A nuclear research and development institute in Japan says the collapse of a structure at its nuclear research institute northeast of Tokyo was caused by rot in its aging wooden supports.
In September, a cooling facility at the agency's institute in Oarai, Ibaraki Prefecture, collapsed during Typhoon Faxai.
The structure, which was about 17 meters high, was built in 1968.
It collapsed when wind gusts reached about 110 kilometers per hour although it had been designed to withstand winds twice as strong.
The Japan Atomic Energy Agency said an examination revealed that wooden supports had partially rotted.
It said a simulation based on the findings confirmed that the structure would collapse in winds weaker than the design standards.
Officials said the 51-year-old facility does not contain radioactive materials. But its outer wall contains asbestos, and there is a possibility small amounts might have been released into the air.
Officials said they conducted regular checks, but the rot went unnoticed because the wooden parts were clad in metal.
The agency said 89 of its facilities nationwide are aging and 70 percent of them are more than 40 years old. It said it will implement measures to prevent a recurrence.