The High Temperature Gas-Cooled Reactor (HTGR) - Safe, Clean and Sustainable Energy for the Future
What are they and how do they work?
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The HTGR is a inherently safe, modular, underground helium-cooled nuclear reactor technology, The reactor and the nuclear heat supply system (NHSS) is comprised of three major components: the reactor, a heat transport system and a cross vessel that routes the helium between the reactor and the heat transport system. The NHSS supplies energy in the form of steam and/or high temperature fluid that can be used for the generation of high efficiency electricity and to support a wide range of industrial processes requiring large amounts of heat or steam. Development of the HTGR in the U.S. is funded by the Energy Policy Act of 2005. The program for the Next Generation Nuclear Plant (NGNP) Project is managed by the Idaho National Laboratory with funding through the Department of Energy.
The NHSS design is modular with module ratings from 200 MWt to 625 MWt, reactor outlet temperatures from 700 ⁰C to 850 ⁰C and heat transport systems that provide steam and/or high temperature fluids. The range of power ratings, temperatures and heat transport system configurations provides flexibility in adapting the modules to the specific application. Safety at the highest levels is designed into the HTGR. No harmful release of radioactive material under any conditions is assured by design.
Multiple assured barriers to the release of radioactive material are provided. These barriers include multiple layers of ceramic coatings on the nuclear fuel, the carbon encasement and the graphite core structure. Additional barriers include the reactor vessel and the reactor building. The high temperature and robust structural capabilities eliminate concerns of fuel damage that could lead to significant release of radioactive materials from the nuclear fuel. The ceramic coated nuclear fuel provides the primary containment for radioactive materials rather than depending on a containment building.
Reactor power levels are limited and the nuclear reactor shuts down if reactor temperatures exceed intended operating conditions. Inherent to the nuclear reactor design is suppression of the nuclear reaction if the operating temperature increases. Complete shutdown is achieved through automatic insertion of control rods into the reactor core by gravity.
No actions by plant personnel or backup systems are required to either ensure shutdown of the reactor or ensure cooling. Conversely, actions of plant personnel cannot achieve conditions that cause the reactor fuel to lose its ability to contain radioactive material.
No power and no water or other cooling fluid is required. Heat removal from the reactor occurs naturally and directly to the earth if normal heat transport systems are not available.The low energy density of the reactor core combined with the large heat capacity of the graphite structure results in the reactor taking days to reach maximum temperatures (still well below temperatures that could cause fuel degradation), even if normal cooling systems are not functional.
Reactor materials including the reactor fuel will not chemically react or burn to produce heat or explosive gases. Helium is inert and the fuel and materials of construction of the reactor core and the nuclear heat supply system are chosen to preclude such reactions.
Intrusion of water or air into the reactor systems does not result in substantive degradation of the capability to contain radioactive materials and maintain a shutdown condition. The presence of water will enhance the heat removal path.
Spent or used fuel is stored in casks or tanks in underground dry vaults that can be cooled by natural circulation of air and shielded by steel plugs and concrete structure. No water is required for either cooling or radiation shielding and no active cooling system is required.