A gas-cooled reactor (GCR) is a

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

that uses

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...

as a

neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely m ...

and a gas (

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

in extant designs) as coolant. Although there are many other types of reactor cooled by gas, the terms ''GCR'' and to a lesser extent ''gas cooled reactor'' are particularly used to refer to this type of reactor. The GCR was able to use

natural uranium Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes ...

as fuel, enabling the countries that developed them to fabricate their own fuel without relying on other countries for supplies of

enriched uranium Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...

, which was at the time of their development in the 1950s only available from the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

or the

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...

. The Canadian

CANDU The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...

reactor, using heavy water as a moderator, was designed with the same goal of using natural uranium fuel for similar reasons.

Design considerations

Historically thermal spectrum graphite moderated gas cooled reactors mostly competed with

light water reactor The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...

s, ultimately losing out to them after having seen some deployment in Britain and France.

Heavy water reactor A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water ( deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The ...

s share some design considerations as both are capable in principle of using unenriched fuel but require online refueling to be viable power reactors.

Advantages

* No void coefficient of reactivity as the coolant is a gas at room temperature and remains gaseous at operating temperature * Able to use natural ( unenriched) uranium as carbon has a lower neutron absorption cross section than light water * High coolant outlet temperature can be achieved, increasing

Carnot efficiency A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...

* Lower pressure than in a

Pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) i ...

* Magnox reactors were designed to be

dual use In politics, diplomacy and export control, dual-use items refers to goods, software and technology that can be used for both civilian and military applications.

producing both power and weapons grade plutonium later designs instead bred reactor grade plutonium * Lower danger of

hydrogen explosion Hydrogen safety covers the safe production, handling and use of hydrogen, particularly hydrogen gas fuel and liquid hydrogen. Hydrogen possesses the NFPA 704's highest rating of 4 on the flammability scale because it is flammable when mixed even ...

as no water is present * High coolant outlet temperature allows better use for process heat if desired * Adding normal (light) water - e.g. as emergency coolant -

scram A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reacto ...

s the reaction allowing better safety in dealing with unforeseen accidents

Disadvantages

* Bulky due to lower

energy density In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or . Often only the ''useful'' or extrac ...

of natural uranium compared to enriched fuel and lower moderating effect of carbon compared to water * Magnox fuel cladding cannot be stored for long times in a

spent fuel pool Spent fuel pools (SFP) are storage pools (or "ponds" in the United Kingdom) for spent fuel from nuclear reactors. They are typically 40 or more feet (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks designed to hold ...

making

nuclear reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, th ...

mandatory * Boudouard reaction between graphite moderator and CO₂ coolant can produce explosive and poisonous

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

* A loss of coolant accident, unlike in a water moderated reactor, does not by itself cause a

* Graphite is flammable and is exposed to high temperatures in operation - a graphite fire is a possible accident scenario *

nuclear graphite Nuclear graphite is any grade of graphite, usually synthetic graphite, manufactured for use as a moderator or reflector within a nuclear reactor. Graphite is an important material for the construction of both historical and modern nuclear reactor ...

is more expensive than light water but less expensive than heavy water

Generation I GCR

There were two main types of generation I GCR: * The

Magnox Magnox is a type of nuclear power/production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The n ...

reactors developed by the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...

. * The UNGG reactors developed by

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

. The main difference between these two types is in the fuel cladding material. Both types were mainly constructed in their countries of origin, with a few export sales: two Magnox plants to

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

and

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...

, and one UNGG to

Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = '' Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , ...

. More recently, GCRs based on the declassified drawings of the early Magnox reactors have been constructed by

North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia. It constitutes the northern half of the Korean Peninsula and shares borders with China and Russia to the north, at the Yalu (Amnok) and T ...

at the

Yongbyon Nuclear Scientific Research Center The Nyongbyon Nuclear Scientific Research Center is North Korea's major nuclear facility, operating its first nuclear reactors. It is located in Nyongbyon County in North Pyongan Province, about 100 km north of Pyongyang. The center produ ...

. Both types used fuel cladding materials that were unsuitable for medium term storage under water, making reprocessing an essential part of the

nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in w ...

. Both types were, in their countries of origin, also designed and used to produce

weapons-grade plutonium Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...

, but at the cost of major interruption to their use for power generation despite the provision of online refuelling.

Generation II GCR

In the UK, the Magnox was replaced by the advanced gas-cooled reactor (AGR), an improved Generation II gas cooled reactor. In France, the UNGG was replaced by the

pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) i ...

(PWR).

Types

Gas-cooled reactor types include: * Gas-cooled reactor (graphite moderated, CO₂ cooled) **

(British design, 28 built, 1956-2015) ** UNGG reactor (French design, 10 built, 1956-1994) ** Advanced Gas-cooled Reactor (Magnox successor, 15 built, 1962-today) * Heavy water gas cooled reactor (heavy water moderated, CO₂ cooled) **

Brennilis Nuclear Power Plant The Brennilis Nuclear Power Plant (EL-4) is a decommissioned site located in the Monts d'Arrée in the commune of Brennilis in Finistère, France. History The Commissariat à l'énergie atomique began construction of this experimental react ...

(1967-1985) **

KS 150 KS 150 is a Gas Cooled Reactor using Heavy Water as a moderator (GCHWR) nuclear reactor design. A single example, A-1, was constructed at the Bohunice Nuclear Power Plant in Jaslovské Bohunice, Czechoslovakia. The power plant suffered a series ...

(1972-1979) * High & Very-high temperature reactor (graphite moderated, Helium cooled) ** Prismatic block reactor *** Dragon reactor (1964-1975) *** Peach Bottom Atomic Power Station (1967-1974) *** Fort Saint Vrain Generating Station (1979-1989) *** High-temperature engineering test reactor (1999-today) ***

Gas Turbine Modular Helium Reactor The Gas Turbine Modular Helium Reactor (GT-MHR) is a class of nuclear fission power reactor designed that was under development by a group of Russian enterprises ( OKBM Afrikantov, Kurchatov Institute, VNIINM and others), an American group headed ...

(

General Atomics General Atomics is an American energy and defense corporation headquartered in San Diego, California, specializing in research and technology development. This includes physics research in support of nuclear fission and nuclear fusion energy. Th ...

design) *** Steam Cycle High-Temperature Gas-Cooled Reactor (

Areva Areva S.A. is a French multinational group specializing in nuclear power headquartered in Courbevoie, France. Before its 2016 corporate restructuring, Areva was majority-owned by the French state through the French Alternative Energies and Atom ...

SMR design) ** Pebble bed reactor ***

AVR reactor The AVR reactor (german: Arbeitsgemeinschaft Versuchsreaktor) was a prototype pebble-bed reactor, located immediately adjacent to Jülich Research Centre in West Germany, constructed in 1960, grid connected in 1967 and shut down in 1988. It was a ...

(1966-1988) ***

THTR-300 The THTR-300 was a thorium cycle high-temperature nuclear reactor rated at 300 MW electric (THTR-300) in Hamm-Uentrop, Germany. It started operating in 1983, synchronized with the grid in 1985, operated at full power in February 1987 and was s ...

(1983-1989) ***

HTR-10 HTR-10 is a 10 MWt prototype pebble bed reactor at Tsinghua University in China. Construction began in 1995, achieving its first criticality in December 2000, and was operated in full power condition in January 2003. Two HTR-PM reactors, scaled ...