In April 1986, Chernobyl' (Chornobyl' in Ukrainian) was an obscure city on the Pripiat' River in north-central Ukraine. Almost incidentally, its name was attached to the V.I. Lenin Nuclear Power Plant located about twenty-five kilometers upstream. On April 26, the city's anonymity vanished forever when, during a test at 1:21 A.M., the No. 4 reactor exploded and released thirty to forty times the radioactivity of the atomic bombs dropped on Hiroshima and Nagasaki. The world first learned of history's worst nuclear accident from Sweden, where abnormal radiation levels were registered at one of its nuclear facilities.
The Chernobyl reactors are of the RBMK type. These are high-power, pressure-tube reactors, moderated with graphite and cooled with water. The operational experience of military reactors testified that they can be reliably operated. However, there were accidents, emergencies occurred, but nothing was known about them. That created the illusion that military reactors can be used for nuclear power plants. This was a strategic mistake.
At the time of the Chernobyl accident there were seventeen RBMKs in operation in the Soviet Union. Since then, five RBMKs have been shut down. All four RBMKs at Chernobyl were shut down: Unit 4 reactor was destroyed in the 1986 accident; Unit 2 reactor was shut down five years later; after a serious turbine building fire; Unit 1 was closed in November 1996, and Unit 3 was closed December 15, 1999 as promised by Ukrainian President Leonid Kuchma. Ignalina Unit 1 in Lithuania, the fifth RBMK, was shut down in December 2004. The remaining 12 operating RBMKs are in Russia (eleven reactors) and Lithuania (one reactor).
The plant lies near the Pripyat River, at the northwest end of a cooling pond. The pond is 12 km long; during normal operation the plant discharges warm water counterclockwise around the pond, taking in cool water near the north end. Just northwest of the plant is the city of Pripyat. The smaller town of Chernobyl lies south of the cooling pond.
According to data in the possession of the KGB of the USSR, design deviations and violations of construction and assembly technology occurred at various places in the construction of the second generating unit of the Chernobyl AES, and the KGB warned that could lead to mishaps and accidents. The structural pillars of the generator room were erected with a deviation of up to 100 mm from the reference axes, and horizontal connections between the pillars are absent in places. Wall panels have been installed with a deviation of up to 150 mm from the axes. The placement of roof plates did not conform to the designer's specifications. Crane tracks and stopways have vertical drops of up to 100 mm and in places a slope of up to 8 degree. The cement plant operated erratically, and its output was of poor quality. Interruptions were permitted during the pouring of especially heavy concrete causing gaps and layering in the foundation. Access roads to the Chernobyl AES were in urgent need of repair.
With complex automated systems, the user's mental model directly impacts operating decisions and emergency response. The user's mental model is his/her concept and mental representation of how the system operates. A correct understanding of the system is particularly needed in response to infrequent or unusual system events. In safety critical systems, lack of a accurate and complete mental model can lead to disastrous results. The meltdown at Chernobyl was attributed to a lack of understanding of how the reactor operated on the part of the operators, technicians, and specifically the engineer who developed the test that required by-passing safety controls.
Unlike the other types of commercial light water reactors, which use water both as coolant and as neutron moderator, conversion of water into steam within the core of the RBMK-1000 reactor results in an increase in reactor power; the core is thus said to have a "positive void coefficient." This characteristic played an important role in the Chernobyl accident.
An experiment was planned to determine the feasibility of utilizing the turbine-generators to supply electricity to selected safety systems while the turbines were coasting down. The shutdown operations were interrupted by a need to keep the reactor on the electrical grid. The reactor continued to operate at 50% power for about 9 hours; during this time, in violation of operating procedure, the emergency core-cooling system remained isolated from the reactor system.
The operators then compounded the problem by permitting the reactor power, through operational error, to decrease considerably below the level intended for the experiment. In so doing, the core power began to fluctuate, and the operators continued in their attempts to increase the power level even though the reactor was now at a power level that was considered unsafe for continuous operation. This circumstance was likewise ignored.
In addition, in an attempt to steady the core and increase the power level, a number of safety systems were disengaged which would have automatically shut down the reactor, and the control rods were removed both in number and in extent that the reactor was now operating in a condition that was in gross violation of the very strictest operating procedures. The operators appeared to have been fully aware of the situation, but ignored it. Even at this stage, the reactor was only at about the 6% power level, considerably less than that intended for the experiment.
Nonetheless, the operators disengaged a final automatic shutdown safety system and, at 1:23 am on April 26, 1986, began the test. About 30 sec later, on observing a sudden increase in power level, the operators attempted to trip the reactor; shortly thereafter, two explosions, one immediately following the other, occurred, and fragments of burning material were thrown into the air above the reactor building.
The accident involved a reactivity excursion which occurred because of the positive void coefficient of the core and its peculiar significance at l«w reactor power operation. Also, it is likely that the situation was initially worsened by a positive insertion of reactivity when the control rods began to be inserted into the core, owing to the extreme positions to which they had been removed, and the use of graphite followers for water displacement.
The Chernobyl accident was neither unique in cause nor in consequence, but the effects completely dwarfed the previous incidents in magnitude. For example, whereas the Windscale accident resulted in the escape of about 2,000 curies of Iodine-131 into the biosphere approximately 7,000,000 curies of Iodine-131 escaped from the Chernobyl reactor. The events at Fukushima Dai-ichi during the period from March 12 – March 20, 2013 released 5,400,000 curies of I-131 into the atmosphere.
While the reactor was still on fire, all settlements within 30 km were evacuated, including Pripyat (1986 population 45,000), Chernobyl (1986 population 12,000), and 94 other villages (estimated total population 40,000). As of 1992, this area remained almost completely abandoned.
To stop the fire and prevent a criticality accident as well as any further substantial release of fission products, boron and sand were dumped on the reactor from the air. In addition, the damaged unit was entombed in a temporary concrete "sarcophagus," to limit further release of radioactive material. Control measures to reduce radioactive contamination at and near the plant site included cutting down and burying a pine forest of approximately 1 square mile. The three other units of the four-unit Chernobyl nuclear power station were subsequently restarted. The Soviet nuclear power authorities presented a report on the accident at an International Atomic Energy Agency meeting in Vienna, Austria, in August 1986.
The Chernobyl accident caused many severe radiation effects almost immediately. Among the 600 workers present on the site at the time of the accident, 134 received high radiation doses and suffered from acute radiation sickness. Of these, thirty-one died in the first four months after the accident. Another 200,000 recovery operations workers received doses of between 0.01 Gy and 0.50 Gy. This group is at potential risk of late consequences such as cancer and their health is being monitored.
The Chernobyl accident also resulted in widespread contamination in areas of Belarus, the Russian Federation, and Ukraine inhabited by several million people. There has been an increase in thyroid cancer among individuals who as children were exposed to radioactive iodine as at the time of the accident. The affected children most likely were exposed by drinking milk contaminated with radioactive iodine. However, no increases in overall cancer have been observed in adults living in contaminated areas that could be attributed to ionizing radiation. The risk of leukemia was not elevated, even among the recovery operation workers. However, there has been a dramatic increase in thyroid cancers (about 1,800 cases), particularly among children living in the severely contaminated areas of the three affected countries. Additional cases of thyroid cancer are expected to occur.
Radiation contamination later forced abandonment even outside the 30-km zone. The 1992 image is overlaid with zones indicating December 1990 levels of cesium-137, which has a half-life of 30 years. Note that the area with more than 40 curies/sq km is almost completely abandoned, but abandonment of fields in the 15-40 curies/sq km zone is highly variable. In all, more than 120,000 people, from 213 villages and cities, were relocated outside contaminated areas. In December 2000, with help from the international community, the last reactor at Chernobyl was shut down.
The radiation also affected wild plants and animals around Chernobyl. Pine forests soon died, cattails grew three heads, and wild animals declined in number. But in the coming years, as the short-lived radionuclides decayed and the longer-lived contaminants settled deep into the soil, the wildlife rebounded. Human abandonment also made habitat available for birds, deer, rodents, wolves, boar and other animals. These populations appear to be increasing despite the extraordinarily high mutation rates caused by contamination in the food chain and by one of the highest background radiation levels in the world.
Ranking as one of the greatest industrial accidents of all time, the Chernobyl' disaster and its impact on the course of Soviet events can scarcely be exaggerated. No one can predict what will finally be the exact number of human victims. Thirty- one lives were lost immediately. Hundreds of thousands of Ukrainians, Russians, and Belorussians had to abandon entire cities and settlements within the thirty-kilometer zone of extreme contamination. Estimates vary, but it is likely that some 3 million people, more than 2 million in Belarus' alone, are still living in contaminated areas. The city of Chernobyl' is still inhabited by almost 10,000 people. Billions of rubles have been spent, and billions more will be needed to relocate communities and decontaminate the rich farmland.
Chernobyl' has become a metaphor not only for the horror of uncontrolled nuclear power but also for the collapsing Soviet system and its reflexive secrecy and deception, disregard for the safety and welfare of workers and their families, and inability to deliver basic services such as health care and transportation, especially in crisis situations. The Chernobyl' catastrophe derailed what had been an ambitious nuclear power program and formed a fledgling environmental movement into a potent political force in Russia as well as a rallying point for achieving Ukrainian and Belorussian independence in 1991.
Construction of the sarcophagus covering the destroyed Chernobyl Unit 4 was started in May 1986 and completed by the Soviet authorities in an extremely challenging environment six months later in November. It was quickly built as a temporary fix to channel remaining radiation from the reactor through air filters before being released to the environment. After several years, uncertainties about the actual condition of the sarcophagus, primarily due to the high radiation environment, began to emerge.
When workers finished the huge steel-and-concrete shell that entombs the intensely radioactive mass of the shattered No. 4 reactor in late 1986, Soviet officials declared the site safe for at least 30 years. Yet within a few years, the sarcophagus was cracked, crumbling and in peril of a disastrous collapse. The melted-down fuel is turning to unstable dust. Contaminated objects are being smuggled out of the poorly guarded 1,092-sq.-mi. exclusion zone. Birds fly into the sarcophagus through holes as big as a garage door; rats breed in the ruin. The structure is so unsteady that a strong windstorm could smash it, sending a plume of radioactive dust into the atmosphere.
In 1997, the countries of the G-7, the European Commission and Ukraine agreed that a multilateral funding mechanism be established to help Ukraine transform the existing sarcophagus into a stable and environmentally safe system through the Chernobyl Shelter Implementation Plan. The Chernobyl Shelter Fund was established to finance the Plan. The European Bank for Reconstruction and Development was entrusted with managing the Fund. The Plan is intended to protect the personnel, population and environment from the threat of the very large inventory of radioactive material contained within the existing sarcophagus for many decades. First, the existing sarcophagus was stabilized and then eventually it was replaced with a new safe shelter (confinement). The new shelter was an arch-shaped steel structure, which will slide across the existing sarcophagus via rails.
In 1997, the G-7, the European Commission and Ukraine agreed to jointly fund the Chernobyl Shelter Implementation Plan to help Ukraine transform the existing sarcophagus into a stable and environmentally safe system. The European Bank for Reconstruction and Development manages funding for the plan, which will protect workers, the nearby population and the environment for decades from the very large amounts of radioactive material still in the sarcophagus. The existing sarcophagus was stabilized before work began in late 2006 to replace it with a new safe shelter called the New Safe Confinement.
The New Safe Confinement structure was an unprecedented project to design a new building that would completely enclose the existing sarcophagus. To protect the construction workers from radiation, the arch-shaped steel structure was assembled away from the damaged reactor building and rolled into place across steel rails. Over 350 feet high and 840 feet wide it was the world's largest transportable building. In 2016, the New Safe Confinement was repositioned over the sarcophagus, and finishing work is expected to be completed in 2018. This new structure is designed to last at least 100 years. In 2017, construction was completed on an Interim Spent Fuel Storage Facility. The facility will process and store the spent fuel assemblies from the undamaged units 1, 2, and 3 in dry, double walled canisters designed to last at least 100 years.
|Join the GlobalSecurity.org mailing list|