by Julian Ryall

in Tokyo
12 May 2011

from Telegraph Website

Nuclear meltdown at Fukushima plant
Workers pour concrete into a pit to stop contaminated water leaking from the reactor building

Photo: TEPCO/AFP/Getty Images
 

Engineers from the Tokyo Electric Power company (Tepco) entered the No.1 reactor at the end of last week for the first time and saw the top five feet or so of the core's 13ft-long fuel rods had been exposed to the air and melted down.

Previously, Tepco believed that the core of the reactor was submerged in enough water to keep it stable and that only 55 per cent of the core had been damaged.

Now the company is worried that the molten pool of radioactive fuel may have burned a hole through the bottom of the containment vessel, causing water to leak.

"We will have to revise our plans," said Junichi Matsumoto, a spokesman for Tepco. "We cannot deny the possibility that a hole in the pressure vessel caused water to leak".

Tepco has not clarified what other barriers there are to stop radioactive fuel leaking if the steel containment vessel has been breached.

Greenpeace said the situation could escalate rapidly if,

"the lava melts through the vessel".

However, an initial plan to flood the entire reactor core with water to keep its temperature from rising has now been abandoned because it might exacerbate the leak.

Tepco said there was enough water at the bottom of the vessel to keep both the puddle of melted fuel and the remaining fuel rods cool.

Meanwhile, Tepco said on Wednesday that it had sealed a leak of radioactive water from the No.3 reactor after water was reportedly discovered to be flowing into the ocean. A similar leak had discharged radioactive water into the sea in April from the No.2 reactor.

Greenpeace said significant amounts of radioactive material had been released into the sea and that samples of seaweed taken from as far as 40 miles of the Fukushima plant had been found to contain radiation well above legal limits. Of the 22 samples tested, ten were contaminated with five times the legal limit of iodine 131 and 20 times of caesium-137.

Seaweed is a huge part of the Japanese diet and the average household almost 7lbs a year. Greenpeace's warning came as fishermen prepared to start the harvest of this season's seaweed on May 20.

Inland from the plant, there has been a huge cull of the livestock left inside the 18-mile mandatory exclusion zone with thousands of cows, horses and pigs being destroyed and some 260,000 chickens from the town of Minamisoma alone. The Environment ministry has announced, however, that it will attempt to rescue the thousands of pets that were left behind when residents were ordered to evacuate.

At least 5,800 dogs were owned by the residents of the zone, although it is unclear how many remain alive, two months after the earthquake struck.

Japan Reactor-Core...

Damage Worse Than Thought
by Joe Carroll and Michio Nakayama

May 12, 2011

from Bloomberg Website
 

Tepco Says Fuel Rods at Fukushima Reactor Are Fully Exposed
 

A worker measures the radiation dose inside the No. 1 reactor building

at Tokyo Electric Power Co.'s Fukushima Dai-Ichi nuclear plant, on May 5, 2011.

Source: Tokyo Electric Power Co. via Bloomberg
 

The No. 1 reactor at Tokyo Electric Power Co.'s Fukushima Dai-Ichi nuclear power plant.

Source: Air Photo Service via Bloomberg
 

Tokyo Electric Power Co. said one of the reactor cores at its stricken Fukushima Dai-Ichi nuclear plant is more seriously damaged than previously thought, setting back the utility’s plan to resolve the crisis.

Fuel rods in the core of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly, Junichi Matsumoto, a general manager at the utility known as Tepco, told reporters at a briefing in Tokyo.

Melted fuel has dropped to the bottom of the pressure vessel and is still being cooled, Matsumoto said.

Japan is trying to contain the worst nuclear disaster since Chernobyl after a quake and tsunami two months ago knocked out power and cooling systems at the Fukushima station.

While authorities have previously suspected a partial meltdown at unit 1, high radiation levels had prevented workers from entering the building to check the damage until last week.

“What this means is this is probably going to be a much more difficult cleanup than they originally planned for,” said Paul Padley, a particle physicist at Rice University in Houston. The government and Tepco “have consistently appeared to be underestimating the severity of the situation.”

There’s no danger of another explosion like the one that blew the roof off the reactor in March, Matsumoto said at the briefing.

That’s because the lack of water means there’s nothing for the zirconium-alloy cladding encasing the uranium to react with to create high-pressure clouds of hydrogen, said Roger N. Blomquist, principal nuclear engineer at the U.S. Energy Department’s Argonne National Laboratory near Chicago.

Holes Discovered

Fuel rods are submerged in water to prevent overheating that would crack the casing and release radiation, said Blomquist, who oversees the nuclear section at Argonne, a federal research center managed by the University of Chicago, birthplace of the atomic industry.

“This is what operators of nuclear reactors want to avoid,” he said in a telephone interview.

Holes have been found in the base of the pressure vessel, and most of the fuel has likely melted, Kyodo News reported yesterday, citing the utility. It’s possible the fuel has leaked into the containment vessel, which was damaged in the explosion, according to the report.

Flooding the chamber was one of the steps Tepco outlined in April to bring the crisis under control.

“The plan needs to be revised,” Matsumoto said. “We can’t deny the possibility that a hole in the pressure vessel caused water to leak.”

Tepco shares fell 8.8 percent to 479 yen in Tokyo yesterday. The stock has declined 77 percent since March 11 when Japan’s worst quake on record triggered a tsunami that left more than 24,000 people dead.

No China Syndrome

The company doesn’t know how long the rods have been exposed, it said in the briefing.

It’s likely they’ve been uncovered since the first days of the disaster, Blomquist said.

The six-reactor complex, Japan’s third-largest by capacity, has been spewing radiation since March 11 and the severity rating of the accident was raised to the same as Chernobyl nearly one month ago. The station is located about 220 kilometers (137 miles) north of Tokyo.

While the risk of explosions still exists if the rods aren’t kept cool, there’s no likelihood of a catastrophic meltdown, with fuel burning through the Earth’s crust, said Tony Roulstone, who directs the University of Cambridge’s masters program in nuclear technology.

“China syndrome’s not a real risk because it would require too much energy.”

Evacuation Zone

The U.S. Nuclear Regulatory Commission, which has officials in Japan monitoring the crisis, hasn’t changed its advice of March 16, urging U.S. citizens to stay 50 miles from the stricken plant, said Eliot Brenner, an NRC spokesman.

Brenner declined to comment on the specific condition of the Fukushima reactors. The agency is holding a public meeting to discuss a safety review of U.S. reactors which was triggered by the events in Japan.

Tepco has said it expects a sustained drop in radiation levels at the entire plant by July, according to the plan announced on April 17.

Following that, a cold shutdown of reactors No. 1, 2 and 3 may take place as early as October, the utility said at the time.