The heroes risking their lives on the front line of Japan's nuclear crisis - some 50 workers at the crippled Fukushima nuclear plant - are being watched by a grateful and anxious nation.

The workers are tasked with keeping cooling water flowing into the six reactors at the Fukushima plant, four of which have overheated since Friday's quake, raising fears of a meltdown.

Explosions and a fire at the plant, 250 kilometres north-east of Tokyo, have unleashed dangerous levels of radiation, forcing the operator to pull out hundreds of workers and leaving just a few dozen behind.

The staff were briefly evacuated on Wednesday after smoke rose above the plant and radiation levels spiked, but then were allowed to return to continue their work - amid jitters about eventual widespread contamination.

"The people working at these plants are fighting without running away," Michiko Otsuki, an employee at the separate Fukushima No.2 plant who also has been evacuated, wrote in a post on Japanese social networking site Mixi.

"Now I can only pray for the safety of everyone... Please don't forget that there are people who are working to protect everyone's lives in exchange for their own lives."

Prime minister Naoto Kan has saluted the efforts and courage of the workers employed by embattled plant operator TEPCO - as his chief spokesman Yukio Edano confirmed their health was at risk.

"Those with TEPCO and related entities are working to pour water, making their best effort even at this moment, without even thinking twice about the danger," Mr Kan said.

Tens of thousands of residents have already been evacuated from within a radius of 20 kilometres of the 40-year-old plant, and Mr Kan urged people living within 10 kilometres of the zone to stay indoors.

David Brenner, the director of radiological research at Columbia Service, said that given the radiation levels detected at the troubled facility, the workers were at "significant risk".

"In many ways they are already heroes... [they] are going to be suffering very high radiation exposures," Mr Brenner told the BBC World Service.

Mr Edano said the level was stable near the plant's front gate by Wednesday afternoon, after spiking to dangerous levels earlier that day and the day before.

Not everyone is singing the praises of the difficult mission.

The Yomiuri Shimbun reported on Tuesday that Japan's defence ministry had hit out at the nuclear safety agency and TEPCO after some of its troops were injured and possibly exposed to radiation.

They had been deployed at the plant's reactor number three when an explosion blew off the outer structure housing the reactor container.

"They said it was safe, which we believed, and we worked there," a senior ministry official was quoted as saying.

"We know about protection from radiation, but we are not specialised in the structure of a nuclear reactor," he said. "When they say it's safe, we just trust them, even if we feel a bit uneasy."

Baku Nishio, head of the Citizens' Nuclear Information Centre which campaigns against nuclear power, said the last batch of workers were probably acting "out of a sense of duty rather than from some heroic sentiment".

"I think it's a grave problem that the country has to give up its fate to the small group of last-ditch workers," he said.

"If the situation gets worse, it'd be certain to cause some deaths."

"We need to realise this is the nature of nuclear plants," he said, adding the crisis could lead to changes in the country's atomic energy policy.