CHICAGO -- Ongoing problems with its nuclear plant could take a toll on the Omaha Public Power District, one of the highest-rated utilities in the country.

Moody’s Investors Service warned it may downgrade OPPD’s Aa1 senior lien rating on $1.5 billion of debt and Aa2 subordinate lien rating on $346 million of debt.

The ratings agency revised its outlook to negative from stable on the utility last September.

The chief problem is the Fort Calhoun Nuclear Station, which sits along the Missouri River and typically provides 25% of the utility’s energy.

The plant has been shut down since April 2011, originally for regularly scheduled maintenance. But a major Missouri River flood followed by increased scrutiny from federal nuclear regulators after the Japan nuclear disaster has repeatedly postponed the plant’s restart date.

The increased scrutiny and delays getting the station back online reflect the risk “associated with full ownership of a single unit nuclear plant in a post-Fukushima environment,” Moody’s said in a ratings report last September when it revised the outlook to negative.

Planned start dates in 2012, then in February and May of 2013 have come and gone. The earliest restart date is now set for July, though it could be later, as the NRC reportedly said at a recent public meeting that the plant had met only eight of 25 performance issues during inspections.

“The ongoing restart delays are indicative of the additional NRC scrutiny,” Moody’s said in its most recent report Wednesday. “Moody’s expects similar scrutiny to continue, negatively impacting OPPD’s credit profile as the restart could be delayed again and future plant operations will be watched closely.”

The extended outage has reduced the utility’s power diversity and forced it to purchase replacement power on the open market at a higher cost than if it had produced it at Fort Calhoun, though low natural gas prices have muted the impact.

The utility is aware of all the steps it needs to take to restart the plant and aims to complete the work by this summer, spokesman Jeff Hanson said.

“We know all of the physical work that needs to be done,” Hanson said.

The commission is scheduled to visit the plant by the end of June, he said.

Moody’s said its review for possible downgrade will focus on its view of OPPD’s management of its generation and business risk profile compared other similarly rated utilities. The OPPD is now Moody’s fourth-highest rated utility.

“The review will focus on the expected financial and operating impact of the extended nuclear outage and the financing schedule and plan for the upgrades at OPPD’s coal plants,” Moody’s said.

Standard & Poor’s rates the district AA with a stable outlook.

The OPPD is the nation’s twelfth-largest utility with 771,000 customers, serving Omaha and 47 other cities across a 5,000-square-mile area. Its competitive position is strengthened by Nebraska’s status as an all-public power state that prohibits investor-owned utilities. It also enjoys the ability to set its own rates.

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