactor (HTGR) is one of the most suitable nuclear reactors for producing the secondary energy carrier hydrogen owing to its capability of producing high-temperature heat at close to 1000°C, which assures an efficient energy conversion.
6.2.2 Advanced gas reactor (AGR)
The AGR is a commercial thermal reactor that has been built in the UK for electricity production in 1550 MWth units, with 14 units still in operation. The AGR core consists of uranium oxide fuel pellets in stainless steel cladding within graphite blocks. The graphite acts as a moderator and carbon dioxide is the coolant. The achievable temperature of the coolant at the reactor exit during normal operation is around 650°C. The carbon dioxide circulates through the core at 4.3 MPa. For future design and implementation, there is the potential to increase the operating pressure of the AGR in order to couple it to a direct cycle supercritical CO2 power conversion system. The temperature of the reactor coolant for a future design can be driven up to 750°C with new designs and analysis. This combination can enable high-efficiency, economic hydrogen production through steam electrolysis at medium temperatures.
6.2.3 Advanced high-temperature reactor (AHTR)
The molten-salt-cooled advanced high-temperature reactor (AHTR) is a new reactor concept designed to provide very high-temperature (750-1000°C) heat to enable efficient low-cost thermochemical production of H2 or production of electricity (Forsberg et al., 2003). The AHTR uses the solid coated-particle fuel in a graphite matrix like the MHR, but a molten-fluoride-salt as coolant. It combines the high-temperature fuel from the HTGR with a denser coolant for the molten salt reactor. The proposed design temperature of the coolant at the reactor exit is 1000°C. The graphite blocks are compatible with fluoride salts as coolant. The reactor concept is designed for atmospheric pressure operation. This design uses Ni-based high-temperature alloys that have been similarly adopted for molten salts. The reactor is proposed to be built in large sizes (2000 MWth) with passive safety systems for decay heat removal.
6.2.4 Modular helium reactor (MHR)
The MHR is a thermal reactor that can be used both for hydrogen and electricity production in modules of 600 MWth. Its core consists of prismatic blocks of graphite that allow coolant flow and contains ceramic fuel. The temperature of the coolant, He gas, at the reactor exit is currently designed to achieve temperatures around 850°C. It is proposed to achieve 1000°C in the future within a new design, based on the same reactor concepts, called the very high-temperature reactor. The operating pressure of the MHR is 7 MPa. The core design can provide passive safety by achieving high temperatures during transients and by large thermal inertia.
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