Gas-Cooled Reactors
Abstract
Gas-cooled reactors formed an early alternative to the more common water-cooled reactors. They are particularly attractive in situations where enriched uranium is not easily available since they can be designed to maintain a sustained nuclear reaction with natural uranium. Reactors of this design include the UK Magnox reactor and its successor the advanced gas-cooled reactor. Both utilized a graphite moderator and carbon dioxide as the coolant. A French design, the UNGG reactor, was similar to the Magnox reactor. This type of reactor can operate at a higher temperature than water-cooled reactors, allowing high efficiency operation. However the designs often proved expensive to build and prone to problems. Some modern advanced high-temperature reactor designs also use gas cooling.
Keywords
Gas-cooled reactor; Magnox reactor; advanced gas-cooled reactor; UNGG reactor; graphite moderator; carbon dioxide coolant; heavy water gas-cooled reactor
The gas-cooled reactor forms a second branch of early nuclear power reactor design. Like the heavy water PWR the aim of this reactor design was to be able to utilize natural, unenriched uranium as fuel. After the Second World War, uranium enrichment technology in the west was virtually all in the hands of the United States. As a consequence other countries looked at ways of developing nuclear reactors that could operate without enriched uranium. The result, in France and in the United Kingdom, was generations of gas-cooled reactors with graphite moderators. These all used carbon dioxide as the coolant gas.
Development in the United Kingdom led to a first generation of gas-cooled reactors called Magnox reactors. These were later superseded by a new design called the advanced gas-cooled reactor (AGR). In France a series of independently designed reactors of a similar type were designed. As in the United Kingdom a second generation was explored but France decided instead to opt for PWR technology.
There were also experiments in the United States with a gas-cooled reactor called the ultrahigh-temperature reactor experiment (UHTREX). This experimental reactor operated from 1959 until 1971 and was part of a program to see if there were advantages to using unclad nuclear fuel instead of fuel contained inside fuel rods. The core of this reactor was constructed of graphite which also acted as the moderator while the coolant was helium. The reactor had a rating of 3 MWth and operated up to 1316°C, a much higher temperature that any commercial reactor. However the design was finally abandoned and development in the United States was stopped.
Russia also experimented with a gas-cooled reactor called the KS 150. A single 143 MW unit was constructed in Czechoslovakia and commissioned in 1972. The reactor was novel in using heavy water as the moderator and carbon dioxide as the coolant. The unit suffered a series of accidents and was decommissioned in 1979.
The Magnox Reactor
The Magnox reactor was a nuclear technology developed in the United Kingdom for both weapons manufacture and power production. The first reactor of this type was built at Calder Hall in Cumbria. It was a dual purpose reactor with a generating capacity of 50 MW and is now considered the first commercial nuclear power station in the world although the power was essentially a by-product of the plutonium production process.
The reactor used carbon dioxide as its coolant and graphite as the moderator. Fuel was natural, unenriched uranium metal which was loaded into magnesium-aluminum alloy fuel rods from which the name Magnox is derived. The control rods were made from boron steel. The carbon dioxide was pressurized to around 20 atm and the hot gas exited the core at a temperature of 360°C. A diagram of a Magnox reactor is shown in Fig. 5.1. Overall efficiency of the Magnox reactor was relatively low at around 18% although the steam cycle efficiency was higher at 31%. The use of unenriched uranium meant that the cores had to be refueled while online in order to maintain the nuclear reaction.
Although a series of Magnox reactors were built, the design was never standardized and each new plant had slight modifications compared to those that had gone previously. This meant that each unit was essentially a prototype, making them costly to build since no standard design could be rolled out. Early units had steel pressure vessels, while some later ones had pre-stressed concrete vessels with steel liners. Shapes varied too; some of the vessels were spherical, others were cylindrical. The earliest plants were all 50 MW in capacity but size later increased so that the final Magnox plant, which began operating at Wylfa in Wales in 1971, had units with a capacity of 490 MW. In total 26 Magnox reactors were built at 11 power plants in the United Kingdom. One was exported to Japan, and another to Italy. Ironically, North Korea also developed a Magnox reactor based on the UK design after the latter had been made public at an Atoms for Peace conference.
The layout of the early Magnox reactors kept the reactor alone inside the pressure vessel while the primary coolant circuit ran through a heat exchanger and steam generator outside the vessel. In later designs the heat exchanger and steam generator were inside the pressure vessel containing the core. Here the hot carbon dioxide was used to heat water and raise steam to drive the plant steam turbine. There was no containment as found in modern nuclear power plants because the design was at the time considered inherently safe. This proved to be an overconfident assessment when there was a fuel melt-down in a channel of the core at a plant in 1967, an accident which led to the release of radiation. In addition the steel vessels in the earlier plants of this design let significant amounts of neutron and gamma radiation pass, creating a hazard for local people.
One of the major limiting factors with the Magnox plants was the magnesium–aluminum alloy. This alloy was used because it had a low level of neutron capture but it could not operate at a very high temperature, limiting the overall thermodynamic efficiency that could be achieved. In addition the cladding reacted with water so that long-term storage in fuel-cooling ponds was not possible and the fuel had to be reprocessed. The last Magnox reactor in the United Kingdom closed in 2015.
The UNGG Reactor
The French réacteur nucléaire à l’Uranium Naturel Graphite Gaz (UNGG)—nuclear reactor with natural uranium, graphite and gas—was a French reactor design that was developed in parallel to the UK’s Magnox reactor programme. As with the latter, the nuclear design used unenriched uranium as fuel, with a graphite moderator and carbon dioxide gas coolant. And as in the United Kingdom, these plants were able to produce plutonium for weapons as well as power.
In the initial UNGG design the nuclear fuel was distributed within the graphite core in the form of slugs of a uranium alloy. Later a design using fuel rods was developed. Fuel rods were loaded horizontally into the core at first, but vertical fuel rods were introduced for later designs. The fuel rod cladding was made from a zirconium–magnesium alloy which had the same flaw as the UK magnesium–aluminum alloy, the material reacted with water.
The cores of the first plants were enclosed in steel vessels. However a concrete containment, several meters thick, was later adopted. In some plants the carbon dioxide circuit and gas/water heat exchanger was within this containment, in others only the core was inside and the heat exchanger was external.
Nine reactors of this type were built in France between 1950 and 1960. The first three were built by the French Atomic Energy Commission. Following that, six further reactors were built by Eléctricité de France (EDF). One UNGG was also built in Spain and another in Israel. As with the UK programme, the design evolved from unit to unit. The largest of them had a generating capacity of 540 MW.
The construction of the French reactor fleet was part of a strategy of the French government at the time, led by General de Gaulle, to become energy independent. However, in 1969, EDF proposed adopting an American PWR design for future nuclear reactors and this became official French policy at the end of that year. The first three of these reactors were shut down between 1968 and 1987. The other six have also been withdrawn from service and are being decommissioned.
The French experience highlighted a number of limitations of this particular design. Carbon dioxide could cause steel corrosion at high temperatures. Meanwhile the graphite core moderator also presented problems depending on the temperature at which it was operated. In this case a higher temperature was better. Finally the steam generators in these plants had shorter lifetimes than the reactors themselves and so needed to be replaced during the plant lifetime.
The Advanced Gas-Cooled Reactor
The AGR was a second generation of graphite moderated, carbon dioxide-cooled reactor developed in the United Kingdom. The Magnox reactors had not proved very efficient and in order to improve efficiency, a new design was developed which operated at higher temperatures and pressures and with a higher power density within the core in order to reduce capital costs and improve the economics. To achieve this the Magnox fuel rod cladding was changed to stainless steel and the uranium metal fuel was changed to uranium dioxide. The fuel change then required that the uranium be enriched and an enrichment level of 2.3% was used.
The design broadly followed that of the former Magnox reactor with a graphite moderator. As with the later reactors of the Magnox design, the steam generator was placed within the concrete containment of the reactor. Fuel rods and control rods were inserted from above as shown in Fig. 5.2. The design also allowed for refueling to be carried out while the reactor was in service.
The operating pressure of the coolant gas was 45 atm and the temperature of the carbon dioxide entering the hottest section of the steam generator was 619°C, much hotter than in the Magnox reactor. This allowed steam to be produced at a pressure of 165 atm and a temperature of 538°C at the inlet to the high-pressure steam generator. The hotter, higher pressure steam meant that the steam cycle could operate at 42% efficiency and this was one of the principal design aims of the AGR. These steam conditions were chosen to be similar to conventional coal-fired power plants in use at the time so that the same steam turbine generators could be used in both.
Uniquely among nuclear power plants, the AGR design included two reactors providing steam to a single set of steam turbines. Each reactor had a core with 332 fuel rod channels and 89 control rod channels. The core contained around 117 tonnes of uranium. The gross power output of each reactor was 660 MW for a total generating capacity for each plant of 1320 MW.
The UK government hoped that the AGR design would prove a competitor for the US PWR and BWR reactors. Five twin reactor power plants were ordered in quick succession but the design proved complex. The first plant at Dungeness was ordered in 1965 but did not enter service until 1983, 18 years later. The design was modified for subsequent plants and these entered service earlier than the first. Two further plants were ordered, so that a total of 7 plants, and 14 reactors, eventually entered service. All continue to operate.
In spite of their relative success the plant turned out to be costly and no orders for plants outside the United Kingdom were taken. When the United Kingdom decided to built its next nuclear power plant, the AGR design was abandoned in favor of a US-designed PWR. This was commissioned in 1995 after a 7-year construction programme.
Heavy Water Gas-Cooled Reactors
In 1962 the Atomic Energy Commission of France began construction of an experimental heavy water moderated gas-cooled reactor (HWGCR). The plant was seen as a prototype for a replacement for the UNGG. The prototype was built at Brennilis in Brittany and entered service in 1967. It had a generating capacity of 70 MW and used water from the local river Ellez for condenser cooling.
The replacement of graphite moderation with heavy water was intended to overcome the limitations of the graphite while allowing a gas-cooled reactor to operate with unenriched uranium. However the French government’s decision in 1969 to adopt the US PWR design as the basis for future French reactors ended research into a new generation of gas-cooled reactors in France. The plant continued to operate until 1985 when it was finally closed and it is now in the process of being decommissioned.
Russia also experimented with an HWGCR, a design called KS 150. Work on the design began in the 1950s and construction, in Czechoslovakia, commenced in 1958. However the construction took 16 years and the plant only entered service in 1972. The prototype power station used uranium metal as fuel, and this was loaded into fuel rods made of a magnesium beryllium alloy. The core was a carbon steel cylinder that contained the heavy water, with channels in it for the fuel rods to pass through. Carbon dioxide could circulate through these fuel rods to carry away the heat generated. Operating pressure was around 54 atm and the carbon dioxide coolant reached 426°C. The overall efficiency of the plant was estimated to be 18.5% and it had a generating capacity of 143 MW.
The plant suffered a major accident during refuelling in 1977. This was the culmination of a series of problems and the plant was decommissioned in 1979 and a second unit of the same type canceled.