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| Fig. 1: Very High Temperature Reactor. (Source: Wikimedia Commons) |
The United States hopes to reduce greenhouse emissions through relying on nuclear energy as a source of nation's electricity supply; currently nuclear energy accounts 20% of nation's electricity and constitutes 70% of low-emissions energy supply. In 2005, the New Generation Nuclear Plant project was formally established on the basis of developing 4th generation nuclear reactors that are greener and safer. There are few phases to the project, and currently it is in the conceptual design phase. This report will briefly introduce the very high temperature reactor (VHTR), one main types of reactors that is currently favored by the department of energy.
Very high temperature reactor, shown in Fig. 1, is loosely defined as any reactor with a coolant outlet temperature of 1000 Celsius or above. In the next generation nuclear plant, the conceptual design is a helium-cooled, graphite moderated reactor with a ceramic core. The design is similar to the high temperature reactor (HTGR) but the goal is to extend to temperatures up to 1000 Celsius, above the 850 Celsius for the HTGR. The increase in temperature will increase electrical generation and better thermal conditions for process heat applications.
VHTR is still been developed, but its core will largely be based on the Generation III predecessor. The inner core is formed from hexagonal rings of fuel blocks (inner reflector), active fuel core around the inner ring (active core), and an outer replaceable reflector ring (outer reflector). Moreover, vessel coolant channels and a barrel core are also in this core. Helium enters by flowing first through the coolant channels and then inters the core barrel cooled, thereby reducing the temperature of the barrel material. The core is ceramic which has a very high thermal capacity and can withstand extremely high temperatures.
VHTR employs a direct Brayton cycle. Previously, indirect Rankine cycle was used. Brayton cycle is much simpler, which leads to better safety and cost reduction. For production of hydrogen, VHTR will use an indirect cycle with an intermediate heat exchanger (IHX) to supply heat to the process application. An indirect cycle isolates the process heat loop from the nuclear reactor, allowing heat systems to be designed to non-nuclear standards. The goal is to have a 600MWth VHTR that will prodice about 2 million cubic meters of hydrogen per day, which is equivalent to about 160,000 gallons of gasoline per day.
The fuel for VHTR will be the triple-isotropic coated fuel (TRISO) particle. These particles are usually dispersed within a graphite matrix to form fuel elements. TRISO fuel consists of 15% uranium oxycarbide kernel surrounded by three layers of pyolytic carbon (PyC) with another layer of silicon carbide ceramic. Fission products contain no free oxygen so they will not degrade the ceramic layer. SiC acts as a pressure vessel that contains all fission products. The fuel could be in pebble form (German designs) or in primsmatic block form (US designs).
The main challenge to VHTR is the materials that are selected under high temperatures and neutron fluxes. Radiation, diffusion, and chemical reactions can all affect the integrity and micro-structure of the material properties. High temperature materials have been developed for gas turbines but a significant amount of validation is needed for these materials to be used in nuclear service.
© Jue Pu. 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.
[1] Next Generation Nuclear Plant, U.S. Office of Nuclear Energy, April 2010.
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