The operator of the tsunami-wrecked Fukushima nuclear plant has begun removing fuel from a cooling pool at one of three reactors that melted down in the 2011 disaster, a milestone in the decades-long process to decommission the plant.

Key points: Workers need to remove 566 nuclear fuel rods from a cooling pool at the reactor site

Workers need to remove 566 nuclear fuel rods from a cooling pool at the reactor site The whole process occurs underwater to prevent radiation leaks

The whole process occurs underwater to prevent radiation leaks New robots will need to be developed to remove debris from the reactors

The process involves taking out spent nuclear fuel rods by using remote-controlled cranes to lift hundreds of radioactive cylinders from a highly contaminated reactor site.

Tokyo Electric Power Co (TEPCO) said on Monday that workers started removing the first of 566 used and unused fuel units stored in the pool at Unit 3.

The fuel units in the pools located high up in reactor buildings are intact despite the disaster, but the pools are not enclosed, so removing the units to safer ground is crucial to avoid disaster in case of another major quake.

"The work is expected to be completed in March 2021, but safety is our first priority," spokesman Joji Hara said.

TEPCO says the removal at Unit 3 would take two years, followed by the two other reactors, where about 1,000 fuel units remain in the storage pools.

If the rods are exposed to air or if they break, radioactive gases could be released into the atmosphere.

The 2011 disaster forced 160,000 people to evacuate areas near the Fukushima plant, and many of them have never returned to the most contaminated areas.

Obstacles to removing melted fuel

The work is carried out remotely because of still-high radiation levels inside the reactor building. ( AP: Kyodo News )

Removing fuel units from the cooling pools comes ahead of the real challenge of removing melted fuel from inside the reactors, but details of how that might be done are still largely unknown.

Experts say the melted fuel in the three reactors amounts to more than 800 tons.

Removing the fuel in the cooling pools was delayed more than four years by mishaps, high radiation and radioactive debris from an explosion that occurred at the time of the reactor meltdown.

Workers are remotely operating a crane to raise the fuel from a storage rack in the pool and place it into a protective cask.

The whole process occurs underwater to prevent radiation leaks.

Each cask will be filled with seven fuel units, then lifted from the pool and lowered to a truck that will transport the cask to a safer cooling pool elsewhere at the plant.

The work is directed remotely from a control room about 500 metres away because of still-high radiation levels inside the reactor building that houses the pool.

Robotic removal

Japanese Prime Minister Shinzo Abe visited Fukushima to inspect the reconstruction effort following the tsunami, quake and nuclear accident in 2011. ( AP: Kyodo News )

In 2014, TEPCO safely removed all 1,535 fuel units from the storage pool at a fourth reactor that was idle and had no fuel inside its core when the March 11, 2011, earthquake and tsunami occurred.

Robotic probes have photographed and detected traces of damaged nuclear fuel in the three reactors that had meltdowns, but the exact location and other details of the melted fuel are largely unknown.

In February, a remote-controlled robot with tongs removed pebbles of nuclear debris from the Unit 2 reactor but was unable to remove larger chunks, indicating a robot would need to be developed that can break the chunks into smaller pieces.

TEPCO and government officials plan to determine methods for removing the melted fuel from each of the three damaged reactors later this year so they can begin the process in 2021.

AP/Reuters