TOKYO — The cleanup of Japan’s devastated Fukushima Daiichi nuclear plant crossed an important milestone on Saturday when the plant’s operator announced it had safely removed the radioactive fuel from the most vulnerable of the four heavily damaged reactor buildings.

The operator, Tokyo Electric Power Company, removed the last remaining fuel rods from the ruined No. 4 reactor building, putting the rods inside a large white container for transportation to another, undamaged storage pool elsewhere on the plant’s grounds. The company, known as Tepco, had put a high priority on removing the No. 4 unit’s some 1,500 fuel rods because they sat in a largely unprotected storage pool on an upper floor of the building, which had been gutted by a powerful hydrogen explosion during the March 2011 accident.

This had led to fears of additional releases of radioactive material if the pool was damaged further, such as by an earthquake. By succeeding in the technically difficult task of extracting those rods, Tepco eliminated one of the plant’s most worrisome vulnerabilities. This is also the first time that the fuel has been removed from one of the four wrecked reactor buildings.

It took almost four years to reach this goal, as the cleanup has been plagued by mishaps and a so far unstoppable flow of groundwater that has flooded the basements of the crippled reactor buildings. The aging Fukushima Daiichi plant suffered a triple meltdown after a huge earthquake and tsunami struck on March 11, 2011, knocking out vital cooling systems.