Some opposition lawmakers blasted Naoto Kan in parliament for his handling of the disaster and for not widening the exclusion zone. Kan said he was seeking advice on such a step, which would force 130,000 people to move in addition to 70,000 already displaced.
The drama at the six-reactor facility has compounded Japan's agony after an earthquake and tsunami on March 11 left more than 28,000 people dead or missing in the devastated northeast.
In a gesture of support, France said it had sent two nuclear experts to Japan to help contain the accident and French President Nicolas Sarkozy will visit on Thursday for a meeting with Kan.
France is the world's most nuclear-dependent country, producing 75 percent of its power needs from 58 nuclear reactors, and selling state-owned Areva's reactors around the world. Sarkozy will be the first foreign leader to visit since the earthquake.
In the latest blow to hopes authorities were gradually getting the plant under control, operator Tokyo Electric Power Co said plutonium was found at low-risk levels in soil samples at the facility.
A by-product of atomic reactions and also used in nuclear bombs, plutonium is highly carcinogenic and one of the most dangerous substances on the planet, experts say.
They believe 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.
Japan's Nuclear and Industrial Safety Agency said while the plutonium levels were not harmful to human health, the discovery could mean the reactor's containment mechanism had been breached.
"Plutonium is a substance that's emitted when the temperature is high, and it's also heavy and so does not leak out easily," agency deputy director Hidehiko Nishiyama told a news conference.
"So if plutonium has emerged from the reactor, that tells us something about the damage to the fuel. And if it has breached the original containment system, it underlines the gravity and seriousness of this accident."
Sakae Muto, a Tokyo Electric vice-president, 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.
"I apologize for making people worried," Muto said.
With towns on the northeast coast reduced to apocalyptic landscapes of mud and debris following the quake and tsunami, 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.