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The control rods, 250 in total, were simultaneously moved back into the reactor. This dramatically increased the already excessive level of neutron activity. Nuclear reactivity soon reached a level where the prompt neutrons alone could cause the nuclear chain reaction, i.e., without requiring any delayed neutrons (a phenomenon described as “prompt criticality”). Within milliseconds, the power output soared to a level a hundred times higher than the nominal value – a type of surge technically described as a “nuclear power excursion”.
The abnormally high temperature quickly warped the control rod channels, preventing the rods from fully entering the reactor core and acting as effective absorbers. Melting in the heat, the fuel elements ruptured, releasing large amounts of hydrogen. This mixed with the oxygen in the air to form oxy-hydrogen, or detonating gas. The gas ignited just seconds after the nuclear excursion, causing a powerful explosion.
The reactor’s sealing cap, weighing more than 1,000 tons, was blown off, destroying the roof of the reactor building, which had been designed to protect the reactor from the weather. As the reactor core was no longer sealed, the graphite in the core started to burn. Over the next ten days, 250 tons of graphite was consumed in the fire; approximately 15 % of the plant’s total supply.
Large amounts of radiation were released. The graphite’s extreme temperature when burning meant that the radioactive materials were propelled high into the atmosphere. There, volatile isotopes such as iodine-131 and caesium-137 formed deadly aerosols. Before these could be washed out of the atmosphere by rain they were blown away as radioactive clouds that travelled hundreds, even thousands of km. Radioactive metals with even higher boiling points were released in the form of dust particles; these settled in the vicinity of the reactor. |
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International
Fallout
