Japan may look to extend the evacuation zone around the quake-damaged Fukushima Daiichi nuclear power plant, the government's top spokesman said Thursday, as towns and villages around the plant continued to feel the impact of a radiation fallout, even after the plant itself showed signs of moving toward stability.

Chief Cabinet Secretary Yukio Edano said the current 20-kilometer evacuation zone may need to be enlarged since the original parameters were established in relation to short-term exposure. The plant emergency caused by an earthquake and tsunami on March 11 is now nearing the end of its fourth week. There is a further 10-km band in which residents are urged to stay indoors as much as possible.

"Current evacuation orders apply to areas where people are in danger of having received 50 millisieverts. We are now looking into what to do with other areas where, with prolonged exposure, people may receive that amount," Edano said. A 50 millisievert amount represents the annual exposure limit for a nuclear plant worker.

The mayor of one town just outside the 30-km radius reacted angrily to the proposed action, saying the government should have thought ahead earlier in the crisis.

The town, Kawamata, located northwest of the plant, has seen elevated radiation levels and is one of seven municipalities from which sales of local produce has been banned.

"We are hearing that it may take months for the plant to settle down, and only now are they talking about expanding the zone?" said Michio Furukawa, the mayor of Kawamata.

"What we want to know is when this will all end?" he said.

Senior nuclear regulator Hidehiko Nishiyama offered apologies for the possible further dislocation to those living in the area. He identified a series of hydrogen explosions at the plant days after the earthquake as the primary cause of the widespread radiation.

"The explosions sent radioactive materials flying to areas far outside the nuclear complex," Nishiyama said in a press conference. "Radioactive materials, once spread, cannot be put back. The best we can do is to stabilize the damaged reactors and prevent further emissions of radiation."

In an effort to head off further hydrogen explosions, the plant operator, Tokyo Electric Power Co., Wednesday began streaming nitrogen gas into Reactor No. 1, one of the less damaged of the three crippled units.

Reactor No. 1's pressure and containment vessels are seen to have come out relatively unscathed by the overheating of the reactor core following the failure of the cooling system on March 11. But as the fuel cooled down and more steam condensed into water, fears grew that the pressure inside the containment vessel would fall sharply, allowing air to come in and create a dangerous mix of hydrogen and oxygen inside the vessel. Nitrogen injection is designed to forestall the entry of oxygen.

"The injection of gas is proceeding smoothly," a Tepco company official said at a press briefing. The process began around 1 a.m. local time Thursday and will continue for about six days, he said, adding that the measure was having the desired effect of slightly raising the pressure within the container vessel.

Tepco also announced that its president, Masataka Shimizu, had returned to work after being hospitalized for more than a week as his company was blamed for power outages and radioactive contamination that swept eastern Japan .

Shimizu will assume the role leading a joint task force with the government to support evacuees, allowing Chairman Tsunehisa Katsumata to continue his role of overseeing the company's effort to bring the plant under control, according to Tepco spokesman Naoyuki Matsumoto.

The other two heavily damaged units--Reactors Nos. 2 and 3--stayed in a relatively stable condition Thursday, due to the continued pumping in of cold water to cool the reactor cores.

But Tadashi Narabayashi, a professor at Hokkaido University and an expert in reactor engineering, said the situation is not sustainable, because the massive water injection results in overflowing of irradiated water from these reactors and the contamination of the facilities. This forces operators to reduce the amount of water pumped in, causing the reactors to heat up again. "Unless a cooling system is restored, in which water is recirculated within the system, the reactors cannot be stabilized on a sustained basis," he said.

Meanwhile, Edano denied a claim by a U.S. lawmaker that part of the No. 2 reactor core at the plant had melted through its pressure vessel.

"I am not sure what the reasoning behind the lawmaker's statement was," he said. "The unfortunate situation (at reactor No. 2), which continues unchanged, is that the fuel rods are partially exposed, and we are doing all that we can to reduce the risks created by this situation and to address the problem."

On Wednesday, Rep. Ed Markey, a Massachusetts Democrat and a critic of nuclear power, said the U.S. Nuclear Regulatory Commission had informed him "the core of Unit Two has gotten so hot that part of it has probably melted through the reactor pressure vessel."

Later Wednesday, an NRC official disputed Markey's remarks, saying that while there was significant fuel damage in all three reactors, "We don't believe at this point in time that the core has left the vessel."

The recovery efforts have received some supportive words from the British government, which said that it was easing its travel restrictions to Tokyo based on its scientific analysis of the current situation.

"Although the situation at Fukushima will remain of concern for some time, the risks are gradually declining as the reactors cool and as facilities to stabilise them are established." the government's Foreign and Commonwealth Office said on its website.

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