久し振りにNew York Timesから記事を抜粋してみました。
By HIROKO TABUCHI Published: March 29, 2012
A handout image supplied Thursday by Tokyo Electric Power Company shows conditions at the Fukushima Daiichi nuclear plant.
By HIROKO TABUCHI
Published: March 29, 2012
TOKYO — The damage to one of three stricken reactors at the Fukushima Daiichi plant could be worse than previously thought, a recent internal investigation has shown, raising new concerns over the plant’s stability and complicating the post-disaster cleanup.
The government has said that the plant’s three badly damaged reactors have been in a relatively stable state, called a cold shutdown, for months, and officials say that continues. But new tests suggest that the plant — which was ravaged last March when a powerful earthquake and tsunami hit the area — might not be as stable as the government or the operator of the plant, Tokyo Electric Power Company, or Tepco, had hoped.
The key to keeping the reactors stable is keeping their fuel rods cool with water.
The company announced this week that an examination of one reactor, No. 2, showed that the water level in an outer containment vessel was far lower than estimated, which could indicate that the already badly damaged uranium fuel might not be completely submerged and, therefore, is in danger of heating up.
Cooling water in that vessel, called the drywell, was just two feet deep, rather than the 33-foot level estimated by Tepco officials when the government declared the plant stable in December. That is probably not a problem for the fuel that the company says has leaked into the drywell from an inner containment vessel because Tepco says that melted fuel is unlikely to be higher than two feet.
But Tepco officials said the low water in the drywell left open the possibility that the water level in the leaking inner containment vessel, where most of the fuel is thought to be, was also low. Experts say that could leave the fuel there exposed and lead to more damage. The fuel would likely then leach more radioactive materials into the water that is flowing through the reactor to cool it.
That scenario would be particularly problematic since the company has long feared that some of the tons of water it is using to cool the reactors is escaping into the ground or into the ocean at the seaside plant.
Throughout the nuclear crisis, both Tepco and the government were accused of playing down the dangers posed by the meltdowns at the plant. Subsequent disclosures that the event was indeed far more severe than they let on have badly damaged their credibility.
Fukushima Daiichi’s vital cooling systems were knocked out in the early stages of the crisis last year. The cooling systems there now were put in place months after the accident. Although they are designed to be closed loops, circulating water in and out of the reactors, the reactors themselves were damaged when operators lost control of the plant and are likely leaking.
The internal investigation also found current radiation levels of 72.0 sieverts inside the drywell, enough to kill a person in a matter of minutes, as well as for electronic equipment to malfunction.
The high readings could be a reflection of the low water level, since the water acts as a shield against radiation.
The high levels of radiation would complicate work to locate and remove the damaged fuel and decommission the plant’s six reactors — a process that is expected to take decades.
Cleanup will probably require flooding the inner reactor vessel and lowering tools into it to scoop up parts of the radioactive rubble. That strategy worked well at Three Mile Island after the 1979 accident there. But at Fukushima, the reactor vessels are known to have cracked, because they were overpressurized. Filling them with water would be difficult, unless the surrounding drywell can also be filled.
The fact that the drywell at No. 2 has so little water could mean that technicians will need to develop a new technique. “With levels of radiation extremely high, we would need to develop equipment that can tolerate high radiation,” Junichi Matsumoto, an executive at Tepco, said Tuesday.
To gauge water levels inside the drywell at reactor No. 2, workers in hazmat suits inserted an endoscope equipped with a tiny video camera, a thermometer, a dosimeter for measuring radiation and a water gauge.
It is unclear if they will be able to perform the same test at the other badly damaged reactors — No. 1 and No. 3 — because nearby radiation levels are higher there.
Experts also worry about a fourth reactor that was not operating at the time of the tsunami, but nevertheless poses a risk because of the large number of spent nuclear fuel rods stored in a pool there.
The spent fuel rods pose a particular threat, experts say, because they lie outside the unit’s containment vessels. Experts have become especially worried in recent weeks, as earthquakes continue to hit the area, that the damaged reactor building could collapse, draining the pool and possibly leading to another large leak of radioactive materials.
Tepco has been working to fortify the crumpled outer shell of the building of that reactor, No. 4.
“The plant is still in a precarious state,” said Kazuhiko Kudo, a professor of nuclear engineering at Kyushu University in southwestern Japan. “Unfortunately, all we can do is to keep pumping water inside the reactors,” he said, “and hope we don’t have another big earthquake.”
Matthew L. Wald contributed reporting from Washington.
A version of this article appeared in print on March 30, 2012, on page A6 of the New York edition with the headline: Damage at Japan Nuclear Plant May Be Worse Than Thought.