The massive tsunami severely damaged four reactors at the Fukushima Dai-ichi nuclear plant north of Tokyo, knocking out cooling systems and triggering meltdowns and radiation leaks. Tens of thousands of people fled their homes in the worst atomic disaster since Chernobyl.
TEPCO vice president Masao Yamazaki, who headed his company's probe of the disaster, acknowledged the company had repeatedly underestimated the risk of a tsunami despite predictions in recent years that such earthquake-generated waves could top seawalls protecting reactors. Officials clung to optimistic views instead of taking the side of safety, he said.
"We must admit that our tsunami anticipation was too optimistic, and our insufficient preparations for a tsunami were the fundamental cause of the accident," Yamazaki said at a news conference.
Wednesday's report provided an incomplete picture of the accident because of the difficulty of inspecting the inside of the buildings housing the melted reactors. The company promised to make the best use of the findings to improve safety at its still-functional reactors, which it hopes to restart quickly as it struggles to finance astronomical compensation costs.
But it said the shortcomings disclosed in the report, both in hardware and crisis management, were still unresolved and needed more improvement. It said further efforts were needed to foster flexibility among plant workers and establish a solid line of communication during crises.
Last week, the government approved restarting two reactors belonging to another utility at the Ohi nuclear power plant in Fukui, western Japan, which will be the first to come back online since last year's disasters.
All 50 of Japan's commercial nuclear reactors have been offline for maintenance or safety checks.