One person died and another was trapped 500 metres underground when a building structure partially collapsed at France's future nuclear waste storage site in Bure, in northeastern France, the local fire service said on Tuesday.
The emergency services were trying to reach the second person, who was injured.
Nuclear waste agency Andra's Cigeo deep geological storage project is designed to bury France's highly radioactive nuclear waste hundreds of metres underground for thousands of years.
The government's final investment decision on the 25 billion euro ($27.08 billion) project is not expected before 2020, but Andra has already dug deep shafts to test the soil and its future installations.
Andra had no immediate comment on Tuesday.