Since the 1970s, construction on new nuclear reactors in the United States has largely ground to a halt, thanks to public protests, regulatory obstacles and tight financing. Yet over that same period, U.S. utilities have managed to increase the amount of electricity they get from nuclear power. By quite a lot, in fact.

How is that possible? Through a process known as "uprating." According to a new analysis by the U.S. Energy Information Administration, the operators of 98 of the country's 104 commercial nuclear reactors have asked regulators for permission to boost capacity from their existing plants. All in all, the Nuclear Regulatory Commission has approved more than 6,500 megawatts worth of uprates since 1977. That's the equivalent of building six entirely new nuclear reactors—and during a period when fresh plants were impossible to build.

There are several ways to boost the capacity of a nuclear power plant. The simplest approach, according to the NRC, is to improve the measurement tools used to gauge the capacity of the reactor, through modern sensors and other digital technology. This option is the cheapest and allows the plant to operate at a slightly higher level. Likewise, "stretch uprates" allow nuclear plants to boost output by up to 7 percent, through replacing older components with newer designs and materials.

In recent years, however, nuclear operators have started applying for much larger "extended uprates," which can increase the output of a plant by as much as 20 percent. This process can include big changes to high-pressure turbines and other equipment. Or it can involve using more potent fuel in the reactor core. The first major expansion was approved in 1998, at the Monticello plant near Minneapolis. Since then, the EIA notes, 26 reactors have received permission from the government for extended uprates, which account for nearly half of the boost in nuclear-generated electricity since the 1970s.

Some nuclear watchdogs have warned that these bigger uprates also carry bigger risks. An investigation by the Los Angeles Times last year explained some of the pros and cons:

In an uprated reactor, more neutrons bombard the core, increasing stress on its steel shell. Core temperatures are higher, lengthening the time to cool it during a shutdown. Water and steam flow at higher pressures, increasing corrosion of pipes, valves and other parts.

"This trend is, in principle, detrimental to the stability characteristics of the reactor, inasmuch as it increases the probability of instability events and increases the severity of such events, if they were to occur," the Advisory Committee on Reactor Safeguards, which is mandated by Congress to advise the NRC, has warned.

Still, the committee has endorsed uprates, based on assurances that any increased risk falls within federal safety standards and is countered by additional safety measures such as plant modifications and more frequent inspections.

Utilities and regulators have used computer modeling to show that "a properly uprated reactor is no more vulnerable than one operating at its original capacity." That hasn't placated critics, however, who warn about unforeseen risks. Still, it's unlikely that uprating will disappear anytime soon, especially since nuclear power companies have few other options for expanding their capacity.

To put this in context, the EIA notes that regulators are now looking over applications for seven extended uprates and nine smaller uprates. If all of these get approved, that would give the country another 1,140 megawatts of nuclear capacity. That's roughly equivalent to one of the two new reactors that Southern Company is building at its Vogtle plant near Augusta, Ga.—a project that's currently at risk of getting entangled in cost overruns and disputes about payment. For the time being, squeezing more electricity out of existing reactors is the easier option for many power companies.