Advanced gas-cooled reactor Totally Explained

Everything about Advanced Gas-cooled Reactor totally explained

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 efficiency, and using enriched uranium fuel so requiring less frequent refueling. The first prototype AGR became operational in 1962 but the first commercial AGR didn't come on line until 1976.
All AGR power stations are configured with two reactors, each reactor with a power output of between 555 MWe and 625 MWe. (External Link

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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 power stations. Thus the same design of turbo-generator plant could be used. In order to obtain high temperatures, yet ensure useful graphite core life (graphite oxidises readily in CO₂ at high temperature) re-entrant flow is utilised, ensuring that the graphite core temperatures don't vary too much from those seen in a Magnox station.
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 (External Link

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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 lead to a fuel rod becoming stuck in a reactor core. Only refuelling at part load or when shut down is now undertaken at AGRs. (External Link

) The prototype AGR at the Sellafield (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

Currently there are seven nuclear generating stations each with two operating AGRs in the United Kingdom. They are all owned and operated by British Energy. These are located at Dungeness B, Hartlepool, Heysham 1, Heysham 2, Hinkley Point B, Hunterston B and Torness.
In 2005 British Energy announced a 10-year life extension at Dungeness B, that will see the station continue operating until 2018. (External Link

) 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 didn't 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". (External Link

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