Radioactive materials may have touched the worker's face on Monday as he took off a full-face mask after finishing his work, operator Tokyo Electric Power (TEPCO) said Thursday.

The employee was not experiencing any adverse health effects and a full body scan showed no internal contamination, but a full analysis will be available next month, TEPCO said.

It was the second such incident in three months after four workers were splashed with water containing radioactive materials in October, with two of them hospitalised as a precaution.

The nuclear power facility was wrecked by a huge earthquake and tsunami in 2011 that killed 18,000 people. It was one of the worst nuclear disasters in history.

The clean-up operation is expected to take decades, with the most dangerous part -- removing radioactive fuel and rubble from three stricken reactors -- yet to begin.

In August, Japan began releasing into the Pacific the equivalent of 540 Olympic swimming pools worth of treated wastewater that has been collected since the catastrophe.

Tokyo insists that the water is harmless, a view backed by the UN atomic watchdog, but China and Russia have banned Japanese seafood imports.