Confirmation this week that all the fuel inside one of Fukushima's broken reactors has long-since melted leaves the site's Japanese operator with the tricky task of eventually scooping it all out, experts say.

Tokyo Electric Power (TEPCO) said Thursday it had performed a sophisticated scan of the reactor core, giving the most detailed picture so far of what is going on in the high-radiation environment.

Nuclear experts said Friday that the test showed the nuclear fuel rods had melted beyond recognition.

"The results reaffirmed our previous understanding that a considerable amount of fuel had melted inside the nuclear pressure vessels," said Hiroshi Miyano, a visiting professor at Hosei University in Tokyo.

"But there has been no evidence that the fuel has melted through the nuclear containment buildings and reached the outer environment," Miyano told AFP.

However, the test -- tomography imaging using elementary particles called "muons" -- did not look at the bottom part of the reactor, with some experts suggesting it was not possible to tell if the fuel was still contained.

Containment buildings enclose the reactor pressure vessel in which fuel rods are installed. These rods generate the heat that is used to drive steam turbines and produce electricity.

"Eventually, TEPCO is aiming to scoop out the melted fuel little by little, rather than burying it in concrete" as was done at Chernobyl in the former Soviet Union, Miyano said.

- Showered from space -

Muons, which are continually being showered all over the earth from space, penetrate solid objects to a greater depth than x-rays.

The rate at which they pass through a material indicates its density and helps scientists to identify it.

Muons move more slowly through relatively dense plutonium and uranium fuel than through the reactor vessel itself, so mapping the particles' trajectory reveals exactly where the fuel is -- or isn't -- lying.

The data from this test should help TEPCO's effort to decommission the plant, which was knocked offline in March 2011 when Japan was hit by a huge tsunami.

The full decommissioning process at Fukushima is expected to take three or four decades.

Experts say the latest results and the operator's assessment of them were in line with earlier expectations.

"We presume that despite the meltdown, the fuel is still in the containment vessel," said Tomohisa Ito, a spokesman for the International Research Institute for Nuclear Decommissioning, a special research unit involved in dismantling the troubled plant.

"But we still need to directly check the situation one day using remote-controlled robots," he said.

Last month the International Atomic Energy Agency said Japan had made "significant progress" in its cleanup efforts but warned the situation "remains very complex" due to the growing amounts of contaminated water generated by the process.

While the quake-tsunami that caused the atomic crisis killed almost 19,000 people, no one is officially recorded as having died as a direct result of the nuclear meltdowns at Fukushima.

However, tens of thousands of people remain displaced because of radioactive contamination around the plant, with scientists warning that some settlements may have to be abandoned forever.