Ukraine fears its Zaporizhzhia nuclear power station could by late summer face a shortage of water to cool reactors because Russian forces have let water out of a reservoir that supplies the plant.

Ihor Syrota, director general of the state-run Ukrhydroenergo hydropower generating company, told Reuters there was no immediate danger to Europe’s largest nuclear plant, which has been occupied by Russian forces for the past year.

But he voiced concern about what would happen if water levels fell further at the Kakhovka reservoir on the Dnipro river, which supplies the plant and millions of people in southern Ukraine, including in Russian-occupied Crimea.

The level has fallen because Russian troops who control the reservoir, and also the Kakhovka hydroelectric power station and dam, have let some water out through sluice gates, he said.

Nuclear plants need enough water to cool their reactors and to help prevent a meltdown. Syrota said the Zaporizhzhia plant still needed water to cool its reactors even though they have been shut down.