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Chernobyl
The Chernobyl nuclear power plant accident in the Ukraine occurred on April 26, 1986 because of bad Soviet reactor design and mistakes by the plant operators. The explosive meltdown at Chernobyl-Unit 4 happened during a regular shutdown 
and test by the reactor crew to determine how long turbines would spin and supply power following a loss of main electrical power supply. A series of operator actions, including turning off automatic shutdown mechanisms, preceded
the attempted test. As flow of coolant water diminished, power output increased. A flaw in the design caused a dramatic power surge when the operator moved to shut down the reactor from its unstable condition. The fuel elements ruptured and the resultant explosive force of steam lifted off the reactor cover plate, releasing fission products into the atmosphere. A second explosion threw out fragments of burning nuclear fuel and graphite from the core. The graphite moderator burst into flames. The graphite burned for nine days, causing the main release of radioactivity into the environment. Helicopters dropped 5000 tonnes of boron, dolomite, sand, clay and lead onto the burning core to put out the fire and limit the release of radioactive particles.
Source: OECD NEA
The accident destroyed the Chernobyl-4 reactor and killed 30 people, including 28 from radiation exposure. A further 209 on site were treated for acute radiation poisoning and among these. The main casualties were firefighters, especially those who fought the initial small fires on the roof of the turbine building. Large areas of Belarus, Ukraine, Russia and beyond were contaminated in varying degrees. The Chernobyl disaster was a unique event and the only accident in the history of commercial nuclear power where radiation-related fatalities occurred.
After the accident was brought under control, the next task was cleaning up the radioactivity at the site so that the remaining three reactors could be restarted, and the damaged reactor shielded more permanently. About 200,000 people ("liquidators") from all over the USSR were involved in the recovery and clean up during 1986 and 1987. They received high doses of radiation, around 100 millisieverts. Some 20,000 of them received about 250 mSv and a few received 500 mSv. Later, the number of liquidators swelled to over 600,000 but most of these received only low radiation doses. The Chernobyl Unit 4 is now enclosed in a large concrete shelter, called a sarcophagus. The sarcophagus was intended as a temporary tomb and there are plans for its reconstruction.
Energy shortages necessitated the continued operation of unit 3 until December 2000. Unit 2 was shut down after a turbine hall fire in 1991, and unit 1 at the end of 1997. Almost 6,000 people worked at the plant every day, and their radiation dose has been within internationally accepted limits. Today, Ukraine depends upon nuclear power stations for 45% of total electricity.
Latest Developments
Bechtel International Systems Corp. engineers are completing plans for a 20,000-ton, 370 feet high steel shell to enclose Chernobyl Reactor 4. An international consortium will finish the conceptual design for a hangar-shaped arch nearly that would be slid into place along greased steel plates to cover the ruined remains in a snug, weather-tight shelter. Inside the 35-story building, robotic cranes and live workers will then begin sorting the wreckage, removing and storing radioactive pieces of the radioactive core in shielded canisters and cutting old steel into manageable lengths. The $768 million 10-year plan was set into motion by the Group of 7 industrialized nations in 1997 and is scheduled for completion in 2007.
The shelter is designed to keep water out and dust in for 100 years, or for as long as it takes the Ukrainian government to designate a permanent storage facility. They will have to dispose of more than 200 tons of uranium and nearly a ton of lethally radioactive plutonium that remain inside the ruins. Most of the fuel-material lies as a solid "lava" formed by the fusion of molten fuel, concrete, 30 tons of fuel dust and 2,000 tons of combustibles. In the basement, rainwater and fuel dust have mixed together in a dangerous radioactive "soup." Lethal chunks of the reactor core lie unseen in the rubble and in the earth alongside the building.
The Chernobyl accident occurred April 26, 1986, when an out-of-control nuclear reaction caused a steam explosion, which blew off the roof of the steel building and the resulting graphite fire spewed radioactive material into the air. It was the worst nuclear accident in history. Thirty workers died immediately at the facility. In the six months immediately following the explosion, the Soviets erected an improvised shelter known as the "sarcophagus," but within 10 years scientists became alarmed because of leaks and the building's threatened collapse. In 1997, the Group of 7, plus Russia, the European Union and Ukraine, set up the Chernobyl Shelter Fund with the European reconstruction bank in charge. The bank established a shelter implementation plan and funded it with donations from 28 nations, ranging from $170 million from the United States to Iceland's $10,000.
The team settled on a steel arch 40 feet thick. The inside dimensions would be 803 feet -- almost three football fields -- across and 330 feet high. Lethal gamma rays escaping from the reactor's damaged core would make the center of the arch too hot for humans to work. Building the arch in place was impossible. Instead, the team decided to construct the arch in four 120-foot sections, then link the sections together and slide the entire structure along a track made of steel plates built on each side of Reactor No. 4.
One end will be fully enclosed, while the other will be a "cutout" that fits snugly over Reactor No. 3's building, which connects to the ruins. Current plans call for the stack (see picture) to be taken down, and the junction between the arch wall and Reactor No. 3 to be sealed. The new shelter will not "contain" the core's radioactivity but will be weatherproof. To help deconstruct parts of the reactor building and the sarcophagus, the new shell will have four ceiling cranes designed to pluck heavy steel beams from the old reactor and to wrestle pieces of twisted metal from the ruins. They will also be equipped with hydraulic cutters to chop wreckage into manageable chunks.
One unusual problem is the need to manage the new shell's microclimate. It's so big, it could rain inside, so we have to keep the moisture down. Source: Washington Post Dec 29, 2002 (edited)
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