"NARRATOR: In the vicinity of the reactor, the radioactive fallout forced a third-of-a-million people to evacuate, never to return. It remains the world's worst-ever nuclear power plant disaster. It left the Soviet authorities with a monumental problem: around 200 tons of shattered uranium fuel rods and other radioactive debris remained inside the damaged reactor building. Left uncovered, it would continually release radioactive dust into the air, a poisonous cloud to threaten the surrounding area. Over the next six months, workers braved extreme radiation to seal the reactor inside a 300,000-ton shelter, made from steel and concrete. It came to be called the "sarcophagus." But it was flawed from the start. The extreme radiation prevented the workers from completing the welds needed to seal the prefabricated sections of the sarcophagus together."