TOKYO — Workers started removing radioactive fuel rods Monday from a reactor building at the crippled Fukushima Dai-Ichi nuclear power plant, Tokyo Electric Power Co. said. The painstaking and risky task is a crucial first step toward a full cleanup of the earthquake and tsunami-damaged plant in northeastern Japan.

The Unit 4 reactor was offline at the time of the March 2011 disaster, and its core didn’t melt as Units 1-3 did. But hydrogen explosions blew the roof and walls off the Unit 4 building and weakened the structure, leaving it vulnerable to earthquakes.

Tokyo Electric, known as TEPCO, has since reinforced the building, but experts say keeping so many fuel rods in a storage pool in the building still poses a major safety risk.

“The operation is an important step toward decommissioning Fukushima Dai-ichi, which would take 30-40 years,” TEPCO President Naomi Hirose said in a video message on the company’s website.

TEPCO has built a massive steel structure next to and partly over Unit 4 to mount cranes for the operation. It will take at least until the end of 2014 to finish moving the 1,533 sets of fuel rods, including 202 unused sets, to a safer location. Each set includes about 60-80 fuel rods containing uranium-based fuel pellets.

TEPCO will remove the unused fuel rods first, and will then move on to the more radioactive spent fuel. At the very end it will remove three sets of rods that are slightly damaged. The storage pools in Units 1-4 contain a total of 80 sets of rods with slight damage, most of which occurred years ago.

TEPCO spokesman Noriyuki Imaizumi said a group of six workers safely stored two sets of fuel rods in a cask on Monday. No problems were reported.

The operation is delicate. Experts say the fuel rod sets may have been damaged or jammed by small pieces of debris that fell into the pool during the explosions. Some have also raised concern about a major earthquake hitting during the removal work.

Two other reactors, Units 5 and 6, were also offline at the time of the disaster and eventually went into normal shutdown. They are also expected to be decommissioned.