Editing Advanced gas-cooled reactor (section)

== AGR design ==
[[File:AGR reactor schematic.svg|upright=1.5|thumb|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. {{ordered list
    |Charge tubes
    |Control rods
    |Graphite moderator
    |Fuel assemblies
    |Concrete pressure vessel and radiation shielding
    |Gas circulator
    |Water
    |Water circulator
    |Heat exchanger
    |Steam
}}]]
[[File:Gen II nuclear reactor vessels sizes.svg|upright=1.5|thumb|AGR reactor size compared to other technologies]]

The AGR was designed such that the final steam conditions at the boiler stop valve were identical to that of conventional [[coal-fired power station]]s, 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 {{convert|648|°C}}. In order to obtain these high temperatures, yet ensure useful graphite core life (graphite [[Boudouard reaction|reacts]] with CO<sub>2</sub> at high temperature) a re-entrant flow of coolant at the lower boiler outlet temperature of 278&nbsp;°C is utilised to cool the graphite, ensuring that the [[Nuclear reactor core|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&nbsp;[[Pounds per square inch#Relation to other measures|psi]] (170 bar) and 543&nbsp;°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,<ref>{{cite journal |doi=10.1016/0022-3115(81)90521-3 |title=Developments in oxide fuels at Harwell |journal=Journal of Nuclear Materials |volume=100 |issue=1–3 |pages=67–71 |year=1981 |last1=Murray |first1=P. |bibcode=1981JNuM..100...67M }}</ref> 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 {{convert|640|C}} 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 absorb thermal neutrons to stop the fission process if the control rods fail to enter the core. A tertiary shutdown system which operates by injecting [[boron]] beads into the reactor is included in case the reactor has to be depressurized with insufficient control rods lowered. This would mean that nitrogen pressure cannot be maintained.<ref name=Nonbel>{{cite report
 | url = https://inis.iaea.org/collection/NCLCollectionStore/_Public/28/028/28028509.pdf
 | last = Nonbel
 | first = Erik
 | date = November 1996
 | title = Description of the Advanced Gas Cooled Type of Reactor (AGR)
 | publisher = Nordic Nuclear Safety Research
 | id = NKS/RAK2(96)TR-C2
 | access-date = 2019-01-02
 | archive-date = 2022-06-09
 | archive-url = https://web.archive.org/web/20220609170021/https://inis.iaea.org/collection/NCLCollectionStore/_Public/28/028/28028509.pdf
 | url-status = dead
 }}{{page needed|date=January 2017}}</ref><ref>{{cite web|title=Nuclear_Graphite_Course-B - Graphite Core Design AGR and Others |url=http://web.up.ac.za/sitefiles/file/44/2063/Nuclear_Graphite_Course/B%20-%20Graphite%20Core%20Design%20AGR%20and%20Others.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110717034453/http://web.up.ac.za/sitefiles/file/44/2063/Nuclear_Graphite_Course/B%20-%20Graphite%20Core%20Design%20AGR%20and%20Others.pdf |archive-date=17 July 2011 }}{{full citation needed|date=January 2017}}</ref>

The AGR was designed to have a high [[thermal efficiency]] (electricity generated/heat generated ratio) of about 41%, which is better than a modern [[pressurized water reactor]] (PWR) with a typical thermal efficiency of 34%.<ref name=shultis_and_faw>{{cite book|last1=Shultis|first1=J. Kenneth|last2=Faw|first2=Richard E.|year=2002|title=Fundamentals of Nuclear Science and Engineering|publisher=Marcel Dekker|isbn=0-8247-0834-2}}{{page needed|date=January 2017}}</ref> This is due to the higher coolant outlet temperature of about {{convert|640|C}} practical with gas cooling, compared to about {{convert|325|C}} for PWRs. However the reactor core has to be larger for the same power output, and the fuel burnup of 27,000 MW(th) days per tonne for type 2 fuel and up to 34,000 MW(th) days per tonne for robust fuel at discharge is lower than the 40,000 MW(th) days per ton of PWRs so the fuel is used less efficiently,<ref>{{cite tech report|title=Appendix 6: Typical design and operating data for currently operating reactors|work=Nuclear energy - the future climate|url=https://royalsociety.org/-/media/Royal_Society_Content/policy/publications/1999/10087.pdf|url-status=live|archive-url=https://web.archive.org/web/20041228121556/http://www.royalsoc.ac.uk/downloaddoc.asp?id=1221|date=1 June 1999|archive-date=28 December 2004}}</ref> countering the thermal efficiency advantage.

Like the magnox, [[CANDU]], [[IPHWR]], and [[RBMK]] reactors, and in contrast to [[light water reactor]]s, 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 pre-stressed concrete pressure vessel contains the reactor core and the boilers. To minimise the number of penetrations into the vessel (and thus reduce the number of possible breach sites) the boilers are of the once through design where all boiling and superheating is carried out within the boiler tubes. This necessitates the use of ultra pure water to minimise the buildup of salts in the evaporator and subsequent corrosion problems.

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 [[very-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.