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Schematic diagram of the Advanced Gas-cooled Reactor.
Note that the heat exchanger is contained within the steel-reinforced concrete combined pressure vessel and radiation shield.

Charge tubes

Control rods

Graphite moderator

Fuel assemblies

Concrete pressure vessel and radiation shielding

Gas circulator


Water circulator

Heat exchanger


An advanced gas-cooled reactor (AGR) is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant. The AGR was developed from the Magnox reactor, operating at a higher gas temperature for improved thermal efficiency, requiring stainless steel fuel cladding to withstand the higher temperature. Because the stainless steel fuel cladding has a higher neutron capture cross section than Magnox fuel cans, enriched uranium fuel is needed, with the benefit of higher "burn ups" of 18,000 MWt-days per tonne of fuel, requiring less frequent refueling. The first prototype AGR became operational in 1962 but the first commercial AGR did not come on line until 1976.

All AGR power stations are configured with two reactors in a single building. Each reactor has a design thermal power output of 1,500 MWt driving a 660 MWe turbine-alternator set. Because of operational restrictions, the various AGR stations produce outputs in the range 555 MWe to 625 MWe. [22075]

AGR design

The design of the AGR was such that the final steam conditions at the boiler stop valve were identical to that of conventional coal fired power stations, thus the same design of turbo-generator plant could be used. The mean temperature of the hot coolant leaving the reactor core was designed to be 648°C. In order to obtain these high temperatures, yet ensure useful graphite core life (graphite oxidises readily in CO2 at high temperature) a re-entrant flow of coolant at the lower boiler outlet temperature of 278°C is utilised to cool the graphite, ensuring that the graphite core temperatures do not vary too much from those seen in a Magnox station. The superheater outlet temperature and pressure were designed to be 2,485 psia and 543°C.

The fuel is uranium dioxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The original design concept of the AGR was to use a beryllium based cladding. When this proved unsuitable, the enrichment level of the fuel was raised to allow for the higher neutron capture losses of stainless steel cladding. This significantly increased the cost of the power produced by an AGR. The carbon dioxide coolant circulates through the core, reaching 640°C (1,184°F)and a pressure of around 40 bar (580 psi), and then passes through boiler (steam generator) assemblies outside the core but still within the steel lined, reinforced concrete pressure vessel. Control rods penetrate the graphite moderator and a secondary shutdown system involves injecting nitrogen into the coolant. A tertiary shutdown system operates by injecting boron balls into the reactor.

The AGR has a good thermal efficiency (electricity generated/heat generated ratio) of about 41%, which is better than modern pressurized water reactors which have a typical thermal efficiency of 34% . This is largely due to the higher coolant outlet temperature of about 640 °C (1,184°F) practical with gas cooling, compared to about 325 °C (617°F) for PWRs.However the reactor core has to be larger for the same power output, and the fuel burnup ratio at discharge is lower so the fuel is used less efficiently, countering the thermal efficiency advantage[22076].

Like the Magnox, CANDU and RBMK reactors, and in contrast to the light water reactors, AGRs are designed to be refuelled without being shut down first. However fuel assembly vibration problems arose during on-load refuelling at full power, so in 1988 full power refuelling was suspended until the mid-1990s, when further trials led to a fuel rod becoming stuck in a reactor core. Only refuelling at part load or when shut down is now undertaken at AGRs. [22077]

The small-scale prototype AGR at the Sellafieldmarker (Windscale) site is in the process of being decommissioned. This project is also a study of what is required to decommission a nuclear reactor safely.

Current AGR reactors

The two power stations with four AGRs at Heysham
AGR power station at Torness

Currently there are seven nuclear generating stations each with two operating AGRs in the United Kingdommarker, owned and operated by British Energy:

AGR Power Station MWe Construction started Connected to grid Commercial operation Accounting closure date
Dungeness Bmarker 1110 1965 1983 1985 2018
Hartlepoolmarker 1210 1968 1983 1989 2014
Heysham 1marker 1150 1970 1983 1989 2014
Heysham 2marker 1250 1980 1988 1989 2023
Hinkley Point Bmarker 1220 1967 1976 1976 2016
Hunterston Bmarker 1190 1967 1976 1976 2016
Tornessmarker 1250 1980 1988 1988 2023

In 2005 British Energy announced a 10-year life extension at Dungeness B, that will see the station continue operating until 2018, and in 2007 announced a 5-year life extension of Hinkley Point B and Hunterston B until 2016. Life extensions at other AGRs will be considered at least three years before their scheduled closure dates.

Since 2006 Hinkley Point B and Hunterston B have been restricted to about 70% of normal MWe output because of boiler-related problems requiring that they operate at reduced boiler temperatures. This output restriction is likely to remain until closure.

In 2006 AGRs made the news when documents were obtained under the Freedom of Information Act 2000 by The Guardian who claimed that British Energy were unaware of the extent of the cracking of graphite bricks in the cores of their reactors. It was also claimed that British Energy did not know why the cracking had occurred and that they were unable to monitor the cores without first shutting down the reactors. British Energy later issued a statement confirming that cracking of graphite bricks is a known symptom of extensive neutron bombardment and that they were working on a solution to the monitoring problem. Also, they stated that the reactors were examined every three years as part of "statutory outages". [22078]

See also


  1. History of Windscale's Advanced Gas-cooled Reactor, Sellafield Ltd.

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