Advanced gas-cooled reactor

Advanced gas-cooled reactor

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 [ [ History of Windscale's Advanced Gas-cooled Reactor] , UKAEA.] but the first commercial AGR did not come on line until 1976.

All AGR power stations up to and including Heysham 2 and Torness are configured with two reactors, each reactor with 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. []

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%cite book
last = Shultis
first = J. Kenneth
coauthors = Richard E. Faw
year = 2002
title = Fundamentals of Nuclear Science and Engineering
publisher = Marcel Dekker
id = ISBN 0-8247-0834-2
] . 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 [] .

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. []

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, owned and operated by British Energy:

In 2005 British Energy announced a 10-year life extension at Dungeness B, that will see the station continue operating until 2018, [citation|url=|title=10-year life extension at Dungeness B nuclear power station|date=15th September 2005|publisher=British Energy|accessdate=2008-06-19] and in 2007 announced a 5-year life extension of Hinkley Point B and Hunterston B until 2016. [citation|url=|title=Life extension of Hinkley Point B and Hunterston B power stations|date=11th December 2007|publisher=British Energy|accessdate=2008-06-19] Life extensions at other AGRs will be considered at least three years before their scheduled closure dates.

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". []

ee also

*Nuclear power in the United Kingdom
*List of nuclear reactors.


External links

* [ Advanced gas-cooled reactors] - IAEA conference paper, September 1980
* [ Project WAGR] - decommissioning the Windscale AGR
* [ AGR estimated closure dates] , House of Lords Hansard column WA232, 24 Feb 2005
* [ Review of Graphite core issues at Hinkley Point B and other AGRs] , Large & Associates (Consulting Engineers) for Greenpeace
* [ British Energy's bifurcation blues] , Nuclear Engineering International, 22 November 2006
* [ Account of visiting Torness AGR] , Charlie Stross

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