Chernobyl nuclear accident, Soviet Union

Chernobyl, What Happened in the Nuclear Accident?

The Chernobyl nuclear accident (1986) is by far the worst nuclear accident in history. Level 7 of the INES scale, the highest value (serious nuclear accident). Although it is the same level at which the Fukushima nuclear accident was classified, the consequences of the Chernobyl accident were still much worse.

Chernobyl, what happened in the nuclear accident?

The Chernobyl Nuclear Power Plant is located next to the city of Pripyat, 18km from the city of Chernobyl; in northern Ukraine.

On September 9, 1982, a partial meltdown of the base occurred in reactor # 1 at the plant. Although due to the secrecy of the Soviet Union, the international community was not informed until 1985. It was repaired and continued to function.

At the time of the accident, the plant had 4 active reactors. Two more were under construction.

The great Chernobyl disaster occurred in 1986. In this new accident the fourth reactor exploded. Then, despite the severity of the accident and due to energy needs, reactors 1, 2 and 3 continued to operate.

Chernobyl nuclear reactor 2 was closed in 1991. Reactor 1 closed in 1996. Reactor 3 stopped working in 2000.

The nuclear accident inspired the miniseries that premiered in 2019.

Chernobyl is a historical drama miniseries created by Craig Mazin. Directed by Johan Renck. The series revolves around the Chernobyl disaster of April 1986 and the unprecedented cleanup efforts that followed.

How Did It Occur in the Chernobyl Nuclear Accident?

The Chernobyl atomic disaster originates during a test at the nuclear power plant. The succession of technical errors of the personnel in charge of the development of the test caused a spectacular atomic explosion.

The disaster occurs overnight from April 25 to 26, 1986 in the plant's fourth reactor. It was a reactor that belongs to the type the Soviets call RMBK-1000, water-cooled and graphite-moderated.

A huge amount of radioactive particles was released into the atmosphere. Radioactivity levels skyrocketed; especially in the city of Prípiat, 18 kilometers from the explosion. The emitted nuclear radiation was registered in countries far removed from the USSR.

Poor management after the accident, especially during the first hours, contributed to aggravating the consequences.

What Caused the Chernobyl Nuclear Disaster?

The motive that triggered the Chernobyl atomic disaster was a test. The test was scheduled for April 25. The address was taken from the Moscow headquarters.

Chernobyl, what happened in the nuclear accident?This test was intended to increase the safety of the reactor. The aim was to find out how long the steam turbine would continue to generate electrical energy once the steam flow had been cut off.

In the event of a breakdown, the emergency coolant pumps required a minimum of power to start (until the diesel generators were started). The plant's technicians were unaware whether, after the flow of steam was cut, the inertia of the steam turbine could keep the pumps running.

The test had to be performed without stopping the chain reaction in the nuclear reactor to avoid a phenomenon known as xenon poisoning. Among the fission products produced within the reactor is xenon135. It is a highly absorbent neutron gas. Neutrons are necessary to maintain nuclear fission chain reactions.

While the plant is operating normally, so many neutrons are produced that absorption is minimal. When the power is too low or the reactor stops, the amount of 135Xe increases and prevents the chain reaction. After a few days, the reactor can be restarted when the 135Xe decays.

Reactor Safety Test Start

One in the morning of April 25. Engineers initiated the entry of the control rods into the reactor core. The goal was to reduce its power.

By 23:00 the monitors had been adjusted to the lowest power levels. But the operator forgot to reprogram the computer to keep power between 700 megawatts electric and 1,000 megawatts thermal. For this reason, the power dropped to the 30 MW level.

Chernobyl, what happened in the nuclear accident?With such a low power level, automatic systems can stop the reactor. It is considered a dangerous situation. Precisely for this reason, the operators switched off the protection systems when the system was about to shut down the reactor. Among them, the power regulation system and the emergency system that cools the core.

Xenon Poisoning

At 30 MW, xenon poisoning begins to occur in the reactor core.

On realizing it, the control rods were removed in order to avoid it by increasing the power of the reactor. Operators manually removed too many control bars.

The reactor core had 170 control bars. The security rules required that there should always be a minimum of 30 bars lowered. This time they left only 8.

The plant's reactor was left in unstable operating conditions. Very insecure. The reason is that the plant's security systems were disabled. Furthermore, almost all of the control bars had been removed.

At that time, a sharp increase in power occurred. The operators did not detect in time.

They wanted to lower the control bars again using the emergency SCRAM button. They did not respond. The most likely reason is that they were possibly already deformed by the heat.

What is a SCRAM? A SCRAM is the emergency shutdown of a nuclear reactor. However in modern nuclear power plant operation, the term used is usually reactor trip.

Reactor Explosion

Eventually, the nuclear fuel disintegrated. It came out of the sheaths coming into contact with the water used to cool the reactor core.

At 1:23 AM, there was a huge explosion.

A few seconds later, a second explosion blew up the reactor slab and the concrete walls of the reactor room. The explosion released fragments of graphite and nuclear fuel outside the plant. Radioactive dust rose through the atmosphere with sky-high radiation levels.

The amount of radioactive material released is estimated to be 200 times greater than that of the atomic bombs dropped on Hiroshima and Nagasaki. At the end of World War II.

The accident was classified as level 7 on the INES Scale. It is the highest possible level, that is, the accident with the worst environmental consequences.

What Was the Political and Social Situation Prior to the Accident?

The accident took place due to clear human error. However, the social and political factors of the Soviet Union must be taken into account. At that time the president was Mikhail Gorbachev.

There was no democratic social structure. There was no control carried out by the company over the operation of nuclear power plants. Nor was there a culture of safety.

Why did operators deactivate security systems? Possibly for fear of not following instructions received from Moscow.

Chernobyl, what happened in the nuclear accident?Nor was there any regulatory body for nuclear safety. There was no one with their own authority and independence. Therefore, no safety inspection and evaluation of nuclear facilities was carried out.

Technical aspects of reactor safety. Please note that there is no containment system in RMBK reactors that covers the primary circuit. There is also no containment building capable of holding fission products in the event of an accident.

What Consequences Did the Chernobyl Nuclear Disaster Have?

The explosion caused the largest catastrophe in the history of civilian exploitation of nuclear energy.

The origin was the execution of a test. At the time of the accident, 31 people died. Around 350,000 people had to be evacuated from the affected 155,000 square kilometers. For many years, vast areas were uninhabited.

Radioactive contamination spread to western Europe as far as France. He did not cross the Pyrenees. In these areas, the levels of radioactivity were above innocuous levels of radiotoxicity for several days.

It is estimated that radiation from the atomic bomb dropped in Hiroshima in 1945 was released about 500 times. The city of Pripyat, closest to the nuclear disaster, was completely abandoned.

Chernobyl, what happened in the nuclear accident?

Right after the accident, the main health problem came from iodine -131, with a half-life of eight days. Currently, however, the main concern is contamination of the soil with strontium -90 and cesium -137, with half-lives of around 30 years.

The highest concentrations of cesium-137 are found in the surface layers of the soil. Plants, insects and fungi absorb radiation. In this way, they enter the food chain. Radioactivity is feared to affect the local population for generations.

What Studies Have Been Carried out to Assess the Consequences of the Accident?

In 2005, the Report of the International Atomic Energy Organization produced the last report that details:

  • Number of deaths directly caused by the accident in 59 people, 48 of them plant workers.
  • The number of cases of thyroid cancer due to radiation exposure recorded has been more than 4,000.
  • An estimated 600,000 people were affected by radiation. Of these, at least 3,500 would die as a consequence of the lethal dose received. Most were among the 2,500 workers and military personnel who built the first cement sarcophagus.

Another study states that the data provided by Ukraine is incomplete. It is estimated that 53,000 people died. Half a million of them died due to the radioactive cloud, which contaminated much of Europe. 30,000 would die in the next few years.

These evaluations differ significantly from the WHO and IAEA research. According to Greenpeace, a total of 30% of the areas where nine million people live have been contaminated with cesium-137.

According to a technician from the Ukrainian government science center, cases of thyroid cancer, leukemias and genetic mutations were recorded in Ukraine. These diseases do not appear in the statistics. They were practically unknown twenty years ago.

What Is the Exclusion Zone?

The exclusion zone is an officially restricted zone around the plant that was established as a result of the accident.

The Soviet Union established this zone militarily after the atomic disaster. Initially it was an area within a radius of 30 km from the nuclear power plant for evacuation and under military control.

Currently the boundaries of the area have changed markedly. From the 30 kilometers around the reactor, a large area of ​​Ukraine has been covered. The sarcophagus that covers the old nuclear power plant is managed separately.

Currently, the exclusion zone covers an area of ​​about 2,600 square kilometers.

The purpose of this exclusion zone is:

  • Restrict access to dangerous areas.
  • Reduce the extent of radiological contamination.
  • Direct radiological activities.
  • Monitor ecological activities.

Red Forest

The Red Forest is a forest area of ​​about ten square kilometers near the plant. The pine forest that grew there became extinct. It turned yellow and red due to high levels of radiation. The trees were cut down and partly buried.

Touristic Interest

Currently, this exclusion zone is one of the most radioactively contaminated areas in the world, but it arouses great scientific and even tourist interest.

Progressive Closure of the Other Three Chernobyl Nuclear Reactors

chernobyl, What Happened in the Nuclear Accident?

Despite the serious accident at reactor number 4, reactors 1, 2 and 3 continued to operate. The reason was energy needs.

In 1991 a turbine in reactor number 2 caught fire. It was thought to repair it using one of the reactor 4 turbines that were not damaged. But by then, the political context had varied, which together with popular pressure caused the definitive closure of reactor 2.

Reactor 1 stopped operating on November 31, 1996. The shutdown occurred after severe cooling deficiencies. These deficiencies resulted in a Level 3 nuclear incident on the INES scale.

Finally, the third Chernobyl nuclear reactor would be closed shortly thereafter, on December 15, 2000. Reactor 3 had already had several fires. The structure was affected by corrosion. After lengthy negotiations with the Ukrainian government, the international community financed the costs of the plant's final closure.


  1. Start of a technical test related to safety.
  2. Automatic security systems will be disconnected.
  3. Xenon poisoning occurs in the reactor core.
  4. Almost all control bars are manually removed.
  5. The reactor power is increased too much.
  6. Fuel breaks down.
  7. Two consecutive explosions occur.
  8. A large amount of radioactive particles is released into the atmosphere.

Published: April 27, 2010
Last review: May 12, 2020