The Chernobyl accident
UNSCEAR's assessments of the radiation effects
•Release of radionuclides | Maps
•Exposure of individuals
The accident at the Chernobyl nuclear reactor that occurred on 26 April 1986 was the most serious accident ever to occur in the nuclear power industry. The reactor was destroyed in the accident and considerable amounts of radioactive material were released to the environment. The accident caused the deaths, within a few weeks, of 30 workers and radiation injuries to over a hundred others. In response, the authorities evacuated, in 1986, about 115,000 people from areas surrounding the reactor and subsequently relocated, after 1986, about 220,000 people from Belarus, the Russian Federation and Ukraine. The accident caused serious social and psychological disruption in the lives of those affected and vast economic losses over the entire region. Large areas of the three countries were contaminated with radioactive materials, and radionuclides from the Chernobyl release were measurable in all countries of the northern hemisphere.
Among the residents of Belarus, the Russian Federation and Ukraine, there had been up to the year 2005 more than 6,000 cases of thyroid cancer reported in children and adolescents who were exposed at the time of the accident, and more cases can be expected during the next decades. Notwithstanding the influence of enhanced screening regimes, many of those cancers were most likely caused by radiation exposures shortly after the accident. Apart from this increase, there is no evidence of a major public health impact attributable to radiation exposure two decades after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality rates or in rates of non-malignant disorders that could be related to radiation exposure. The incidence of leukaemia in the general population, one of the main concerns owing to the shorter time expected between exposure and its occurrence compared with solid cancers, does not appear to be elevated. Although those most highly exposed individuals are at an increased risk of radiation-associated effects, the great majority of the population is not likely to experience serious health consequences as a result of radiation from the Chernobyl accident. Many other health problems have been noted in the populations that are not related to radiation exposure.
The Scientific Committee has been involved from early on in the assessment of radiation exposures and health effects from the accident.
In 1988 it issued its first study of Acute radiation effects in victims of the Chernobyl accident (21 pages), which reviews experience gained in treating the immediate radiation injuries of workers and firefighters who dealt with the initial emergency.
It also published an account of the accident, and its global fallout and exposures: Exposures from the Chernobyl accident (74 pages). Soon after the accident, the dispersion of radionuclides and the resulting radiation exposures had been measured and evaluated throughout the region. UNSCEAR made use of those data to evaluate the average individual and population doses for various countries and regions, and for the northern hemisphere as a whole.
In 2000 UNSCEAR issued a more detailed assessment of radiation levels and effects: Exposures and effects of the Chernobyl accident (115 pages). Evaluating the exposures received by the people who had been evacuated or who were still residing in the areas most affected by the accident required much time and effort. The initial measurements were supplemented by information on such matters as the location and diet of the people in each settlement. The accumulation of data on any late health effects also required time.
In 2001 UNSCEAR also published findings related to possible genetic effects of radiation exposures resulting from the Chernobyl accident (4 pages) .
From 2003 to 2005 some of the Committee's experts participated in the work of the Chernobyl Forum, whose conclusions essentially concur with those of the UNSCEAR 2000 report. >> Summary report of the Chernobyl Forum
In 2008, at its 56th session, the Committee approved a document, entitled "Health effects due to radiation from the Chernobyl accident", as a scientific annex (advance copy issued 2011) to its Report to the General Assembly ( A/63/46). The findings are based on more than two decades of experimental and analytical studies of the radiation consequences of the Chernobyl accident for health of the exposed populations and for the environment. The data reviewed, including numerous dosimetric measurements and results of analytical epidemiological studies, allowed a comprehensive evaluation to be made of the human exposure levels and radiation-induced health effects to date.
In 2010, in accordance with General Assembly resolution 62/9 on the strengthening of international cooperation, the Secretary-General submitted to the General Assembly the report "Optimizing the international effort to study, mitigate and minimize the consequences of the Chernobyl disaster" ( A/65/341).
Release of radionuclides
The accident at the Chernobyl reactor happened during an experimental test of the electrical control system as the reactor was being shut down for routine maintenance. The operators, in violation of safety regulations, had switched off important control systems and allowed the reactor, which had design flaws, to reach unstable, low-power conditions. A sudden power surge caused a steam explosion that ruptured the reactor vessel, allowing further violent fuel-steam interactions that destroyed the reactor core and severely damaged the reactor building. Subsequently, an intense graphite fire burned for 10 days. Under those conditions, large releases of radioactive materials took place.
The radioactive gases and particles released in the accident were initially carried by the wind in westerly and northerly directions. On subsequent days, the winds came from all directions. The deposition of radionuclides was governed primarily by precipitation occuring during the passage of the radioactive cloud, leading to a complex and variable exposure pattern throughout the affected region, and to a lesser extent, the rest of Europe.
Exposure of individuals
The radionuclides released from the reactor that caused exposure of individuals were mainly iodine-131, caesium-134 and caesium-137. Iodine-131 has a short radioactive half-life (eight days), but it can be transferred to humans relatively rapidly from the air and through consumption of contaminated milk and leafy vegetables. Iodine becomes localized in the thyroid gland. For reasons related to the intake of milk and dairy products by infants and children, as well as the size of their thyroid glands and their metabolism, the radiation doses are usually higher for them than for adults.
The isotopes of caesium have relatively longer half-lives (caesium-134 has a half-life of 2 years while that of caesium-137 is 30 years). These radionuclides cause longer-term exposures through the ingestion pathway and through external exposure from their deposition on the ground. Many other radionuclides were associated with the accident, which were also considered in the exposure assessments.
Average effective doses to those persons most affected by the accident were assessed to be about 120 mSv for 530,000 recovery operation workers, 30 mSv for 115,000 evacuated persons and 9 mSv during the first two decades after the accident to those who continued to reside in contaminated areas. (For comparison, the typical dose from a single computed tomography scan is 9 mSv). Maximum individual values of the dose may be an order of magnitude and even more. Outside Belarus, the Russian Federation and Ukraine, other European countries were affected by the accident. Average national doses there were less than 1 mSv in the first year after the accident with progressively decreasing doses in subsequent years. The average dose over a lifetime in distant countries of Europe was estimated to be about 1 mSv. These doses are comparable to an annual dose from natural background radiation (the global average is 2.4 mSv) and are, therefore, of little radiological significance.
The exposures were much higher for those involved in mitigating the effects of the accident and those who resided nearby. Those exposures are reviewed in great detail in the UNSCEAR assessments.