May 12 (Reuters) - The U.S. Nuclear Regulatory Commission $(NRC)$ has launched a special inspection at the Quad Cities Generating Station in Marseilles, Illinois, following discovery of inoperable safety-related vacuum breakers, the agency said on Monday.

The two-unit nuclear plant is operated by Constellation Energy CEG.O.

Operators discovered that vacuum breakers intended to maintain the structural integrity of the containment system during major events were inoperable because certain valves were not reopened after testing during a recent refueling outage, according to the NRC press release.

It added that the event compromised the system's ability to regulate containment pressure, warranting a special investigation, and that the system has been restored.

"While this did not affect safe plant operation, an independent review of the issue by the regulator is warranted given questions related to the performance of plant personnel that compromised the ability of a safety system to fulfill its function," said Region III Administrator Jack Giessner.

The NRC said its inspectors will evaluate Constellation’s response, the company's understanding of the event, the scope of its assessment actions, and the adequacy of procedures and system design.

Findings from the inspection will be documented in a publicly available report, which will be distributed electronically and posted on the NRC’s website, it added.

(Reporting by Anjana Anil in Bengaluru; Editing by David Gregorio)

((Anjana.Anil@thomsonreuters.com;))