A safety test’s failure made Chernobyl a maximum-scale accident
On 26 April 1986 reactor 4 at Chernobyl near Pripyat exploded—one of only two nuclear accidents rated level 7 on the International Nuclear Event Scale. Two engineers died at once; among 237 hospitalized workers, 134 showed acute radiation syndrome and 28 died within three months.
Even a shut-down reactor keeps producing heat: fission supplies most of it, but more than 6 percent comes from radioactive decay that carries on regardless. Chernobyl's RBMK reactors were cooled by water driven through electric pumps, and reactor 4 alone had 1,661 fuel channels needing over 45 million litres of coolant an hour. If the plant lost power, three diesel generators per reactor took 60 to 75 seconds to reach the 5.5 MW needed for a single main pump. Engineers hoped the spinning-down steam turbine could power the pumps for about 45 seconds and narrow that gap.
Nobody had proved it. A 1982 trial showed the generator's excitation voltage was too low; a modified 1984 attempt also failed; a 1985 run produced nothing because its recording equipment broke. A fourth test was set for a planned power-down of reactor 4 before maintenance. Its authors treated it as a purely electrical exercise, did not understand how the RBMK-1000 would behave in those conditions, and under the rules needed no sign-off from the reactor's designers or the safety regulator. The programme even called for disabling the emergency core cooling system.
On 26 April 1986, after an accidental drop in power, operators ran the test. When they shut the reactor down, design flaws triggered a power surge, components ruptured, and steam explosions and a meltdown destroyed the building. The fire that followed spread radioactive material over the Soviet Union and Europe. A 10 km exclusion zone was set up 36 hours later, moving about 49,000 people, and its expansion to 30 km displaced roughly 68,000 more. Moscow admitted the accident only two days on, after Sweden detected rising radiation.
Only Chernobyl and Fukushima in 2011 carry the top rating on the International Nuclear Event Scale. More than 500,000 people took part in the response, and total costs are estimated at US$700 billion, the most expensive disaster on record. By 2005 some 6,000 childhood thyroid cancers had occurred in affected populations, 15 of them fatal; projections of eventual deaths vary widely, from as many as 4,000 to roughly 16,000. Pripyat was replaced by the new town of Slavutych, a sarcophagus went up by December 1986, and the New Safe Confinement built over it in 2016 to 2018 should allow clean-up to finish by 2065.
Source: Chernobyl disaster
