Moody's Investors Service said Thursday it may take a more negative view of power companies looking to build new nuclear-power plants.

The credit-ratings firm in a report said companies developing new reactors haven't taken steps to strengthen their balance sheets ahead of the massive projects. Although federal loan guarantees will provide low-cost financing, they only "modestly" reduce the risk plant developers face.

"It has become increasingly likely that the pursuit of new nuclear power projects will lead to some near-term rating actions or outlook changes," Moody's said.

Several U.S. power companies are pursuing nuclear projects for the first time in two decades. But the plants are expected to take years to build, and cost estimates range from $5 billion to $12 billion. They also are relying on favorable state regulations, which Moody's warns that while positive now, could change over the lengthy construction period.

NRG Energy Inc. (NRG), Scana Corp. (SCG), Southern Co. (SO) and UniStar Nuclear Energy, a partnership that includes Constellation Energy Group Inc. (CEG) and Electricite de France SA (EDF.FR), are developing the leading projects. Duke Energy Corp. (DUK), Progress Energy Inc. (PGN) and PPL Corp. (PPL) will likely make up the next wave, Moody's said.

Because new nuclear construction will increase the business and operating risk of power companies, they will need to take steps to preserve their ratings. Possible moves include partnerships, increased reliance on equity as part of financing plans and moderating their dividend policies to retain cash flow.

A Moody's analysis of the last build-out of nuclear generation in the U.S. found 40 out of 48 companies that developed plants were downgraded, with the average cut being four notches.

-By Mark Peters, Dow Jones Newswires; 212-416-2457;