The worst nuclear accident in history occurred on April 26, 1986 at the 1000 megawatt Reactor IV at the multiunit Chernobyl Power station near Kiev on the Black sea. A violent explosion had followed a series of experiments. The explosion brusted through a 4000 tonne steel concrete cover. The reactor core temperature reached more than 2000°C. The graphite in the core caught fire and melted part of the fuel. The pressure of hydrogen built up led to the explosion, which removed the top of the building above the reactor. The fire produced a cloud of radioactive particles which went about 5000 ft. in the atmosphere.
The radioactive cloud spread north-west wards and affected a large area of Ukraine, By elorussia, Russia and Northern Europe. When high levels of radioactivity were recorded by workers of the nuclear power plants in Sweden, the accident became known to the public. Among the radioactive elements released were cesium -137 and iodine 131.
Iodine -131 is a substance that can enter the human diet by falling on grasses that milk producing cows can eat and then concentrate in the human thyroid gland. Estimates of total radiation from fall out have been varied, one source compared it to the total of radiation from all atmosphere weapon tests. According to another source, the radiation released to the atmosphere compared that from about 300 atomic bombs.
Once it was realized that the fire could not be controlled by conventional means, helicopters were used for dumping sand, boron, dolomite and clay on to the open reactor. These neutrons absorbing materials slowed the fission of Uranium -235 and decreased the production of heat and radioactivity. Subsequently several foot layer of concrete was used to entomb the reactor and isolate it from the ground water and the nearby river. The studies on long-term effects on exposure to radiation are still going on.
It was reported that 237 people suffered fro.m acute radiation sickness and 31 people died. Approximately 115,000 people were evacuated in the 30 km. Zone. About 24,000 people received an average radiation of 0.43 Sv. However, a total of about 3 billion people in the Northern Hemisphere received varying amounts of radiation.
It is estimated that the radiation exposure would cause about 16,000 more deaths in 50 years time due to long-term effect of radiation. It has also been observed that a number of people suffered from disease like leukemia, thyroid, cancer etc. due to radiation exposure. The vegetation area within 7 km. of the power plant was severely damaged. The contamination of soil and water made the area inhabitable.
Several questions have been raised about the safety and proper handling of the nuclear power plants. Even with the best of the solutions, the risk of accidents would always be there.