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Windscale Graphite Pile Reactors, site of
Sellafield, Seascale, Cumbria
Windscale Graphite Pile Reactors, site of
associated engineer
Ministry of Public Buildings & Works
date  November 1947 - 1950
UK era  Modern  |  category  Power Generation  |  reference  NY026043
ICE reference number  HEW 2639
photo  Sellafield Limited
As part of an international drive towards nuclear armament after World War II, two Graphite Reactors — called Pile Reactors at the time — were built at Sellafield in Cumbria. They were developed by order of the Ministry of Works to supply plutonium for British nuclear weapons but were in use for less than ten years.
Nuclear energy derives from fission — the breaking up of certain atoms, such as those of uranium-235, by bombarding their nuclei with neutrons, and the heat that this produces. In the case of electricity generation, that energy, developed in reactors, is used to drive turbines. Through a slightly different process, it can be used to generate the man-made material called plutonium, which is used in nuclear weapons. Graphite is one of the moderators used to control the speed at which fission takes place.
The Sellafield site, classified as a nuclear fuel plant, was open farmland until 1940. It was taken over by the Courtauld family, who planned to develop a rayon factory. However, soon after, the Ministry took it over and used its direct labour arm — the Mobile Labour Force — as the first contractor for the Windscale project. Excavation for the construction of the two reactors and their tall chimneys began in November 1947. Sellafield Tarn was mostly filled in, despite problems with a peaty soil substratum.
The pile reactors were sited 183m apart, set within 44m high hangars. Each hangar formed a 2m thick steel-lined reinforced concrete biological shield around its reactor core. Reinforced concrete raft foundations, 61m long by 30m wide and 3m deep, were constructed using concrete made in an on-site batching plant. Concrete was fed to the works using a narrow-gauge railway and placed using cranes and skips. The highest daily placing rate achieved was 268 cubic metres.
Each 14m diameter pile chimney used 50,800 tonnes of reinforced concrete and rose to a height of 125m above ground level. Around each was a diffusion section above which was a filter gallery, 18m wide externally. Above each gallery was concentrator section. Each pile had a blower house from which coolant air was sent to the graphite cores and then on to be dispersed via the pile's chimney. A pond between the piles supplied coolant water, which was discharged to the sea via pipelines after use.
Each graphite core weighed 2,030 tonnes and was fuelled by 183 tonnes of uranium. The 15m high and 7.6m thick octagonal core was formed from 50,000 graphite blocks, 800mm long and 200mm square. An array of ducts right through the core held the nuclear fuel. Rods of aluminium-covered uranium 300mm long were pushed along the ducts, undergoing neutron irradiation as they passed through the core and emerged into a 90m long water-filled channel. Rods were collected later, once cooled.
The reactors produced plutonium-239, polonium-210, tritium and radioisotopes. Plutonium from Windscale was used in Britain’s first nuclear weapons test, on 3rd October 1952 near the Monte Bello Islands (north west coast of Australia).
The first reactor began operating in 1950 and the second nine months later. However, less than ten yars later, disaster struck.
Fire broke out in Pile No.1 on the 10th October 1957 after a failed attempt to release stored energy (a ‘Wigner energy’ release). Uranium fuel rods in some of the ducts were found to be red hot with the graphite core on fire. Flames were visible around the reinforced concrete containment shell. The temperature inside the core reached 1,300 degrees Celsius as 11 tonnes of uranium burned.
Initial attempts to extinguish the fire by turning on the cooling fans and injecting liquid carbon dioxide into the core only made matters worse. Over 11th and 12th October, the core was flooded with water, which caused the molten metal to oxidise and release hydrogen. All air intakes were closed and water was fed gradually into the fuel ducts to reduce the possibility of an explosion. Water was pumped into the core until the reactor was completely cold.
Pile No.1 was ruined. Some fuel rods were salvaged and the concrete shield was sealed. Pile No.2, although undamaged, was judged to be potentially unsafe and was shut down soon after the accident and its fuel removed. There have been no more air-cooled nuclear reactors.
This fire was the world’s worst nuclear accident up until the incidents at Three Mile Island, Pennsylvania (1979), and Chernobyl in Russia (1986). Pile No.1 was entombed within its concrete bio-shield.
Decommissioning of the Pile Reactors did not begin until 1988. The core of Pile No.1 was inspected using an endoscope in 2007, which enabled a plan for its final phase of decommissioning to be drawn up. Work on the implemenation of this plan began in 1993 and will be completed in 2015 at an estimated cost in excess of £100 million. Pile No.2 will be decommissioned at a later date.
The first task was to clean the 2.7m square 90m long water channel leading away from the reactor core. Some 300 discarded fuel rods and five cubic metres of radioactive waste including sludge and chunks of graphite were retrieved.
Core dismantling began in 1997-98. The 15 tonnes of recoverable fuel is said to have only one percent of the radioactivity it had at the time of the accident. All graphite blocks from the core will be annealed (heated gently) to release Wigner energy. Every piece from the reactor will be stored as waste in 3m long drums inside a specially constructed storage facility.
In September 1998, ten ageing concrete tanks containing radioactive waste were enclosed by a 110m long steel-framed building, weighing 2,800 tonnes. The deteriorating tanks could not be emptied safely until they were encased. The portal frame was manufactured nearby, jacked up 15mm and then slid into place along 15m of PTFE coated rails.
Main contractor: McAlpine
Turbine and generator supply: Associated Electric Industries
Turbine and generator supply: C.A. Parsons & Co
Reactors: The Nuclear Power Group
Reactor decommissioning engineering (1995-8): W.S. Atkins
Reactor decommissioning (1997 onwards): British Nuclear Fuels Ltd
Steel frame (1998): Balfour Beatty Construction, Hevilift
Research: ECPK
"Windscale — Problems of civil construction and maintenance"
by Stuart Sinclair, George Newnes Limited, London, 1960
"Going Critical — An Unofficial History of British Nuclear Power"
by Walter C. Patterson, Paladin Books, 1985
"Fifty years on, the deadly legacy of atomic accident to be cleaned up"
by Russell Jenkins, The Times, 5th October, 2007
Obituary — Sir John Hill, Chairman of UKAEA 1967-81
The Times, 30th January, 2008
"Nuclear Energy Past, Present and Future" by Malcolm Grimston
Nuclear Industry Association, London 2004

Windscale Graphite Pile Reactors, site of