(CNN) -- The water that three men were recently exposed to while working at the Fukushima Daiichi nuclear power plant had 10,000 times the amount of radiation typical for that locale, an official with Japan's nuclear and industrial safety agency said Friday, adding that the high levels indicated the nuclear fuel inside the No. 3 reactor "is damaged."
The incident raised questions about radiation control measures in place at the plant, at a time when 536 people -- among them government authorities, firefighters, soldiers and utility company employees -- continued working there on Friday, an official with its owner, the Tokyo Electric Power Company, told CNN. They are undertaking a broad array of measures aimed at preventing the further release of radioactive substances into the air and beyond.
Some 17 people already have been exposed to 100 or more millisieverts of radiation since the crisis began two weeks ago, when a 9.0-magnitude earthquake and subsequent struck and, specifically, wreaked havoc at the northeastern Japan power plant, according to the Japan Atomic Industrial Forum industry group.
A person in an industrialized country is naturally exposed to 3 millisieverts of radiation a year. But Japan's health ministry recently raised the maximum level of exposure for a person working to address the crisis at the nuclear plant from 100 millisieverts to 250 millisieverts per year.
The three workers exposed to radiation Thursday had the highest levels recorded so far, Tokyo Electric said.
They had been laying cables in the No. 3 reactor turbine building's basement when they stepped in the water. It seeped into the ankle-height boots of two of the men, according to the power company, with the workers remaining in the 15-centimeter (5-inch) deep water for about 40 to 50 minutes.
Two of them were admitted to the hospital: one in his 30s who was exposed to 180.7 millisieverts and the other in his 20s who tested at 179.37 millisieverts. A third man, who was exposed to 173 millisieverts, did not go to the hospital, as his boots were high enough to cover his skin, the power company said.
The water in this locale typically has been boiled and has low levels of radiation, Hidehiko Nishiyama of Japan's nuclear and industrial safety agency explained Friday.
The high radiation measure prompted a top official with Nishayama's agency to contact authorities at Tokyo Electric to urge that company to "improve its radiation management measures."
These and other dangers remained real at the Fukushima Daiichi plant, though there were no signs of billowing smoke or other obvious perils Friday to continued their multifaceted, often frustrated efforts to control the crisis.
The No. 1 reactor remains a chief concern, with the Japan Atomic Industrial Forum noting Friday that its containment vessel was experiencing "increased" pressure. Earlier, buildups of hydrogen gas had driven up pressure that led to explosions at three of the nuclear plant's reactors, including the No. 1 unit.
Nishayama, from Japan's nuclear safety agency, conceded that "controlling the temperature and pressure has been difficult" for that reactor. Still, he told reporters Friday that the situation then was "rather stable," given indications the pressure was decreasing.
As to that unit's spent nuclear fuel pool, Nishayama said the hope is to start pumping in fresh water -- rather than seawater, as has been done. Such pools, which are distinct but tied to a given reactor, have nuclear fuel rods that can emit radiation especially if they heat, which is more likely to happen without any functional cooling system in place and when the rods are not fully covered in water.
Switching to fresh water, instead of seawater, is also a priority for the No. 2 reactor's core (as well as for its spent fuel pool), said Nishayama. The aim is to prevent further corrosion and damage inside, which may be worsened by the buildup of salt.
The No. 3 reactor has been another pressing concern, especially after black smoke was seen wafting from its east side on Wednesday. (By the next day, that smoke had subsided and its cause still wasn't known.)
Thursday's incident has further made it a focus, and Nishayama conceded Friday that "radiation levels are high" in some locales near that unit. He said that authorities were considering "other routes" to accomplish their goals of restarting its cooling systems, keeping its spent nuclear fuel pool in check and other aims.
To this end, firefighters from Tokyo and Kawasaki were expected to resume spraying toward the No. 3 reactor and its fuel pool on Friday afternoon, according to Nishiyama.
Efforts are ongoing, too, on the Nos. 4, 5 and 6 reactors -- each of which have their own concerns, though they tend to be less pronounced because all of these units were on scheduled outages when the quake struck. Thus, none of these three units had nuclear fuel then inside their reactors, though efforts are ongoing to control temperatures inside the spent fuel pools.
CNN's Whitney Hurst contributed to this report.