The announcement comes after operator Tokyo Electric Power Company (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.

Removing this debris is seen as the most daunting challenge in the decades-long decommissioning project, because of the dangerously high radiation levels.

Despite delays caused by technical snags, the trial debris removal from the plant in northeastern Japan went ahead with the aim of studying the sample for clues about conditions inside the reactors.

TEPCO said in an email to reporters on Tuesday that "the transportation of the fuel debris collected during the test removal operation... was completed" during the afternoon.

The company had not previously given details of when the 160 kilometre (100 mile) journey to the Oarai Nuclear Engineering Institute of the Japan Atomic Energy Agency (JAEA) would take place, citing safety concerns.

According to Japanese media reports, while the sample retrieved by the robot weighs 0.7 grams (0.02 ounces), the JAEA has said it would need at least one gram to conduct a proper analysis.

Three of Fukushima's six reactors went into meltdown in 2011 after the huge tsunami swamped the facility.

Japan last year began releasing into the Pacific Ocean some of the 540 Olympic swimming pools' worth of reactor cooling water amassed since the disaster.

The step sparked a diplomatic row with China and Russia, both of which banned seafood imports, although Japan insists the discharge is safe, a view backed by the UN atomic agency.

Beijing, however, said in September it would "gradually resume" importing seafood from Japan.

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TEPCO - TOKYO ELECTRIC POWER