Volodymyr Vechirko, 62, was sent to Chernobyl in summer 1986 to clear away topsoil, clean buildings and make the other still-functioning reactors on the site safe. He has been ill for much of his life, which he attributes to working at the disaster site, including chronic dizziness, weakness and frequent pain.

Because of his poor health, he said, he lost touch with colleagues. "Being back here now, the emotions are overwhelming," he said. "It's incredibly sad to compare what this place was to what it is now."

Oleksii Harbuz, 72, a medical officer during that time, was close to the human costs of the disaster. Returning for the first time in decades, he recalled colleagues who have since died and the bonds formed among survivors.

"This 40th anniversary represents both a deep tragedy and a vital chance to reunite with my brothers-in-arms," he said. "So many of our colleagues have passed away over the years, but those of us remaining hold on."