A 400-kilogramme (880-pound) machine part fell into a nuclear fuel pool at Japan's crippled Fukushima plant Friday, the operator said.
Tokyo Electric Power (TEPCO), said it had not detected any significant changes in radiation readings or in the level of pool water at the No. 3 reactor.
A massive earthquake and tsunami in March 2011 ravaged Japan's northeastern coast and wrecked the plant, sparking meltdowns at three of its six reactors.
Friday's incident occurred shortly after noon during a remotely controlled operation to remove debris from the fuel pool at the unit where the broken reactor still lies untouched. The pool contains 566 fuel rods, most of which are spent.
The operating console of the fuel handling machine slipped loose and fell into the pool as it was about to be lifted by a crane, TEPCO said in a statement.
The console weighed 400 kilogrammes and measured 160 centimetres (63 inches) by 90 cm by 100 cm, a TEPCO official said.
Radioactivity readings at the pool remained unchanged at 3.2 millisieverts per hour after the incident, the statement said.
"The operation was being remotely controlled and there were no injuries caused to workers," the official said.
The meltdowns at Fukushima were the world's worst nuclear mishap since the 1986 Chernobyl disaster, and forced hundreds of thousands of local residents to evacuate nearby areas.
In a vivid reminder of the fragility of the area, a magnitude-5.0 quake struck off the Fukushima coast hours after Friday's incident.
But there were no fears of tsunami and no immediate reports of damage.