Japan’s nuclear safety agency raised the severity rating of the crisis at its nuclear plant to the highest level today, on a par with the 1986 Chernobyl disaster.
A Nuclear Safety Commission of Japan official, speaking on national television, said the rating was raised from five to seven.
The official said the amount of radiation leaking from the Fukushima Dai-ichi nuclear plant was around 10% of that in the Chernobyl accident in the former Soviet Union.
Level seven signifies a “major accident” with “wider consequences” than the previous level, according to the standards scale.
“We have upgraded the severity level to seven as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean,” said Nuclear and Industrial Safety Agency (Nisa) official Minoru Oogoda.
Nisa officials said one of the factors behind the decision was that the total amount of radioactive particles released into the atmosphere since the incident had reached levels that applied to a Level seven incident.
The action lifts the rating to the highest on an international scale designed by a group of experts in 1989 and is overseen by the International Atomic Energy Agency.
In Chernobyl, Ukraine, a reactor exploded on April 26 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 30km around the plant was declared uninhabitable, although some plant workers still live there for short periods and a few hundred other people have returned despite government encouragement to stay away.
Meanwhile, setbacks continued at Japan’s tsunami-stricken nuclear power complex, with workers discovering a small fire near a reactor building at 8.38pm BST on Monday. The fire was extinguished quickly, the plant’s operator said.
Tokyo Electric Power Co (Tepco) which operates the disabled Fukushima Dai-ichi nuclear power plant, said the fire at a box that contained batteries in a building near the No 4 reactor was discovered at about 6.38am Tuesday, local time, and was put out seven minutes later.
It was not clear whether the fire was related to a magnitude-6.3 earthquake that shook the Tokyo area yesterday. The cause of the fire is being investigated.
“The fire was extinguished immediately. It has no impact on Unit 4’s cooling operations for the spent fuel rods,” said Tepco spokesman Naoki Tsunoda.
The plant was damaged in a massive tsunami on 11 March that knocked out cooling systems and back-up diesel generators, leading to explosions at three reactors and a fire at a fourth that was undergoing regular maintenance and was empty of fuel.
The magnitude-9.0 earthquake that caused the tsunami immediately stopped the three reactors, but overheated cores and a lack of cooling functions led to further damage.
Engineers have been able to pump water into the damaged reactors to cool them down, but leaks have resulted in the pooling of tons of contaminated, radioactive water that has prevented workers from conducting further repairs.
Aftershocks yesterday briefly cut power to back-up pumps, halting the injection of cooling water for about 50 minutes before power was restored.
The revision was based on cross-checking and assessments of data on leaks of radioactive iodine-131 and caesium-137, said Nisa spokesman Hidehiko Nishiyama.
“We have refrained from making announcements until we have reliable data. The announcement is being made now because it became possible to look at and check the accumulated data assessed in two different ways,” he said, referring to measurements from Nisa and the Nuclear Security Council.
Nishiyama noted that unlike in Chernobyl there had been no explosions of reactor cores at the Fukushima Dai-ichi plant, although there were hydrogen explosions.
“In that sense, this situation is totally different from Chernobyl,” he said.
Tepco was still estimating the total amount of radioactive material that might be released by the accident, said company spokesman Junichi Matsumoto.
He acknowledged the amount of radioactivity released might even exceed the amount emitted by Chernobyl.