They're already being dubbed the Fukushima Fifty: the men who are trading shifts to try to prevent the full meltdown of the Fukushima Dai-ichi nuclear facility.

On their shoulders rests the safety -- and perhaps the lives -- of thousands of Japanese citizens. When the facility's 800 employees were evacuated last week following the earthquake and tsunami, these were the men who stayed behind to try to cool down the reactors, to fight the fires and to prevent further explosions.

Details about the men and their work are sketchy. In fact, there may be more than 50 of them.

Because they need to rotate shifts to rest and to decontaminate periodically, it's likely the full contingent numbers closer to 200. However many there are, they are performing intensely dangerous work.

Some have said the only term to describe their work is a suicide mission. Others believe there is a good chance these men can still emerge from this work without any ill effects.

Japanese Prime Minister Naoto Kan offered a tribute to the men this week, saluting their efforts and courage.

"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," Kan said.

TEPCO, the Tokyo Electric Company, has not provided any details about the men, so it's not clear who they are. Nuclear power consultant Arnold Gundersen, a consultant who worked in American plants similar to Fukushima, says it's likely they include regular workers as well as retirees and workers from unaffected plants who were called in to help.

He told The New York Times that plant managers "may also be asking for people to volunteer to receive additional exposure."

Surging radiation levels Wednesday forced a delay in the work to cool down the overheating Dai-ichi reactors, as workers were forced to temporarily withdraw from the crippled plant. It was not clear how far away they withdrew or whether the same workers returned when radiation levels fell later on Wednesday and work resumed.

In a surprising move on Wednesday, Japan's Ministry of Health Labour and Welfare decided to raise the maximum radiation dose allowed for nuclear workers. It said the hike was needed to prevent the crisis at the power plant from worsening.

The Ministry raised the maximum allowable exposure for nuclear workers to 250 millisieverts from 100 millisieverts. It described the move as "unavoidable due to the circumstances." Still, even at 250 mSv, that's still below the level at which people would show symptoms of radiation poisoning.

Dr. Lew Pepper, a professor at the Boston University School of Public Health who has studied the effects of radiation on nuclear weapons workers, notes that there's only so much radiation that any one person can absorb.

"If they exceed a certain amount, they can't go back in for a day or a week or longer," he told The New York Times.

The workers are likely wearing full bodysuits and air packs to reduce the exposure. But Gundersen said some forms of radiation can penetrate any gear.

Because so little information is being released, it's not clear whether the Fukushima Fifty have fully accepted the danger they're in.

Shan Nair, a British nuclear safety expert who was part of a panel that advised the European Commission on its response to the Chernobyl disaster in 1986, tells Time.com that during the Chernobyl disaster, many first responders to the incident knew they faced certain death.

Nair says the helicopter pilots who dumped sand on the burning core at Chernobyl knew they were going to die. And in fact, all did die.

But this situation may be different, he says. At Chernobyl, there was a massive explosion, and a resulting radioactive cloud. So far at Fukushima, even the spike of radiation being reported may not be fatal.

"We don't know what the radiation levels are inside the plant but reports of a 400 millisievert figure suggests that it's not a suicide mission for the 50 workers who have stayed.

He notes that the rotation of shifts allows for "dose sharing" so that no one team of workers receives an unsafe dose for any longer than needed.

"It is still a risky operation, however," Nair added.