TOKYO — 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.

Tokyo Electric Power Co. said 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 pool 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.

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.

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

Workers are remotely operating a crane built underneath a jelly roll-shaped roof cover 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 carried out remotely from a control room about 500 meters (yards) away because of still-high radiation levels inside the reactor building that houses the pool.

About an hour after the work began Monday, the first fuel unit was safely stored inside the cask, TEPCO said.

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. Removing fuel from the cooling pools will help free up space for the subsequent removal of the melted fuel, though details of how to gain access to it are yet to be decided.

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

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. Toshiba Corp.'s energy systems unit, which developed the robot, said the findings were key to determining the proper equipment and technologies needed to remove the melted fuel, the most challenging part of the decommissioning expected to take decades.

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