Tokyo (CNN) — Authorities in Japan raised the prospect Friday of a likely breach in the all-important containment vessel of the No. 3 reactor at the stricken Fukushima Daiichi nuclear power plant, a potentially ominous development in the race to prevent a large-scale release of radiation.
Contaminated water likely seeped through the containment vessel protecting from the reactor’s core, said Hidehiko Nishiyama of the Japan nuclear and industrial safety agency.
Three men working inside the No. 3 reactor stepped into water this week that had 10,000 times the amount of radiation typical for that locale, Nishiyama said. That water likely indicates “some sort of leakage” from the reactor core, signaling a possible break of the containment vessel that houses the core.
The containment vessel is designed to prevent radioactive material from escaping into the atmosphere, even if other parts of the reactor are damaged. A rupture in the containment vessel could pose problems for workers who are trying to prevent that, depending on its severity.
The three workers who were exposed to radiation by stepping in the contaminated water had the highest levels of radiation recorded so far, said Tokyo Electric Power Co., which owns the plant.
The incident raised questions about radiation control measures at the plant as 536 people — including government authorities and firefighters — continued working there Friday, according to an official with Tokyo Electric.
Workers are undertaking various measures to prevent the further release of radioactive substances into the air and beyond.
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Some 17 people have been exposed to 100 or more millisieverts of radiation since the plant’s crisis began two weeks ago following a 9.0-magnitude earthquake and subsequent tsunami struck.
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 workers 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, according to the power company.
The workers remained in the 15-centimeter (5-inch) deep water for about 40 to 50 minutes.
Two of them were admitted to Japan’s National Institute of Radiological Sciences: one in his 30s who was exposed to 180.7 millisieverts and the other in his 20s who tested at 179.37 millisieverts.
Nishiyama said the third man — who was exposed to 173 millisieverts but at first did not go to the hospital because his boots were high enough to prevent water from touching his skin — has also gone to the same research hospital out of “an abundance of caution.”
The water in this location is typically boiled and has low levels of radiation, Nishiyama said.
The high measure prompted a top official with Nishiyama’s agency to urge Tokyo Electric to “improve its radiation management measures.”
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.
Nishiyama 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, Nishiyama 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 Nishiyama. The aim is to prevent further corrosion and damage inside, which may be worsened by the buildup of salt.
Beyond the seawater/saltwater issue, water in and around the Nos. 1 and 2 reactors had “high radiation levels,”Nishiyama said Friday — though not as high as that of the No. 3 unit.
Thursday’s incident has further made the latter reactor a prime focus, and Nishiyama said 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 around No. 3, keeping its spent nuclear fuel pool in check and other aims. Later in the day, Nishiyama said authorities hadn’t yet determined how to get around the obstacle.
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 less pronounced because the units were on scheduled outages when the quake struck. None of these three units had nuclear fuel inside their reactors, though efforts are ongoing to control temperatures inside the spent fuel pools.
On Friday morning, a concrete pump truck was used once again to inject seawater into the No. 4 unit’s fuel pool.
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