Chernobyl Accident

I

Introduction

Chernobyl Accident, major accident at the Chernobyl nuclear power plant in the Ukrainian republic of the former Union of Soviet Socialist Republics (USSR) on April 26, 1986, which produced a plume of radioactive debris that drifted over parts of the western Soviet Union, Eastern Europe, and Scandinavia. Large areas of the Ukrainian, Belorussian, and Russian republics of the Soviet Union were contaminated, resulting in the evacuation and resettlement of more than 200,000 people. The accident raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years, while forcing the Soviet government to become less secretive. The later independent countries of Ukraine and Belarus were burdened with substantial costs for decontamination and health care because of the Chernobyl accident.

II

The Accident

The Chernobyl nuclear power station was one of the largest in the Soviet Union. It was located just outside the town of Pripyat’, about 18 km (11 mi) north-west of the town of Chernobyl. The plant was only 16 km (10 mi) from the border between the Ukrainian and Belorussian republics, and roughly 110 km (70 mi) north of Kiev, the capital and largest city of Ukraine. Construction of the plant began in the 1970s, with reactor no. 1 commissioned in 1977, followed by no. 2 (1978), no. 3 (1981), and no. 4 (1983). Each reactor had an electricity-generating capacity of 1,000 megawatts, and the four together produced about 10 per cent of Ukraine's electricity at the time of the accident. Two more reactors (no. 5 and no. 6, also capable of producing 1,000 megawatts each) were under construction at the time of the accident.

In the early morning of April 26, 1986, reactor no. 4 was operating at very low capacity (6 to 7 per cent) during a planned shutdown. Plant personnel intended to monitor the performance of turbine generators, which supplied electrical power for the plant’s own operation, during a changeover from standard to a back-up source of power. The reactor’s design made it unstable at low power and the operators were careless about safety precautions during the test. After a sudden power surge, two explosions destroyed the reactor core and blasted a large hole in the roof of the reactor building. Radioactive debris moved up through this hole to heights of 1,000 m (3,300 ft), carried by a strong updraught. Fires caused by the explosion and the heat of the reactor core fed the updraught.

An estimated 100 to 150 million curies of radiation (primarily radioactive isotopes of iodine and caesium) escaped into the atmosphere before clean-up crews were able to bring the fires under control and stabilize the situation some two weeks later. Initially, prevailing winds carried the radioactivity north-west from the plant across Belorussia and into Poland and Sweden, where heightened radiation levels detected on April 28 first brought the accident to the world's attention. Subsequently, from May 1 to 5, wind patterns shifted so that the bulk of radioactivity was carried more directly north and north-east, over Belorussia and south-western Russia.

After the explosion, firefighters and other workers arrived on the scene in an attempt to contain the blast. To reduce emissions, the team bombarded the reactor with 5,000 tonnes of shielding material consisting of lead, boron, sand, and clay. A second concrete foundation was constructed under the reactor to prevent contamination of groundwater. Finally, workers erected an enormous concrete-and-steel shell or “sarcophagus” over the damaged reactor to prevent radioactive materials, including gases and dust, from escaping. Initially, Soviet officials placed the death toll at 2 (both workers killed during the explosion at the no. 4 reactor) but by mid-August revised the figure to 31, reflecting deaths of workers from acute radiation exposure during the clean-up.

By mid-July, roughly three months after the accident, containment and clean-up had proceeded to the point where the plant’s management had moved back into the administration building just 300 m (1,000 ft) from the no. 4 reactor. In addition to reducing the radiation threat, a key objective to the clean-up effort was resumption of electric power generation at Chernobyl before the onset of winter. The no. 1 and no. 2 reactors, in fact, were returned to service in November 1986, and the slightly damaged no. 3 unit was restarted in December 1987.

III

Evacuation

More than 100,000 people were evacuated during the first few weeks after the accident. Evacuation of Pripyat’ (where 35,000 people lived at the time of the accident) and the immediate surrounding area began roughly 36 hours after the accident, on the afternoon of April 27. Evacuation within a larger, officially designated evacuation zone of 2,800 sq km (1,100 sq mi), including parts of Belorussia, began on May 3. That area became known as the “30-km zone” because it is a circle with a 30-km (18-mi) radius from Pripyat’. At least 50,000 people were relocated in Ukraine and 25,000 in Belorussia during this second-stage evacuation, which continued into June.

As officials, especially in Belorussia, determined that areas of serious contamination extended well beyond the official evacuation zone, additional people were relocated. In 1986 and 1987, 50,000 people were evacuated from areas outside the 30-km zone in Belorussia, and over 30,000 more were moved between 1991 and 1993. In Ukraine, over the period from 1991 to 1996, approximately 50,000 were evacuated. The total number of people evacuated in Ukraine and Belorussia as a result of Chernobyl was more than 200,000.

IV

Aftermath

The principal environmental effect of the Chernobyl accident was the accumulation of radioactive fallout in the upper layers of soil, where it destroyed important farmland. The second most important impact was the threat to surface water and groundwater. The clean-up in some of the most heavily contaminated areas within the evacuation zone, such as Pripyat’, involved the stripping and burying of topsoil and vegetation, the sealing of wells, and the building of structures designed to prevent surface water from entering streams and rivers that drain into the Dnepr river system, which provided Kiev's water supply.

By most measures, the country most seriously affected by the accident was Belarus (which changed its name from Belorussia after it, along with the other Soviet republics, became independent with the collapse of the Soviet Union in 1991). Almost 20 per cent of the republic’s farmland was removed from production during the years immediately after the accident. Half of the vast 27,850-sq km (10,750-sq mi) area described as being “seriously contaminated” by radiation (with levels of radioactive caesium in topsoil exceeding 5 curies) was in Belarus. The regions commonly identified as experiencing the greatest contamination included the oblasts (districts) of Homel, Mahilyow, and Brest in southern and eastern Belarus; Kiev, Zhytomyr, and Chernihiv in northern Ukraine; and Bryansk in south-western Russia.

Effects on public health were more difficult to determine and were subject to considerable controversy. It is not clear which health problems were caused directly by radiation, and which were caused by poor nutrition, the general low level of health, and anxiety and stress produced by fear of radiation exposure. These issues surrounded the debate over the causes of higher death rates among the more than half a million workers who participated in the Chernobyl clean-up.

However, at least one type of cancer can be attributed directly to Chernobyl. There was a significant rise in the incidence of thyroid cancer among children in the areas where radiation levels had been highest; a study in 2001 suggested that almost 2,000 had developed this form of cancer as a result of the accident, and thyroid cancer rates in the Homel oblast alone increased 22-fold from 1986 to 1990 compared to the period from 1981 to 1985.

Many observers have argued that the accident at Chernobyl accelerated the transformation of the Soviet Union towards a more open society. Soviet officials, unable to conceal the accident from the world, reluctantly acknowledged the accident during an evening news broadcast in Moscow on April 28, and in brief newspaper accounts on April 30. This was followed by regular coverage focused on the clean-up efforts in the months that followed. This reporting sharply contrasted with the lack of coverage of previous catastrophic events (an accident at a nuclear weapons plant in the Ural Mountains in 1957 and major earthquakes in Central Asia in 1948 and 1964).

Also, after the accident several key officials in the Soviet nuclear power industry were dismissed, punished, or both, and a new Ministry of Nuclear Power was created in 1986. Before then, officials in the general electrical power ministry had overseen nuclear power. Chernobyl also called into question the basic safety of nuclear power in both the USSR and in several Eastern European countries whose power plants contained reactors based on the RBMK reactor design used at Chernobyl. (In the RBMK design there was no containment shell, the graphite blocks used to moderate the fission reaction were flammable, and excess steam in the reactor core would cause the nuclear reaction to increase.) As a consequence, international organizations, such as the International Atomic Energy Agency, became involved in programmes to improve safety procedures and upgrade the design of RBMK reactors in the USSR and Eastern Europe.

The accident, coupled with a general economic decline that set in during the final years of the USSR, also resulted in a dramatic scaling back of Soviet plans to use nuclear power to generate the bulk of electrical power in Soviet regions remote from oil and gas energy resources. In Ukraine, opposition to further nuclear construction in the immediate post-Chernobyl years was particularly intense. In August 1990, for example, the Ukrainian parliament declared a moratorium on nuclear power plant construction, although this was subsequently lifted in 1993 because of severe energy shortages there.

Earlier in 1990 the Ukrainian parliament had voted to close the Chernobyl plant permanently within five years, but closure was repeatedly postponed because of the country’s shortage of electricity-generating capacity. Eventually, after a turbine fire in October 1991, the no. 2 reactor at Chernobyl was shut down, leaving only two of the original four reactors at the plant in service. Reflecting mounting safety concerns in the international community, an agreement was concluded in April 1996 between the Ukrainian government and the G-7 countries (Group of Seven major industrial nations) to decommission the Chernobyl plant by 2000. In conjunction with the agreement, the G-7 countries pledged US$300 million to finance programmes to strengthen the sarcophagus, which some feared may collapse, and for additional clean-up work. In November 1996, as part of the schedule for the decommissioning, the no. 1 reactor at Chernobyl was taken out of service, leaving only the third unit operational. In December 2000 the third unit was taken out of service and the Chernobyl nuclear plant was closed down permanently.