A damaged gas cylinder was the source of a loud bang outside a reactor building at the Fukushima Daiichi nuclear plant.
Plant operator Tokyo Electric Power Co (TEPCO) said the cylinder had been damaged as rubble was being cleared away but that no one was injured at the earthquake-damaged plant.
There was no change in radiation levels at the plant's monitoring posts and no one was injured, TEPCO officials added.
"We found out that a heavy machine caught a gas cylinder in the rubble, like a crab uses its claw, and damaged the gas cylinder," Tepco spokesman Junichi Matsumoto said.
Clearing rubble is a pressing task for Tepco as it tries to avoid any further spread of radiation, including that from the debris, in its struggle to regain control of the Fukushima plant.
Three of six reactors suffered meltdowns within days after the massive quake and tsunami on March 11 knocked out the power to the plant, triggering the world's worst nuclear accident since Chernobyl in 1986.