OMAHA, Neb. (AP) – Federal regulators are investigating the failure of a part at the Fort Calhoun nuclear power plant during this spring’s maintenance outage. The Nuclear Regulatory Commission said Monday that it had launched a special inspection at the plant that sits along the Missouri River about 20 miles north of Omaha.

The NRC says workers discovered the problem earlier this month after replacing a seal in a valve that controls cooling water flow into one of the plant’s steam generators. The valve wouldn’t open because the new seal material couldn’t handle the high operating temperatures. The seal was replaced a second time to resolve the problem, but regulators want to better understand how it happened and whether workers’ response was adequate.

Fort Calhoun is owned by the Omaha Public Power District utility.