Tokyo Electric Power Company (Tepco) aims to retrieve a tiny sample of the estimated 880 tons of radioactive debris that is believed to sit inside reactors at the tsunami-hit nuclear plant.

The sample will be studied for clues about the condition of the inside of the reactors and their hazardous contents, a crucial step towards decommissioning the plant.

"We will proceed carefully by putting safety as our highest priority," a Tepco official told a news conference Monday.

The debris has radiation levels so high that Tepco has had to develop specialised robots that can withstand them to function inside.

Removing it has long been dubbed the most daunting challenge in the decades-long project to decommission the Fukushima Daiichi nuclear plant.

Three of Fukushima's six reactors were operating when the tsunami hit on March 11, 2011, knocking down cooling systems and sending them into meltdown in what became the worst nuclear disaster since Chernobyl.

In three units of the Fukushima plant, fuel and other material melted and then solidified into highly radioactive "fuel debris".

Tepco deployed in February two mini-drones and a "snake-shaped robot" into one of the three nuclear reactors, as part of the preparations for the removal task.

The latest probe, equipped with a robotic arm, is expected to take about a week to reach radioactive debris inside the reactor and should emerge again with the sample next month.

Japan began almost a year ago to release wastewater from the stricken plant into the Pacific Ocean.

The step has 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.