Hydrogen buildup spurs more concern at Japanese nuclear plant

By Matt Smith, CNN

Engineers began injecting non-flammable nitrogen into the No. 1 reactor at the crippled Fukushima Daiichi nuclear power plant Thursday to counter a buildup of potentially explosive hydrogen, the plant’s owner reported.

The Tokyo Electric Power Company said the danger of another hydrogen explosion, like the one that blew the roof and upper walls off the reactor building in March, was “extremely low.” But it warned that more hydrogen could build up in the damaged reactor and that it planned a similar procedure for reactors 2 and 3.

Hydrogen buildup is a symptom of overheated fuel rods in the cores of the reactors, which plant workers have been struggling to keep under control since the March 11 earthquake and tsunami. The nitrogen injections are aimed at displacing oxygen in the reactor shell, reducing the possibility of an explosion.

In addition to the March 12 blast at Unit 1, a hydrogen explosion blew apart the No. 3 reactor building on March 14 and a suspected hydrogen explosion is believed to have damaged the No. 2 reactor the following day.Tokyo Electric and Japanese regulators believe No. 2 is the source of the highly contaminated water they are now struggling to contain at Fukushima Daiichi, 240 kilometers (150 miles) north of Tokyo. Plant workers are pouring 8 tons of water (2,100 gallons) into that reactor every hour to keep it cool, and the water that flows out carries extremely high concentrations of radioactive particles.

That highly radioactive fluid is building up in the turbine plant and the service tunnels around the unit, leaving Japanese officials grasping for ways to contain it.

Until Wednesday, some of that water had been bleeding out into the Pacific Ocean through a cracked utility shaft behind the plant. On Saturday, the day the leak was discovered, concentrations of the reactor byproduct iodine-131 in seawater next to the shaft was as 7.5 million times higher than the legal limit, according to sampling data taken by the utility.

Those levels prompted Japanese authorities to start dumping nearly 10,000 tons of less-radioactive water into the Pacific on Monday night, largely to make room in a waste treatment reservoir for the No. 2 reactor coolant. The move enraged the country’s fishing industry and drew protests from neighboring South Korea, but Japan’s government called it an emergency move to prevent a worse discharge.

“They have to be injecting water,” said Michael Friedlander, a former senior operator at U.S. nuclear power plants. “They have to keep the core covered. And I can tell you, you do not want to take that water out of containment right now, because it’s screaming with radioactivity and you have absolutely no capabilities whatsoever to process it.”

Japan is currently consulting with Russian authorities on whether a shipborne decontamination plant has the capability to handle the wastewater, the Japanese Foreign Ministry told CNN on Thursday. But Tokyo has not yet asked for the vessel to be brought into the fight, said Tomosaburo Esaki, an official with the ministry’s arms control and disarmament division.

The ship, the Suzeran (“Lily of the Valley”), can process up to 35 tons of radioactive waste a day and store about 800 tons. Japan built the vessel for Russia in the 1990s to help Moscow take aging nuclear submarines out of service.

Sergey Novikov, a spokesman for Russia’s state-run nuclear energy company Rosatom, told CNN that, “The ball is now in their court.”

“We have responded to their questions regarding the plant, and they sent us their additional questions, to which we responded as well,” Novikov said. “They are still studying this issue, and we hope to hear from them soon.”

Reactors 1 to 3 are all believed to have suffered damage to the fuel assemblies at their cores from overheating when the quake and tsunami knocked out cooling systems at Fukushima Daiichi.

Stopping the flow of radioactive water from No. 2 was a victory for plant workers, but Tokyo Electric and a top Japanese official warned the fight was far from over.

Hidehiko Nishiyama, a spokesman for Japan’s Nuclear and Industrial Safety Agency, said Wednesday that the now-contained water “may lead to more leakage somewhere else,” and the utility said Thursday that the water level in a service tunnel leading out of the unit’s turbine plant had gone up about 2 cm (0.8 inches) since the leak was plugged.

Contamination levels behind the plant were falling sharply even before the leak was plugged, with iodine-131 down to about 4 percent from Saturday’s peak on Tuesday, according to the utility’s sampling data. The level remained 280,000 times higher than the legal limit, but those concentrations were dropping sharply as the water flowed out into the Pacific.

Levels of longer-lived cesium-137 were down sharply as well — but they remained 61,000 times the legal standard behind the plant.

Experts have said the releases likely won’t pose any long-term health risks to humans or sea life despite the eye-popping levels. Iodine-131 loses half its radiation every eight days, and the vast Pacific will quickly dilute the contaminated water, they said.

Samples from a monitoring point 20 kilometers (12.5 miles) southeast of the plant found iodine-131 levels down to 1.5 times legal levels, with no reading for cesium, Tokyo Electric reported Wednesday. Meanwhile, levels of airborne radiation nearby and further away have been declining steadily, government readings show.

“So hopefully the churning of the ocean and the currents will quickly disperse this so that it gets to very dilute concentrations relatively quickly,” said Timothy Jorgensen, chairman of the radiation safety committee at Georgetown University Medical Center.

The company has added three monitoring stations offshore to keep track of the spread of contamination. It also has been rebuilding a damaged sea wall around the back of the plant with large sandbags to limit the flow of seawater out of the area.

CNN’s Junko Ogura in Tokyo and Maxim Tkachenko in Moscow contributed to this report.

(From :http://www.cnn.com/2011/WORLD/asiapcf/04/07/japan.nuclear.reactors/index.html?hpt=T2)


Komentar ditutup.