An Advanced Gas-cooled Reactor is a specific 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, and operates at a higher gas temperature for improved
thermal efficiency, but requires 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 refuelling. The
first prototype AGR became operational in 1962 but the first commercial AGR did
not come on line until 1976. All existing AGR power stations 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.
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 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 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 and a pressure of around 40 bar, and then passes through boiler
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 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 practical with gas cooling, compared to about 325 °C 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.
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 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
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 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. The small-scale prototype AGR at the
Sellafield site is 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 EDF Energy: 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. See also
Nuclear power in the United Kingdom List of nuclear reactors
References 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 for Greenpeace British Energy's bifurcation blues,
Nuclear Engineering International, 22 November 2006
Account of visiting Torness AGR, Charlie Stross