A small amount of radioactive debris removed by a robot from the stricken Fukushima nuclear plant has arrived at a research lab near Tokyo, the plant operator said Tuesday, after a journey kept secret for safety reasons.

The announcement comes after operator Tokyo Electric Power Co. (Tepco) said last week that a tricky trial debris removal operation using a specially developed extendible device had been completed.

More than 13 years after a catastrophic tsunami caused by a 9.0 magnitude earthquake triggered one of the world's worst nuclear accidents, around 880 tons of hazardous material remain.