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= Chapelcross nuclear power station =
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Introduction
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Chapelcross nuclear power station is a former Magnox nuclear power
station undergoing decommissioning. It is located in Annan in Dumfries
and Galloway in southwest Scotland, and was in operation from 1959 to
2004. It was the sister plant to the Calder Hall nuclear power station
plant in Cumbria, England; both were commissioned and originally
operated by the United Kingdom Atomic Energy Authority. The primary
purpose of both plants was to produce weapons-grade plutonium for the
UK's nuclear weapons programme, but they also generated electrical
power for the National Grid. Later in the reactors' lifecycle, as the
UK slowed the development of the nuclear deterrent as the cold war
came to a close, power production became the primary goal of reactor
operation.
The site is being decommissioned by Nuclear Decommissioning Authority
subsidiary Magnox Ltd. The station's four cooling towers were
demolished in 2007. The reactors are spent-fuel free and are currently
undergoing dismantlement of primary loop equipment such as heat
exchangers and hot gas ducts. Once complete, the reactors will enter a
care and maintenance stage to allow radiation levels to decline before
the reactors themselves are demolished.
Location
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Chapelcross occupies a 92 ha site on the location of former World War
II training airfield, RAF Annan, located 3 km north east of the town
of Annan in the Annandale and Eskdale district within the Dumfries and
Galloway region of south west Scotland. The nearest hamlet is Creca.
History
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Chapelcross was the sister plant to Calder Hall in Cumbria, England.
Construction was carried out by Mitchell Construction and was
completed in 1959. The primary purpose was to produce plutonium for
the UK's nuclear weapons programme, for weapons including the WE.177
series. Electricity was always considered to be a by-product. Both
Chapelcross and Calderhall were the only nuclear power stations built
as part of the UK's gas reactor fleet to use cooling towers as a heat
sink as opposed to using the sea.
The Chapelcross Works was officially opened on 2 May 1959 by the Lord
Lieutenant of Dumfriesshire, Sir John Crabbe. It was initially owned
and operated by the Production Group of the United Kingdom Atomic
Energy Authority (UKAEA) until the creation of British Nuclear Fuels
Limited (BNFL) in 1971 by an act of Parliament. The site then operated
in conjunction with Calder Hall under the banner of BNFL's Electricity
Generation Business (EGB) until rebranding, relicensing and
restructuring of the various nuclear businesses operated by HM
Government under the umbrella legal entity of BNFL took place in April
2005.
Chapelcross had four Magnox reactors capable of generating 60MWe of
power each. The reactors were supplied by the UKAEA and the turbines
by C.A. Parsons & Company.
Ownership of all of the site's assets and liabilities was transferred
to the Nuclear Decommissioning Authority (NDA), a new regulatory body
created by the Energy Act 2004. The site was then operated under the
two-tier Site Management Company/Site License Company (SMC/SLC) model,
with British Nuclear Group's Reactor Sites business as SMC and Magnox
Electric Ltd as the SLC. In June 2007, EnergySolutions bought the
Reactor Sites Management Company Ltd (RSML, consisting of two
operational divisions, Magnox North and Magnox South) from British
Nuclear Group. RSML subsequently became Magnox Ltd and is now a
wholly-owned subsidiary of the NDA.
Several significant events in 2001 persuaded BNFL to upgrade the fuel
routes of both Calder Hall and Chapelcross to near modern standards at
a cost of tens of millions of pounds, to guarantee that a License
Instrument would be granted by the Nuclear Installations Inspectorate
(NII) to permit final defuelling: the engineering work was carried out
by BNS Nuclear Services (formally Alstec).
Generation ceased in June 2004.
Decommissioning and the cooling towers
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The Health and Safety Executive (HSE) granted consent to carry out
decommissioning projects at Chapelcross under the regulations to
Magnox Electric Ltd on 26 September 2005. The first visible sign of
decommissioning was controlled demolition at 09:00 BST on 20 May 2007
of the four natural-draught concrete cooling towers, which were of the
same hyperboloid design as conventional inland power stations such as
Didcot, Drax, Ferrybridge and Fiddlers Ferry. The explosions were
designed to remove a section of the towers' shells. Approximately
two-thirds of the circumference and two-thirds of the shell legs were
removed by the blasts, causing a controlled collapse of each tower.
The charges were fired sequentially, reducing the 300 ft high towers
to an estimated 25,000tons of rubble in less than 10seconds. Those at
Calder Hall were demolished on 29 September 2007.
Some local people (including site employees) were opposed to the
obliteration of a symbol of the region's industrial heritage. The
towers were considered a local landmark that could be seen from a
distance of up to 50 mi in good weather conditions. British Nuclear
Group and the NDA prioritised conventional demolition over deplanting
and post-operational clean-out (POCO) of the nuclear facilities on the
site. A large part of the shell of tower 1 managed to resist the
explosives despite having a visible bulge that resulted from a
construction anomaly.
By December 2012, three of the four reactors had been defuelled.
Defuelling was completed in February 2013.
Removal of most buildings is expected to take until 2023-2024,
followed by a care and maintenance phase from 2024 to 2089. Demolition
of reactor buildings and final site clearance is planned for 2089 to
2095.
Plant design
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The plant design was essentially the same as Calder Hall, comprising
four 180MWth graphite moderated, carbon dioxide cooled nuclear
reactors fuelled by natural abundance uranium (0.71% 235U) enclosed in
magnesium-alloy cans, the principal difference being in plant layout.
Since Chapelcross was commissioned from the outset as a four-reactor
site (the option for a further four reactors was not exercised) rather
than separate two-reactor sites as at Calder 'A' and 'B' stations, the
site layout was more compact. There is a single turbine hall housing
all eight turbines, which were originally rated at 23MWe but
progressively uprated to 30MWe as the reactor thermal output was
uprated to nominally 265MWth.
Reactor 1 had the same core design as Calder Hall (i.e. unsleeved),
but the fuel channels of reactors 2, 3 and 4 were fitted with graphite
sleeves to allow the bulk moderator to run 80 C-change hotter, to
limit the effects of in-service graphite damage due to irradiation.
Two of the reactors were used to produce tritium for the UK's nuclear
weapons and required enriched uranium fuel to offset the neutron
absorbing effect of the lithium target material.
Layout and facilities
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The south part of the site consists of a modular administration
building, four reactor buildings, turbine hall, maintenance workshops,
stores, fuel element cooling pond building, tritium processing plant
(CXPP) and new flask handling facility (FHB). The part of the site
referred to as north site consists of legacy buildings including
aircraft hangars, a graphite handling laboratory and a large building
that originally housed some 10,000drums of yellow depleted uranium
trioxide arising from reprocessing at Sellafield. In the mid-2010s,
the NDA removed and shipped all of the depleted uranium drums from
Chapelcross to Sellafield.
Liquid effluent is disposed of via a 5 km long pipeline to the Solway
Firth. All environmental discharges are subject to an annual discharge
authorisation which is regulated by the Scottish Environment
Protection Agency (SEPA).
Chapelcross produced tritium for the Polaris and Trident strategic
nuclear weapon systems from about 1980 until 2005. This was achieved
by neutron bombardment of lithium target material, with the tritium
gas extracted in the Chapelcross Processing Plant (CXPP). This
facility was managed by BNFL on behalf of the Ministry of Defence
(MoD). The material was transferred to Atomic Weapons Establishment at
Aldermaston via secure road convoys. Because of its involvement in the
nuclear weapons programme, the site was not subject to international
safeguards until 1998.
Single channel fuel clad melt (May 1967)
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Fuel in a single channel in Reactor 2 that was loaded with fuel
elements under evaluation for the commercial reactor programme
experienced a partial blockage, attributed to the presence of graphite
debris (see fuel element failure). The fuel overheated and the Magnox
cladding failed, causing contamination to be deposited in one region
of the core. The reactor was restarted in 1969 after successful
clean-out operations and was the final reactor to cease operation in
February 2004.
Fatal accident (ca. 1978)
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BNFL was fined £200 in 1978 for a fatal accident at Chapelcross.
Boiler shell defect (June 1997)
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Cracks associated with brackets in Heat Exchanger 6 on Reactor 2 were
discovered during routine ultrasonic testing. Metallurgical
examination of samples of the defect showed that:
* It originated during fabrication in the workshop and prior to an
over-pressure test of 2.35 times the design pressure (a loading
significantly in excess of a modern pressure vessel code requirement).
* There was no evidence of in-service fatigue crack growth.
* The material in which the crack was located was different from that
specified in the design. Similar material was also identified in other
heat exchangers, and no additional cracks of structural significance
were revealed during comprehensive inspections. The NII considered the
material to be adequate and within the bounds of the heat exchanger
safety case.
Exposure of workers to an irradiated fuel element (first quarter 2001)
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During refuelling operations on Reactor 2, an irradiated fuel element
failed to release from the grab which holds an element while it is
withdrawn from a reactor. Routine methods were used to release the
grab. However, the irradiated fuel element snagged during the
operation and was lifted out of its shielding, resulting in the
operators on the pile cap being exposed to the intense radiation being
emitted from the irradiated fuel element. Personnel responded quickly,
and the radiological dose they received was small.
The event revealed shortfalls in the safety of the refuelling
operation and the licensee took the immediate step of halting all
refuelling while it investigated the event and reviewed the safety of
the equipment. The NII investigated the event and judged that it was
due to inadequate design and operation of the equipment.
The incident was classified as Level 1 (anomaly) on the International
Nuclear Event Scale (INES).
Leak of Magnox depleted uranium trioxide (July 2001)
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A small amount of Magnox depleted uranium (MDU) leaked from some
corroded mild steel drums due to rainwater ingress and leaching. MDU
is a dense yellow powder that is less radiologically toxic than
naturally occurring uranium but chemotoxic in a similar manner to
lead. Owing to its high density and low solubility, it does not tend
to disperse far and dry spills are easy to clean up. This material was
stored at the larger sites, including Capenhurst in mild steel drums.
BNFL upgraded the fabric of the building and the original drums were
overpacked into stainless steel drums and dispatched to Capenhurst for
long-term storage.
Dropped basket of irradiated fuel elements (July 2001)
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During routine defuelling activities on Reactor 3, a basket containing
24 low-rated irradiated Magnox fuel elements fell a few feet within
the discharge machine onto the door at the top of the fuel discharge
well. Remote TV camera inspections revealed that twelve of the
elements had fallen just over 80 ft down the discharge well into a
water-filled transport flask at the bottom. The NII initiated an
investigation because dropping irradiated fuel elements is a serious
issue even when, as in this event, BNFL had advised NII that there had
been no release of radiological activity.
Charge pan movement relative to the core (September 2001)
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Because of known shrinkage of the graphite moderator bricks in the
core due to in-service irradiation effects, some of the steel charge
pans on top of them had become dislocated from their design position
in the interstitial channel and were suspended from the Burst Can
Detection (BCD) pipework. This was most prevalent in Reactor 1 because
of the different core design to Reactors 2, 3 and 4. BNFL were unable
to make an adequate safety case or effect an economic repair and
therefore, Reactor 1 did not return to power from its annual outage in
August 2001. The core of Reactor 4 was repaired but this reactor did
not return to power after the repair.
Interim storage facility
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An interim storage facility for storing intermediate level radioactive
waste from Chapelcross came into operation in 2021. Construction began
in 2014 of the 57 by facility, which can store 700 waste packages for
120 years.
See also
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* Nuclear weapons and the United Kingdom
* Nuclear power in Scotland
* Nuclear power in the United Kingdom
* Energy policy of the United Kingdom
* Energy use and conservation in the United Kingdom
External links
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*
[https://web.archive.org/web/20080203174010/http://www.chapelcrosssite.co.uk/
Official website] archived in February 2008
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Original Article: http://en.wikipedia.org/wiki/Chapelcross_nuclear_power_station