In May, the S.C. Public Service Commission allowed Duke to spend up to $230 million through 2009 on preconstruction work at the site in Cherokee County.
In petitions filed last month, Friends of the Earth and NC Warn said that the federal Nuclear Regulatory Commission had not approved the reactor design that Duke plans, creating uncertainty about costs.
Commissioners said that the groups' petition was not filed quickly enough after the May decision and that it brought up no new issues that merited reconsideration.
"I'm not surprised they ruled against our motion," said Tom Clements, the Southeast nuclear-campaign coordinator for Friends of the Earth. "But this issue is going to come up again. The PSC is going to have to face up to the fact that that reactor design is not final."
Five companies, including Duke Energy and S.C. Electric and Gas, are proposing to use the same Westinghouse Electric Co. design for new reactors. SCE&G is planning two at its nuclear station in Fairfield County, with a request pending before the Public Service Commission to begin preconstruction work at the site.
The NRC and attorneys for Duke Energy said that Westinghouse's original design has been certified, but revisions are still being reviewed by the NRC.
Clements contends that means that "there is no final buildable design" and that costs could vary widely from current estimates.
Duke Energy has not released a cost estimate for its proposed reactors.
SCE&G, which is planning to build and operate its reactors in partnership with Santee Cooper, a state-owned utility, has estimated that its new reactors will cost about $10 billion.