Many believe that these extended life spans -- longer than almost any coal-fired plant and rivaling hydroelectric projects like the Hoover Dam -- are a necessity at a time when the vast majority of the United States' energy is generated from carbon-intensive fossil fuels. Nuclear power supplies some 20 percent of the nation's electricity.
Without the recent extensions, the electricity market would have faced a severe shortage of supply that would have been "nothing short of catastrophic" and resulted in more coal- or gas-fired power plants being built and increased greenhouse gas emissions, Was said.
"Without relicensing, we go off a cliff five years from now," Was said.
While much debate in Washington focuses on the construction of new nuclear power plants, these plants will only replace the electricity produced by existing facilities unless further extensions are sought, according to Ronald Szilard, the technical director of DOE's Light-Water Reactor Sustainability Program at Idaho National Laboratory.
"The focus right now is very intense on building new nuclear power plants, because we have come to realization that [reducing] greenhouse gas emissions in the future cannot be achieved without pushing nuclear further," he said. "Both existing and new plants will have to contribute."
The original 40-year licensing period for power plants in the United States was never a question of the technical life of the plant, added John Gaertner, the lead technical manager on EPRI's long-term operations project.
"The engineers didn't throw up their hands and say, 'That's the lifetime I can get out of these things,'" he said. "It wasn't a technical reason."
Engineers have learned much about how to operate plants, reducing the time reactors remain offline during routine operations. And, while it varies on a case-by-case basis, the "assumption is generally correct that it is getting more cost-effective" to replace plant parts, said Scott Brooks, a spokesman for the Tennessee Valley Authority, which operates several nuclear sites.
"With 30 years of operating experience, we feel we can reassess a lot of the original assumptions" involved in running plants, Gaertner said.
It has been nearly 70 years since the world's first artificial fission reaction, created by Enrico Fermi and collaborators beneath abandoned athletic-field stands at the University of Chicago. In that time, scientists have come to understand much about how neutrons -- the uncharged particles that provide ballast to the atom's nucleus -- alter the composition of materials.
Neutrons are effectively the trigger for nuclear power. Each time uranium splits in a nuclear reactor, neutrons are shot out at high energies. These neutrons in turn cause more uranium splits, resulting in a self-sustaining reaction. But while causing these divides, the neutrons also relentlessly pummel the steel and other metals that enfold the nuclear reactor, known as the pressure vessel.
"From a physicist's standpoint, [neutrons] are like bowling balls," Gaertner said.
"There are millions of millions of millions of impacts per year. At some point, it begins to impact the reactor vessel," he added.
After some time, decades or more, the radiation causes changes to the microstructure of metals, Was said. The relentless bombardment creates minute flaws, such as dislocation loops or precipitates, that "tend to harden the material," Was said. "When it gets hard, the trade-off is ductability -- the ability to transform."