The Chernobyl disaster was a nuclear accident that occurred at Chernobyl nuclear power plant in on April 26, 1986. A nuclear meltdown in one of the reactors caused a fire that sent a plume of radioactive fallout that eventually spread all over Europe.
The most terrific and disastrous accident took place in 1986 at Chernobyl, which left the world in shock. Chernobyl nuclear reactor plant, built at the banks of Pripyat river of Ukraine, had four reactors, each capable of producing 1,000 MW of electric power.
On the evening of April 25th, 1986, a group of engineers, planned an electrical-engineering experiment on number 4 reactor. With their little knowledge of Nuclear physics, they thought of experimenting with how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply.
Actually, the reactor unit 4 was to be shut down for routine maintenance on 25 April 1986. But, it was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the main core cooling water circulating pumps, until the diesel emergency power supply became operative. The aim of this test was to determine whether cooling of the core could continue to be ensured in the event of a loss of power.
Due to the misconception that this experiment belongs to the non-nuclear part of the power plant, was carried out without a proper exchange of information between the testing department and the safety department. Hence the test was started without adequate safety precautions and the operating personnel were not alerted to the nuclear safety implications of the electrical test and its potential danger.
According to the test planned, the Emergency Core Cooling System (ECCS) of the reactor, which provides water for cooling the reactor core, was shut down deliberately.
For the test to be conducted, the reactor has to be stabilized at about 700-1000 MW prior to shut down, but it fell down to 5000 MW due to some operational phenomenon. Then, the night shift operator committed an error, by inserting the reactor control rods so far. This caused the reactor to go into a near-shutdown state, dropping the power output to around 30 MW.
Since this low power was not sufficient to make the test and will make the reactor unstable, it was decided to restore the power by extracting the control rods, which made the power stabilize at 200 MW. This was actually a violation of safety law, due to the Positive void co-efficiency of the reactor. The positive void coefficient is the increasing number of reactivity in a reactor that changes into steam. The test was decided to be carried out at this power level.
Actually, the reactors were highly unstable at a low power level, primarily owing to control rod design and positive void coefficient factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water.
The figure below shows the reactor 4 where the experiment was conducted. This picture was taken after everything was restored.
At 1:23, on April 26th, 1986, the engineers continued with their experiment and shut down the turbine engine to see if its inertial spinning would power the reactor’s water pumps. In fact, it did not adequately power the water pumps and without the cooling water, the power level in the reactor got surged.
The water pumps started pumping water at a slower rate and them together with the entry to the core of slightly warmer feed water, may have caused boiling (void formation) at the bottom of the core. This, along with xenon burn out, might have increased the power level at the core. The power level was then increased to 530 MW and continued to rise. The fuel elements were ruptured and lead to steam generation, which increased the positive void coefficient resulting in high power output.
The high power output alarmed the engineers who tried to insert all the 200 control rods, which is a conventional procedure done in order to control the core temperature. But these rods got blocked half the way, because of their graphite tip design. So, before the control rods with their five-meter absorbent material, could penetrate the core, 200 graphite tips simultaneously entered the core which facilitated the reaction to increase, causing an explosion that blew off the 1,000-ton heavy steel and concrete lid of the reactor, consequently jamming the control rods, which were halfway down the reactor. As the channel pipes begin to rupture, mass steam generation occurred as a result of depressurization of the reactor cooling circuit.
As a result, two explosions were reported. The first one was the initial steam explosion. Eventually, after two to three seconds, a second explosion took place, which could be possibly from the build-up of hydrogen due to zirconium-steam reactions.
All the materials such as Fuel, Moderator, and Structural materials were ejected, starting a number of fires and the destroyed core was exposed to the atmosphere. In the explosion and ensuing fire, more than 50 tons of radioactive material were released into the atmosphere, where it was carried by air currents. This was 400 times to the number of radioactive materials released at the time of Hiroshima bombing.
The Chernobyl nuclear power plant in Ukraine is the only accident in the history of commercial nuclear power to cause fatalities from radiation.
There were many fatal effects due to the radiation released.
This nuclear accident has been the most disastrous one that the place was so inflicted that it is not safe to live there even today.