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Windscale fire

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For an episode in the BBC "Disaster" series, see The Windscale Fire.

On October 10, 1957, the graphite core of a British nuclear reactor at Windscale, Cumbria, caught fire, releasing substantial amounts of radioactive contamination into the surrounding area. The event, known as the Windscale fire, was considered the world's worst reactor accident until the Three Mile Island accident in 1979. Both incidents were dwarfed by the magnitude of the Chernobyl disaster in 1986.

Background

The design of Windscale Pile no. 1.

After the Second World War, in 1946, despite the participation of many British scientists in the Manhattan Project, and formal agreement of a joint technology-exchange program, the United States government passed legislation that closed its nuclear weapons program to all other countries.

The British government, not wanting to be left behind as a world power in an emerging arms race, then embarked on a programme to build its own atomic bomb as quickly as possible.

The Windscale Piles

The reactors were built in a short time near the tiny village of Seascale, Cumbria, and were known as Windscale Pile 1 and Windscale Pile 2, housed in large, concrete buildings a few hundred feet from one another. The reactors were graphite-moderated and air-cooled. Because nuclear fission produces large amounts of heat, it was necessary to cool the reactor cores by blowing cold air through channels in the graphite. Hot air was then exhausted out of the back of the core and up the chimney. Filters were added late into construction at the insistence of Sir John Cockcroft and these were housed in galleries at the very top of the discharge stacks. They were deemed unnecessary, a waste of money and time and presented something of an engineering headache, being added very late in construction in large concrete houses at the top of the 400-ft chimneys. Due to this, they were known as 'Cockcroft's Folly' by workers and engineers. As it was, 'Cockcroft's Folly' probably prevented a disaster from becoming a catastrophe.

Core design

The reactors themselves were built of a solid graphite core, with horizontal channels through which cans of uranium and isotope cartridges could be passed, to expose them to neutron radiation and produce plutonium and radioisotopes, respectively. Fuel and isotopes were fed into the channels in the front of the reactor, the 'charge face', and spent fuel was then pushed all the way through the core and out of the back—the 'discharge face'—into a water duct for initial cooling prior to retrieval and processing to extract the plutonium.

Fuel

Unenriched uranium metal in aluminium cans with fins to improve cooling was used for the production of plutonium. As this plutonium was intended for weapons purposes, the burnup of the fuel would have been kept low to reduce production of the heavier plutonium isotopes (240Pu, 241Pu etc).

Isotope cartridges

The following substances were placed inside metal cans and subjected to neutron irradiation to create radioisotopes. Both the target material and some of the product isotopes are listed below. Of these, the Polonium-210 release made the most significant contribution to the collective dose on the general population.[1]

Wigner energy

When the reactors were being built, little was known about the behavior of graphite when exposed to neutrons. Hungarian physicist Eugene Wigner discovered that graphite, when bombarded by neutrons, suffers dislocations in its crystalline structure causing a build up of potential energy. This energy, if allowed to accumulate, could escape spontaneously in a powerful rush of heat. Once commissioned and settled into operations, Windscale Pile 2 experienced a mysterious rise in core temperature, and this was attributed to a sudden Wigner energy release. This worried British scientists, so a means of safely releasing the stored energy was sought. The only viable solution was also extremely simple: an annealing process, in which the graphite core was heated to 250 degrees Celsius to allow the displaced molecules to slip back into place and gradually release their stored energy (as heat) as they did so, causing a uniform release which spread throughout the core. Annealing succeeded in preventing the buildup of Wigner energy, but the monitoring equipment, the reactor itself and all of its ancillaries such as the cooling system were never designed for this. Each annealing cycle was slightly different and progressively more difficult as time went on; many of the later cycles had to be repeated, and higher and higher temperatures were required to start the annealing process. It was also found that some pockets of Wigner energy remained that had not been released on previous occasions. The annealing was performed with the reactor charged or fully fueled.

Because they were built hastily and during a time when little was known about reactor design, the reactors had a number of serious design flaws that contributed to the disaster. Graphite is flammable in air and air was being fed into the reactors constantly for cooling, so there was a constant fire hazard. During the accident, however, the graphite in the reactor did not actually catch fire. The only graphite moderator damage was found to be localized around burning fuel elements.[2] The direct venting of the cooling air to the atmosphere meant that any radioactive material released by the core which slipped through the filters would be released into the countryside. The annealing phases were not part of the original plan, so thermocouples were placed in positions in the reactor to monitor normal operations, but not to monitor the annealing process. This allowed unknown hot spots to form. The reactor's fuel, metallic uranium, burns if it becomes too hot, unlike the uranium dioxide used in modern reactors.

The accident

Ignition

On October 7, 1957, operators began an annealing cycle for Windscale Pile no. 1 by switching the cooling fans to low power and stabilizing the reactor at low power. The next day, to carry out the annealing, the operators increased the power to the reactor. When it appeared that the annealing process was taking place, control rods were lowered back into the core to shut down the reactor, but it soon became apparent that the Wigner energy release was not spreading through the core, but dwindling prematurely. The operators withdrew the control rods again to apply a second nuclear heating and complete the annealing process. Because some thermocouples were not in the hottest parts of the core, the operators were not aware that some areas were considerably hotter than others. This, and the second heating are suspected to be the deciding factors behind the fire, although the precise cause remains unknown. The official report suggests that a can of uranium ruptured and oxidised causing further overheating and the fire, but a more recent report suggests that it may actually have been a magnesium/lithium isotope cartridge. All that was visible on the instruments was a gentle increase in temperature, which was to be expected during the Wigner release.

Early in the morning on October 10, it was suspected that something unusual was going on. The temperature in the core was supposed to gradually fall as Wigner release ended, but the monitoring equipment showed something more ambiguous was going on and one thermocouple indicated that core temperature was instead rising. In an effort to help cool the pile, more air was pumped through the core. This lifted radioactive materials up the chimney and into the filter galleries. It was then that workers in the control room realised that the radiation monitoring devices which measured activity at the top of the discharge stack were at full scale reading. In accordance with written guidelines, the foreman declared a site emergency. No one at Windscale was now in any doubt that Pile Number 1 was in serious trouble.

The fire

Operators tried to examine the pile with a remote scanner but it had jammed. Tom Hughes, second in command to the Reactor Manager, suggested examining the reactor personally and so he and another operator went to the charge face of the reactor, clad in protective gear. A fuel channel inspection plug was taken out close to a thermocouple registering high temperatures and it was then that the operators saw that the fuel was red hot.

"An inspection plug was taken out," said Tom Hughes in a later interview, "and we saw, to our complete horror, four channels of fuel glowing bright cherry red."

There was no doubt that the reactor was now on fire, and had been for almost 48 hours. Reactor Manager Tom Tuohy[3] donned full protective equipment and breathing apparatus and scaled the 80 feet to the top of the reactor building, where he stood atop the reactor lid to examine the rear of the reactor, the discharge face. Here he reported a dull red luminescence visible, lighting up the void between the back of the reactor and the rear containment. Red hot fuel cartridges were glowing in the fuel channels on the discharge face. He returned to the reactor upper containment several times throughout the incident, at the height of which a fierce conflagration was raging from the discharge face and playing on the back of the reinforced concrete containment—concrete whose specifications insisted that it must be kept below a certain temperature to prevent its disintegration and collapse.[4]

Initial fire fighting attempts

Operators were unsure what to do about the fire. First, they tried to blow the flames out by putting the blowers onto full power and increasing the cooling, but predictably this simply fanned the flames. Tom Hughes and his colleague had already created a fire break by ejecting some undamaged fuel cartridges from around the blaze and Tom Tuohy suggested trying to eject some from the heart of the fire, by bludgeoning them through the reactor and into the cooling pond behind it with scaffolding poles. This proved impossible and the fuel rods refused to budge, no matter how much force was applied. The poles were withdrawn with their ends red hot and, once, a pole was returned red hot and dripping with molten metal. Hughes knew this had to be molten irradiated uranium and this caused serious radiation problems on the charge hoist itself.

"It [the exposed fuel channel] was white hot," said Hughes' colleague on the charge hoist with him, "it was just white hot. Nobody, I mean, nobody, can believe how hot it could possibly be."

Carbon dioxide

Next, the operators tried to extinguish the fire using carbon dioxide. The new gas-cooled Calder Hall reactors next door had just received a delivery of 25 tonnes of liquid carbon dioxide and this was rigged up to the charge face of Windscale Pile 1, but there were problems getting it to the fire in useful quantities. The fire was so hot that it stripped the oxygen from what carbon dioxide could be applied.

"So we got this rigged up," Hughes recounted mockingly in interview, "and we had this poor little tube of carbon dioxide and I had absolutely no hope it was going to work."

The use of water

On the morning of Friday October 11 and at its peak, 11 tonnes of uranium were ablaze. Temperatures were becoming extreme (one thermocouple registered 1,300 degrees Celsius) and the biological containment around the stricken reactor was now in severe danger of collapse. Faced with this crisis, the operators decided to use water. This was incredibly risky: molten metal oxidises in contact with water, stripping oxygen from the water molecules and leaving free hydrogen, which could mix with incoming air and explode, tearing open the weakened containment. But there was no other choice. About a dozen hoses were hauled to the charge face of the reactor; their nozzles were cut off and the lines themselves connected to scaffolding poles and fed into fuel channels about a meter above the heart of the fire.

Tom Tuohy then ordered everyone out of the reactor building except himself and the Fire Chief. All cooling and ventilating air entering the reactor was shut off. Tuohy once again hauled himself atop the reactor shielding and ordered the water to be turned on, listening carefully at the inspection holes for any sign of a hydrogen reaction as the pressure was increased. Tuohy climbed up several times and reported watching the flames leaping from the discharge face slowly dying away. During one of the inspections, Tuohy found that the inspection plates—which are removed with a metal hook to facilitate viewing of the discharge face of the core—were stuck fast. This, Tuohy reported, was the fire trying to suck air in from wherever it could.

"I have no doubt it was even sucking air in through the chimney at this point to try and maintain itself," he remarked in interview.

Finally he managed to pull the inspection plate away and was greeted with the unfathomable sight of the fire dying away.

"First the flames went, then the flames reduced and the glow began to die down," he described, "I went up to check several times until I was satisfied that the fire was out. I did stand to one side, sort of hopefully," he went on to say, "but if you're staring straight at the core of a shut down reactor you're going to get quite a bit of radiation."

Water was kept flowing through the pile for a further 24 hours until it was completely cold.

The aftermath

Damage caused

The fire itself released an estimated 700 terabecquerels (20,000 curies) of radioactive material into the nearby countryside, although recent reworking of contamination data has shown national and international contamination to have been much higher than previously estimated.[5] Of particular concern at the time was the radioactive isotope iodine-131, which has a half-life of only 8 days but is taken up by the human body and stored in the thyroid. As a result, consumption of iodine-131 often leads to cancer of the thyroid. It had previously been estimated that the incident caused 200 additional cancer cases, although this figure has recently been revised upwards to 240.[5]

No one was evacuated from the surrounding area, but there was concern that milk might be dangerously contaminated. Milk from about 500km² of nearby countryside was destroyed (diluted a thousandfold and dumped in the Irish Sea) for about a month.

There is evidence to suggest, however, that the official Meteorological records may have been altered in an attempt to cover up the fact that, throughout the radiation leak, the wind was blowing out to sea, significantly increasing the contamination dose to Ireland and the Isle of Man.[6]

Public reaction

Public reactions varied. There were no reports of widespread panic. Reported comments were overwhelmingly calm and downplayed the seriousness of the accident. Many were involved with the plant, while others could see no ill effects and assumed they were safe. Several of those interviewed by reporters said that they were unhappy about how the media was accusing them of panic. The most serious sign of local distress was a 15% drop in milk sales in nearby Carlisle.

Nationally, the accident was generally reported with fear in the tabloids and with restraint by the broadsheets. The News Chronicle described it as "the accident experts said could not happen", and boasted an interview with "Britain's first atom-dust casualty". The Manchester Guardian, on the other hand, emphasized the strength of safety measures already in place. Coverage of the milk ban tended to stress how little damage it could do to an adult. In spite of occasionally hyperbolic coverage, the accident had no appreciable long term impact on British attitudes to nuclear power.

Salvage operations

The reactor was unsalvageable; where possible, the fuel rods were removed, and the reactor bioshield was sealed and left intact. Approximately 6,700 fire-damaged fuel elements and 1,700 fire-damaged isotope canisters remain in the pile. The damaged reactor core was still slightly warm as a result of continuing nuclear reactions.[7] Windscale Pile no. 2, though undamaged by the fire, was considered too unsafe for continued use. It was shut down shortly afterward. No air-cooled reactors have been built since. The final removal of fuel from the damaged reactor is scheduled to begin in 2008 and continue for a further four years.[8]

Board of Inquiry

The Board of Inquiry met under the chairmanship of Sir William Penney from 17-25th October 1957. Its report (the "Penney Report") was submitted to the Chairman of the United Kingdom Atomic Energy Authority and formed the basis of the Government White Paper submitted to Parliament in November 1957. The report itself was released at the Public Record Office in January 1988. In 1989 a revised transcript was released, following work to improve the transcription of the original recordings.[9][10]

Penney reported on 26 October 1957, 16 days after the fire was extinguished[11] and reached four conclusions:

  • The primary cause of the accident had been the second nuclear heating on 8 October, applied too soon and too rapidly.
  • Steps taken to deal with the accident, once discovered, were "prompt and efficient and displayed considerable devotion to duty on the part of all concerned".
  • Measures taken to deal with the consequences of the accident were adequate and there had been "no immediate damage to health of any of the public or of the workers at Windscale". It was most unlikely that any harmful effects would develop. But the report was very critical of technical and organisational deficiencies.
  • A more detailed technical assessment was needed, leading to organisational changes, clearer responsibilities for health and safety, and better definition of radiation dose limits.

Those who had been directly involved in the events were heartened by Penney's conclusion that the steps taken had been "prompt and efficient" and had "displayed considerable devotion to duty". Some felt, nevertheless, that the determination and courage shown by Thomas Tuohy, as well as the critical role he played in the aversion of complete disaster, had not been fully recognised. Tuohy died on 12 March 2008 having never received any kind of public recognition for his efforts.[3]

The Windscale site was decontaminated and is still in use. Part of the site was later renamed Sellafield, after being transferred to BNFL the whole site is now owned by the Nuclear Decommissioning Authority.

Comparison with other accidents

Windscale was greatly exceeded by the Chernobyl disaster in 1986, and the fire has been described as the worst reactor accident until Three Mile Island (TMI) in 1979. This later description is dubious, however, as estimates put the number of additional cancers caused by the TMI accident at less than one, or at most one. Since only Chernobyl produced immediate casualties these assertions are dependent upon epidemiological assessments. Also, Chernobyl was primarily a civilian reactor and TMI was a civilian reactor, both being used for electrical power production, whereas Windscale was for purely military purposes. Other military reactors have produced immediate, known casualties such as the 1961 incident at the SL-1 plant in Idaho which killed three operators, or the criticality accident which killed Louis Slotin at the Los Alamos National Laboratory in 1946. The reactors at TMI, unlike those at Windscale and Chernobyl, were in buildings designed to contain radioactive materials released by a reactor accident.

See also

References

  1. ^ Crick, MJ (1984). "An assessment of the radiological impact of the Windscale reactor fire, October 1957". Int J Radiat Biol Relat Stud Phys Chem Med. 46 (5): 479–506. doi:10.1080/09553008414551711. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  2. ^ Template:PDFlink
  3. ^ a b "Windscale Manager who doused the flames of 1957 fire - Obituary in The Independent 2008-03-26". Retrieved 2008-03-27.
  4. ^ Arnold, L. (1992). Windscale 1957: Anatomy of a Nuclear Accident. Macmillan. p. 235. ISBN 0333650360.
  5. ^ a b "Windscale fallout underestimated". BBC News. 6 October 2007.
  6. ^ The Low Level Radiation Campaign
  7. ^ Details of the levels and nature of the radioactivity remaining in the core can be seen at Template:PDFlink
  8. ^ Template:PDFlink
  9. ^ Paul Dwyer (5 October 2007). "Windscale: A nuclear disaster". BBC News.
  10. ^ "Proceedings into the fire at Windscale Pile Number One (1989 revised transcript of the "Penney Report")" (PDF). UKAEA. 18 April 1989.
  11. ^ When Windscale burned

Further reading

  • Windscale, 1957: Anatomy of a Nuclear Accident, Lorna Arnold
  • An Assessment of the Radiological Impact of the Windscale Reactor Fire, Oct., 1957, Nov., 1982 (NRPB Reports) M J Crick, G.S. Linsley
  • An airborne radiometric survey of the Windscale area, October 19-22nd,1957 (A.E.R.E. reports;no.R2890) Atomic Energy Research Establishment
  • The deposition of strontium 89 and strontium 90 on agricultural land and their entry into milk after the reactor accident at Windscale in October, 1957 (A.H.S.B) United Kingdom Atomic Energy Authority
  • Accident at Windscale No.1 Pile on 10 October,1957 (Cmnd.302)
  • Chernobyl: worst but not first, Walter C. Patterson, Bulletin of the Atomic Scientists August/September 1986
  • Windscale fallout blew right across Europe, Rob Edwards, New Scientist, October 6,2007 (contains further references)
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