Chernobyl Nuclear Power Plant in northern Soviet Ukraine suffered a critical failure on April 26, 1986. On the night of the incident, at 1.23am local time, Reactor Four of the Soviet power plant suffered two deadly explosions. The Chernobyl explosion tore the roof of the reactor’s building, flipped the reactor’s nuclear core and exposed the whole of Europe to radioactive isotopes. The nuclear disaster instantly killed one Chernobyl plant worker and another 28 people in a matter of weeks but at least 4,000 people have developed radiation-related cancer in the years that followed.
How did the Chernobyl nuclear explosion happen?
The exact details of what happened on the night of April 26 are forever lost to history but scientists have a very good approximation of how the incident unfolded.
Chernobyl Power Plant in north Ukraine, near the border of Belarus and 81 miles (130km) from Kiev, housed four RBMK-1000 nuclear reactors.
The Soviet reactor design emerged in the 1970s and according to the World Nuclear Association, “had several shortcomings” that played part in the 1986 disaster.
The nuclear rector operates as a “light water graphite reactor” by boiling water in fuel channels to create steam, which then powers turbines and generates electricity.
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Chernobyl explained: The catastrophy was a mix of human error and design flaws
Graphite in the nuclear reactor acts as a moderator used to help sustain the ongoing nuclear chain reaction in combination with water acting as a coolant.
This was a major design flaw found “in no other power reactors in the world”.
The Nuclear Association said: “As the Chernobyl accident showed, several of the RBMK’s design characteristics – in particular, the control rod design and a positive void coefficient – were unsafe.
“A number of significant design changes were made after the Chernobyl accident to address these problems.”
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On April 25, 1986, the reactor crew operating Chernobyl’s Reactor Four was instructed to drastically power down the reactor to perform a safety test.
The test would asses the reactor’s ability to keep water flowing through the reactor during a power outage, solely by the reactor’s spinning down turbines before a backup generator would come on.
The safety test was carried out at the plant previously but has failed to produce positive results.
The Nuclear Association explained: “A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test on early April 26.
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“By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition.
“A peculiarity of the design of the controls rods caused a dramatic power surge as they were inserted into the reactor.”
Inside of a nuclear reactor, control rods limit or increase the rate of nuclear fission of uranium or plutonium by absorbing rogue neutron particles.
During nuclear fission, radioactive fuel rods of uranium eject neutrons in the hopes they hit other uranium particles and split into lighter elements, releasing energy.
Chernobyl explained: The power plant housed four RBMK-1000 reactors
The control rods used in Chernobyl were made from neutron absorbent boron carbide but were tipped with graphite – a material that initially caused the rate of fission to spike.
A peculiarity of the design of the controls rods caused a dramatic power surge
As the nuclear fuel rods got hotter, they turned the reactor core’s water into steam.
The pressure inside of the reactor then reached a critical point, destroying the rector and blowing off the 1,000-tonne reactor cover plate.
This, in turn, jammed all of the control rods while they were only halfway down to the core.
Chernobyl explained: The control room of the destroyed Chernobyl Reactor Four
Chernobyl explained: The reactor explosion obliterated the Reactor Four building
The Nuclear Association said: “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.”
According to the association, the exposed reactor core released about five percent of its nuclear material into the atmosphere.
Chernobyl explained: Thousands of people suffered radiation-related ailments in the aftermath
But human error also played a role in the nuclear disaster.
When the safety test was initiated, the Chernobyl reactor was brought down to a power output of 720MWt, despite operations below 700MWt being strictly forbidden.
The reactor then proceeded to drop to around 500MWt before suddenly plummeting to just 30MWt.
As the reactor core’s crew scrambled to bring the power back up to around 200MWt, safety precautions were violated.
For instance, the reactor was required to always have a minimum of 15 control rods extended into the core but only eight were inserted during the failed test.