During a subcritical counting experiment, a power buildup went undetected at the Vinca Nuclear Institute's zero-power natural uranium heavy water-moderated research reactor. Saturation of radiation detection chambers gave the researchers false readings and the level of moderator in the reactor tank was raised, triggering a criticality excursion which a researcher detected from the smell of ozone. Six scientists received radiation doses of 2–4 sieverts (200–400 rem). An experimental bone marrow transplant treatment was performed on all of them in France and five survived, despite the ultimate rejection of the marrow in all cases. A single woman among them later had a child without apparent complications. This was one of the first nuclear incidents investigated by then-newly formed IAEA.