In a 3-2 vote, trustees gave the go-ahead for CPS to acquire at least an additional 12 percent share of the South Texas Project nuclear plant, which the city-owned utility and three other entities built in 1988.

Such a move is not expected to hit local power customers harder in the pocketbook because the potential rise in the electric rate is only one component of a customer's bill.

An increase in the electric rate would be offset by a corresponding decrease in fuel costs that would spring from the expanded use of the nuclear plant, utility officials said.

Texas Central Co., a successor to one of the original partners in the South Texas Project, has found a buyer for its 25.2 percent interest in the plant.

But before it can accept the offer, it first must offer that share -- at the same price -- to the companies that built the plant.

CPS, which already owns 28 percent of the nuclear plant's generating capacity, is the first of the partners to accept the offer.

The decision by the utility, which sprung from a motion made by trustee Stephen Hennigan, commits the utility to purchasing at least 12 percent, or almost half, of the 630 megawatts of nuclear- generated power Texas Central has up for sale.

But if another co-owner of the plant, Texas Genco, decides not to participate in the purchase, that percentage could increase.

The proposed purchase price was not available Wednesday because the final percentages were not worked out.

But Texas Central has been offered $332.6 million for its share of the nuclear plant, and the partners' offer cannot be less than that.

That price tag is fairly cheap when compared with the original price tag for the 2,500-megawatt generation facility, Hennigan said.

Estimates show the purchase price is less than a third of what the 25.2 percent interest originally cost partners when they constructed the plant 16 years ago, he said.

Still, additional staffing and other maintenance costs associated with the city's expanded nuclear interest are expected to drive up CPS' base electric rate by as much as 5 percent, officials said.

That increase must be approved by the CPS board and the City Council.

Mayor Ed Garza, who was part of the majority that voted for the purchase, said he expects the increased electric rate to be negated by lower fuel costs.

CPS officials said the new share of the nuclear plant will not eliminate the need for a $1 billion, coal-fired power plant planned at Calaveras Lake.

That plant, which has aroused outrage from environmentalists, is still in the permitting stage.

Utilities pay less to generate power from nuclear plants than from their other primary energy sources, natural gas and coal, when construction and other fixed costs are excluded.

CPS staff, after a lengthy evaluation, had recommended against the purchase.

Jim Nesrsta, director of generation planning for CPS, said there were too many unknowns, including maintenance costs and the potential risks associated with a major operational breakdown at the plant.

On the plus side, officials said if natural gas prices stay high, the additional investment in the nuclear plant could pay for itself in less than five years through cost savings passed on to customers and extra energy sales available to CPS.

"Management felt it was a close call," Nesrsta said, explaining the staff's recommendation. "If it's that close, maybe this is not the right time."

Trustees also were divided, but they came down on the side of additional nuclear power based in part on their confidence in the performance of the nuclear plant.

CPS' citizens advisory panel backed the proposed purchase, as did many of the utility's engineers, who are most familiar with the reliability of the nuclear plant, Hennigan said.

One technical consultant estimated, at most, a 1 percent chance the plant could go down for an extended period of time.

Hennigan, Garza and board Chairwoman Aurora Ortega-Geis supported the purchase, while trustees Alvaro Sanchez Jr. and Clayton Gay Jr. opposed it.

Also questioning the board's decision was the consumer watchdog group Public Citizen, the environmental advocacy group Sustainable Energy and Economic Development Coalition and the local chapter of the Sierra Club.

Those groups said the utility should be pursuing energy efficiency savings and renewable energy options rather than investing in nuclear power.

"It's never been a well managed plant," Karen Hadden, executive director of Sustainable Energy, said about the STP. "CPS would do better to send a message of cleaner energy."