Standing in a 1960s industrial building by the German Baltic Sea coast, Florian Grose, holds a dosimeter as he moves towards a corner of the room. It starts beeping furiously.

"That's around 10 microsieverts," he says, dressed in protective overalls. A normal dose rate is under 0.2 microsieverts. "This is an area where I'd say we maybe should move a meter away. You shouldn't stand or lie here for an hour," Grose, a radiation protection worker, warns in a disarmingly jovial tone.

Inside special building one of the former nuclear power plant, parts of the wall are uneven and pockmarked — the result of workers hammering off layers of concrete, hunting for radioactive contamination.…