A tricky operation to remove a second sample of radioactive debris from inside Japan's stricken Fukushima nuclear plant has been completed, the site operator said on Wednesday.

Dangerously high radiation levels mean that removing melted fuel and other debris from the plant hit by a huge tsunami in 2011 is seen as the most daunting challenge in the decades-long decommissioning project.

Around 880 tonnes of hazardous material are inside the power station – the site of one of history's worst nuclear accidents after the tsunami, which was triggered by a 9.0-magnitude earthquake.

Plant operator Tepco said its second trial debris removal operation, which began just over a week ago, "has been completed" as of Wednesday morning.

The debris was "removed from a different location from the previous sampling location" to better understand the material's "characteristics and distribution", government spokesman Yoshimasa Hayashi said.

Trpco completed its first trial removal, using a specially developed extendible device, in November.

That first sample weighing just under 0.7 grams – roughly equivalent to one raisin – was delivered to a research lab near Tokyo for analysis.

"Detailed analysis of the debris collected in the first sampling" will inform future decommissioning work as Tepco studies how to conduct a "full-scale removal", Hayashi said.

US nuclear expert Lake Barrett, a special adviser on the Fukushima clean-up, said removing more debris would be challenging but not impossible.

"We're going to find all of these complexities of almost a witch's brew down underneath there" in the reactors, Barrett said.

"They've got to develop robots we've never done before.

"But the fundamentals are there for the technology to do it."

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

In 2023, Japan began releasing into the Pacific Ocean some of the 540 Olympic swimming pools' worth of treated wastewater that had been collected at the plant.

The move was endorsed by the International Atomic Energy Agency but China banned Japanese seafood imports as a result, and Russia later followed suit.

This month China said it had found no abnormalities in seawater and marine life samples that it independently collected near the Fukushima plant in February.

But Beijing indicated more tests would be needed before it lifts the ban. (AFP)