Advanced gas-cooled reactor

The AGR design retained the Magnox's graphite moderator and carbon dioxide coolant but increased the cooling gas operating temperature to improve steam conditions.

However, steel has a higher neutron cross section and this change required the use of enriched uranium fuel to compensate.

When this proved unsuitable due to brittle fracture,[4] the enrichment level of the fuel was raised to allow for the higher neutron capture losses of stainless steel cladding.

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.

This necessitates the use of ultra pure water to minimise the buildup of salts in the evaporator and subsequent corrosion problems.

[9] 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.

For political reasons the CEGB was instructed to spread the 'first generation' orders between three different 'design & build' consortia and a variety of major subcontractors.

The final two AGRs at Torness and Heysham 2 returned to a modified and 'debugged' Hinkley design with much greater seismic margin, and have proved to be the most successful performers of the fleet.

[10] Former Treasury Economic Advisor, David Henderson, described the AGR programme as one of the two most costly British government-sponsored project errors, the other being Concorde.

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.

[12] In 2018 inspections ordered by the ONR at Dungeness B showed that seismic restraints, pipework and storage vessels were "corroded to an unacceptable condition", and that would have been the state when the reactor was operating.

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.

[17] In 2006 AGRs made the news when documents were obtained under the Freedom of Information Act 2000 by The Guardian which claimed that British Energy were unaware of the extent of the cracking of graphite bricks in the cores of their 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.

[19] In February 2012 EDF announced it expected 7-year life extensions on average across all AGRs, including the recently life-extended Heysham 1 and Hartlepool.

[26][27] 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.

[29] On 7 June 2021, EDF announced that Dungeness B, which had been in an extended outage since September 2018, would move into the defuelling phase with immediate effect.

[33] On 3 December 2024 EDF announced an extension to the life of the UK's remaining 4 AGR nuclear power plants.

[34] In January 2025, EDF announced that their "ambition is to generate beyond these dates, subject to plant inspections and regulatory oversight", although they expect "that by the mid-2030s all seven of the AGR power stations will be owned by the UK Government" (for decommissioning).