The disclosure by Tepco raises the stakes in a race to complete by next month a system to decontaminate a massive pool of radioactive water at the site that critics see as a growing risk to both the nearby Pacific and groundwater.
A magnitude 9.0 earthquake and the massive tsunami that followed killed about 24,000 people and knocked out the Fukushima plant on March 11, triggering the world's worst nuclear accident since Chernobyl.
The crisis, which has displaced some 80,000 residents from around the plant, prompted a review of Japan's energy policy and growing calls for efforts to step up health monitoring for a crisis now in its 11th week.
Experts from the International Atomic Energy Agency began an inspection on Thursday of equipment damaged by the tsunami at a second nuclear plant, the Tokai complex about 120 km (75 miles) north of Tokyo, as part of an investigation prompted by the Fukushima accident. (read more)