bloomberg.com | Radiation readings at Japan’s Fukushima Dai-Ichi station rose to the highest since an earthquake and tsunami knocked out cooling systems, impeding efforts to contain the worst nuclear crisis since Chernobyl.

Two robots sent into the reactor No. 1 building at the plant yesterday took readings as high as 1,120 millisierverts of radiation per hour, Junichi Matsumoto, a general manager at Tokyo Electric Power Co., said today. That’s more than four times the annual dose permitted to nuclear workers at the stricken plant.

Radiation from the station, where four of six reactors have been damaged by explosions, has forced the evacuation of tens of thousands of people and contaminated farmland and drinking water. A plan to flood the containment vessel of reactor No. 1 with more water to speed up emergency cooling efforts announced yesterday by the utility known as Tepco may not be possible now.

“Tepco must figure out the source of high radiation,” said Hironobu Unesaki, a nuclear engineering professor at Kyoto University. “If it’s from contaminated water leaking from inside the reactor, Tepco’s so-called water tomb may be jeopardized because flooding the containment vessel will result in more radiation in the building.”

Decontaminating Robots

Tepco plans to decontaminate the two iRobot Corp. Packbot robots before sending them into a building tomorrow or later to further investigate the damage, spokesman Takeo Iwamoto said. High radiation in the reactor buildings prevents engineers from working inside them, Iwamoto said.

The cores in reactors 1, 2 and 3 and the spent fuels rods in reactor 4 have been damaged. Tepco has been using fire trucks, concrete pumps and other emergency measures for nearly seven weeks to pour millions of liters of water to cool the units after the accident.

Tepco started moving the radioactive water, which leaked to the basements and trenches, to a waste storage facility on April 19. Tepco transferred 1.89 million liters of the water from the trenches near reactor No. 2 as of 7 a.m. today, Iwamoto said. The utility plans to install a second pump after transferring 2.5 million liters.

Less Damage

Tepco shares fell 3.3 percent to 412 yen today in Tokyo. The shares are down about 80 percent since the quake and tsunami struck on March 11, leaving almost 26,000 people dead or missing.

Reactors 1 and 2 are less damaged than estimated, Tepco said in a statement today.

As much as 55 percent of the No. 1 reactor core at the Fukushima Dai-Ichi station was damaged, compared with its earlier estimate of 70 percent.

“We revised the core damage data because some readings on the containment vessel monitors were wrong,” Matsumoto said. “There was also a recording mistake. We are investigating why this happened.”

The assessment for the No. 2 reactor was cut to 25 percent from 35 percent, while that for the No. 3 unit was raised to 30 percent from 25 percent.

“It seems a reasonable estimate that three reactor cores may be damaged to a similar extent,” said Unesaki. The new estimate “doesn’t indicate lower or higher risks at the plant.”

Radiation in Tokyo’s water supply fell to undetectable levels for the first time since March 18, the capital’s public health institute said today.

The level of iodine-131 in tap water fell to zero yesterday, and cesium-134 and cesium-137 also weren’t detected, the Tokyo Metropolitan Institute of Public Health said today.

Tokyo residents were told on March 23 that the city’s water was unsafe for infants after iodine and cesium levels exceeded guidelines.

To contact the reporters on this story: Tsuyoshi Inajima in Tokyo at tinajima@bloomberg.net; Michio Nakayama in Tokyo at mnakayama4@bloomberg.net