Delays at a key supplier are pushing back by one year the $10 billion expansion of the nuclear power plant that South Carolina Electric & Gas Co. and Santee Cooper own in the Midlands.

The financial impact on the two utilities and, ultimately, their ratepayers is not yet known, officials said Tuesday.

SCE&G was recently notified by the construction consortium at the V.C. Summer Nuclear Station that "substantial completion" of the first of two new reactors is now expected by late 2018 or the first half of 2019. The other will follow 12 months later.

"These expected substantial completion dates do not reflect all efforts that may be possible to mitigate delay, nor has SCE&G accepted this new schedule," the Cayce-based company said in a statement.

The builders haven't yet provided cost overruns tied to the latest delay, SCE&G and state-owned Santee Cooper said.

Details about the financial ramifications probably will be released by the end of the year, said Steve Byrne, chief operating officer of SCE&G.

The utility has initiated a series of rate increases that are being phased in over 10 years to finance its portion of the Fairfield County nuclear expansion.

Byrne said negotiations will get underway soon to determine whether the building consortium is entitled to seek more money. The talks are likely to begin with a high dollar number, or as Byrne put it, "Something I'm not going to like," and proceed from there.

"And we've seen this before," he said during a conference call Monday with financial analysts.

SCE&G also must go back to the S.C. Public Service Commission for approval of a new construction schedule because some of the 46 remaining "milestone" projects won't be finished in time, Byrne said.

Last year, SCE&G pushed back the completion of the first of the new reactors at V.C. Summer from March 2017 to late 2017 or early 2018, blaming manufacturing delays at a vendor in Lake Charles, La.

That same supplier, CB&I, continues to hold up the project, Byrne said.

At issue are huge, heavy components known as "submodules" that go inside the main containment vessels. Until those are delivered and lifted into place, other critical construction projects at V.C Summer can't proceed, Byrne said.

"It is constraining other milestones," he said.

A representative of the owner of CB&I could not be reached for comment Tuesday.

SCE&G owns 55 percent of the nuclear plant 25 miles north of Columbia, in Jenkinsville. Moncks Corner-based Santee Cooper owns the rest but is selling a portion of its stake to SCE&G.

The construction delays are not expected to affect that deal, said Jimmy Addison, chief financial officer of SCANA Corp, which owns SCE&G.

Contact John McDermott at 937-5572.