The Chernobyl Nuclear Accident occurred in Ukraine (while it was still part of the former Soviet Union) on April 26, 1986. The accident occurred at one of the four Chernobyl reactor where a test procedure to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply was being carried out. During the previous attempt at this test in the previous year the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested (World Nuclear Association).

Partly due to the faulty design of the control rods and inexperienced operators, the test of April 26, 1986 resulted in a power surge leading to a powerful steam explosion and fire that released a significant portion of core material into the environment. The disaster killed and maimed several people. The exact number of deaths were not known but some have put the number of death immediately after the accident and afterwards at close to 60. Also it was estimated that over 4,000 people exposed to the to the elevated doses of radiation from the accident surfer cancer.

As a result, the city of Chernobyl (population of ~ 14,000) was largely abandoned, the larger city of Pripyat (population ~ 49,400) was completely abandoned, and a 30 km exclusion zone was established. Several years after the incidence, some people have argued that thousands of deformed babies were reportedly born of those people exposed to the radiations at Chernobyl accident. Chernobyl accident is considered the worst nuclear accident in human history and it was given INES ratiing of 7.0 (the maximum). The other nuclear accident with INES rating 7.0 is the Fukushima Daiichi Nuclear Accident of March, 2011. There have been changes to the nuclear reactor designs, since the Chernobyl accident, to prevent such accidents from occurring in the future.

Chernobyl reactor is said to be of the ex-soviet union old design called RBMK. RBMK’s are boiling water reactors using graphite as the liquid moderator. These reactors do not have a secondary circuit and are not equipped with a containment structure. That is why the accident at Chernobyl had such damaging effects on people and the environment. After the Chernobyl accident, pressurized water reactor (PWR), capable of preventing or minimizing the effect of similar accidents in the future, were designed and manufactured.

Other Sites:

• Fukushima Nuclear Accident Update Log
• World Nuclear Association - Chernobyl Disaster
• Nuclear Energy
• International Nuclear and Radiological Event Scale. Other links: INES HTML - (IAEA Website) or INES pdf  - (IAEA Website)
• Japan Travel and Living Guide
• Japan's Nuclear Crisis Raised to INES Scale 7.0, same scale as Chernobyl