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

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

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

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

In September 9, 1982, a partial melting of the base occurred in the No. 1 reactor plant. Although due to the secrecy of the Soviet Union, the international community was not informed until 1985. It was repaired and continued working.

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 followed up.

The Chernobyl nuclear reactor number 2 was closed in 1991, the reactor number 1 in 1996 and the reactor number 3 stopped working in 2000.

Chronology of the Chernobyl nuclear accident

The nuclear accident in Chernobyl (Ukraine) occurs during the night of 25 to 26 April 1986 in the fourth reactor of the nuclear power plant. It was a nuclear reactor that belongs to the Soviets call type RBMK-1000 water-cooled graphite moderated.

Origin nuclear accident: the completion of a test

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

This test was intended to increase the safety of the nuclear reactor. It was to find out how long the steam turbine would continue generating electricity once cut the flow of steam.

Chernobyl nuclear power plant before the nuclear accident

In case of failure, the emergency coolant pumps require minimal power to get going (until torn diesel generators) and plant technicians didn't know whether, once cut the flow of steam the inertia of turbine could keep running the pumps.

The test was performed without stopping the chain reaction in the nuclear reactor to avoid a phenomenon known as xenon poisoning. Among the fission products produced in the reactor, is the Xenon135, a gas with a very high capacity to absorb neutrons (neutrons are needed to sustain nuclear fission reactions in chain). While in normal operating mode, there are so many neutrons produced that the absorption is minimal, but when the power is too low or the nuclear reactor is stopped, the amount of Xenon135 increases and prevents the chain reaction for a few days. The reactor can be restarted when the Xenon135 decays.

Starting the test

At one in the morning of April 25, engineers began the entry of the control rods in the core of the nuclear reactor in order to reduce its power.

Towards the 23 hours they were set monitors to lower power levels. But the operator forgot to reprogram the computer to mantain the power between 700 MW and 1,000 MW thermal. For this reason, the power down to the level of 30 MW.

With a low level, automatic systems can stop the reactor due to its danger and therefore the operators disconnected the system power regulation, the coolant system emergency from the core and other protective systems when the system was ready to shut down the nuclear reactor.

Control room of the Chernobyl nuclear power plantWith 30MW xenon poisoning began. When they realized it the control rods were extracted in order to avoid increase the nuclear reactor power. The operators manually removed too many control rods. The reactor core had 170 available control rods. Safety rules had always required that at least 30 were down but at this time they left only 8.

Due the security systems of the plant were disabled and almost all control rods had been removed , the nuclear reactor conditions remained extremely unstable and unsafe operation. At that time, a sharp increase in power that operators did not detect in time took place.

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

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

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

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 level, ie, the nuclear accident with worst environmental consequences.

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

Although the accident took place by a clear human error, social and political factors of the Soviet Union at that time must be taken into account. The lack of a democratic social structure led to a lack of control of the company on the operation of nuclear power plants and a "safety culture". Perhaps the fear of traders not to comply with instructions received from Moscow, led them to dismantle essential safety systems for reactor control.

Nor was there any Regulatory Organization Nuclear Safety which occupy with their own authority and independence of the inspection and assessment of the safety of nuclear installations.

As for the technical aspects of nuclear reactor safety, keep in mind that in the RBMK reactors there is no confinement system covering the primary circuit and there is no containment building which can retain fission products in case if nuclear accident, as in western nuclear reactors.

Consequences of the Chernobyl nuclear accident

The nuclear accident led to a subsequent fire, which was not achieved off until 9 May. The fire increased the effects of dispersion of radioactive materials, and the heat energy stored in the graphite still gave greater magnitude to own fire and the atmospheric dispersion.

Radioactive products released were especially hazardous iodine-131 (whose half-life is 8.04 days) and cesium-137 (with a half-life of about 30 years), of which, approximately half, they left the amount contained in the nuclear reactor. Furthermore, it was estimated that around the xenon gas was discharged outside the Chernobyl nuclear reactor. These products were deposited unevenly, depending on their volatility and the rain during those days.

the heavier ones were found within a radius of 110 km, and the more volatile ones achieved great distances. Thus, besides the immediate impact on Ukraine and Belarus, the radioactive contamination reached areas of the European part of the former Soviet Union, and the United States and Japan.

International Programme on the Health Effects of the Chernobyl Accident

To determine the effects of radiation on people's health, the World Health Organization developed the IPHECA (International Programme on the Health Effects of the Chernobyl Accident), so that they could be investigated the possible health consequences of the Chernobyl nuclear accident. These consequences include related effects relate with the anxiety produced in the inhabitants of the areas most contaminated as the result of evacuation from their homes, and the fear of possible future damage to health by the biological effects of radiation. In addition, the program provided technical assistance to the national health system in Belarus, the Russian Federation and Ukraine, to alleviate the health consequences of the Chernobyl nuclear accident.

Aliementos contamination in ChernobylThe results obtained with IPHECA pilot projects have greatly improved scientific understanding of the effects of a radioactive accident on human health, so that they can lay the foundations of planning guidelines and the development of future research.

The immediate consequences of the accident on the health of the people were:

  • 237 people showed symptoms of Acute Radiation Syndrome (ARS), confirming the diagnosis in 134 cases. 31 people died in the nuclear accident, of which, 28 (firefighters and operators) were victims of high doses of radioactivity, and 3 from other causes. After the acute phase, 14 more people have died in the ten years following the Chernobyl nuclear accident.
  • Between 600,000 and 800,000 people (skilled workers, volunteers, firefighters, military and others), called liquidators, charged with the tasks of control and cleaning, died in different periods.
  • 16,000 local residents were evacuated several days after the Chernobyl nuclear accident, as a protection against high levels of radioactivity, establishing an exclusion zone in the most contaminated areas within a radius of 30 km around the facility.
  • 565 cases of thyroid cancer in children mainly (aged between 0 and 14) and some adults living in the most contaminated areas (208 in Ukraine, 333 in Belarus and 24 in the Russian Federation) of which, 10 cases proving fatal due to radiation.
  • Other cancers, including leukemia, have been no statistically significant deviations from the expected incidence in normal conditions.
  • Psychosocial effects from causes unrelated to radiation, due to lack of information, evacuation of affected and fear of biological effects of long-term radiation. These effects were due to the shocked reaction of national authorities to the Chernobyl nuclear accident, as to the extent, duration and pollution over long distances. As emergency procedures were nonexistent, little information was available, it being noted mistrust and public pressure for action to be taken, but the official decision did not take into account the psychological effects of the population, carrying out misinterpretation of recommendations the International Commission on Radiological Protection (ICRP) for intervention levels in food. All this was translated into a number of health disorders, such as anxiety, depression and various psychosomatic effects. The World Health Organization (WHO) has purchased medical equipment and supplies for the 3 countries (Belarus, Russia and Ukraine) worth about 16 million dollars. The remaining costs of the pilot project is dedicated to aid programs, scientific meetings, training courses in foreign research institutions and clinical institutions to 200 specialists, and provide capital to continue with the activities of IPHECA program.

According to the Atomic Energy Agency (AEA) of the OECD, the ranges of radiation dose received by the various groups were as follows:

  • Liquidators: of all the liquidators, about 200,000 received varying doses from 15 to 170 millisieverts ( mSv).

Liquidators of the Chernobyl nuclear accident Liquidators of the Chernobyl nuclear accident Liquidators of the Chernobyl nuclear accident Liquidators of the Chernobyl nuclear accident

  • Evacuated: 116,000 evacuees, most of a radius of the central of 30 km, received high doses (10% more than 50 mSv and 5% more than 100 mSv), especially in the thyroid by the  incorporation of iodine-131. The area was evacuated Prypiat, aproximatelly 2 km from the Chernobyl nuclear power plant scarce, becoming a "ghost town"; to leave the city the 60,000 people that was living there.
  • Inhabitants of contaminated areas: about 270,000 people continued liveing in contaminated areas so that children received high thyroid doses due to the ingestion of milk contaminated with iodine-131 during the first weeks after the nuclear accident. After the food control, during the period 1986-1989, the range of cesium-137 in soil was 5-250 mSv / year, with an average of 40 mSv / year.
  • Rest of the population: the volatile radioactive materials spread throughout the Northern Hemisphere, although the doses received by the population were very low and unimportant from the point of view of radiation protection. Radiation doses in the first year in Europe ranged between 0.005 and 0.5 mSv in Asia between 0.005 and 0.1 mSv, and in North America were the order of 0.001 mSv.

Phasing the other three Chernobyl nuclear reactor

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

Despite the serious nuclear accident in Chernobyl nuclear reactor 4, due to the energy needs reactors 1, 2 and 3 followed up.

In 1991 a turbine No. 2 nuclear reactor caught fire. He thought of repair using a turbine from the nuclear reactor number 4 which were not damaged. But by then, the political context had changed along with the popular pressure caused the closure of the reactor 2.

Reactor 1 stopped working on November 31, 1996, after refrigeration serious deficiencies that led to a nuclear incident level 3 on the INES scale.

Finally, the third nuclear reactor at Chernobyl was shut bit later, on 15 December 2000. The nuclear reactor 3 had already several fires and the structure was affected by corrosion. After protracted negotiations with the Ukrainian government, the international community financed the costs of decommissioning the plant.

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Last review: July 10, 2015