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 ).

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.