With little fanfare, workers at the Limerick Generating Station are laying the foundation for a major overhaul of the Montgomery County nuclear-power plant.

Construction crews last week prepared a concrete slab to serve as a staging area for the replacement of Limerick's six huge transformers, a $90 million job that will take about two years to complete.

The improvements to the transformers, which convert electricity for transmission on big power lines, are only one component of a complicated effort to "uprate" the plant's output, adding 170 megawatts of generating capacity to each unit. Along with earlier upgrades, the improvements will expand Limerick's total capacity to 2,600 megawatts - 23 percent more power than it produced when the two units were completed in 1989.

The upgrades at Limerick, and a similar project at the twin Peach Bottom reactors in York County, are part a larger Exelon Corp. plan to add up to 1,500 megawatts of capacity to its fleet of 17 reactors, which are concentrated in Pennsylvania and Illinois.

"There is surely a place for new nuclear build, but if you look at uprates from a cost-competitive point of view, they compare very favorably," said Charles "Chip" Pardee, Exelon's chief nuclear officer.

With the youngest U.S. reactor now 13 years old, and no new nuclear plants expected to be built for another decade, the industry has focused on squeezing as much juice as it can from the nation's 104 reactors by expanding capacity and becoming more efficient.

The industry's capacity factor - a measure of how efficiently the nuclear assets produce power - has increased from 62 percent to almost 92 percent in two decades. According to the Nuclear Energy Institute, the industry's policy arm, no other electrical generators come close. Coal-fired plants operate at 71 percent capacity; wind turbines operate at 31 percent capacity, and solar panels generate power only 21 percent of the time.

Owners also are extracting more life from their reactors by extending their 40-year operating licenses. The Nuclear Regulatory Commission has granted 20-year extensions to more than half the nation's commercial reactors - on Monday, PPL Corp.'s two Susquehanna units in Luzerne County became the latest to get extensions. Exelon plans to apply for Limerick extensions in 2011.

Uprates, which must be approved by the NRC, also have become the norm.

Thanks to uprates, the nation's nuclear-generating capacity is about the same now as it was 12 years ago, even though five reactors have been decommissioned since 1997.

Last year, PPL completed major upgrades at the Susquehanna station, and Public Service Enterprise Group boosted output 15 percent at its Hope Creek unit in Salem County, N.J. Those facilities have the same type of General Electric Co. reactors as Limerick.

Limerick can provide about two million homes with power, and supplies about a third of the power consumed in Peco Energy Co.'s service area on a typical summer day. Exelon says renewable sources such as wind, solar, and hydroelectric dams cannot supply that kind of load because they operate intermittently.

"We're going to add 170 megawatts per unit," said Chris Mudrick, Exelon's site vice president at Limerick. "What's a windmill give you? Maybe three megawatts maximum, and they're only running a third of the time. You'd need wind farms half the size of Montgomery County to produce as much power as we do."

For all the effort the industry has made to operate efficiently and safely in recent decades, nuclear advocates complain that they still receive little respect from lawmakers, even though reactors account for 70 percent of the nation's electricity that does not result in greenhouse gases.

Despite the unresolved issue of nuclear-waste disposal, the industry has campaigned relentlessly to recast itself as green. Its lobbyists are seeking special treatment in federal climate-control legislation.

And on a state level, operators have pushed for a friendly amendment to Pennsylvania House Bill 80, which aims to reduce emissions of greenhouse gases by requiring 18 percent of power to come from renewable and alternative-energy sources by 2024. The nuclear industry wants uprates to count as new emission-free generation.

Whatever lawmakers decide, Exelon plans to go ahead and overhaul the Limerick and Peach Bottom power stations because it makes economic sense.

Uprates cost about $2,400 per kilowatt, which would put the Limerick upgrades in the neighborhood of $800 million. Uprates are competitive with building a new fossil-fuel or renewable plant because they require few increases to operating costs - no additional staffing or maintenance costs, just more fuel.

The initial uprate planned for Limerick is a "measurement uncertainty upgrade," which involves installing equipment to more precisely measure the water flowing into the reactor, and therefore the quantity of steam produced, Exelon says. With more accurate metering, the plant can operate closer to its legal capacity, yielding an improvement of up to 2 percent. Limerick plans to apply to the NRC for that uprate in February.

More complicated "extended power" uprates involve substantial increases in output - changes in fuel configuration and the amount of water boiled to make steam. The effects on the reactor-safety systems must be thoroughly examined, said Pardee, Exelon's nuclear officer.

The extended upgrades also require hardware replacements on the nonnuclear side of the plant, which converts the reactor steam into electrical energy. Limerick would need new turbines, generators, and transformers to handle the greater electrical loads.

The uprates will take years to accomplish and are further complicated because Exelon will try to do the job incrementally - during the plant's annual refueling outages, when one unit is shut down for a few weeks to replace part of its uranium fuel.

The work must be precisely coordinated ahead of time to minimize the reactor's idle time.

Pardee said safety would not be compromised for commercial interests.

"What we have learned over the years, if we focus on running the units absolutely the best we can, the reliability follows, and as reliability improves, the machines become more valuable to us," he said.

"It turns out that a well-run reactor is also a profitable reactor."