Unit 4 was offline at the time of the March 2011 disaster, so its core didn't melt down as the other three did. But hydrogen explosions blew the roof off the 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.
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 to a safer location. Each set includes about 60-80 rods.
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.
The full decommissioning of the plant is expected to take decades.