How does Fukushima differ from Chernobyl?

Japanese authorities have raised the severity rating of the nuclear crisis at the damaged Fukushima Daiichi power plant to the highest level, seven.

The decision reflects the ongoing release of radiation, rather than a sudden deterioration. Level seven previously only applied to the 1986 Chernobyl disaster, where 10 times as much radiation was emitted.

But most experts agree the two nuclear incidents are very different. Explore the table below to find out how they compare.

Fukushima and Chernobyl

Fukushima and Chernobyl compared

Category Fukushima Daiichi Chernobyl

Source: Nuclear and Industrial Safety Agency, Japanese authorities, UNSCEAR. *Becquerels are a measurement of radiation

Date of accident

11 March 2011

26 April 1986

Accident details

A magnitude-9.0 earthquake and resulting tsunami damaged the plant's power systems, causing cooling systems to fail. A series of gas explosions followed

A sudden power output surge during a systems test caused a reactor vessel to rupture, leading to a series of blasts. An intense fire burned for 10 days

Severity rating

Level 7 - major accident

Level 7 - major accident

Number of reactors

Six; but only three of concern, plus pools storing spent fuel

Four; but only one reactor involved

Type of reactors

Boiling-water reactors. Japanese authorities stress that unlike at Chernobyl, the containment vessels at Fukushima remain intact. Also, unlike Chernobyl, the reactors at Fukushima do not have a combustible graphite core

Graphite-moderated boiling water reactor. The graphite made it highly combustible. The reactor also had no containment structure and nothing stopped the trajectory of radioactive materials into the air

Radiation released

370,000 terabecquerels* (as of 12 April)

5.2 million terabecquerels*

Area affected

Officials say areas extending more than 60km (36 miles) to the north-west of the plant and about 40km to the south-southwest have seen radiation levels exceed annual limits

Contamination of an area as far as 500 km (300 miles) from the plant, according to the UN. But animals and plants were also affected much further away

Evacuation zone

20km; 20-30km voluntary zone. Five communities beyond the existing evacuation zone have also been evacuated

30km

People evacuated

Tens of thousands

The authorities evacuated, in 1986, about 115,000 people from areas surrounding the reactor and subsequently relocated, after 1986, about 220,000 people from Belarus, the Russian Federation and Ukraine

Related deaths

No deaths so far due to radiation

A UN report places the total confirmed deaths from radiation at 64 as of 2008. Disputes continue about how many will eventually die

Long-term health damage

Not yet known, but risks to human health are thought to be low

Among the residents of Belarus, the Russian Federation and Ukraine, there had been up to the year 2005 more than 6,000 cases of thyroid cancer reported in children and adolescents who were exposed at the time of the accident, and more cases can be expected during the next decades

Current status

Engineers have brought the plant to a "cold shutdown condition", a key milestone in bringing it under control. It will take decades to dismantle it completely however.

The damaged reactor is now encased in a concrete shell. A new containment structure is due to be completed by 2014

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