A gas-cooled reactor or GCR is a type of nuclear thermal nuclear fission reactor.
The neutron moderator of the GCR reactor is graphite. The coolant in the gas nuclear reactor technology is carbon dioxide in the gaseous state.
According to the classification made by the International Atomic Energy Agency of the United Nations (IAEA), this type of reactors includes the advanced gas type or AGR and Magnox (from Magnesium Non-OXidising), both from British technology.
Within the history of nuclear energy, there was also a French GCR type called UNGG (from the French Uranium Naturel Graphite Gaz). Still, it is an obsolete type, and of which there is currently no operational nuclear power plant in the world. They were the first generation of nuclear reactors in France, developed in the wake of the Second World War.
The essential differences between the Advanced Gas Reactor (AGR), Magnox, and UNGG models are their fuel and the coating around the pellets.
The Magnox and the UNGG were developed simultaneously; they are the oldest and quite similar. Both reactors use natural uranium as nuclear fuel and differ in that the Magnox surrounds the pellets with an alloy of magnesium and aluminum. At the same time, the UNGG did it with one of magnesium and zirconium.
The new generation of GCRs, ACRs (or advanced gas-cooled reactors), use enriched uranium in its fuel cycle.
Magnox Nuclear Reactor
Magnox is a series of nuclear reactors with research and development made in the UK. As the nuclear fuel used is natural metallic uranium, the coolant's role carries carbon dioxide as moderator graphite.
The name "magnox" coincides with the name of the magnesium-aluminum alloy brand used in these reactors to make cladding for fuel cells.
Like most first-generation reactors, Magnox is a dual-purpose reactor designed for both plutonium-239 production and electricity generation. As in other plutonium-producing reactors, an important feature is the weak absorption of neutrons by the core materials.
The efficiency of the graphite moderator makes it possible to operate on natural uranium fuel without the need for enrichment.
Graphite is easily oxidized in the air; therefore, CO2 is used as a heat carrier. Heat transfer from the first circuit to the second is carried out in steam generators, and the resulting steam drives a conventional turbine to generate electricity. The design of the reactor allows refueling on the fly.
The dual-purpose feature of the Magnox reactors has allowed the UK to create significant stockpiles of reactor plutonium by reprocessing spent nuclear fuel at the B205 plant.
Despite the modernization aimed at increasing electricity production efficiency, after plutonium production faded into the background, Magnox reactors have not been equal to pressurized water reactors in terms of fuel efficiency due to their design features and operation on unenriched uranium.
Only a small number of reactors of this type have been built in the UK. Besides, fewer of them have been exported to other countries. The first reactor was built at Calder Hall in 1956 and is often regarded as "the first commercial power reactor in the world,” while the last one in the UK was closed Wilf in 2015.
For 2016, North Korea remains the only country to operate Magnox reactors at the Yongbyon Nuclear Research Center. Further development of gas-graphite reactors was improved gas-cooled reactors having the same coolant, but with several changes that increase economic performance.
Advanced Gas-cooled Reactor
Advanced gas-cooled reactors (AGR) is the second generation of British gas-cooled nuclear reactors, using graphite as a neutron moderator and carbon dioxide as a coolant. The Magnox reactors were the nuclear engineering base of the AGR.
AGR retained the Magnox graphite retarder and CO2 to cool the reactor core but increased its operating outlet temperature to improve efficiency when converting to steam. The steam he produced was deliberately identical to that generated in coal-fired power plants, allowing the same turbines and equipment to be used for generation.
At the initial stages of the system design, the designers were forced to replace the beryllium used to contain uranium fuel cells with stainless steel. Steel has a higher nuclear cross-section, and this change entailed a change in fuel from natural uranium to enriched uranium fuel to maintain criticality.
As part of this change, the new project had a higher burn-up rate of 18,000 MW / d per ton of fuel, requiring less frequent refueling.
The first AGR prototype was launched in 1963, but the first only commercial reactor in 1976. A total of 14 reactors were built at six sites from 1976 to 1988. They are all configured with two reactors in the same building. Each reactor has a design thermal power of 1,500 MW, driving a 660 MW turbine generator.
Various AGR stations produce outputs ranging from 555 MW to 670 MW, some of which operate below design capacity due to operational constraints. They all use Westinghouse fuel.
Gas Turbine - Modular Helium Reactor (GT-MHR)
A gas turbine, modular helium reactor (GT-MGR, GT-MHR) is an international project to create a nuclear power plant that meets the safety requirements of the 21st century. These requirements are based on a high-temperature gas-cooled reactor with a helium coolant operating in a natural gas turbine cycle.
The creation of two reactors of this type, along with the BN-600 and BN-800 fast reactors, is included in the Russian-American program to dispose of weapons-grade plutonium, which is not necessary for defense purposes.
The project is funded on a parity basis by Rosatom (RF) and the Department of Energy and NNSA (USA).
The Afrikantov OKBM, RSC KI, VNIINM, General Atomics (USA), Framatome (France), Fuji Electric (Japan) are participating in the project.
Disadvantages of the Gas-cooled Nuclear Reactor
The heat capacity and thermal conductivity of the refrigerant gas is low. Obtaining the necessary thermal energy is ensured by increasing the gas pressure.
However, there is also a problem that the reactor inevitably becomes large due to the small thermal power density compared to the light water reactor.
With the Magnox reactor as a prototype, many gas-cooled power generation reactors were commissioned.
Since the excess reactivity is initially small, it is difficult to burn the fuel efficiently in the Magnox nuclear reactor. It is necessary to replace nuclear fuel frequently.