The U.S. Nuclear Regulatory Commission gave the OK for a unit at the Davis-Besse nuclear power plant in Ohio to restart after a 3-month-long outage.

FirstEnergy Nuclear Operating Co. (FENOC) officials told the NRC that it completed an outage at the plant to replace a reactor vessel head and perform other scheduled maintenance. FENOC also said it performed a thorough engineering analysis and assessment of its shield building and confirmed that the building’s structural integrity was sound.

This comes after the company found hairline cracks in the concrete outer shell at the one-unit plant on Oct. 10. More cracks were found on Nov. 1. The plant had been shut down since Oct. 1 to install the new reactor vessel head.

The cracks were mostly found near the main steam lines and portions of the top 20 feet of the building also contained similar cracks.

Davis-Besse officials said they would continue to look for the cause behind the cracks in the shield building and developing a long-term building monitoring plan.

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