In the early morning of April 24, 1986, the Chernobyl Nuclear Power Plant exploded during a routine maintenance check. The accident, which saw the Chernobyl 4 reactor destroyed, is what many consider the worst nuclear disaster in history. More than 30 years after the accident, there are still many unanswered questions about the disaster, which caused the largest uncontrolled radioactive release into the environment ever recorded for any civilian operation.
What caused the Chernobyl explosion?
The Chernobyl explosion was the result of a “flawed Soviet reactor design”, operated by “inadequately trained personnel”, according to the World Nuclear Association.
The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors — a design that’s now universally recognised as inherently flawed.
The nuclear plant operators made serious mistakes – a consequence of “Cold War isolation and the resulting lack of any safety culture”, the association adds.
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According to the organisation, the steam explosion and fires from the Chernobyl 4 reactor “released at least five percent of the radioactive reactor core into the atmosphere”.
On April 25, the day before the accident and prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply.
The same test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were tested.
A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on April 26.
By the time the operator moved to shut down the reactor, it was already in an extremely unstable condition.
The World Nuclear Association said: “The overpressure caused the 1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down.
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“Intense steam generation then spread throughout the whole core (fed by water dumped into the core due to the rupture of the emergency cooling circuit) causing a steam explosion and releasing fission products to the atmosphere.”
About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite.
There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.
The accident destroyed the Chernobyl 4 reactor, killing at least 30 operators and firefighters within three months of the explosion.
While there is rough agreement that a total of either 31 or 54 people died from blast trauma or acute radiation syndrome (ARS) as a direct result of the disaster, there is considerable debate concerning the accurate number of deaths due to the disaster’s long-term health effects.
Long-term death estimates range from 4,000 (per the 2005 and 2006 conclusions of a joint consortium of the United Nations and the governments of Ukraine, Belarus, and Russia), to more than 93,000.
Who was to blame?
Anatoly Stepanovich Dyatlov was the deputy chief-engineer of the Chernobyl Nuclear Power Plant, and the supervisor of the catastrophic safety test which resulted in the Chernobyl disaster.
Dyatlov was overseeing a test of the nuclear power plant’s emergency safety mechanisms on the night the disaster took place.
According to reports following the incident, the chief-engineer threatened plant workers with termination if they failed to carry out the tests.
As core supervisor, he shouldered most of the blame, but his involvement is only one part of the story.
Chernobyl investigators tried Dylatov alongside chief Chernobyl engineer Nikolai Fomin and plant manager Viktor Bryukhanov, for mishandling the event and failing to follow safety protocols.
All three were convicted of their alleged crimes and sentenced to 10 years in prison.
But serious mistakes made by what the World Nuclear Association have called “inadequately trained personnel” have also been made part of the blame.