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 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. By succeeding in the technically difficult task of extracting those rods, Tepco eliminated one of the plant's most worrisome vulnerabilities. It took almost four years to reach this goal. 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 [into] lumps on the bottom of the reactor vessels.