On October 10, 1957, the graphite core of a British nuclear reactor at the Windscale site, near Sellafield, caught fire, releasing substantial amounts of radioactive contamination into the surrounding area. It was considered the worst nuclear accident in the world until it was dwarfed by the Chernobyl accident in 1986.
The reactor
After the end of the Second World War, in 1946, in spite of the participation of many British scientists in the Manhattan Project, the United States of America closed its nuclear program to all other countries. The British government embarked on a race to build the "British Bomb" as quickly as possible. For a full nuclear weapons program, it is necessary to be able to produce plutonium in quantity. The British government therefore built, as quickly as possible, a nuclear reactor. The Windscale site was chosen because of the ready availability of cooling water, the pre-existing buildings, and the distance from large population centers should an accident occur.
Two reactors were built at the Windscale site, to the same basic design, called "Windscale Pile no. 1" and "Windscale Pile no. 2". The reactors were graphite-moderated and air-cooled. The air was filtered (to remove stray radioactive materials) and then released through large smokestacks. The reactors had horizontal channels through which cans of unenriched uranium and lithium could be passed to expose them to neutron radiation and produce plutonium and tritium, respectively.
When the reactors were built, little was known about the behaviour of graphite when exposed to neutron radiation. Both reactors experienced unplanned rises in core temperature, which were ultimately determined to be caused by sudden releases of Wigner energy. This energy is stored as dislocations in the crystal structure of graphite, caused by exposure to neutron radiation. When enough accumulates, it is suddenly released. As a safety measure, it was necessary to anneal the graphite before enough Wigner energy accumulated to spontaneously release. The annealing process was simple: the temperature was allowed to rise to a point where the graphite was hot enough for the Wigner energy to be released. Annealing succeeded in preventing the buildup of Wigner energy, but it was poorly understood and each annealing cycle was different. The annealing cycles were also growing more difficult; many of the later cycles had to be repeated.
Because they were built hastily and during a time when little was known about reactor design, the reactors had a number of dangerous features. Since graphite is flammable in air, and air was being fed to the reactors constantly for cooling purposes, fire was a constant danger. Since the cooling air was vented to the atmosphere, any radioactive material released by the core had only to slip through the filters to be released into the countryside. Since the annealing phases were not planned, there were not enough thermocouples to monitor the core temperature accurately. What thermocouples there were, were placed in the zones of maximum temperature under normal operation of the reactor, but these were not necessarily the hottest zones while annealing was going on. The fuel was uranium metal, rather than uranium dioxide, so its melting point was lower.
The accident
On October 7, 1957, operators began an annealing cycle for Windscale Pile no. 1 by shutting off the cooling systems and setting the reactor to low power. The temperature sensors indicated a falling (rather than rising) temperature. Although the operators did not realize, the temperature was indeed rising, but in a part of the pile not measured by the thermocouples. The next day, to carry out the annealing, the operators increased the power to the reactor. The reactor temperature increased further until the reactor was hot enough to catch fire. It is not known exactly what started the fire. It may have been simple overheating, or a can of fuel or lithium may have burst.
All that was visible on the instruments was a small variability in temperature.
On October 10, air samplers about a kilometer away detected a rise (to 10 times the usual value) of radioactivity in the air. Operators tried to examine the pile with a remote scanner but it had jammed. Operators donned protective gear and tried to exmine the reactor, only to discover that it was red hot; the fuel rods had melted and the reactor had been ablaze for nearly two days.
The fire carried a great deal of radioactive material out of the pile; the filters could remove only a small fraction of this material, and the rest was carried out the smokestack to fall as radioactive contamination on the surrounding countryside.
Operators were unsure what to do about the fire. Men were sent with sticks to push fuel cans out of the reactor in the hopes that this would help cool it. The reactor was flushed with carbon dioxide, but the temperature was so high this proved ineffective.
On the morning of Friday October 11, temperatures were rising 20 degrees per minute, and operators decided to try showering the pile with water. While this might extinguish the fire, it might also evolve hydrogen and acetylene gases, which could then lead to an explosion. In the event, the gamble paid off; the fire came under control, although a cloud of radioactive steam was released to drift across all of England and over to Europe.
The aftermath
The fire itself released an estimated 20000 curies (700 terabecquerels) of radioactive material into the nearby countryside. Of particular concern 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.
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 (by dumping in local rivers) for about a month.
The reactor was unsalvageable; all the fuel rods that could be removed were, and the reactor was buried in concrete. Approximately 6700 fire-damaged fuel elements and 1700 fire-damaged isotope canisters remain in the pile. The damaged reactor core is still heated by continuing nuclear reactions. 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 Windscale site was decontaminated and is still in use; several more modern nuclear reactors are there now. To avoid bad associations, the UKAEA renamed the site Sellafield.
In the 1990s, the UKAEA began plans to decommission (disassemble and clean up) both piles; the decommissioning is now partially complete. Plans are being explored to safely remove the fire-damaged core, which is still radioactive and could burst into flame.
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