Around 880 tonnes of hazardous material remain at the Fukushima site, 13 years after a catastrophic tsunami caused by a 9.0-magnitude earthquake triggered one of history's worst nuclear accidents.

Removing the debris is seen as the most daunting challenge in a decommissioning project due to last decades, because of the dangerously high radiation levels.

Last month, operator Tokyo Electric Power Company (TEPCO) said a trial debris removal operation using a specially developed extendible device had been completed.

The sample weighing just below 0.7 grams (0.02 ounces) -- equivalent to about one raisin -- was delivered to a research lab near Tokyo for analysis.

A TEPCO official told a press conference they are now gearing up for a second sample removal due in "March to April".

The company is "upgrading" the telescopic device used for the first experiment by attaching a new camera to its tip, according to documents released Thursday.

"Based on our previous experience, we will also make progress on training our workers to better familiarise them with the process" before tackling the additional sample collection, the document said.

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

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