Chernobyl

The Chernobyl Nuclear Power Plant after the explosion on April 26. Reactor 4 can be seen to the right of the chimney, with its concrete housing blasted open. Damaged debris and reactor-moderating graphite can be seen strewn around. The partially-collapsed roof of the turbine hall is not visible.

Chernobyl, formally known as the V. I. Lenin Nuclear Power Plant, is a closed (but not decommissioned) nuclear power station in Ukraine near the border with Belarus.

River Monsters

In the River Monsters episode Atomic Assassin, Jeremy Wade is allowed access to the exclusion zone, and finishes the episode by catching a wels catfish in Reactor Number 4's cooling pond.

Location

It is about 110 kilometers, or 68 miles, from the city of Kyiv, and 2 kilometers or 1.24 miles from Pripyat, a city built to house the families of the plant workers. On April 26, 1986 at 1:23:40 am, the Chernobyl plant became the site of the worst nuclear disaster in history when Reactor Unit 4 exploded, spewing almost eight tons of highly radioactive material into the atmosphere.

Construction

Construction began on the V.I. Lenin Nuclear Power Plant in 1970, with Reactor Number 1 becoming operational in 1977. It was the third power plant in the Soviet Union to use the RBMK reactor system, a graphite-moderated Generation-II reactor type. It was originally to consist of four reactors RBMK-1000s, each rated at 1000 Megawatts. By 1983, four reactors had become operational, with units 5 and 6 under construction and almost completed.

Electrical and Turbine Systems

An aerial view of the plant after the explosion. The concrete 'sarcophagus' covering reactor 4 is visible.

Connected by a single transformer to the 750 kV electrical grid, the power plant had its generators connected by two switches in series, and were capable of connecting to the transformer to power the plant's electrical systems in case of power failure.

Electricity was generated through the use of large 500 MW turbines, cooled by Hydrogen and hooked to TBB-500 generators. The turbines and generators were connected by common shafts, which weighed around 200 tons when combined with their rotors and revolved at 3000 rpm. The turbo generator weighed 1200 tons, with the hydrogen for cooling the rotor being manufactured on site. The stator, however, was cooled by water.

Disaster

At the time of the disaster, the plant was testing emergency procedures for power outages, a process that disabled emergency systems. Due to a high positive void coefficient, a fatal flaw in RBMK-type reactors, and mismanagement contrary to safety regulations, an uncontrolled reaction lead to superheated water instantaneously expanding into steam. This violent expansion, or steam explosion, blew open the upper biological shield of the reactor and exposed the core, starting an open-air graphite fire.

Diagram of radiation spread from Chernobyl.

Updrafts from the fire's heat carried radioactive materials high into the air for days before containment occurred on May 4, 1986, 8 days after the explosion. This radioactive ash was wafted over much of the USSR and Western Europe.

The total number of deaths from the disaster are unknown and highly disputed, with some suggesting that they number as few as 4000, where as others claim that over a hundred thousand died as a result of radiation. Two workers were killed instantly, due to the force of the explosion. 28 first responders (mostly fire fighters) died in the immediate aftermath due to Acute Radiation Poisoning.

Considered as the most catastrophic nuclear disaster of all time, Chernobyl is ranked as a level 7 accident, the highest possible level in the International Nuclear Event Scale. In the months following the explosion, construction crews and minors labored in conditions of extreme contamination to contain the spread of radiation and seal off the reactor.

Summary

Upon observing a dangerous power surge after the disabling of multiple safety features, the engineers of Reactor 4, including Leonid Toptunov and Anatoly Dyatlov, pressed the SCRAM button, or reactor shutdown. However, immediately after this button was pressed, a second and massive power surge occurred, instantly vaporizing the light-water in the reactor and leading to the first explosion, which ruptured the containment vessel and exposed the nuclear fuel to the atmosphere. A defect in the graphite-tipped control rods caused them to displace water, leading to the uncontrolled reaction. The 49,000 residents of Pripyat were not evacuated until 36 hours after the explosion. The original exclusion zone was 10 km. After radioactive matter continued to spread, the zone was expanded to 30 km, and a further 68,000 were evacuated.

The disaster at Chernobyl lead to the modification of all RBMK reactors in the Soviet Union, leading to a negative void coefficient and more control rods, as well as corrections to the rods themselves.

The events of the disaster are briefly described in the Atomic Assassin episode before Jeremy Wade enters the exclusion zone.