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.
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.
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
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.
With 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.
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.
The 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).
- 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.
Current status and future prospects of Chernobyl
During the seven months following the accident, the wreckage of the nuclear reactor 4 accident were buried by the liquidators, through the construction of a "sarcophagus" with 300,000 tons of concrete and metal structures lead to prevent the spread of nuclear fission products. In principle, this sarcophagus was a temporary solution and should be under strict control because of its long-term instability because a collapse could occur.
Recovery from the crash site and cleaning products has resulted in a large amount of radioactive waste and contaminated equipment, stored in about 800 different sites inside and outside the exclusion zone of 30 km around the nuclear reactor 4 of Chernobyl.
These are partially nuclear waste stored in containers or buried in trenches and can cause risk of contamination of groundwater.
It has been assessed that the sarcophagus and the proliferation of storage sites residues represent a dangerous source of radioactivity in the nearby areas, and some experts from the NEA feared the collapse of rugged reactor would cause severe damage to the only operating reactor until December 15, 2000, the reactor 3.
The following images correspond to the appearance of abandonment which currently has the city of Prypyat, the town closest to the Chernobyl power plant.
International Conference in Vienna
In the Vienna International Conference, held in April 1996, it was concluded that the total rehabilitation of the area was not possible due to the existence of "hot spots" of pollution, risk of groundwater contamination, restrictions on food and risks associated with the possible collapse of the sarcophagus, since deterioration in the years following the accident. It was noted that it was necessary to carry out a full program of research to develop a suitable design to serve as a safe confinement system from an ecological point of view, avoiding rainwater seeps inside and preventing the collapse of the existing sarcophagus, which would cause the escape of radioactive dust and debris nuclear fuel (uranium and plutonium) into the environment.
International aid programs
In this situation, the authorities and the nuclear industry in Western countries are making remarkable efforts to help the Eastern European countries to improve the safety of its reactors, including the RBMK, and we can say that at present, the situation of these countries is much better than in 1986.
Among the aid programs of the European Union highlights the TACIS (1989) and PHARE (1990) programs. All financial contributions are transferred to a fund managed by the EBRD (European Bank for Reconstruction and Development) known as "Chernobyl Shelter Fund (CSF)". The EBRD will administer the fund on behalf of the contributing countries and donors, being responsible to the Assembly, which meets 3 or 4 times a year. At present, it has 22 members, including the European Union and Ukraine.
The TACIS Programme funded, in 1996, the first study to analyze, in a first phase, the possible short- and long-term to remedy the deplorable state of the sarcophagus, and eventually transform it in a safe location.
At first, there were two alternatives: to bury the coffin in a concrete block and build a new enclosure that completely cover the injured reactor 4 and the reactor 3.
In May 1997, a group of European, American and Japanese experts, funded by the program, prepared the SIP (Shelter Implementation Plan). The objectives of the plan to convert the sarcophagus in a safe location were as follows:
- Reduce the risk of collapse of the sarcophagus.
- In the event of sinking, limiting the consequences.
- Improve the nuclear safety of the sarcophagus.
- Improve worker safety and environmental protection in the sarcophagus.
- Convert the location of the sarcophagus into a safe area from the environmental point of view.
In addition, the SIP set three goals to achieve
- Strategic decision to follow in terms of stability and security.
- Strategy to follow regarding the problem of damaged and scattered nuclear fuel inside the sarcophagus.
- Decision of the new type of enclosure to be built.
According to the program, the project should be completed in 2007. In May 2001, carried out the tasks of stabilization and other short term measures, constituting the first phase of the SIP. Preliminary technical studies were also performed to determine a strategy for improving security systems and prepare, in a second phase, the sarcophagus as a safe place.
As for the type of protective enclosure, it was finally decided to build a wide arc of metal dome inside which would be the damaged 4 damaged because it offered many advantages in reducing radiation dose, safety during construction, the current release of unstable structures, greater space for dismantling and flexibility to cope with the uncertainties of removal of the damaged nuclear fuel and dispersed.
This metal archway, under construction since 2002 and until 2005, with a cost of $ 700 million, will house units 3 and 4 of the Chernobyl plant, under the waterproof wall internally pressurized double wall with a foundation of 27 meters deep.
Unit 3 of the Chernobyl nuclear power plant, definitively stopped on 15 December 2000. Both the Ukrainian and foreign experts, set closing costs between 2,000 and 5,000 million dollars, to remove the radioactive fuel remaining in the nuclear plant deadline in 2008. This decision completed the total closure of the nuclear facility that had led the April 26, 1986, the worst nuclear disaster in the history of nuclear power.
Phasing the other three Chernobyl nuclear reactor
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.
- World Nuclear Assosiation - Chernobyl Nuclear Accicent
- SSE Chernobyl Nuclear Power Plant
- The long shadow of Chernobyl
- 25 años de imagenes satelitales de Chernobyl
- El accidente de Chernobil y sus causas
Last review: July 10, 2015