ST. LOUIS —From oil fields to wind turbines to coal mines, size and scale rule the economics of energy.

But the nuclear industry is thinking small these days.

The latest evidence came in April, when Ameren Corp. and Westinghouse Electric Co. announced plans to pursue a $452 million federal subsidy to advance development of small modular reactors that could be built alongside the utility’s much larger Callaway nuclear plant near Fulton, Mo.

While some utilities are still pursuing full-scale plants, there is a parallel push for smaller reactors that could be easier for utilities to finance and minimize sticker shock for regulators and consumers. But despite a lower total cost, there’s no evidence yet that tiny fission factories would be able to produce electricity at a competitive cost in an era of abundant, cheap natural gas.

The so-called small nuclear reactors promise the same benefits as larger ones: namely, an option for around-the-clock, low-carbon electric generation that could be a key in replacing aging coal plants.

For utilities considering nuclear technology, the smaller size means a smaller price. Even using the most generous cost estimates, a new nuclear plant the size of Ameren Missouri’s existing Callaway plant could rival or exceed the $7.5 billion market value of the utility’s entire parent company.

But the differences go beyond size. For one, the small reactors envisioned would be modular, able to be manufactured at a central factory, shipped by rail, ships or truck and assembled on site. That means a potentially larger market for vendors like Westinghouse.

“This (small) plant will appeal to a very broad market,” Kate Jackson, a Westinghouse senior vice president and chief technology officer, said recently.

The pursuit of small reactors represents a new path to the oft-referenced nuclear renaissance.

It was only a few years ago that the industry focused strategy on certification of a few large reactor designs that would, in theory, eliminate the risk and uncertainty, cost overruns and construction delays that tainted the last nuclear plant boom.

While new reactors are going forward in South Carolina and Georgia, a full-tilt nuclear revival hit a wall for several reasons. Among them: the inability of utilities to finance projects that cost multiple billions of dollars.

In fact, more than half of the new reactors for which construction and operating licenses were sought have been deferred or canceled, including Ameren Missouri’s proposed 1,600-megawatt Callaway 2 plant.

Andrew Klein, a nuclear engineering professor at Oregon State University, sees small reactors as part of a new strategy that could help utilities get over the hump by adding new capacity in small bites.

President Barack Obama’s administration, which is pushing for development of low-carbon energy technologies, sees potential, too. And the president wants the United States to take the lead in developing the industry.

In March, Obama proposed $452 million to help speed up development of small modular reactors. The federal funding has yet to be appropriated by Congress.

Westinghouse says it believes it has an advantage because the 225-megawatt reactor it’s developing is an offshoot of the company’s full-size reactor that has already been certified by the NRC.

At least two other groups have indicated they will seek a share of the federal grant. Both are eyeing the Department of Energy-owned Savannah River site in South Carolina, and are being backed by NuHub, an economic development initiative in South Carolina.

Holtec International Inc. last week said it intends to seek a share of the federal funding to speed up development of its 160-megawatt small modular reactor. Two weeks before that, NuScale Power, based in Corvallis, Ore., announced plans to seek funding to accelerate development of its 45-megawatt nuclear modules at Savannah River site.