Most of the radioactivity of the destroyed reactor is contained within the Sarcophagus built around the destroyed unit - at present about 200 tonnes of irradiated and fresh nuclear fuel, mixed with other materials in various forms. The total activity of this material is estimated to be 700 x 1015 Bq of long lived radionuclides radionuclides (137Cs, 90Sr, Pu isotopes, 241Am).
The Sarcophagus has fulfilled its protective function for over the past sixteen years. In the long term, however, its stability and the quality of its confinement are in doubt.
It has been established that the Sarcophagus is currently safe from the point of view of a self-sustaining chain fission reaction (SCFR) in the nuclear fuel remnants. The Ukrainian Government signed an agreement with the countries of the G-7 and the European Community Commission on co-operation in making the Sarcophagus ecologically safe object, and the "Shelter Implementation Plan" (SIP) was developed.
Radionuclide in the Exclusion Zone
The Exclusion Zone around the Chernobyl NPP (hereinafter Exclusion Zone) is the area most contaminated by the accident. Its population was evacuated outside the area in 1986, and all economic activity not related to the Chernobyl NPP (ChNPP) was stopped. The Exclusion Zone covers a surface area of 4300 kmІ.
At present, the main radionuclides causing Exclusion Zone contamination are 137Сs, 90Sr and a -emitters of transuranium elements (TUE) 238, 239, 240Pu, 241Am. The amounts deposited in the Exclusion Zone (on the ground surface) are about 11PBq.
The territory of the Exclusion Zone is the most heavily contaminated by 137Cs, 90Sr and TUE dose-created radionuclides. It is the main depot of "Chernobyl" radionuclides and as such a dangerous source of radionuclide transfer to neighbouring areas. But Exclusion Zone is also a very effective complex barrier against the migration of deposited radionuclides to populated areas of Ukraine and Belarus.
Contamination outside the Exclusion Zone
The territories of Belarus, Russia and Ukraine have undergone the heaviest contamination on account of the Chernobyl disaster. Since air masses containing airborne radioactive substances transited over the north of the globe for several weeks, the contamination spread to almost all of the countries in Europe with the greatest impact in the Scandinavian and Alpine regions. The formation of radioactive fields of contamination outside the borders of the former Soviet Union began during the night of 27 April 1986 and was actually completed in the first ten days. Rainfall at that time caused the formation of zones with high levels of radionuclide fallout on the territories of Sweden, Finland, Germany, Austria, Switzerland, Greece, Bulgaria, Romania and Georgia.
Consequences of the accident for public health
Exposure doses received by "liquidators"
About 200,000 persons who participated in 1986-1987 in the "liquidation" of the accident's consequences received average doses of around 100 mSv. Around 10% of them received doses of around 250 mSv; a smaller percentage received doses exceeding 500 mSv. Several dozen of those providing immediate response to the accident received potentially lethal doses of a few thousands of millisieverts which, unfortunately, was not documented.
Exposure doses received by the population
The 116,000 people evacuated from the Exclusion Zone in 1986 had already been exposed to radiation. Fewer than 10% had received doses of more than 50 mSv and fewer than 5% had received doses of more than 100 mSv.
The radioiodines released delivered radiation doses to the thyroid gland. Iodine was absorbed into the bloodstream, generally by ingestion in foodstuffs, mainly contaminated milk, and also by inhalation of the initial radioactive cloud, and accumulated in the thyroid gland. Doses to the thyroid were anticipated to be particularly high compared with those to other body organs, especially for children. Equivalent doses to the thyroid were estimated at up to several Gy (made primarily on the basis of measurements reported for 150,000 people in Ukraine and also in Belarus and the Russian Federation) .
A total of 237 individuals exposed to radiation in the course of their work were stated to be suffering from clinical syndromes attributable to radiation exposure and were admitted to hospital. Acute radiation syndrome (ARS) was diagnosed in 134 cases. Of these 134 patients, 28 died as a consequence of radiation injuries, all within the first three months.
A highly significant increase in the incidence of thyroid cancer among those persons in the affected areas who were children in 1986 is the clear evidence to date of public health impact of radiation exposure as a result of the Chernobyl accident.
Besides the confirmed increase in the incidence of thyroid cancer, there have been some reports of increases in the incidence of specific malignancies in some populations living in contaminated territories and in the "liquidators". These reports, however, may require further investigation.