New inspections found low water levels, high radiation, other dangers.
TOKYO - The damage to the core of at least one of the meltdown-stricken reactors at the Fukushima Daiichi Nuclear Power Plant could be far worse than previously thought, raising fresh concerns over the plant's stability and gravely complicating the post-disaster cleanup, a recent internal investigation has shown.
The results of the inquiry, released this week by the operator of the plant, also cast doubt over the Japanese government's declaration three months ago that the ravaged site was under control.
Throughout the crisis that ensued after a powerful earthquake and tsunami in March 2011, both the plant's operator, Tokyo Electric Power (TEPCO), and the government were accused of playing down the dangers posed by the nuclear meltdown. Subsequent disclosures that the event was indeed far more severe than they let on have badly damaged their credibility, to the point that almost any statement from the authorities is now regarded as suspect by a dubious Japanese public.
Fukushima Daiichi's vital cooling systems were knocked out in the early stages of the crisis last year. The uranium cores at three of the plant's six reactors quickly melted down, breaching their containment vessels and triggering a massive radiation leak.
Three reactors were later rocked by hydrogen explosions that blew out their outer walls.
What followed was a frantic effort to keep the inner parts of the reactors flooded with cooling water to prevent their cores from again overheating. Officials at TEPCO had previously said that operation was succeeding, and the damaged fuel rods were safely submerged in water.
Water level a worry
But earlier this week, an examination at one of the reactors showed the water level at its core to be far lower than levels previously estimated, raising fears that the broken-down remnants of the uranium fuel rods there may not be completely submerged and in danger of heating up again.
Cooling water at the plant's No. 2 reactor came up to just 2 feet from the bottom of the reactor's containment vessel, a beaker-shaped structure that encases the fuel rods. That was far below the 10 meter-level (33 feet) estimated by officials when the government declared the plant stable in December.
The low water level also raises concerns that radioactive water may be leaking out of the reactor at a higher rate than previously thought, possibly into a part of the reactor known as the suppression chamber, and into a network of pipes and chambers under the plant -- or into the ocean. At the No. 2 reactor, workers still pump about 9 tons of water an hour into the core to keep it cool.
The investigation also found current radiation levels of 72.0 Sieverts inside the containment vessel, enough to kill a person in a matter of minutes, as well as for electronic equipment to malfunction.
Kazuhiko Kudo, a professor of nuclear engineering at Kyushu University in southwestern Japan, said it was now suspect whether the nuclear fuel was being adequately cooled. And if some parts of the fuel remained above water, there was a risk the fuel could again heat up and melt, he said.
That could trigger a dangerous spike in the pressure inside the containment vessel, and lead to more radiation escaping the reactor, he said.
The high levels of radiation would greatly complicate work to locate and remove the damaged fuel and decommission the plant's six reactors -- a process that is expected to take decades.
"With levels of radiation extremely high, we would need to develop equipment that can tolerate high radiation," Junichi Matsumoto, a TEPCO executive, said Tuesday.
To gauge water levels inside the containment vessel at the No. 2 reactor, workers clad in hazmat suits inserted an endoscope equipped with a tiny video camera, thermometer, a dosimeter for measuring radiation and a water gauge.
Problems at other reactors
Two other badly damaged reactors -- Nos. 1 and 3 -- could be in even worse condition. Hydrogen explosions blew out the outer walls of those reactors, and officials believe that more nuclear fuel has breached the containment vessel at the No. 1 reactor than the others.
Experts also worry about a fourth reactor that was not operating at the time of the accident but nevertheless poses a risk because of the large number of spent nuclear fuel rods stored in a water coolant tank there. The No. 4 reactor was also hit by a hydrogen explosion in the early days of the crisis, possibly due to hydrogen that leaked into the reactor from the adjacent No. 3 unit.
The spent fuel rods stored at the No. 4 reactor pose a particular threat, experts say, because they lie unprotected outside the unit's containment vessel. TEPCO has been racing to fortify the crumpled outer shell of the reactor and to keep the tank fed with water.
But should a problem also arise with cooling the spent fuel, the plant could run the risk of another colossal radiation leak, experts say.
The many aftershocks that continue to hit the Fukushima region are also a source of worry.
"The plant is still in a precarious state," said Kudo of Kyushu University. "Unfortunately, all we can do is to keep pumping water inside the reactors and hope we don't have another big earthquake."