Story highlights Decommissioning the Fukushima nuclear plant is estimated to take up to 40 years

The plant is now stable, according to TEPCO, its operator

Biggest challenge is to take out the molten nuclear fuel debris

Tokyo (CNN) Cleaning up Japan's Fukushima Daiichi nuclear plant, which suffered catastrophic meltdowns after an earthquake and tsunami hit in 2011, may take up to 40 years.

The crippled nuclear reactor is now stable but the decommissioning process is making slow progress, says the plant's operator Tokyo Electric Power Co, better known as TEPCO.

"If I may put this in terms of mountain climbing, we've just passed the first station on a mountain of 10 stations," said Akira Ono, head of the Fukushima plant.

It's almost five years since the earthquake and the tsunami it triggered killed more than 15,000 people and destroyed coastal towns on March 11, 2011.

TEPCO has attracted fierce criticism for its handling of the disaster.

A worker takes notes in front of storage tanks for radioactive water at the Fukushima Daiichi nuclear plant on February 10.