PARIS (AP) — As Europe struggles through a major heatwave, the French energy company EDF says it has halted a fourth nuclear reactor, this time one at the country’s oldest nuclear plant at Fessenheim in eastern France.

In a statement, EDT said the Fessenheim nuclear reactor was temporarily shut down Saturday.

Since Thursday, four French nuclear reactors in three power plants near the Rhine and the Rhone Rivers, including Fessenheim, have had to be temporarily shut down. EDF said the decision was made to avoid overheating the rivers.

Nuclear power plants use water from the rivers to cool down the temperatures of their reactors before sending the water back into the rivers. Rivers that are unusually warm can experience mass fish die-offs, which has happened in Germany in the past week.