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The Windscale Disaster

info on Windscale:
Windscale is situated on the Sellafield site, Cumbria, although it has its own site license. The site includes Windscale Piles I and II, the Windscale Advanced Gas-Cooled React
or, and extensive post-irradiation examination facilities. The Windscale site occupies 35 acres.
Both Windscale Piles were shut-down following the Windscale fire and Pile I is now being decommissioned. The Windscale AGR was shut-down in 1981 and is now the UK's demonstration project for complete decommissioning of a power-generating reactor.
Around 383 people work at Windscale from UKAEA, AEA Technology and Johnson Controls.
Windscale Piles One and Two were early production reactors built in the 1940s and used for the production of plutonium for the UK weapons programme.
In 1957 Windscale Pile One was damaged by a fire in the reactor core and nuclear material released to the environment.

Since 1957 the Piles have been kept in a state of safe care and maintenance, regularly monitored to ensure the safety of the structure and to ensure that no radioactivity can escape.
In 1957, the graphite moderator of one of the air-cooled plutonium production reactors at Windscale (now Sellafield), had a fire which resulted in the first significant release of radioactive material from a reactor. The reactor served a second purpose at the time - production of Po-210 (polonium) from bismuth. Po-210 was also released. These gas cooled reactors were operated by the British government at the time.
During the incident, radioactive releases included:

Radionuclide Estimated Range of Release (Curies)
I-131 16200 - 20000
Cs-137 600 - 1240
Sr-89 80-137
sr-90 6-9
Po-210 ~ 240

Reference: M. Eisenbud, Environmental Radioactivity (1987)

The fire started during the process of annealing the graphite structure. During normal operation, neutrons striking the graphite result in distortion of the crystal structure of the graphite. This distortion results in a buildup of stored energy in the graphite. The controlled heating annealing process was used to restore the graphite structure and release the stored energy. Unfortunately, in this case, excessive energy was released resulting in fuel damage. The metallic uranium fuel and the graphite then reacted with air and started burning.

The first indication of an abnormal condition was provided by air samplers about 1/2 mile away. Radioactivity levels were 10 times that normally found in air. Sampling closer to the reactor building confirmed radioactivity releases were occurring. Inspection of the core indicated the fuel elements in ~ 150 channels were overheated. After several hours of trying different methods to extinguish the fire, the reactor core was flooded with water. The plant was cooled down. Factors contributing to the event were:
  1. Inability to adequately monitor the core for damage
  2. Use of uranium metal, rather than uranium dioxide, as fuel. The metal has a lower melting point than the oxide.
Extensive sampling of milk, drinking water, and foods was conducted offsite following the event. The key radionuclides noted in the table above were analyzed.

Major conclusions were:
  1. The slow burning of the core resulted in the preferential release of radioactive iodine.
  2. The dose to any individual was greater from consumption of dairy products than from inhalation or direct exposure to the plume.
  3. Iodine contamination can be estimated from the gamma radiation levels in the area.
  4. If I-131 levels exceed 0.1 microcuries per liter in milk consumed, a child's thyroid dose could exceed 20 Rem.

Commercial reactors today use containments to reduce the likelihood of release. Uranium dioxide fuel is used in the majority of reactors. Emergency planning methods include better sampling and communication than in the 50's. The lower threshold for public concern would likely result in more restrictions on the use of dairy products.

Reference: M. Eisenbud, Environmental Radioactivity (1987)

Detailed Description

Most solids exposed to neutron radiation undergo change in physical properties. Graphite swells, its thermal and electrical conductivity decrease, and it tends to store thermal energy (also called Wigner Energy). By heating the graphite slowly, the Wigner energy can be released.

Graphite was used as the moderator in the core design of Windscale 1. Standard procedure after a normal reactor shutdown was to release the Wigner energy and restore the graphite to its original form. Eight such releases had been carried out successfully. The process is slow and time consuming.

On October 7, 1957 after a normal reactor shutdown, operators started the procedure to release the Wigner energy. The procedure involved starting the reactor at low power with the cooling blowers shut off. This heated the graphite and released the energy under controlled conditions. Following the first heat addition, operators noted the temperature was falling rather than rising. 

Subsequent investigation found that, in fact, in some parts of the reactor temperatures were decreasing, but a substantial number were increasing. This would indicate substantial areas were releasing energy, but some were not.

The next day, the operators added more power to release the Wigner energy. Because of a faulty power, the power was added too quickly. The temperature instruments were located in the positions of maximum temperature when the reactor was normally operating at full power. In this case, the maximum temperatures were elsewhere in the reactor. The releases of the Winger energy in the unmonitored areas got high enough to allow the graphite to catch fire. There were no unusual indication except for some variability in temperature. Steps were taken to cool and stabilize the reactor. 

On the fourth day, there were indications of radioactivity release through the offgas stack. Graphite temperatures also started to increase. Suspecting a ruptured fuel rod, the operators used a remote scanning device but the operating mechanism gear was jammed. Donning protective clothing, workers opened a plug on the front of the reactor and found the fuel was red hot. This was the fire indication of a fire that had been smoldering for about 2 days. Attempts were made to extinguish the fire which did not work. Finally on the 5th day, the reactor was flooded with water and the fire was extinguished.

The reactor was ruined. During the event, radioactive gases (primarily iodine and noble gases [krypton and xenon]) had been released. Meteorological conditions had varied throughout the event. Subsequent investigation showed that about 20,000 Curies of Iodine 131 had been released from the 405 foot stack.

Surveys in the surrounding countryside indicated the highest level had been at about 4 millirem per hour. Vegetative sampling indicated the stack filter had removed almost all of the radioactive particulates, but permitted the radioactive gases to be released. As a result, some gases as I-131 were transported to animal feed, which resulted in subsequent contamination of milk - the only effect on the public.

Radioactive analysis of milk over a larger area showed that the ban on milk distribution had to be extended to a total area of 200 square miles, beginning 2 or 3 miles north of the plant, extending over a strip 7 to 10 miles wide to a distance of 30 miles from the plant. The use of milk by the population in the restricted area was prohibited for 25 days. For the most highly contaminated areas, the prohibition was maintained for 44 days.

The Medical Research Council Committee concluded "that it is in the highest degree unlikely that any harm has been done to the health of anybody, whether a worker in the Windscale plant or a member of the general public." Except for the restrictions on milk usage, no other environmental action was required.

Descriptions of Selected Accidents that have occurred at Nuclear Reactor Facilities, H.W. Bertini and members of the staff of the Nuclear Safety Information Center, April 1980, pp 93-95

In October 1957 Britain spread a plume of radioactive contamination into the atmosphere from a nuclear reactor fire at Sellafield.
Having helped the US Manhattan Project develop the atom bomb at the end of the Second World War, the British government felt it had to develop its own A bomb to be able to stay “at the Top Table” as a world power. The Americans had refused to allow Britain to have the weapons technology its own scientists had helped develop.
Without any reference to Parliament great energy was poured into producing a British bomb. (The first time MP s were told officially was through a brief announcement in 1947) One key requirement were reactors to burn uranium and produce plutonium. It was decided to use an old ammunition factory at Windscale (now called Sellafield). The site had plenty of cooling water from Wastwater lake and was remote from population in case of any accidental nuclear incidents. At a time of post war austerity two huge heavily shielded reinforced concrete “piles” were built at break neck speed and by 1950 the piles were operating. Alongside the first nuclear reprocessing plant (B204) had also been built to extract the precious plutonium.
It was in February 1952 that the first salmon tin sized billets of plutonium were ferried south in the boot of a taxi to the new Aldermaston weapons factory near Oxford. Britain’s first A bomb, code named Hurricane was detonated off the cost of Australia in October 1952. These early plutonium reactors were crude affairs with the main objective being to get the weapons material as quickly as possible. Each “pile” was a honeycomb of carved graphite blocks. Hundreds of horizontal channels ran from the front (charge face) of the reactor to the rear discharge face. Some 35,000 aluminium cans of uranium were pushed into these channels to assemble the critical mass for the chain reactions to burn away.
As theygenerated the intense heat and neutron flux of a nuclear chain reaction some of the uranium converts into plutonium. The fuel cans which had undergone this fiery transformation were a few weeks later pushed through to drop out of the discharge side of the reactor. They then travelled by a mini boat along a water duct into the adjoining reprocessing plant. All this had to take place behind several feet of concrete shielding to cut down the intense penetrating radiation. Each reactor weighed a total of 57,000 tonnes. Because the graphite could release its own latent heat suddenly and unexpectedly the entire reactor had to be deliberately heated up to aneal the graphite.
On October 8, 1957 a technician was heating up the reactor to release this so called Wigner energy. Because of the inadequacy of the temperature measuring instrumentation the control room staff mistakenly thought the reactor was cooling down too much and needed an extra boost of heating. Thus temperatures were actually abnormally high when at 11.05am the control rods were withdrawn for a routine start to the reactor's chain reaction. A canister of lithium and magnesium, also in the reactor to create tritium for a British H bomb, was probably the first can to burst and ignite in the soaring temperatures. This coupled with igniting uranium and graphite sent temperatures soaring to 1,300 degrees centigrade.
These early plutonium "piles" were cooled by massive fans blowing air through them. The heat and some contamination was then carried up the famous concrete chimneys that are such a symbol of the Sellafield skyline. As the fire raged radioactivity was carried aloft. Blue flames shot out of the back face of the reactor and the filters on the top of the chimneys could only hold back a small proportion of the radioactivity. An estimated 20,000 curies of radioactive iodine escaped along with other isotopes such as plutonium, caesium and the highly toxic polonium.
In the days that followed a dangerous cloud of 'fallout' was carried in a south easterly direction towards cities in the North of England. The scientists were unsure how to deal with the raging fire. Workers were sent in relays to use scaffolding polies to frantically push out hundres of fuel cans to try and make a fire break around the fire. Then they tried to pump in carbon dioxide gas to try and smother the flames, but the heat was such that oxygen was produced from the gas and thus fed the flames higher. The scientists then had to gamble on flooding the reactor with cooling water. The risk they were aware of was that explosive hydrogen and or acetylene gas could be created and then flash over into an explosion. As this critical decision was being taken the temperatures were climbing by 20 degrees a minute.
Luckily the gamble paid off and the water starved the fire of oxygen and the reactor was brought under control. Yet even today as the fateful chimneys are slowly taken down by shielded robots the centre of the fire crippled reactor of Pile one still contains molten uranium and still gives off a gentle heat. There is still unreleased Wigner energy in the graphite and water hoses are still left connected to the charge face as a final safety precaution.
Despite reassurances given to the public at the time the official National Radiological Protection Board estimated in a 1987 study that at least 33 people are likely to die prematurly from cancers as a result of the accident. At 4.30pm on October 10th 1957, a fire was discovered inside Britain's first nuclear reactor at Windscale.
Built at the start of the Cold War, in a race to manufacture plutonium for Britain's first atomic weapon, the Windscale Fire occured at a time when little existed in the way of emergency procedures.
Never having faced a nuclear emergency before, Atomic Inferno tells the extraordinary story of how the fire was eventually put out and unravels the causes behind it.
The medical and scientific legacies are still being felt to this day, with the UKAEA (United Kingdom Atomic Energy Authority) admitting that as many as 100 people may have been fatally affected by the ensuing contamination.