The Nuclear Safety Commission of Japan issued a preliminary calculation that reveals the crippled nuclear plant had been releasing as much as 10,000 terabecquerels of radioactive materials per hour for hours after the 9-magnitude earthquake and tsunami March 11, Kyodo News reported.
The assessment led the government to consider raising the accident's severity level from 5 to 7, the highest on the international scale, the Japanese news service said.
The 1986 Chernobyl nuclear power plant meltdown in Ukraine is considered the worst civilian nuclear accident in history.
Commission Chairman Haruki Madarame said it has estimated the release of 10,000 terabecquerels of radioactive materials per hour went on for several hours. The commission says the release has fallen to under 1 terabecquerel per hour.
The commission also released preliminary numbers for the cumulative amount of external exposure to radiation, saying it exceeded the yearly limit of 1 millisieverts in areas extending more than 38 miles to the northwest of the plant and about 30 miles to the south-southwest of the plant.
The announcement came the same day another powerful earthquake struck the country's northeastern region. The U.S. Geological Survey initially reported the quake registered 7.1-magnitude, but later lowered the magnitude to 6.6.
A tsunami warning issued by Japan's Meteorological Agency was canceled.
The quake's epicenter was about 101 miles northeast of Tokyo and about 50 miles south of the Fukushima Daiichi nuclear power plant, the USGS said.
Workers at the plant were asked to evacuate, CNN said.
The Tokyo Electric Power Co., operator of the plant, said power was knocked out for about 220,000 households and businesses in Fukushima after Monday's earthquake.
The temblor came as Japanese officials said they were considering extending the evacuation zone around the nuclear plant because of radiation concerns, the BBC reported. The 12-mile zone would be widened to include five communities beyond the current boundary, officials said.
Chief Cabinet Secretary Yukio Edano said the new evacuations would occur in the next month.
"There is no need to evacuate immediately," he said during a news conference.
Japan's monthlong atomic crisis is far from being stabilized but Edano gave an optimistic assessment.
As the critical work of containing radiation leaks and keeping the reactors from overheating proceeded at the Daiichi plant, with remote-controlled machinery doing some of the radioactive debris clearing, Edano told reporters there was a lesser risk now of massive radiation emission than a month ago.
"The risk that the situation will worsen and that there would be new massive emissions of radioactive materials is becoming considerably smaller," Kyodo News quoted Edano as saying.
The March 11 earthquake, Japan's strongest, was followed by a 30-foot high Pacific Ocean tsunami that devastated much of northeastern Japan, killing thousands of people and leaving thousands of others missing. The epic episode caused economic damage already running into the hundreds of billions of dollars at a time when Japan was only beginning a slow economic recovery after years of a deflationary slump.
At the six-reactor plant, Tokyo Electric used a drone chopper to take pictures of the damaged Nos. 1, 3 and 4 reactors and their pools holding spent fuel rods, which must be kept cool to prevent a meltdown, even as other robotic machinery helped remove the radioactive materials.
Workers were channeling some of the 60,000 tons of radioactive water in the basements of the reactors into holding pits. Plant workers also have been pumping inert nitrogen gas into the No. 1 reactor to prevent a hydrogen explosion, while at the same time battling high-level radioactive water flooding the basements of some of the other reactors.
The utility has been dumping low-level radioactive water from a nuclear waste disposal facility into the Pacific Ocean -- a process described as unavoidable -- to create more storage room for the high-level contaminated water, which otherwise would overflow and interfere with other critical restoration work.
The process has raised serious concerns abroad about marine life contamination and the Japanese fishing industry.
A 7.1-magnitude aftershock hit the northeast region Thursday but didn't appear to have caused any further damage to the Fukushima plant. The aftershock spilled some radioactive water at another plant in neighboring Miyagi prefecture.
Tokyo Electric Power President Masataka Shimizu visited the Fukushima prefectural government office Monday to apologize for the disaster, but Kyodo News quoted officials as saying prefectural Gov. Yuhei Sato again declined to meet him.
Bank of Japan Gov. Masaaki Shirakawa said smaller businesses were encountering problems raising operating capital since the disaster. He expressed concern as the central bank released its quarterly report on the country's regional economies.