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Chernobyl

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At the Chernobyl location, 130 km north-west of Kiev, four reactor units of type RBMK 1000 entered service between 1977 and 1983. On 26th April 1986, the most serious accident to date in the peaceful use of nuclear energy occurred in unit 4. Although the accident at the Chernobyl Nuclear Power Plant was due to a series of wrong decisions and forbidden interventions by the operating staff, it was the inefficient reactor safety concept and the lack of a pressure-tight containment enclosing the reactor plant which were ultimately responsible for the occurrence of the accident and the release of such large quantities of radioactive substances. The reactor accident developed during an experiment with the turbine generator set of the power plant. A number of operating mistakes including the bridging of shutdown signals entailed a major power increase - up to 100 times the nominal power. The overheating of fuel caused the rupture of fuel rod claddings, resulting in a heavy fuel/water reaction with shock-like pressure build-up and destruction of the reactor building. Considerable sections of the graphite moderator and the plant were set on fire. During the destruction phase, approximately eight tonnes of radioactive fuel from the core were hurled into the building and the environment. Thanks to the immediately initiated fire fighting, it was possible to extinguish the fires outside the reactor building and in the engine house within four hours. To smother the moderator graphite fire in the reactor and limit the consequences of the accident, unit 4 was filled in by air with a total of 5 000 tonnes of lead, sand and clay in the following days. By November 1986, the Chernobyl reactor unit 4 stayed „buried“ under a structure of metres of concrete - called sarcophagus.

The massive release of radioactive fission products from the destroyed reactor continued for more than ten days. Due to the thermal lifting effects the release, in particular that of the volatile fission products such as iodine and caesium, took place up to considerable altitudes (1 500 m and more). This led to dispersion of the activity of 4 . 1018 Bq released into the atmosphere over broad areas of Europe. The radioactive substances released on 26th April reached Sweden on 28th April due to the prevailing north-west wind. The activity increase measured there in the air was the first indication of the accident in the west. Because of the weather conditions, the activity emission of the 27th April reached Poland and that of the 29th and 30th April reached Central Europe via the Balkans. On 29th April the radioactive cloud reached the territory of the Federal Republic of Germany.

The power plant personnel and in particular the fire-fighting staff were seriously affected by radiation. The dose values amounted up to 16 Gy. 203 persons with acute radiation syndrome were treated in hospitals. 31 persons died as a result of burns and radiation overexposure. The radiation exposure in the town of Pripyat 4 km to the west of the location with
45,000 inhabitants amounted to up to 6 mSv/h on the day after the accident. The population was subsequently evacuated. In the following days, a further 90,000 persons were evacuated from the 30-km zone around the location. A resettlement of the 10-km zone is not intended; the agricultural use of the 10- to 30-km zone depends upon the success of decontamination programmes and the result of radiological examinations.

Due to meteorological influences, the activity quantities from the radioactive cloud deposited in the regions of the Federal Republic are quite different - in the north and west clearly less than in the south and south-east. Therefore, no uniform values for the resulting radiation dose in Germany are possible, which also depends considerably on the individual dietary habits. The inhalation dose was almost exclusively determined by the aerial activity between 1st and 5th May 1986. The ingestion dose results almost exclusively from I 131, Cs 134 and Cs 137. The radiation exposure in the following years was remarkably lower than in the first year after the accident.

Person group

period

North

South

foothills of the Alps

   

effective dose, mSv

infants

1986

0.12

0.35

0.6

lifetime

0.4

1.3

2.4

adults

1986

0.1

0.3

0.5

lifetime

0.4

1.1

2.1

Average radiation exposure due to the Chernobyl accident in different areas of Germany

For individual persons with extreme living and eating habits, maximum dose values of up to twice or three times of these values may result.

In the areas of the Ukraine and Belarus affected by the accident, children and young adults showed a considerable increase in thyroid gland cancer in the following years, which is to be explained by the radiation exposure. Above all the intake of iodine 131, a radioactive isotope with a half-life of about 8 days, via the nutrition chain and its storage in the thyroid gland led to high radiation doses in this organ. In the countries concerned, more than one thousand thyroid gland cancer cases have occurred to date in children and young people. Based on the risk observations, a total of up to 4 000 thyroid gland cancer cases are expected.

Unit 2 of the four reactor units at the Chernobyl location was finally shutdown in October 1991. Unit 1 followed in November 1996. On 15th December 2000 unit 3, the last reactor in Chernobyl, was finally shutdown.

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