very-high-temperature reactor

very-high-temperature reactor
Name	Very-high-temperature reactor
Generation	IV
Status	Research and development
Moderator	Graphite
Coolant	Helium
Fuel	TRISO
Neutron spectrum	Thermal

very-high-temperature reactor. A very-high-temperature reactor is a Generation IV reactor design characterized by its use of helium as a coolant and graphite as a moderator, enabling core outlet temperatures significantly higher than those of conventional nuclear reactors. This class of reactor, which includes designs like the pebble-bed reactor and prismatic block reactor, aims to provide highly efficient electricity generation and enable industrial process heat applications. The technology is primarily developed through international collaborations such as the Generation IV International Forum and has seen significant research programs in nations including Japan, China, South Africa, and the United States.

Design and operation

The fundamental design of a very-high-temperature reactor centers on a graphite-moderated core that is cooled by pressurized helium. This inert gas is circulated through the core, where it absorbs heat before passing through a turbine for power generation or to a heat exchanger for industrial use. Core configurations typically involve either moving pebble fuel elements, as in the pebble-bed reactor, or stationary hexagonal prismatic blocks containing fuel compacts. Key operational components include the reactor pressure vessel, helium circulator, and steam generator or gas turbine, with the entire primary circuit often housed within a prestressed concrete reactor vessel for added safety. The high outlet temperatures, often targeting 750°C to 950°C or higher, necessitate advanced materials like silicon carbide and specialized alloys for core internals and heat exchanger components.

History and development

Early conceptual work on gas-cooled, high-temperature systems began with experiments like the Dragon reactor in the United Kingdom and the AVR reactor in West Germany during the 1960s. The United States Department of Energy later constructed the Fort St. Vrain Generating Station, which operated from 1979 to 1989. Major development programs emerged in the late 20th and early 21st centuries, including Japan's High Temperature engineering Test Reactor, South Africa's Pebble Bed Modular Reactor project, and China's HTR-10 and subsequent HTR-PM demonstration plants. International coordination was formalized through the Generation IV International Forum, with ongoing research supported by institutions like the Idaho National Laboratory and the European Union's Euratom framework.

Safety features

The safety philosophy relies heavily on inherent, passive characteristics rather than active engineered systems. The robust TRISO fuel form itself constitutes a primary containment barrier, capable of retaining fission products at extreme temperatures. The chemical inertness and single-phase nature of the helium coolant eliminate risks associated with coolant boiling or chemical reactions. Furthermore, the large thermal capacity of the graphite core and low power density allow for passive decay heat removal through natural mechanisms like thermal radiation and conduction, even in a complete loss of forced cooling, as demonstrated in tests on the AVR reactor and HTR-10. The design often incorporates a containment building, with some concepts proposing below-grade placement for additional protection.

Fuel and fuel cycle

The standard fuel is TRISO (TRIstructural-ISotropic) particle fuel, where a kernel of uranium dioxide or uranium oxycarbide is coated with layers of pyrolytic carbon and silicon carbide. These microscopic particles are then embedded within a graphite matrix to form either spherical pebbles or cylindrical compacts. The fuel cycle can utilize various fissile materials, including low-enriched uranium, with some designs considering the use of plutonium or thorium fuels. The exceptional integrity of the TRISO coatings allows the fuel to withstand temperatures far beyond normal operating conditions, potentially enabling long burnup cycles and contributing to a reduction in high-level waste volume compared to some traditional light-water reactor fuels.

Applications and economics

The primary advantage is the supply of high-temperature process heat for industrial applications beyond electricity generation. Potential uses include hydrogen production via thermochemical cycles like the sulfur-iodine process, petroleum refining, coal gasification, and desalination. For electricity production, the high temperatures enable greater thermal efficiency, potentially exceeding 50% when coupled with a Brayton cycle gas turbine. Economically, the modular nature of proposed designs aims to achieve economies of series production, while passive safety features could simplify licensing requirements and reduce costs associated with containment structures. The technology also aligns with goals for load following operation to support grids with high penetrations of renewable energy sources like wind power and solar power.

Comparison with other reactor types

Compared to conventional light-water reactors such as the pressurized water reactor, very-high-temperature reactors operate at much higher temperatures and use helium instead of water as a coolant, eliminating risks of steam explosions. Versus other Generation IV reactor concepts like the sodium-cooled fast reactor, it operates on a thermal neutron spectrum and does not involve challenging liquid metal coolant chemistry. When contrasted with earlier gas-cooled reactor designs like the Magnox reactor or Advanced Gas-cooled Reactor, it employs more advanced TRISO fuel and targets significantly higher outlet temperatures. Its safety approach differs from that of boiling water reactors by emphasizing inherent passive features over active safety systems.

Category:Nuclear reactors Category:Generation IV reactors