Chubu Electric Power Co. Inc. said on Tuesday that turbine problems were the likely cause of an unplanned June shutdown of one of its nuclear power units but the company did not immediately assign blame. 

The turbines were designed by Hitachi Ltd.

Chubu, Japan's third-biggest utility, briefed media on its continuing investigation into cracks found in low-pressure turbines at its No. 5 nuclear unit at its Hamaoka plant.

Chubu said that random swirling currents inside the turbines and the flashback reverse steam flow were two possible causes that contributed to the problem.

"We have almost narrowed down the causes to the two possibilities," Takahiro Kumazaki, a Chubu official told reporters. "But it's still too early to say it was due to a mistake in the design, or talk about seeking damages."