CHERNOBYL, Ukraine — Twelve times a month — the maximum number of shifts the doctors will allow — Sergei A. Krasikov takes a train across the no man’s land and reports for work at a structure enclosing Reactor No. 4 known as “the sarcophagus.”

Among his tasks is to pump out radioactive liquid that has collected inside the burned-out reactor. This happens whenever it rains. The sarcophagus was built 25 years ago in a panic, as radiation streamed into populated areas after an explosion at the reactor, and now it is riddled with cracks.

Water cannot be allowed to touch the thing that is deep inside the reactor: about 200 tons of melted nuclear fuel and debris, which burned through the floor and hardened, in one spot, into the shape of an elephant’s foot. This mass remains so highly radioactive that scientists cannot approach it. But years ago, when they managed to place measurement instruments nearby, they got readings of 10,000 rem per hour, which is 2,000 times the yearly limit recommended for workers in the nuclear industry.

Mr. Krasikov, who has broad shoulders and a clear, blue-eyed gaze, has been baby-sitting this monster for eight years. He’ll stay until he is pensioned off and then leave his job to another man, who will stay until he is pensioned off. Asked how long this will continue, Mr. Krasikov shrugged.