Four thousand temporary workers will cycle in and out of Vicksburg and Warren and Claiborne counties in the next six to eight months, as Grand Gulf Nuclear Power Station completes a scheduled refueling and an upgrade that will make it the most powerful plant in the country, a local civic group was told Thursday.

Darron Case, Entergy Mississippi’s manager for resource planning, told the Vicksburg Rotary Club that the upgrade, which will increase Grand Gulf’s power output by about 178 megawatts or 13 percent, is under way at the nuclear plant about 30 miles south of Vicksburg in Claiborne County.

“It will be the same facility, with the same personnel working there, but will produce increased electricity,” Case said. “This will be the single largest upgrade ever completed and will make Grand Gulf again the single largest power producer in the U.S.”

Case said the improvements will primarily increase its “steam handling” capability, not its nuclear components.

About 700 full-time employees, many of whom live in Vicksburg, normally staff Grand Gulf. The refueling outage and power upgrade will require about 4,000 specialists working temporarily at various times, said Case.

The estimate is nearly double what had previously been predicted to fill travel trailer parks, hotels, motels and short-term rentals in the area and increase receipts at local shops, restaurants and other retail outlets.

Grand Gulf’s last refueling outage early in 2010 — bringing about 1,000 temporary workers — was credited for a nearly 15 percent increase in Vicksburg hotel and bed and breakfast occupancy rates over the same period in the previous year.

The 2012 refueling shutdown is scheduled for mid-February. Refueling outages normally last about six weeks.

The power upgrade is expected to be completed by summer to early fall, Case said.

Entergy Mississippi is also completing a $200 million purchase of a natural gas-fired plant in West Jackson, Case said. The purchase allows the utility to increase power production at one-third the cost of building a new plant, he told Rotarians.

Entergy will spend about $750 million in the next five years and up to $2 billion in the next 10 years to maintain its current equipment and plants, which include Vicksburg’s Baxter Wilson facility as well as Grand Gulf.

Baxter Wilson, which can burn natural gas and fuel oil in electricity production, was built in 1966.

Grand Gulf station was built in the 1970s and 1980s, and began operating in 1985. The plant uses a controlled nuclear reaction to generate heat to boil water.

The resulting steam is piped under pressure to a turbine, which spins to generate electricity that goes onto a grid with power from other plants.

The station is in the process of obtaining license renewal to operate for 20 more years.