Thick, black smoke pouring from one of the cooling towers at Ukraine’s Zaporizhzhia nuclear power plant over the weekend raised alarm about safety at the facility as the military conflict with Russia continues.

On-site staff from the International Atomic Energy Agency witnessed the smoke and reported hearing multiple explosions at Zaporizhzhia, which is the largest nuclear plant in Europe and one of the largest worldwide.

Ukraine’s Zaporizhzhia nuclear power plant temporarily lost the connection to its sole remaining 750 kilovolt (kV) off-site power line last week due to a reported short circuit, leaving it reliant on a single backup line for more than three hours.