Government authorities offered reassuring words after raising the severity rating of the Fukushima No. 1 nuclear power plant crisis to the maximum level. But the continued leak of radioactive substances and disruptive aftershocks are compounding the problems in containing the nuclear crisis.
Government authorities offered reassuring words after raising the severity rating of the Fukushima No. 1 nuclear power plant crisis to the maximum level. But the continued leak of radioactive substances and disruptive aftershocks are compounding the problems in containing the nuclear crisis.
The government Tuesday raised the seriousness of the situation at the plant, which was severely damaged by the March 11 earthquake and tsunami, to level 7 on the International Nuclear and Radiological Event Scale (INES), the same ranking as the 1986 Chernobyl nuclear disaster.
Prime Minister Naoto Kan stressed that progress is being made in the fight to bring the reactors under control.
"As we look at the situation today, we can say we have been moving forward step by step," Kan said at a news conference. "The release of radioactive substances has declined."
But Kan also warned against premature optimism. "We have not yet reached a stage where we can say for sure (when the crisis will end)."
A level 7 accident is defined as a "major accident" resulting in an environmental release of radioactive substances equivalent to more than tens of thousands of terabecquerels of iodine 131.
The Nuclear and Industrial Safety Agency (NISA), the nation's nuclear industry regulator, has estimated the radioactive leak so far from the Fukushima plant at 370,000 to 630,000 terabecquerels, about one-tenth of the release in Chernobyl, or 5.2 million terabecquerels.
The Japanese government was criticized by experts and officials abroad after March 18 for failing to revise the accident assessment upward.
In a separate news conference Tuesday, Hidehiko Nishiyama, deputy director general at NISA, said upgrading the severity assessment doesn't mean the accident at the Fukushima nuclear station is as serious as the Chernobyl disaster.
"Level 7 is an assessment indicating (the accident's) seriousness, but it is considerably different from Chernobyl," Nishiyama said.
He cited several reasons, including: no patients showing acute symptoms due to exposure to radiation from the plant; reactors still maintaining their original structures; and workers in the compound are dealing with the situation.
The Nuclear Safety Commission of Japan concurs, maintaining that there is, at the moment, no need for the government to take additional emergency measures like expanding the evacuation zone.
The government has ordered people living within a 20-kilometer radius of the plant to evacuate.
However, there is no end in sight to the nuclear crisis.
The radioactive release from the Chernobyl plant lasted for about 10 days. But the leak in Fukushima still continues one month after the emergency began. Efforts continue to restore the systems for cooling the decay heat emitted from the nuclear fuel rods in the reactors and the fuel pools.
In addition, a far larger amount of radioactivity remains within the nuclear plant's buildings than the total amount released so far.
The plant is currently releasing several terabecquerels of radiative substances per hour, about one-10,000th of the peak level, according to an estimate by the Nuclear Safety Commission.
"If this level of release continues, the total wouldn't surpass the Chernobyl figure," said Seiji Shiroya, a member of the commission.
But a serious development like the destruction of a container vessel surrounding a reactor to prevent the escape of radioactive materials would dramatically change the situation.
In particular, the No. 1 reactor is in a precarious state, with high temperatures building within the pressure vessel.
Continued seismic activity in eastern Japan has compounded the problems for the workers at the site.
On both Monday and Tuesday, earthquakes with a seismic intensity of a lower 6 jolted the eastern part of Fukushima Prefecture, where the nuclear power plant is located. The strong aftershock Monday temporarily disrupted the work to inject cooling water into the reactors.
Restoring the fuel cooling systems is considered the key to preventing the crisis from worsening.
Chief Cabinet Secretary Yukio Edano said Wednesday that NISA had notified the government by the end of last month of a possibility of raising the severity rating of the Fukushima No. 1 nuclear power plant crisis to the maximum level of 7.
Edano told a news conference that the government did not release the revised assessment immediately because it could have been wrong.
"The government was under no circumstance to announce the assessment with certainty because it was based on data of radioactive materials at three locations," he said.
(This article was written by Keisuke Katori, Ryoma Komiyama, Toshio Kawada)