Sweden’s biggest nuclear reactor will be offline for more than two weeks after suffering an unplanned outage early on Tuesday, just as it was restarting after an earlier halt.

Uniper SE’s Oskarshamn-3 unit in the southeast of the nation is set to resume output on Sept. 15, the German utility said in a filing. The plant had stopped on Friday.

There is a “problem with pump in turbine system. Corrective actions ongoing,” the firm said in the filing with the Nord Pool power exchange.

While demand is relatively low because of the late summer lull, the plant is located in southern Sweden, a region that needs all the capacity it can get because of permanent closures to other reactors in the past decade.

The plant has a capacity of 1,400 megawatts, according to Nord Pool.