Chernobyl nuclear accident, Soviet Union

Abandoned City or Prypiat,
Chernobyl nuclear accident

Chernobyl control room.
Before the nuclear accident

Current status of the control room.
Chernobyl nuclear accident

Chernobyl Nuclear Accident

Chernobyl Nuclear Accident

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

The Chernobyl nuclear power plant is located next to the city of Prypyat, 18km from the city of Chernobyl.

At the time of the accident, the Chernobyl nuclear power plant had 4 reactors in operation and two more were under construction.

On September 9, 1982, a partial fusion of the base took place in the reactor No. 1 of 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.

Map of the Chernobyl nuclear power plant.  Chernobyl nuclear accident.

The serious accident occurred in 1986, when reactor number 4 exploded. Subsequently, despite the severity of the accident and due to the energy needs, reactors 1, 2 and 3 continued.

The Chernobyl nuclear reactor 2 was closed in 1991, reactor 1 in 1996 and reactor three ceased operation in 2000.

Chernobyl accident timeline

The Chernobyl nuclear accident (Ukraine) occurs overnight from April 25 to 26, 1986 in the fourth reactor of the nuclear plant. It was a nuclear reactor that belongs to the type that the Soviets call RMBK-1000, cooled by water and moderated by graphite.

Origin of the nuclear accident: conducting a test

The reason that triggered the Chernobyl nuclear accident was the realization of a test scheduled for April 25 under the direction of the Moscow headquarters.

This test was intended to increase reactor safety. It was about finding out how long the steam turbine would continue to generate electricity once the steam influx was cut off.

In the event of a breakdown, the emergency cooling pumps required a minimum of power to start up (until the diesel generators were started) and the plant technicians did not know if, after the steam influx was cut, the inertia of the turbine could keep the bombs running.

Chernobyl nuclear power plant before the nuclear accidentThe test should be performed without stopping the chain reaction in the nuclear reactor to avoid a phenomenon known as xenon poisoning. Among the fission products that are produced inside the reactor, there is xenon 135 , a very neutron-absorbing gas (neutrons are necessary to maintain nuclear fission chain reactions). While in normal operation, so many neutrons are produced that absorption is minimal, but when the power is very low or the reactor stops, the amount of 135 Xe increases and prevents the chain reaction for a few days. The reactor can be restarted when 135 Xe disintegrates .

Start of the test

At one o'clock in the morning on April 25, the engineers began the entry of the control rods into the core of the nuclear reactor in order to reduce their power.

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

With such a low level, the automatic systems can stop the reactor due to its danger and for this reason the operators disconnected the power regulation system, the core cooling emergency system and other protection systems when the system was already at point of shutting down the nuclear reactor.

Control room of the Chernobyl nuclear power plantWith 30 MW the xenon poisoning begins. Upon realizing, the control rods were removed in order to avoid increasing the power of the nuclear reactor. Operators manually removed too many control bars. The reactor core had 170 control rods. The safety rules required that there be always a minimum of 30 bars lowered and this time left only 8.

Since the safety systems of the plant were rendered useless and almost all the control rods had been removed, the plant's reactor was in unstable and extremely unsafe operating conditions. At that time, there was a sharp increase in power that operators did not detect in time.

When they wanted to lower the control rods again using the emergency SCRAM button, they did not respond because they were possibly already deformed by heat and disconnected them to allow them to fall by gravity.

Finally, the nuclear fuel disintegrated and left the pods, coming into contact with the water used to cool the reactor core. At one and 23 minutes, there was a big explosion, and a few seconds later, a second explosion blew up the slab of the reactor and the concrete walls of the reactor room, throwing fragments of graphite and nuclear fuel out from the plant, the radioactive dust rising through the atmosphere.

It is estimated that the amount of radioactive material released was 200 times higher than the atomic bombs dropped on Hiroshima and Nagasaki at the end of World War II.

Chernobyl nuclear accident

The nuclear accident was classified as level 7 (“serious nuclear accident”) on the International Nuclear Event Scale ( INES Scale) of the International Atomic Energy Agency (IAEA). This is the highest possible level, that is, the accident with the worst environmental consequences.

Political, social and technical considerations prior to the Chernobyl nuclear accident

Although the accident took place due to a clear human error, the social and political factors of the Soviet Union at that time must be taken into account. The lack of a democratic social structure implied an absence of control of society over the operation of nuclear power plants and a "culture of security." Possibly, the fear of the operators not to comply with the instructions received from Moscow, led them to disassemble the safety systems essential for the control of the reactor.

Nor was there any Nuclear Safety Regulatory Body that carried out the inspection and evaluation of the safety of nuclear facilities with its own authority and independence.

Regarding the technical safety aspects of the nuclear reactor, it must be taken into account that in RMBK reactors there is no confinement system that covers the primary circuit and there is no containment building capable of retaining fission products in the event of an accident. , as in western reactors.

Consequences of the Chernobyl nuclear accident

The explosion caused the greatest catastrophe in the history of civil exploitation of nuclear energy. Allegedly caused by the performance of a test, at the time of the accident 31 people died, and around 350,000 people had to be evacuated from the 155,000 km² affected, remaining large uninhabited areas for many years. The radiation extended to western Europe to France, without crossing the Pyrenees, so that in these areas the radioactivity indexes were above harmless levels of radiotoxicity for several days. It is estimated that the radiation of the atomic bomb launched in Hiroshima was released about 500 times in 1945.

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 soil contamination with strontium -90 and cesium -137, with half-life of about 30 years. The highest concentrations of cesium-137 are found in the surface layers of the soil, where it is absorbed by plants, insects and fungi and thus enter the food chain. It is feared that radioactivity affects the local population for generations.

In 2005, the IAEA produced the latest report detailing the number of deaths directly caused by the accident in 59 people, 48 of them at the plant. The cases of thyroid cancer accounted for have been more than 4,000. It is estimated that 600,000 people were affected by radiation, of which at least 3,500 will die as a result of it, including most of the 2,500 workers and military who built the first cement sarcophagus.

Another study obtains different results regarding Chernobyl. According to this, half a million people have died and the data provided by Ukraine is not complete. This would be the number of people (500,000) who would have lost their lives, due to the radioactive cloud, which polluted much of Europe. And another 30,000 would die in the coming years. These evaluations present an important difference with the WHO and IAEA research. According to Greenpeace in total, 30% of the areas in which nine million people live have been contaminated with cesium -137. According to a technician from the scientific center of the Ukrainian government, cases of thyroid cancer, leukemia and genetic mutations that are not listed in the WHO statistics are registered in Ukraine and were virtually unknown twenty years ago.

Progressive closure of the other three Chernobyl nuclear reactors

Monument to the victims of the nuclear accident at the Chernobyl nuclear power plant

Despite the serious nuclear accident of Chernobyl nuclear reactor 4, due to the energy needs reactors 1, 2 and 3 continued.

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

Reactor 1 ceased operation on November 31, 1996, following serious refrigeration deficiencies that resulted in a level 3 nuclear incident on the INES Scale.

Finally, the third Chernobyl nuclear reactor would close a little later, on December 15, 2000. Nuclear reactor 3 had already had several fires and the structure was affected by corrosion. After prolonged negotiations with the Ukrainian government, the international community financed the costs of the final closure of the plant.

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Last review: December 4, 2019