The Advanced Gas-cooled Reactor (AGR) is a type of nuclear reactor designed and operated in the United Kingdom. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant. They have been the backbone of the UK's nuclear generation fleet since the 1980s.
The AGR was developed from the Magnox reactor, the UK's first-generation reactor design. The first Magnox reactors had been optimised for generating plutonium, and for this reason it incorporated a number of features that are not necessarily the best for economic performance. Primary among these requirements was the Magnox's ability to run on natural uranium, which demanded the use of a coolant with a low neutron cross section, in this case CO2, and efficient neutron moderator, graphite. Magnox also ran relatively cool compared to other power-producing designs, which made it less efficient extracting power from the reactor core.
The AGR retained the Magnox's graphite moderator and CO2 coolant but increased its operating temperature to improve efficiency when converted to steam. The steam it produced was deliberately identical to that from a coal fired plant, allowing the same turbines and generation equipment to be used. During the initial design stages the system was forced to switch the tubing enclosing the fuel pellets from beryllium to stainless steel. Steel has a higher cross section and this change demanded the switch to enriched uranium fuel to maintain criticality. As part of this change, the new design had higher burnup of 18,000 MWt-days per tonne of fuel, requiring less frequent refuelling.
The first prototype AGR became operational in 1962, but the first commercial AGR did not come online until 1976. A total of fourteen AGR reactors at six sites were built between 1976 and 1988. All of these 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. The various AGR stations produce outputs in the range 555 MWe to 670 MWe though some run at lower than design output due to operational restrictions.
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 psi (170 bar) 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 due to brittle fracture, 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 system involves injecting nitrogen into the coolant to hold the reactor temperature down. A tertiary shutdown system which operates by injecting boron balls into the reactor is included in case the reactor has to be depressurized with insufficient control rods lowered. This would mean that nitrogen pressure can not be maintained. 
The AGR was designed to have a high 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 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.
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 (see Online refuelling). This on-load refuelling was an important part of the economic case for choosing the AGR over other reactor types, and in 1965 allowed the Central Electricity Generating Board (CEGB) and the government to claim that the AGR would produce electricity cheaper than the best coal-fired power stations. 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.
The AGR was intended to be a superior British alternative to American light water reactor designs. It was promoted as a development of the operationally (if not economically) successful Magnox design, and was chosen from a multitude of competing British alternatives - the helium cooled High Temperature Reactor, the Steam Generating Heavy Water Reactor and the Fast Breeder Reactor - as well as the American light water pressurised and boiling water reactors (PWR and BWR) and Canadian CANDU designs. The CEGB conducted a detailed economic appraisal of the competing designs and concluded that the AGR proposed for Dungeness B would generate the cheapest electricity, cheaper than any of the rival designs and the best coal-fired stations.
There were great hopes for the AGR design. An ambitious construction programme of five twin reactor stations, Dungeness B, Hinkley Point B, Hunterston B, Hartlepool and Heysham was quickly rolled out, and export orders were eagerly anticipated. However, the AGR design proved to be over complex and difficult to construct on site. Notoriously bad labour relations at the time added to the problems. The lead station, Dungeness B was ordered in 1965 with a target completion date of 1970. After problems with nearly every aspect of the reactor design it finally began generating electricity in 1983, 13 years late. The following reactor designs at Hinkley Point and Hunterston significantly improved on the original design and indeed were commissioned ahead of Dungeness. The next AGR design at Heysham 1 and Hartlepool sought to reduce overall cost of design by reducing the footprint of the station and the number of ancillary systems. The final two AGRs at Torness and Heysham 2 returned to a modified Hinkley design and have proved to be the most successful performers of the fleet. Former Treasury Economic Advisor, David Henderson, described the AGR programme as one of the two most costly British government-sponsored project errors, alongside Concorde.
When the government started on privatising the electricity generation industry in the 1980s, a cost analysis for potential investors revealed that true operating costs had been obscured for many years. Decommissioning costs especially had been significantly underestimated. These uncertainties caused nuclear power to be omitted from the privatisation at that time.
The small-scale prototype AGR at Sellafield (Windscale) has been decommissioned as of 2010 – the core and pressure vessel decommissioned leaving only the building "Golf Ball" visible. This project was also a study of what is required to decommission a nuclear reactor safely.
In October 2016 it was announced that super-articulated control rods would be installed at Hunterston B and Hinkley Point B because of concerns about the stability of the reactors' graphite cores. The Office for Nuclear Regulation (ONR) had raised concerns over the number of fractures in keyways that lock together the graphite bricks in the core. An unusual event, such as an earthquake, might destabilise the graphite so that ordinary control rods that shut the reactor down could not be inserted. Super-articulated control rods should be insertable even into a destabilised core.
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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.
From 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. In 2013 these two stations' power increased to about 80% of normal output following some plant modifications.
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".
On 17 December 2010, EDF Energy announced a 5-year life extension for both Heysham 1 and Hartlepool to enable further generation until 2019.
In February 2012 EDF announced it expects 7 year life extensions on average across all AGRs, including the recently life-extended Heysham 1 and Hartlepool. These life extensions are subject to detailed review and approval, and are not included in the table above.
On 4 December 2012 EDF announced that Hinkley Point B and Hunterston B had been given 7 year life extensions, from 2016 to 2023.
On 5 November 2013 EDF announced that Hartlepool had been given a 5-year life extension, from 2019 to 2024.
In 2013 a defect was found by a regular inspection in one of the eight pod boilers of Heysham reactor A1. The reactor resumed operation at a lower output level with this pod boiler disabled, until June 2014 when more detailed inspections confirmed a crack in the boiler spine. As a precaution Heysham A2 and the sister Hartlepool station were also closed down for an eight weeks' inspection.
In October 2014 a new kind of crack in the graphite moderator bricks was found at the Hunterston B reactor. This keyway root crack has been previously theorized but not observed. The existence of this type of crack does not immediately affect the safety of a reactor – however if the number of cracks exceed a threshold the reactor would be decommissioned, as the cracks cannot be repaired.
In January 2015 Dungeness B was given a ten-year life extension, with an upgrade to control room computer systems and improved flood defences, taking the accounting closure date to 2028.
In February 2016, EDF extended the life of four of its eight nuclear power plants in the UK. Heysham 1 and Hartlepool will have their life extended by five years until 2024, while Heysham 2 and Torness will see their closure dates pushed back by seven years to 2030.