The Japanese operator of the tsunami-wrecked Fukushima Daiichi nuclear power plant on Tuesday said the start of full-scale removal of melted fuel debris will be delayed for several years until 2037 or later, the latest setback underscoring the challenges ahead.

The Tokyo Electric Power Company Holdings, or TEPCO, said it will need 12 to 15 years of preparation before starting full-scale removal of melted fuel debris at the No. 3 reactor. That preparation includes reducing radiation levels and building necessary facilities in and around the reactor.

Overall, at least 880 tons of melted nuclear fuel has mixed with broken parts of internal structures and other debris inside the three reactors that suffered meltdowns at the plant following the 2011 earthquake and tsunami.

The delay again sets back the 2051 target set by Japan’s government and TEPCO for decommissioning the plant.

A test retrieval of a tiny sample of melted fuel debris in November was already three years behind, and some experts estimate that the decommissioning work could take more than a century.

TEPCO said it plans to stick to the current completion target of 2051.

“Realistically, we are aware of the difficulty (to achieve the target) but we will not drop the goal just yet, as we still don’t have a clear work schedule after the full-scale removal begins,” said Akira Ono, chief decommissioning officer at TEPCO.

Ono said TEPCO plans to examine preparation work necessary at the two other reactors within the next couple of years ahead of full-scale melted fuel retrieval.

After small missions by robots to gather samples, experts will determine a larger-scale method for removing melted fuel, first at the No. 3 reactor.