But some say the shortened timetables are unrealistic, given safety and other concerns and the need to test new designs before seeking approval from the commission.

“It’s a 25-year process, no matter what,” said Michael McGough, the chief commercial officer of NuScale Power, which is the furthest along among companies working on less conventional reactors. NuScale’s design, called a small modular reactor, uses water as a coolant, but the units are far smaller than current reactors and have advanced safety features. They could be built largely in a factory, saving money, and up to 12 of them could be installed at one site.

Mr. McGough knows all about long timetables; NuScale’s design has been under development since 2000. It has lined up a potential first customer, Utah Associated Municipal Power Systems, or Uamps, which operates in the Intermountain West, and hopes to have 12 of the small reactors operating at a site in Idaho by the mid-2020s. “It’s improbable to me that you will see any new design developed or licensed in a much shorter time frame than the kind we’re on,” Mr. McGough said.

NuScale has been testing its design for 13 years, using a nonnuclear prototype. Later this year, it plans to submit an 11,000-page application to the N.R.C. to have its design certified. The commission then has up to 40 months to review the application.

The certification process, and a later application by Uamps for a construction and operating license, could be delayed if the N.R.C. asks for more information. But even if all goes smoothly, the plant will produce only about half the electricity of many existing reactors. About 50 of these 12-reactor plants would be needed to replace the generating capacity that could be lost by 2035.

Many in the industry hope that extending the licenses of existing reactors will forestall at least some closings. Nuclear plants were originally licensed for 40 years, but almost all have sought and received 20-year extensions.