Sometimes older is better.

Although Toshiba and others have promoted fast breeder reactors for the nuclear industry and still other companies have promoted things like small, modular nuclear reactors, the quickest, most efficient way to add nuclear capacity lay in concentrating on light water reactors, according to Ernie Moniz, the director of the MIT Energy Initiative during a visit to San Francisco this week.

"For the next decades, it is light water reactors," he said. "The key question for the U.S. is getting them built."

Why? "We know how to build them," he added. Hyman Rickover developed the first light water reactors in the 1950s and variants of light water reactors constitute the majority of reactors around the world. The "light" in light water reactors refers to the fact that plain water, versus oxygen combined with deuterium (the heavier isotope of hydrogen), serves as a coolant.

Fast breeder reactors were developed with the idea that uranium and nuclear fuel would be in short supply. "It turns out that that pathway was based on assumptions that aren't relevant," he added.

Fusion? Not in our lifetimes, he stated after eyeballing me. (I'm a youthful 48.) He then added that that meant not likely before 2050. The Energy Initiative is MIT's multimillion dollar push into developing technologies and companies around solar, energy storage and other fields. Hence, his opinion carries weight.

While there are no nuclear power plants under construction at the moment in the U.S. many believe that nuclear will play a larger role in the future. Energy secretary Steve Chu has come out in favor of expanding nuclear. Several European nations are also looking at nuclear. The U.S. has also changed the approval process (reactor designs can be certified by the manufacturer separately from the site approval).

"I don't see a sensible solution [toward reducing carbon emissions] without having nuclear as part of the mix," said Dan Kammen, the UC Berkeley professor who also runs the Renewable and Appropriate Energy Lab at the school. Nuclear power plants can gobble up billions in capital and take years to build, but they also can act as a very low-carbon source of energy for decades. The carbon comes from the energy expended to build them.

Nonetheless, many, including Rocky Mountain Institute's Amory Lovins, argue that nuclear, with its chronic cost overruns, delays and waste issues, isn't nearly as economical as advocates claim.