CNN Wire Staff
Tokyo (CNN) - The problems at Japan's earthquake-damaged Fukushima Daiichi nuclear plant are more serious than initially thought, the country's nuclear safety agency indicated Friday as it adjusted its assessment of the disaster.
The Nuclear and Industrial Safety Agency raised its rating for the most serious issues from 4 to 5 - putting those problems on par with those in the 1979 incident at Pennsylvania's Three Mile Island.
According to the International Nuclear Events Scale, a level 5 indicates the likelihood of a release of radioactive material, several deaths from radiation and severe damage to a reactor core. Each step on the scale indicates an increase of 10 times the severity of the step below it, the International Atomic Energy Agency says.
The Chernobyl nuclear accident in the former Soviet Union rated a 7, the highest level on the scale, while Japan's other nuclear crisis - a 1999 accident at Tokaimura in which workers died after being exposed to radiation - was rated a 4. The partial meltdown of a reactor core at Three Mile Island was deemed the worst nuclear accident in U.S. history.
But the rating change was not due to new problems at the plant, said Hidehiko Nishiyama, deputy head of the nuclear safety agency.
In fact, the situation at the plant - while still serious - did not worsen for the second consecutive day, according to the IAEA.
The change instead came after engineers reviewed images showing damage to fuel rods and other structures inside the reactor buildings, Nishiyama said.
Despite the more serious assessment, no expansion of the 20-kilometer (12.4-mile) evacuation zone was necessary, Nishiyama said Friday at a briefing.
Earlier evacuation orders took the possibility of greater damage to the plant into account, he said.
IAEA Director General Yukiya Amano cautioned against reading too much into the raised disaster assessment. He said it's too early to compare the plant's situation to Three Mile Island, and he said the disaster unfolding at Fukushima is not like what happened at Chernobyl.
But Peter Bradford, a member of the U.S. Nuclear Regulatory Commission when the Three Mile Island incident occurred, said Fukushima is worse.
"In terms of severity, this accident left Three Mile Island in the rear-view mirror several days ago," he said.
Amano also appeared to defend Japan's evacuation response, saying that IAEA guidelines call for exactly what Japan has ordered - an evacuation radius of 20 kilometers in the event of a reactor meltdown and a suggestion that people 20 kilometers to 30 kilometers stay inside.
U.S. officials have urged Americans to evacuate to a radius of 50 miles.
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