Braving explosions and invisible hydrogen fires as well as bursts of radiation at least eight times higher than government hourly safety standards, a cohort of 50 or so workers has returned to the embattled Fukushima Daiichi nuclear power plant in Japan. The workers represent the last line of defense in cooling the overheating reactors and spent fuel pools, such as reactor No. 3, which is still billowing white smoke or steam—a possible indication of a breach in the thick steel and concrete that contains the nuclear core or of water boiling off its spent fuel pool.

Tokyo Electric Power Company (TEPCO) has evacuated all non-essential personnel due to radiation pulses spiking as high as 400 milli-Sieverts per hour—and readings at the border of the power plant site of roughly 6 milli-Sieverts per hour, enough for an individual worker to reach the maximum exposure allowed by standard Japanese regulation for a full year in less than 9 hours. In fact, the Japanese Health Ministry has raised the legal limit on radiation exposure to 250 millisieverts a year—five times that standard level, which is also the legal limit in the U.S.—to enable the 50 or so workers efforts to bring the nuclear power plant back under control. Those workers continue to struggle to cool overheating nuclear reactors by pumping seawater through fire extinguishing system lines—while necessarily exposing themselves to the dangerous radioactive particles released by hydrogen explosions and even a series of fires on the power plant grounds.

"There are basically three strategies for reducing radiation exposure: time, distance and shielding," explains health physicist Peter Caracappa of Rensselaer Polytechnic Institute, who is also the radiation safety officer for that university's research reactor and radioactive materials and equipment. "If you spend less time, you get a smaller dose. The further away you are from the source, the intensity drops off quickly. If there is a shielding material between you and the radiation source, the radiation dose is reduced."

In the case of the workers at Fukushima Daiichi, it is likely only time—if anything—that can be controlled, via limiting the amount of time workers spend working in radioactive areas, which have now likely spread to cover much of the nuclear power plant. Of course, the Japanese operators at the controls of these or any nuclear reactors benefit from the shielding provided by thick concrete construction of the power plant buildings. In addition, operators at the Browns Ferry nuclear power plant in Alabama—which employs a modified version of the same reactors that are at the Fukushima power plant—enjoy a ventilation system with both a filter to keep radioactive particles out and pressurization so that air always flows out rather than in, producing a stiff breeze—a protection that may also be available to the Japanese workers. "If there was a material like smoke or radioactive material, the tendency would be for it to be repelled," says Ray Golden, a spokesman for the Tennessee Valley Authority (TVA), a U.S. government-owned corporation that runs the nuclear power plant.

Nuclear power plant workers in the U.S. and Japan also have access to self-contained breathing apparatus—like scuba gear—that prevents them from inhaling radioactive particles, such as cesium-137 and iodine-131, when working outside the control room and other more secure areas. "You can walk away from an external source," Caracappa says. "Radioactive material deposited in the body you carry around with you, you can't walk away from that."

The skeleton crew of workers flit like ghosts through the nuclear power plant, wearing simple white suits used to keep them from tracking radioactive material beyond certain areas. These suits also have the benefit of partially blocking alpha radiation, essentially helium nuclei released during the breakdown of other, larger radioactive elements.

But it would take cumbersome lead suits or the like to block more energetic radiation, the most familiar of which are known as X-rays and gamma rays. So the TEPCO workers are likely relying on either shielding themselves behind other equipment—though all the equipment involved in power generation at a boiling water reactor like those at Fukushima is radioactive to some extent—or limiting the time spent receiving radiation doses.

Even those TEPCO workers who receive a significant dose of radiation—more than 2 Sieverts, or high enough to cause immediate radiation sickness characterized by skin redness, hair loss, nausea and even burns—could be treated with drugs. But all those still in the nuclear power plant will face at least a slight increase in risk for cancers, according to Caracappa. "It's not simple to quantify," he notes. "The most commonly accepted value for the risk is that the increased risk of cancer is 4 percent per Sievert."

The radioactive material that has escaped from one or more of the reactors will be around for years after the Fukushima Daiichi power plant crisis is resolved. For example, cesium-137 has a half-life of 30 years, meaning it will take three decades for it to lose half its radiation punch. At the same time, however, such radioactive material can be simply washed off before it penetrates the skin or cleaned off surfaces. "It's just stuff, you can wash off dirt or whatever it is," Caracappa continues. "A surface that is contaminated can be cleaned."

Regardless, the TEPCO workers' presence at Fukushima Daiichi nuclear power plant remains critical, as partial meltdowns have cracked as much as 70 percent of the fuel rods in reactor No. 1, according to NHK, the Japanese government broadcaster. And the fuel rods in reactor No. 3 were exposed to the air for hours—causing a build up of steam and hydrogen that raised pressures and may have cracked the massive steel suppression pool designed to cool the radioactive core and trap radioactive materials. Without the workers' efforts, cooling could not continue at the stricken nuclear power plant and a full fuel meltdown would very likely ensue.

As of the end of the day on March 16, according to the Japan Atomic Industrial Forum, fuel rods remain exposed in all three reactors and the workers continue to pump seawater into all of them to prevent any further melting—venting radioactive steam and gases as necessary to ensure that no further cracks develop. And the spent fuel pools at Reactors No. 3 and 4 have lost water, potentially exposing those still hot, nuclear fuel rods—another source of radiation and meltdown risk. "What they're dealing with in Japan is beyond the design," says spokesman Terry Johnson of the TVA, which owns and operates the similar Browns Ferry nuclear power plant in Alabama. "They have to improvise as they go along."