August 1, 2011

San Francisco Chronicle

Tokyo Electric Power Co., operator of Japan’s crippled Fukushima Dai-Ichi nuclear plant, said it detected the highest radiation to date at the site.

Geiger counters, used to detect radioactivity, registered more than 10 sieverts an hour, the highest reading the devices are able to record, Junichi Matsumoto, a general manager at the utility, said today. The measurements were taken at the base of the main ventilation stack for reactors No. 1 and No. 2.

The Fukushima plant, about 220 kilometers (137 miles) north of Tokyo, had three reactor meltdowns after the March 11 magnitude-9 earthquake and tsunami knocked out power and backup generators. Radiation leaks displaced 160,000 people and contaminated marine life and agricultural products.

The utility, known as Tepco, tried to vent steam and gas the day after the earthquake as pressure in reactor No. 1 exceeded designed limits. A buildup of hydrogen gas subsequently caused an explosion that blew out part of the reactor building.

"I suspect the high radiation quantity was an aftermath of venting done," Matsumoto told reporters in Tokyo. "The plant is not running. I don’t think any gas with high radiation level is flowing in the stack."

Tepco sent three workers around the ventilation stack today after a gamma camera detected high radioactivity levels in the area yesterday, Matsumoto said. The workers were exposed to as much as 4 millisieverts during the work, he said.

The utility will create a no-go zone around the stack and cover the area with protective material, he said.

–With assistance from Shunichi Ozasa in Tokyo. Editors: Amanda Jordan, Reed Landberg