In parliament, Japanese Prime Minister Naoto Kan was lambasted for his handling of the disaster, which was triggered by a massive earthquake and tsunami that slammed into the coastline north of Tokyo.
Already deeply unpopular, Kan assured angry lawmakers that that the government was making public all the information it had to hand and he apologised for flying over the stricken site one day after the quake, which media reports said had delayed crucial operations to cool the reactors.
Opposition MP Yosuke Isozaki blasted Kan for not ordering evacuation from a zone 20-30 km (12-19 miles) beyond the nuclear plant, asking "is there anything as irresponsible as this?".
Kan said the government was seeking advice on whether to extend the zone beyond its current perimeter of 20 km.
The drama at the six-reactor facility has compounded Japan's agony after the double disaster left more than 28,000 people dead or missing in the devastated northeast.
PLUTONIUM LEVELS NOT HARMFUL
Plant operator Tokyo Electric Power Co (TEPCO) said the plutonium -- a by-product of atomic reactions and also used in nuclear bombs -- had been found at low-risk levels in five places at the plant, hit by a March 11 quake and tsunami.
Muto said the traces of plutonium-238, 239 and 240 were in keeping with levels found in Japan in the past due to particles in the atmosphere from nuclear testing abroad.
"It's not at the level that's harmful to human health."
Experts believe that at least some of the plutonium may have come from spent fuel rods at Fukushima or damage to reactor No. 3, the only one to use plutonium in its fuel mix.
The United Nations' International Atomic Energy Agency said the find was expected due to known fuel degradation.
But Japan's own nuclear safety agency was concerned at the plutonium samples, whose levels of radioactive decay ranged from 0.18 to 0.54 becquerels per kg.
"While it's not the level harmful to human health, I am not optimistic. This means the containment mechanism is being breached so I think the situation is worrisome," agency official Hidehiko Nishiyama was quoted as saying by Jiji news agency.
Workers at Fukushima are resigned to a struggle of weeks or months to re-start cooling systems vital to control the reactors and avert disaster. Their conditions are extremely dangerous, earning them sympathy and admiration round the world.
On Monday, highly contaminated water was found in concrete tunnels extending beyond one reactor, while at the weekend radiation hit 100,000 times over normal in water inside another.
That poses a major dilemma for TEPCO which wants to douse the reactors to cool them, but not worsen the radiation spread.
Fires, blasts, smoke and steam have posed other hazards.
Japan says a partial meltdown of fuel rods inside reactor No. 2 has contributed to the radiation levels.
The crisis, the worst nuclear accident since Chernobyl in 1986, has contaminated vegetables and milk from the area, as well as the surrounding sea. U.S. experts said groundwater, reservoirs and the sea all faced "significant contamination."
With towns on the northeast coast reduced to apocalyptic landscapes of mud and debris, more than a quarter of a million people are homeless. The event may be the world's costliest natural disaster, with estimates of damage topping $300 billion.
The environmental group Greenpeace said its experts had confirmed dangerous radiation of up to 10 microsieverts per hour in Iitate village, 40 km (25 miles) northwest of the plant.
It called for the extension of a 20-km (12-mile) evacuation zone. "It is clearly not safe for people to remain in Iitate, especially children and pregnant women," Greenpeace said, urging Japan to "stop choosing politics over science".
In most countries the maximum permissible annual dose for radiation workers is 50 millisieverts, or 50,000 microsieverts, according to the World Nuclear Association Industry body.
Tens of thousands of people have been evacuated from a 20-km (12 mile) radius around the plant. Those within a further 10-km radius have been told by the government to stay indoors or, better still, leave too.
Beyond the evacuation zone, traces of radiation have been found in tap water in Tokyo and as far away as Iceland.
Japanese officials and international experts have generally said the levels away from the plant were not dangerous for human beings, who in any case face higher radiation doses on a daily basis from natural sources, X-rays or flying.
In downtown Tokyo, a Reuters reading on Tuesday showed 0.20-0.22 microsieverts per hour, within the global average of natural ambient radiation of 0.17-0.39 microsieverts per hour given by the World Nuclear Association.
"It's worth remembering that millions of Americans in the west of the country would have been exposed to tiny traces of plutonium during the above-ground bomb tests prior to the 1960s," he said.
The risk of a core meltdown had receded, he added, and hopefully water pumps would be restarted in the next couple of weeks to get the reactors into a safe "cold shutdown" stage.
"The eventual clean-up still looks like it should be easily manageable," he added.
The crisis has put huge pressure on TEPCO, criticised for safety lapses and a slow disaster response, and its boss, Masataka Shimizu, has barely been seen. Chief Cabinet Secretary Yukio Edano denied a newspaper report that the government was considering a temporary nationalisation of the company.
Kan, leading Japan during its worst crisis since World War Two, has also been low-profile.
Even though Japan's culture stresses group efficiency over individual charisma, many are unhappy and a weekend poll showed a majority feel Kan has not shown good leadership.