Japan’s 8.9 magnitude earthquake and tsunami that devastated the island nation on Friday nearly precipitated a nuclear meltdown when the natural disasters knocked out electricity and prevented officials at the Fukushima Dai-ichi nuclear power plant from cooling the reactor core. According to the Christian Science Monitor, subsequent explosion at the reactor building on Saturday turned out to not have damaged the core housing and, fortunately, the reactor began cooling.
Talk of Chernobyl and Three Mile Island surfaced in the media while Japanese authorities worked to control the situation. Memories of Japan’s most recent disaster scare, just a little over a decade gone, resurfaced as well.
But how is accident severity rated?
The International Atomic Energy Agency (IAEA) introduced the International Nuclear and Radiological Event Scale in 1990 to better describe nuclear accidents. The scale measures accidents on a scale from one, an anomaly, to seven, a major accident.
The five worst follow:
* Chernobyl, Soviet Union (present Ukraine), 1986. The worst accident in the history of nuclear power occurred when a routine test resulted in a power surge and a shutdown of the reactor’s cooling system, which in turn produced a meltdown. The reactor exploded and delivered massive amounts of radiation into the surrounding environment. Casualty numbers from the accident are unknown because of the Soviet Union’s attempts to cover up the accident. Some 200,000 people were evacuated from the area. INES rating: 7, major accident.
* Kyshtyn, Soviet Union (present Russia), 1957. In the Soviet Union’s nascent nuclear power program, poor construction procedures resulted in a cooling unit for a waste storage tank failing at the Mayak River Power Plant. The subsequent explosion saw the environment irradiated and 10,000 people relocated, but no immediate deaths. INES rating: 6, serious accident.
* Windscale, United Kingdom, 1957. Radioactive material from the Windscale plant spewed into the air from a fire and explosion. It is estimated that perhaps 200 people died in Britain from cancer caused by accident. INES rating: 5, accident with wider consequences.
* Three Mile Island, United States, 1979. A valve malfunction and a faulty monitoring device nearly caused a meltdown just miles from Pennsylvania’s state capital in 1979. The nuclear plant’s designers instructed the operators how to avert the danger. INES rating: 5.
* Chalk River, Canada, 1952. A reactor core was damaged at the power plant in Ontario, Canada. INES rating: 5.
On the INES scale, Japan’s worst nuclear disaster rated a 4, which is an accident with local consequences. The event took place when operators overfilled a reactor precipitation tank with uranium. According to Discovery News, although the tank was drained to stop the reaction, three workers died of radiation exposure. Fewer than 100 people from the plant and living nearby were treated for radiation exposure.