The operator of Japan's crippled Fukushima nuclear power plant on Monday began removing spent fuel assemblies from the building housing Reactor No. 4, the most delicate operation since a devastating earthquake and tsunami touched off the worst nuclear crisis in a quarter century.

The operation, which is expected to take about a year, marks the start of a new phase in the decades-long process of decommissioning the entire facility.

The operator, Tokyo Electric Power Co., which has been harshly criticized for its numerous errors in managing the crisis, will be under heavy scrutiny by Japanese authorities and various nuclear-safety organizations as it carries out this task.

At 3:18 p.m., TEPCO technicians began using cranes to extract the first of 1,533 fuel assemblies - most containing spent uranium fuel - from storage racks and move them into a large steel chamber.

The initial transfer takes place inside the spent fuel pool that has housed the fuel assemblies and kept them cool since the March 2011 natural disaster.

Later, using another crane, the steel chamber will be taken out of the pool and lowered to the ground floor of Reactor No. 4's building, where it will be mounted on a trailer and transferred to another nearby pool that is much closer to the ground and considered a safer storage place.

The operation is fraught with challenges because Reactor No. 4's building was damaged by a hydrogen-air explosion triggered by the earthquake and tsunami.

"There is enormous potential risk associated with the spent fuel," the president of the Nuclear Regulation Authority, Shunichi Tanaka said a few months ago.

He said then that he was more worried about this operation than the problem of a radioactive water accumulating at the plant and leaking into the sea.

The Citizen's Nuclear Information Center and other anti-nuclear groups say TEPCO is not prepared to carry out this delicate operations and that using a crane to lower the steel chamber, which weighs 90 tons when full, is a very risky operation.

The casks must be vertically lowered a distance of 32 meters (105 feet) from the highest floor of the building to the ground floor, and the CNIC fears one of them could fall, break open and release large amounts of radioactive gases.

The 2011 earthquake- and tsunami-triggered crisis was serious enough to rate a Level 7 designation, a distinction the Fukushima disaster shares only with the 1986 meltdown of the Chernobyl plant in Ukraine. EFE