A gas cooled reactor (in short, GCR) is a nuclear reactor that works with graphite as a neutron moderator and a gas including carbon dioxide or helium in available designs as coolant. Although there are different types of reactor cooled by gas, the terms GCR and, to a lesser extent, gas cooled reactor is used specifically to refer to this reactor type.
The GCR could utilize natural uranium as fuel, and the countries that have developed them were able to produce their fuel without any dependence on other countries to supply enriched uranium. It was available at the time of their development in the 1950s only in the United States or the Soviet Union.
At present, gas cooled reactors account for about three percent of all reactors in commercial operations around the world. All of them are advanced carbon-dioxide gas cooled reactors in the UK that will be phased out by the mid-2020s.
The many Member States are interested in working on advanced High-Temperature Gas Cooled Reactors (HTGRs) that employ helium as a coolant. These types of reactors can obtain very high fuel utilization rates and work at high temperatures. They also generate process heat used in hydrogen production and low-temperature applications, including seawater desalination and district heating.
Gas Cooled Reactor Basics
Gas cooled reactors utilize graphite as a neutron moderator and carbon dioxide as the coolant. With the three percent market share, all are installed in the United Kingdom. These reactors apply natural or somewhat enriched uranium as fuel.
As shown in the following figure, carbon dioxide circulates inside the core, absorbs the heat from the fuel parts, and reaches 650 °C.
It then flows to the heat exchangers located outside of the pressure vessel of the reactor concrete. These are of the gas-to-water heat exchanger types that use the once-through fundamental to boil the flowing water. The water is then applied in the conventional steam cycle.
The once-through boiler operates based on the critical point of water. With an increase in pressure in the Rankine cycle, the saturation temperature corresponding to that pressure increases. Thus, as the pressure increases, the quantity of latent heat needed decreases. At a critical point, no latent heat is required, and therefore, the water directly evaporates into steam. The once-through boiler works at pressures above the critical water point pressure. Therefore, they are also called “supercritical boilers”.
In this design, to penetrate the moderator and control the reaction, boron control rods are utilized. In addition, there may be a secondary shutdown system that involves the injection of nitrogen into the coolant.
However, in the second generation of the gas cooled reactors, the steam generators are installed inside the concrete pressure vessel, which needs a much larger structure and, therefore, more capital costs.
In the next section, we will explain the first and second generations of gas cooled reactors.
The First-Generation GCRs
The first-generation gas cooled reactions were made in the United Kingdom and France in which natural uranium fuel and magnesium or magnesium alloys were used for the cladding. Following plants used low-enriched uranium–oxide fuel with stainless steel cladding. All 15 active GCRs are located in the United Kingdom at the end of 2014.
There were generally two types of first-generation GCRs:
The Magnox Reactors
Magnox is a type of nuclear gas cooled reactor designed to work with natural uranium with graphite as the moderator, and CO2 as the coolant. The name is due to the magnesium-aluminum alloy utilized to clad the fuel rods in the reactor. Like most other first-generation nuclear reactors, the Magnox was designed to produce electrical power and plutonium-239 for the nuclear weapons program in Britain. The name refers especially to the design done in the UK but is sometimes applied generically to any similar reactor.
Like other reactors running to produce plutonium, conserving neutrons is a significant element of the design. In Magnox, the neutrons are moderated in great graphite blocks. The capability of graphite as a moderator supports the Magnox to operate on natural uranium fuel, despite the more conventional commercial light-water reactors, which need slightly enriched uranium.
Graphite can be easily oxidized in the air, so the core cooled with carbon dioxide gas. Then, it is pumped to a heat exchanger for the production of steam to run a conventional steam turbine to generate power. The core is open on one side, so the elements of fuel can be added or eliminated while the reactor is still running.
Totally, only a few dozen Magnox reactors have been built. Most of them were constructed from the 1950s to the 1970s in the UK, and few were exported to other countries. The first reactor of this type was Calder Hall in 1956, which was often considered as the first commercial-scale electricity-producing reactor worldwide. However, the last one in Britain was shut down in 2015. Since 2016, North Korea remained the only operator to run on Magnox reactors in a scientific research center.
The Magnox design is replaced by the advanced gas cooled reactor with a similar cooling system but some changes to improve the economic performance.
The following figure shows a schematic diagram of a Magnox nuclear reactor with the gas flow. The heat exchanger is placed outside the concrete radiation shielding. This figure demonstrates an early design of Magnox with a cylindrical steel pressure vessel.
The UNGG Reactors
The UNGG, abbreviated form of Uranium Naturel Graphite Gaz, is an outdated design of nuclear power reactor developed in France. It was graphite-moderated, cooled by CO2, with natural uranium metal fuel. The first generation of French nuclear power plants was UNGGs. Of the ten units, all ended by the end of 1994, usually due to economic reasons.
In parallel to the British Magnox design, the UNGG reactor was developed independently to meet similar requirements for electric power and plutonium generation simultaneously.
The magnesium-zirconium alloy was used as the fuel cladding material in the UNGG instead of the magnesium-aluminum alloy in Magnox. Since both claddings react with water, they may be stored in a spent fuel pool only shortly. Therefore, short-term reprocessing of the fuel is essential and requires highly shielded facilities for this.
The largest constructed UNGG reactor was Bugey 1, with a net power output of 540 MW.
Difference between Magnox and UNGG Reactors
The major difference between these two types of first-generation of GCRs is in the fuel cladding material. They both were mainly built in their countries of origin, with several export sales: two Magnox plants to Japan and Italy and a UNGG to Spain.
Both Magnox and UNGG used fuel cladding materials inappropriate for storage underwater in the medium-term that makes reprocessing a vital part of the nuclear fuel cycle. Both types were designed and used in their countries of origin to create weapons-grade plutonium, but at the cost of the main interruption to their use for power production despite the preparation of online refueling.
The Second-Generation GCRs
The second-generation reactor is a design class of nuclear reactors referring to the commercial reactors constructed by the end of the 1990s. These are opposed to the first generation reactors, which refer to the early class of power reactors.
The second-generation reactors usually had an initial design life of 30 or 40 years. This period was set so that taken loans for the plant would be paid off. But, many second-generation reactors live up to 50 or 60 years. Also, a second life extension of up to 80 years may be economical in some cases. By 2013 about three-quarters of operating U.S. reactors had been admitted life extension licenses to 60 years.
The exploded Chernobyl’s No.4 reactor was a second-generation reactor.
Advanced Gas Cooled Reactor
The Advanced Gas Cooled Reactor (AGR) is a designed and operated nuclear reactor in the UK. This is the second generation of British gas-cooled reactors, utilizing graphite as the neutron moderator and CO2 as coolant. Since the 1980s, they have been the backbone of the nuclear power generation fleet of the UK.
The AGR was the developed form of the Magnox reactor, the first-generation reactor design in the UK. The first Magnox plant had been run for plutonium production, so it had characteristics that were not the most economical for electricity generation.
The main requirement among these was to operate with natural uranium and the need for a coolant with a low neutron cross-section, here carbon dioxide, and an effective neutron moderator, graphite. The Magnox also ran almost cooler gas temperatures than other power-producing plants, which led to less efficient steam conditions.
The AGR design kept the graphite moderator and carbon dioxide coolant of the Magnox. However, it increased the cooling gas working temperature to enhance steam conditions. They were made the same as those of a coal-fired power plant, providing the same design of applied turbines and generation facilities.
During the early stages of design, it was necessary to turn the beryllium cladding into stainless steel. Neutron cross-section of steel is higher than beryllium, and this shift requires the application of enriched uranium fuel to balance. This change led to a greater burnup of 18,000 MW-days per fuel tonne, requiring less regular refueling.
The AGR prototype was practiced in 1962 at Windscale, but the first commercial AGR did not start working until 1976. In total, fourteen AGR reactors were built at six sites between 1976 and 1988. All of them are configured with two reactors in a building. Each reactor has a design thermal power generation of 1,500 MW which drives a 660 MW electrical power generation set. Different AGR plants generate electrical outputs from 555 MW to 670 MW. However, some of them work at a lower output than design due to operational limitations.
The AGR was designed so that the final conditions of steam at the boiler stop valve were equal to that of conventional coal-fired power plants. Therefore the same design of turbo-generator could be applied.
A schematic diagram of the advanced gas cooled reactor is illustrated below. The heat exchanger is located within the steel-reinforced concrete pressure vessel and radiation shield.
According to the numbers indicated in the figure, the names of the components of this reactor are as follows:
- Charge tubes
- Control rods
- Graphite moderator
- Fuel assemblies
- Concrete pressure vessel and radiation shielding
- Gas circulator
- Water circulator
- Heat exchanger
The hot coolant means temperature exiting the reactor core was designed to be 648°C. To reach these high temperatures, while ensuring useful graphite core life (due to readily oxidation of graphite in CO2 at high temperatures), a re-entrant coolant flow at the lower temperature of boiler outlet of 278 °C is used to cool the graphite. It makes the graphite core temperatures to have not too much different from those of a Magnox station. The outlet temperature and pressure of the super heater were designed to be 170 bar and 543 °C.
The fuel is enriched uranium dioxide pellets, to 2.5-3.5%, in stainless steel tubes. Due to the brittle fracture of the original design concept of beryllium-based cladding, the idea of stainless steel cladding for the higher neutron capture losses was raised. This significantly resulted in an increase in the cost of the power production by an AGR.
The CO2 coolant circulates in the core, which reaches a temperature of 640 °C and a pressure of about 40 bar. Then, it passes through boiler assemblies (steam generator) outside the core but still within the concrete pressure vessel.
The graphite moderator penetrates via control rods, and a secondary system injects nitrogen into the coolant to take thermal neutrons to prevent the fission process when the control rods cannot enter the core. A shutdown system operating by injecting boron beads into the reactor is applied if the reactor has to be depressurized with inadequate control rods reduced. This means that the pressure of nitrogen cannot be maintained.
The advanced gas cooled reactors were designed to provide high thermal efficiencies (defined as the ratio of electricity generated to the heat generated) of about 41%. This value is better than modern Pressurized Water Reactors (PWRs), with a typical thermal efficiency of about 34%. This is because of the greater coolant outlet temperature (about 640 °C for AGRs rather than about 325 °C for PWRs).
However, the reactor core must be larger in size for the same output. Also, at discharge, the fuel burnup ratio is lower. Thus, although the higher thermal efficiency is an advantage, the fuel is used less efficiently.
The Very High Temperature Reactor (VHTR) is a type of high-temperature gas cooled reactors (HTGRs) that, in concept, can reach high outlet temperatures, up to 1000 °C.
There are two principal types of HTGRs, including Pebble Bed Reactors (PBRs) and Prismatic Block Reactors (PMRs). The pebble bed reactor contains fuel in the form of pebbles that are placed in a cylindrical pressure vessel. The prismatic block reactor has a configuration of prismatic block core, in which hexagonal graphite blocks are placed in a cylindrical pressure vessel. Both reactors may put the fuel in an annulus section with a graphite center spire, according to the design and desired power of the reactor.
Advantages of Gas Cooled Reactors over Water Cooled Reactors
Gas cooled reactors present potential operational and safety advantages over water cooled reactors. A principal operational motivation for working on this technology is enhanced energy conversion efficiency provided by a higher reactor operating temperature. For example, water cooled reactors have a possible maximum temperature limit of around 350°C, which allows a conversion efficiency (the ratio of output electricity to heat) of about 32–34%.
In comparison, a GCR can run at temperatures up to 800–850°C and yield a heat-to-electricity conversion efficiency of more than 40% using conventional steam turbine facilities or as high as 50% using a more advanced gas turbine apparatus.
From the safety perspective, gas cooled reactors usually employ lower core power density and higher heat capacity core, supporting the limited fuel temperatures after a loss-of-coolant accident.
Since they use various forms of fuel and cladding, they avoid chemical reactions of steam/zirconium cladding which can release explosive hydrogen gas under unintended conditions in light water reactors. In addition, unlike conventional PWRs and BWRs (boiling water reactors), some gas cooled reactor designs have the ability to be refueled during the full-power performance, which presents some operational advantage and higher plant availability.
Some gas cooled reactors have been constructed and operated using either carbon dioxide gas or helium for the reactor core cooling. The application of graphite neutron moderators in commercial gas cooled reactors leads to the absorption of fewer neutrons compared to water-moderated reactors.