Small High Temperature Gas Cooled Reactors (HTGRs) continue to be developed by a number of organizations throughout the world. The growing international interest in the HTGR concept is the direct result of HTGR’s features that are suitable for deployment at remote sites that are off-grid and which currently rely on deliveries of diesel fuel to meet power needs.
The HTGR does not require active safety systems to ensure public and worker safety. Reactor decay heat is removed by means of natural heat transfer mechanisms to maintain reactor fuel temperatures low for assurance of nuclear safety.
The HTGR is capable of safely producing temperatures of up to 950°C. This high-temperature capability translates into higher energy conversion efficiency for a variety of energy outputs, which can also include applications that require process heat.
The higher-energy conversion efficiency of the HTGR, combined with the elimination of any need for active safety systems, results in a design that is more economically competitive than other non-passively safe reactor concepts.
The thermal neutron spectrum of the HTGR, combined with the robust, ceramic-coated TRISO particle fuel, allow for very high burn up in a single pass through the reactor. This flexible fuel cycle capability, combined with its flexible energy output capability, result in a design concept that is very well suited for a wide variety of energy-growth scenarios.
The helium coolant, graphite moderator and coated TRISO particle fuel are key HTGR characteristics that provide several important inherent safety features. More specifically:
Helium is an inert gas, in both chemical and nuclear aspects, which remains in a single phase for all possible reactor conditions.
The TRISO fuel, graphite moderator and graphite structural components form an all-ceramic core that provides for tolerance of very high temperature operating conditions which make consequences of loss of coolant an insignificant event.
More specifically, the microsphere, ceramic coated particle fuel plays an important role in assuring safety of all HTGR designs. The ceramic coatings retain fission products during both normal operation and extraordinary conditions. The coating temperature limit is higher than can be achieved during even the most severe conditions. Therefore, no significant quantities of radioactive fission products can be released under any credible condition.
These practically indestructible ceramic fuel coatings assure the safety of the public and prevent any releases of radioactivity to the environment.
HTGR safety is also enhanced by a combination of passive and inherent safety features:
Core decay heat removal during transients is achieved passively by natural conduction, radiation and convection to the environment.
A highly negative temperature coefficient of reactivity inherently terminates power excursions and shuts the reactor down in the event of operator error or failure of the normal control system, a key safety parameter in remote operations.
The inert helium coolant precludes the possibility of chemical reactions with the coolant. There are no high-pressure water sources that could inject water into the reactor system. No motive powered active safety features or operator action are required to ensure safety.