CHERNOBYL

On April 26, 1986, a catastrophic accident occurred at the Chernobyl-4 reactor near the town of Pripyat, Ukraine, 100 kilometers northwest of Kiev. Figure 1 shows the reactor location and the regions of most intense radioactive contamination. The accident destroyed the reactor and released a large amount of radioactivity into the atmosphere, particularly radioactive iodine (I-131) and radioactive cesium (Cs-137), both of which have the potential to cause cancer. Thirty-one workers at the plant died within a few weeks, most of them from receiving lethal doses of radiation while putting out fires and responding to other emergencies.

Radiation fallout caused significant contamination in parts of Belarus, Russia, and the Ukraine, resulting in the resettlement of more than 350,000 people from 4,300 square kilometers. An approximate five- to ten-fold increase in thyroid cancer has been observed in children from Belarus, Russia, and Ukraine who received a large exposure to I-131. The economic impact has also been large, not only from the direct costs of accident cleanup, decontamination, and entombing the reactor, but also for lost agricultural production from the evacuated areas, and from regions throughout Europe where the radioactive fallout resulted in restrictions on eating certain foods and on limiting imports. Continued health monitoring over many years will be required for citizens who had lived in or are currently living in contaminated areas, and for cleanup workers who received significant doses of radiation.

At the time of the accident, the Chernobyl reactors were owned and operated by the Soviet Ministry of Power and Electrification. The reactor design was a unique Soviet design called an RBMK. A schematic diagram is shown in Figure 2. Like reactors in the United States, RBMK reactors use ordinary water to cool the fuel. Unlike U.S. reactors, which use water to slow down or moderate the neutrons produced in fission, the RBMK uses graphite as the moderator. In this case the water used for the coolant is actually a neutron absorber and reducing the density of the water increases the neutron production. In addition, the ratio of uranium isotopes U-235 to U-238 in the fuel is less in the RBMK than in U.S. reactors. The effect of these differences was that at low power operation, under the right conditions, the power in the RBMK could increase in an uncontrolled manner. Reactor designs that allow power increase in an uncontrolled manner are prohibited by regulation in the United States. The type of accident that occurred was unique to the Soviet-designed RBMK reactor. Another important difference is that Soviet reactors did not have a steel-lined, thick concrete-walled containment building like those in Western Europe, North America, and Asia, using instead an industrial-type building. This final difference had profound consequences.

The Accident

The accident occurred while the operators were conducting a test simulating loss of power at the plant. The goal was to determine if power from the spinning turbines could be used to operate the pumps while backup diesel generators were brought on line. In order to conduct the test, most of the safety systems that would have provided a safe shutdown were disconnected. A test of this type that disconnected the safety systems would never be allowed in the United States, Western Europe, or Asia. The test was to be conducted at about 25 percent power, but when the power level was reduced from 100 percent to 50 percent, the test was delayed for nine hours because the electricity was needed in Kiev. While the operators waited, a strong neutron-absorbing isotope, Xenon-135, built up in the reactor. The operators did not recognize this and did not incorporate the effect into the control computer. When the test resumed, the operators could not control the reduction of power because the Xenon-135 was absorbing neutrons needed for fission and consequent power production. To keep the reactor from shutting itself down, they pulled out most of the neutron-absorbing control rods that are used to control the reactor power. This was in violation of Soviet operating procedures. Unknown to the operators, they now found themselves operating under conditions where the reactor could increase in power in an uncontrolled manner.

When the operators continued the test procedure by turning off the water coolant pumps and re-inserting the slow-moving control rods, there was still enough power to cause the water to start boiling, thereby reducing the water density and increasing the neutron production. In addition, there were graphite tips on the bottom of the control rods that added moderator when the control rods were initially inserted, and this further increased neutron production. Instead of the power level decreasing, as the operators expected, it increased rapidly, reaching approximately one hundred times full power in just a few seconds. The increased power resulted in a massive steam buildup inside the reactor leading to an explosion.

A second explosion that followed shortly lifted the large top shield above the reactor, blew off the roof and walls of the building, and dispersed burning fuel and graphite. The steel shield resettled at an angle, allowing air to enter the reactor and the argon gas that normally covers the reactor to escape. Contact with the air caused the hot graphite to ignite, propelling the volatile radioactive materials high up into the atmosphere. Firefighters who went to the room to put out the fires received a lethal dose of radiation. It took ten days to control the fire, and by that time 5 to 10 percent of the radioactive material in the core had been released to the atmosphere.

Evacuation and Health Effects

Evacuation of residents from the nearby town of Pripyat took place the following day, but evacuation from adjacent contaminated areas did not take place for several days, nor did the Soviet government quickly inform the residents or the world what had happened. The radiation release was first made public after the airborne radiation from the accident was detected in Sweden. In 1986 about 116,000 people were relocated from areas surrounding the reactor, and an additional 220,000 people from Belarus, the Russian Federation, and Ukraine were relocated after 1986.

The World Health Organization (WHO) has continually monitored those exposed to radiation, including the residents and 600,000 "liquidators" who came to clean up the accident. Two hundred thousand liquidators built a cooling system under the reactor and a shield building—commonly called the sarcophagus—around the damaged reactor. They received doses of 100 millisieverts (10 rem) or more, with 20,000 receiving doses of at least 250 millisieverts. For comparison, this is five times the U.S. total effective dose limit for radiation workers. Another 400,000 liquidators, who arrived after 1987, received much lower doses.

Extensive analyses of the public health effects from Chernobyl have been conducted by United Nations organizations including WHO. A comprehensive summary is available in a report by the United Nations Scientific Commission on the Effects of Atomic Radiation (UNSCEAR) published in 2000, which concludes that the only radiation-related effect to that date was an increase in thyroid cancer, largely in children. Through the years 1990–1998 there were about 1,800 cases of thyroid cancer in the contaminated areas of Belarus, Russia, and Ukraine in children 0–17 years old at the time of the accident. The majority of these were related to the accident.

Additional cases of thyroid cancer are expected in the future. Thyroid cancer is generally treatable if caught early. Nonetheless, ten deaths were reported as of 2000. It is not possible to make any accurate prediction about the number of deaths that will ultimately result, because there are no models that accurately predict deaths from low levels of radiation exposure. Many reports indicating much larger numbers of cancers and deaths from the Chernobyl accident were found in the UNSCEAR review to contain misinterpretations of data or use of unsubstantiated data.

While the heaviest radiation doses were received in Belarus, Russia, and Ukraine, the release of radioactivity from the Chernobyl reactor went high into the atmosphere and spread throughout Europe and then around the whole northern hemisphere. Figure 3 shows the dispersion of the radioactive cloud. The fallout in Europe varied considerably among the countries, depending on wind patterns and rain during the ten days the reactor was releasing radioactivity to the atmosphere. The largest doses were received in Poland, followed by Sweden, Germany, Italy, Finland, and Czechoslovakia. While such fallout caused great concern with the governments and the population, the doses received by the populations were relatively low, although there were some localized "hot spots."

Response and Lessons

Engineers from the former Soviet Union have made changes to all RBMK reactors to eliminate the possibility of repeating this type of accident. Nonetheless, in the West the RBMK is considered too unsafe to continue operation. All the Chernobyl units have been shut down. Other RBMK reactors outside Russia are being phased out of operation. However, the Russians still consider it a safe reactor and plan to continue operating all existing units. Plans to build new ones have been nevertheless cancelled.

In addition, the Soviet-designed water-cooled reactors, the VVER, built in the former Soviet Union and its satellite states, have either been shut down for inadequate safety features or modified to enhance safety. The United States and European countries have contributed millions of dollars in equipment and expertise to upgrade the safety of the existing reactors and their operation. The accident also stimulated the creation of the World Association of Nuclear Operations (WANO), whose goal is to improve safety in operations. WANO is an extension of the Institute for Nuclear Power Operations (INPO) that was formed after the accident at Three Mile Island in 1979 and was instrumental in improving safety in the operation of nuclear power plants in the United States.

While the design of the RBMK was flawed, a far greater problem was failures in human performance. How was it possible for the managers to allow the safety systems that would have prevented the accident to be disconnected during the test? How could all the control rods have been removed in violation of fundamental safety procedures? One answer is that written safety and accident response procedures actually did not exist in most RBMK control rooms before the accident. Furthermore, the operators were not trained to respond to different accident scenarios, and surely not to an accident that might occur during an experimental procedure. Importantly, there was no effective safety review of the proposed test. Moreover, the accident occurred in a society where secretiveness rather than openness was standard operating procedure, and this resulted in a lack of communication within the organization and with the public. After the accident, the Soviet government attempted to conceal it and the dangers posed to the local population and to the world.

Western nations learned the lessons of Three Mile Island, but states of the former Soviet Union did not. Specifically, they did not incorporate the fundamental lesson that safety is the most important responsibility of the operators, and that management from the top down must emphasize, encourage, and incorporate this thinking into plant operation. A culture that fosters "safety-first thinking" throughout the organization is necessary if nuclear power is to reach its potential to benefit humanity.

Since Chernobyl, nations of the former Soviet Union (FSU) have made significant improvements in both operations and design. Assistance from the United States and Europe led to the establishment of new training facilities, enhanced operator training, improved procedures for responding to accidents, and upgraded plant equipment. New Russian designs of the VVER type have added safety features and a containment building so they now meet safety standards used elsewhere in the world. There has been a change in management philosophy and an increased emphasis on operations safety. Regulatory agencies are improving in their capabilities. Nonetheless, the culture change needed to reach the safety standards of the United States and Western Europe will be a continuing challenge in the FSU countries.

The Chernobyl accident showed dramatically that an accident anywhere represents an accident everywhere, for it reflects on the ability of nuclear power to serve society as a trustworthy technology. This is a high standard and it raises the question: Can nations throughout the world that desire to use nuclear power maintain this level of attention to safety? Even though future reactors may be designed and built that prevent a catastrophic accident, it is important that an emphasis on a culture of safety be maintained. Ultimately it will reflect on the capacity of the world nuclear industry to serve civilization.

The accident was also global in the sense that radioactive fallout was present throughout the northern hemisphere and caused local contamination in many European countries that were not prepared for such an accident. The reactions of national authorities varied greatly on issues such as restrictions on consumption and marketing of foodstuffs. There was no uniformity in standards for implementation of protective actions. This could be especially disconcerting to the public in the border region when the nations on each side of the border took significantly different actions. National authorities sometimes used interpretations that responded to public fears rather than being based on sound science. This resulted in unnecessarily increasing public confusion and possibly public fears, and caused unnecessary government expense and economic loss. International efforts have been undertaken to produce more uniform regulations and criteria related to radiation accidents, and for emergency management of transnational accidents. Whether these efforts will be effective may not be known unless they are put to the test. In light of the level of terrorism that now exists in the world, and the possibility that biological and chemical agents can cross national boundaries as well as nuclear agents, it has become ever more critical that this type of emergency management be carefully developed and practiced. This could be one of the most important lessons from the Chernobyl accident.

EDWARD H. KLEVANS
DARRYL L. FARBER

SEE ALSO Nuclear Ethics;Three-Mile Island.

BIBLIOGRAPHY

Knief, Ronald Allen. (1992). Nuclear Engineering: Theory and Technology of Commercial Nuclear Power, 2nd ed. New York: Hemisphere. Nuclear engineering textbook with a good technical discussion of the RBMK reactor, the chronology of the accident, the causes of the accident, the effects of the accident, and comparisons with the Three Mile Island accident.

Medvedev, Grigori. (1991). The Truth About Chernobyl, trans. Evelyn Rossiter. New York: Basic Books. Written by a physicist who had been a chief engineer at Chernobyl and was an investigator of the accident; presents both background material and an intimate account of the events leading to the accident and the accident itself.

Organisation for Economic Co-operation and Development, Nuclear Energy Agency. (1995). Chernobyl—Ten Years On: Radiological and Health Impact. Paris: Nuclear Energy Agency, OECD. Covers radiation releases, dose estimates, health impacts, agricultural impacts, reactions of national authorities, and other issues.

INTERNET RESOURCES

Nuclear Regulatory Commission. (2000). "Fact Sheet on the Accident at the Chernobyl Nuclear Power Plant." Available from http://www.nrc.gov. Briefly discusses the accident and presents information on the Nuclear Regulatory Commission's post-accident activities to assist the Ukraine in dealing with the accident.

United Nations Scientific Commission on the Effects of Atomic Radiation (UNSCEAR). "Exposures and Effects of the Chernobyl Accident." In Volume II of the UNSCEAR 2000 Report. Available from www.unscear.org/chernobyl.html. Comprehensive and authoritative report on the health effects of the Chernobyl accident with data through 1998.

World Nuclear Association. (2001). "Chernobyl." Available from www.world-nuclear.org/info/chernobyl. (Go to Document inf07.) Site has a set of detailed and authoritative documents discussing many facets of the accident and its consequences. There are also maps of the area. Document inf07 describes the accident and its consequences in detail and gives direct access to three short reports on health effects from the accident.