Japan has lifted the crisis rating for the crippled Fukushima nuclear power plant to seven, the maximum international level.
Previously only the 1986 Chernobyl nuclear disaster had been given a rating of seven.
Fukushima is being upgraded from level five after emitting radiation for more than a month.
“This is a preliminary assessment and is subject to finalisation by the International Atomic Energy Agency,” Japan’s nuclear safety watchdog said on Tuesday.
“In terms of volume of radioactive materials released, our estimate shows it is about 10 per cent of what was released by Chernobyl.”
The meltdown at Chernobyl, the world’s worst peacetime nuclear event, in the then-Soviet Union spewed forth a large volume of toxic radiation, poisoning large areas of land and affecting thousands of lives.
Nuclear industry specialist Murray Jennex, an associate professor at San Diego State University in California, dismissed the comparison with Fukushima.
“It’s nowhere near that level. Chernobyl was terrible – it blew and they had no containment, and they were stuck,” he said.
“[Japan’s] containment has been holding. The only thing that hasn’t is the fuel pool that caught fire.”
But not long after the crisis was upgraded, the Fukushima operator said it was concerned the radiation leakage could eventually exceed that of the Chernobyl disaster.
“The radiation leak has not stopped completely and our concern is that it could eventually exceed Chernobyl,” an official from operator Tokyo Electric and Power told reporters.
Plant workers were then forced to evacuate, as a magnitude-6.3 earthquake hit the Fukushima prefecture – the third large aftershock to strike Japan in 24 hours.
According to the International Nuclear Events Scale, level seven incidents are ones with a “major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures”.
The Fukushima Daiichi nuclear plant was badly damaged by the March 11 earthquake and tsunami that hit Japan and has been leaking radiation since, despite efforts to cool and stabilise its reactors.
Tokyo said on Monday it was augmenting the evacuation area around the plant because of long-term health worries, even as the government said the danger of a large leak of radioactive materials was fading.
“The possibility that the situation at the nuclear plant will deteriorate and lead to new leakage of massive radioactive materials is becoming significantly smaller,” chief government spokesman Yukio Edano said.
He later said the government was concerned about the effect of long-term exposure to radiation and would be ordering people to leave certain areas more than 20 kilometres from the plant, which are currently outside the exclusion zone.
Another fire was reported in reactor four this morning, but it was later extinguished.
The reactor fire was reported just moments after Japan was hit by yet another powerful aftershock.
This morning’s magnitude-6.3 quake halted Tokyo trains and caused a brief delay at Narita Airport.
A magnitude-7.1 tremor last night killed four people, including a 16-year-old girl trapped in a mudslide.