By NICHOLAS K. GERANIOS

Associated Press

SPOKANE, Wash. (AP) -- The Grand Coulee Dam, a product of the Great Depression, is getting a major facelift as federal operators seek to squeeze even more power out of the nation's largest hydroelectric producer.

Design work has already started to renovate six massive electrical generating units in the Third Powerhouse of the dam located about 100 miles west of Spokane. The goal is to increase the amount of power produced, and to extend the life of the generating units by up to four decades.

"The Third Powerhouse is incredibly important to this region and to California," said Chris Vick, project manager for the U.S. Bureau of Reclamation, which operates the dam.

Indeed, the three smaller generating units each produce 690 megawatts of power, while the three bigger ones each produce 805 megawatts, Vick said.

The powerhouse was finished in 1974, and all six units are operating well beyond their 25-year design life, Vick told the Lake Roosevelt Forum, an annual conference named for the reservoir behind the dam.

"We want 30 to 40 more years out of the units," Vick said.

The old units show plenty of wear and tear, including leaking pipes and other woes, Vick said.

Part of the rehabilitation work will be efforts to boost the three smaller power generators to a capacity of 770 megawatts of power each, Vick said. That would produce 240 more megawatts of power. The three larger generator plants cannot have their capacity expanded in a cost-effective way, Vick said.

Construction on Grand Coulee Dam ran from 1933 to 1942, and it originally had two power plants.

The dam's electricity helped power airplane and nuclear weapons production in World War II, and it also provides irrigation water for hundreds of thousands of acres of farmland in arid Eastern Washington.

Updating the generating plants will take about 17 months per unit, so one of the six power plants will be down for a period of 10 years, Vick said. Public tours of the powerhouse will be suspended while that work occurs, he said.

The work is estimated to cost more than $300 million, he said.

Meanwhile, the Reclamation Bureau is also replacing the aging underground cables that carry the dam's electricity to the western power grid, Vick said.

The 18 underground cables are oil-filled and pose a danger to catch fire as they age, Vick said.

An $18.5 million project currently under way will replace those underground cables with overhead power lines, suspended from towers that are up to 350 feet tall, Vick said. The new lines will be finished by December, he said.

Numerous smaller projects are also under way to modernize power production at the dam, including renovations of elevators, cranes, transformers and other equipment. A 200,000-square-foot storage building is under construction.

The Reclamation Bureau is also looking down the road at renovations to the older power plants at the dam to increase their electrical production, officials said.

Copyright 2012 The Associated Press.