|Fig. 1: Very-High-Temperature Reactor Design (Source: Wikimedia Commons)|
The Ultrahigh Temperature Reactor, also refereed to as Very-High-Temperature reactor VHTR, is a Generation IV concept reactor that differentiates itself from other reactors by both it operation temperature, and its fuel source. The VHTR can theoretically operate in upwards of 1000°C, and is mainly limited by the current technology of steel which can prevent reactor from reaching this operating temperature. As seen in Fig. 1, the design of VHTRs centers around a helium coolant and a graphite reactor core. 
The VHTR will employ a direct Brayton cycle for the generation of electricity, this is due to the recent developments in the Brayton cycle making it much safer than the previous Rankine cycle that was utilized, in addition to added safety it will also be cheaper to operate. When it comes to production of hydrogen the VHTR will utilize an indirect cycle with an intermediate heat exchanger to supply heat. Because of the heat involved in VHTR, an indirect cycle is needed in order to provide a thermal interface between the reactor and the chosen process heat application. 
The fuel used in VHTR will build upon previous high temperature gas reactors, but will use the triple-isotropic coated fuel particles, but instead arranged in a graphite matrix to form the fuel elements. These triple-isotropic fuel particles are are either dispersed in a pebble, for the pebble bed design, or molded into compacts/rods that are then inserted into the hexagonal graphite matrix.  The fuel particle can been seen in Fig. 2. This results in a very flexible fuel arrangement, that can accommodate other types of fuel cycles.
|Fig. 2: Particle Bed Reactor Element (Source: Wikimedia Commons)|
The graphite core structure of the VHTR, helium coolant, and coated fuel particles allow the VHTR to withstand accident temperatures without structural damage or fission product release. The reduces the need for more active safety measures found in traditional Light Water Reactors where you need to assure that the fission product does not leak. The high heat capacity of the graphite core also reduces the safety concerns, while the high exit temperature of the reactor permits emissions-free production of process heat. 
While a limited by the current technology of steel, which can prevent reactors from reaching the ideal exit-temperature of around 1000°C, when coupled with hydrogen production plants using high temperature electrolysis, VHTRs can help provide a carbon neutral energy solution for the future. 
© Craig Jones. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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