Even as hopes were raised that a permanent presence of United Nations inspectors would help reduce the risk of disaster at the Zaporizhzhia nuclear power plant in Ukraine, the war once again endangered the plant’s safe operation.

After shelling Friday evening, the plant lost the connection with its only remaining primary external power line, forcing it to use a lower-voltage reserve line to power the cooling equipment needed to prevent meltdowns, the International Atomic Energy Agency said in statement Saturday.

Still, the director of the agency, Rafael Grossi, expressed cautious optimism that a plan to station two nuclear experts at the plant indefinitely would help lower the risk of a catastrophe.