The Japan Nuclear and Industrial Safety Agency has provisionally increased its estimate of the scale of the Fukushima Daiichi nuclear power plant crisis to the level of Chernobyl, level 7 on the International Nuclear and Radiological Event Scale.

Reactor-by-reactor, system-by-system summary from JAIF as of 12 April

Reactor-by-reactor, system-by-system summary from JAIF as of 12 April; yellow indicates abnormal/unstable; red means damaged/nonfunctional/unsafe

INES level 7, ‘major accident’, involves ‘major release of radioactive material with widespread health and environmental effects requiring implementation of palnned and extended countermeasures,’ according to the International Atomic Energy Agency’s INES general description.

The new rating covers units 1, 2 and 3 as a group; previously, each unit was rated level 5, ‘accident with wider consequences’, which means ‘limited release of radioactive material likely to require implementation of some planned countermeasures,’ and ‘several deaths from radiation.’

Provisional ratings of INES level 3 for units 4 and 5 remains, according to the IAEA, although a chart published by the Japan Atomic Industrial Forum has changed the INES rating of unit 4 to the higher level.

The revision of the rating was based on radiological information gathered since 18 March. New values based on airborne radiation values have prompted the reevaluation. The value representing radiation impact, which is converted into an equivalent amount of iodine 131, exceeds 10^16 Bq. Still, NISA said that the total amount of discharged radioactive materials is about 10% of the Chernobyl accident.

Provisional calculations of the total amount of radioactive discharge to the air from the Japan Nuclear Safety Commission, which were similar, also suggested the need for a reevaluation.

A final rating will be issued after technical impact, once more is known about the incident, NISA said.