|Fig. 1: The Windscale Piles, as seen from a distance. (Source: Wikipedia Commons)|
In the mid 1940s, the United Kingdom government designed a policy that authorized Great Britain to independently develop nuclear weapons.  Between 1947 and 1951, two nuclear reactors, called the Windscale Piles, were built at Windscale Works, located on the Cumbrian coast of northwest England.  These reactors were chiefly built to produce Pu-239, but were also used to generate other nuclides such as Po-210, tritium, Th-232, Np-237, and Co-59.  Each of the reactors had graphite moderators, weighing 2,000 metric tons, and were fuelled by 180 metric tons of uranium metal. In 1952, the completed Windscale Piles were tested, and proved to be successful in extracting the plutonium necessary for the UKs first nuclear weapons test. The Windscale Piles can be seen in Fig. 1.
In October of 1957, the UK faced its worst nuclear power accident in history.  The accident ranked a high level of 5 on the International Nuclear Event Scale.  It all started with a routine annealing process that was carried out to regulate the levels of Wigner energy stored in the graphite moderators. Wigner energy gathered because of the displacement of carbon atoms from their original positions in the lattice, and this energy needed to be regularly annealed through a heating process that released the energy.  On October 7, this process was undertaken in Pile 1, where Wigner energy was to be slowly released until the temperature of the uranium reached 250°C. By the next day, however, the temperature wasn't high enough - this led the operators to reattempt the annealing process. The following day, on October 9, the temperature was at an alarming 400°C, marking the highest temperature the reactor ever reached. In the following two days, fans failed to cool down the pile, fuel cartridges were found to be incredibly hot, and flames were discovered at the back of the pile. On October 11, numerous efforts were taken to extinguish the fire. At 4:30 am, carbon dioxide was pumped into the pile, and this proved to be ineffective. At 8:55, water was turned on, changing from an initial water flow of 300 gallons per minute, to 800 gallons per minute, to 1,000 gallons a minute at 12:00pm. The water flow continued until the next day, when the reactor finally cooled down to a stable temperature.
Following the fire, the British government downplayed the severity of the accident.  British nuclear physicist Sir William Penney's report on the incident, which was censored, blamed the staff rather than the plant's structures and operations themselves. Harold Macmillan, then Prime Minister of the UK, mandated that only the summary of the report be published, and it wasn't until 30 years following the catastrophe that the full report was made available.  The Windscale Piles closed for good, and an investigation by Sir Alexander Fleck prompted the 1971 creation of the National Radiological Protection Board (NRPB).
Following the fire, environmental measurements were taken to determine the radiological impact of the disaster.  The measurements showed that event led to the atmospheric dispersion of radioactive materials throughout England and Wales and parts of northern Europe.  The results indicated the highest concentrations of emitted fission products to be I-131 and Cs-137. Of the two, the bigger focus was on the I-131 in examinations of the hazardous consequences of the fire.  It was found that the total amount of iodine that could have been emitted totaled 7000 TBq, but the actual amount of emitted reactive and particulate I-131 was 1800 TBq.  If cows graze on grass thats been contaminated by Ie-131, the radioactive material can easily be absorbed by the human thyroid gland upon consumption of cows milk.  Thus, the government instituted a milk ban in the surrounding Windscale areas, ranging from 10 km north of the Piles to 20 km to the south.  This proved to be a good decision, as the thyroid dose levels would have been higher than the safe threshold without it.  There are several estimates for the level of Cs-137 that was emitted, and these values range from 90 to 350 TBq.  The Cs-137 would have been released in the form of particulates, and air filters were used to sample the data. Other techniques also included measuring activity in fuel and estimated release fraction. Aside from Cs-137 and I-131, it was estimated that 3,000 to 5,000 TBq of tritium, 14 to 110 TBq of Po-210, and 0.003 to 0.1 TBq of Pu-239 were released.
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