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.

Tepco still faces the far more challenging task of removing the ruined fuel cores from the three reactors that melted down in the accident. These reactors were so damaged — and their levels of radioactivity remain so high — that removing their fuel is expected to take decades. Some experts have said it may not be possible at all, and have called instead for simply encasing those reactors in a sarcophagus of thick concrete.

The fuel cores from those three reactors, Nos. 1-3, are believed to have melted like wax as the uncooled reactors overheated, forming lumps on the bottom of the reactor vessels. Scientists have warned that the hot, molten uranium may have even melted through the metal containment vessels, possibly reaching the floor of the reactor buildings or even the earth beneath.

However, it was the storage pool at the No. 4 unit, and particularly its highly radioactive spent fuel rods, that had caused the most intense concern in the first weeks after the accident. While the No. 4 reactor itself had been safely shut down when the accident happened, hydrogen released by the meltdowns at the other reactors caused an enormous explosion that blew off the reactor building’s roof and walls, leaving its storage pool exposed to the air.

Japanese and American nuclear officials at first worried that the pool may have been cracked in the explosion, but this proved not to be the case. Still, falling water levels in the storage pool caused anxiety that the fuel rods within could be exposed to the atmosphere. This would have caused a far larger release of radioactive materials than what occurred during the actual accident, which spewed contamination across a wide swath of northern Japan.