Last week, a nuclear power plant in Tennessee went critical.

Don’t be alarmed. Achieving criticality is a necessary step for a nuclear reactor to power up and prepare to supply electricity to the grid. The Tennessee Valley Authority’s Watts Bar Unit 2, a 1,150-megawatt behemoth, will soon be producing as much electricity as a couple large coal-fired plants.

This marks a major milestone for the TVA, which is trying to reduce its dependence on coal and move to a lower-carbon future. And it marks a major milestone for the U.S. nuclear power industry. This is the first new plant to go live in America this millennium. With four other plants under construction, and the pressure to produce emissions-free electricity on the rise, Watts Bar could represent the first stirrings of a nuclear renaissance in America. Or, looked at another way, it could represent a rare rearguard victory for a beleaguered industry.

Nuclear has long been the overlooked workhorse of America’s electricity stable. In the U.S., you can produce energy in a carbon-intensive manner, by burning coal, natural gas, or wood; or you can produce energy in a carbon-free manner by harnessing wind, sun, or water—or by smashing atoms. In recent years, America’s electricity mix has rapidly shifted—much less coal, much more natural gas, wind, and solar. Amid this transition, nuclear has quietly done its part. Despite the potential for negative externalities—radioactive waste, the possibility of leaks—nuclear offers a tested means of generating emissions-free energy. In a typical year, America’s 99 nuclear reactors provide about 18 percent of the electricity used in the U.S; in the 12 months that ended in February, they accounted for nearly 20 percent of the total.

Nuclear has certain advantages over wind and solar. It works reliably around the clock and doesn’t take up as much space. A single plant has 1,100 megawatts in capacity—four to five times the size of a typical wind farm, and about 10 times the size of a very large solar farm. Nuclear plants account for 63 percent of America’s emissions-free electricity. If we’re really serious about moving beyond fossil fuels, we’ll likely need more of them.

But construction of new plants has been dead in the water since the 1990s. After the Three Mile Island debacle of 1979, development ground to a halt. Compared with other means of generation, nuclear is expensive to construct, takes a long time to plan, permit, and build, and arouses lots of opposition. The result: Two decades passed without a single nuclear plant coming online.

That’s changing. Watts Bar Unit 2 marks the beginning of a small wave of nuclear construction. In Georgia, a consortium of power companies, backed by some $8.3 billion in federal loan guarantees, is constructing Vogtle Unit 3 and Vogtle Unit 4, the first U.S. uses of a new Westinghouse reactor design. In South Carolina, construction crews are busy building two plants, V.C. Summer Unit 2 and V.C. Summer Unit 3. All are expected to come online within the next four years. These five plants together will add about 6,000 megawatts of new capacity, boosting the U.S. total by about 6 percent.

And there’s potential for more. In the past year, the government has granted licenses for developers to construct nuclear power plants in Michigan and in Texas. The Nuclear Regulatory Commission is reviewing applications for five more plants.

But that doesn’t mean the nuclear industry will be expanding. Demand for electricity isn’t growing that much in the U.S. thanks to efficiency gains and the continuing decline of the economy’s energy intensity. The fracking revolution has liberated huge supplies of natural gas, which is cheap and difficult to export. That gives power producers a huge economic incentive to shut down coal plants and construct plants that burn natural gas. For their part, the solar and wind industries are getting a significant boost from federal tax credits and from the desire of corporations to burnish their green credibility. (You won’t see a consumer-facing brand touting an investment in nuclear power as part of its sustainability efforts.) And it simply takes longer—up to 10 years—to plan and build a nuclear plant than it does to build other types of generating facilities.

In fact, even with this small wave of expansion, the U.S. nuclear industry is shrinking. In a best-case scenario, the plants under construction will replace capacity being lost. Most U.S. nuclear plants are several decades old. It costs a lot to maintain and repair them, and many politicians and landowners are only too eager to see them go. In the past three years, companies have either shut down or announced their intent to close eight reactors with 6,300 megawatts of generating capacity, according to the Nuclear Energy Institute. The Kewaunee plant in Wisconsin was retired in 2013. In 2014, the venerable Vermont Yankee plant shuttered. Operators in California (San Onofre) and Florida decided not to restart nuclear plants because they couldn’t justify the cost of required repairs. In December, Entergy said it would mothball a nuclear plant in upstate New York, and the plant at Oyster Creek, New Jersey, was set on a path for closure in 2019. In April, the owners of the Pilgrim nuclear plant in Massachusetts said it would shutter in 2019. Last week, the CEO of the Omaha Public Power District recommended that the utility shut down the Fort Calhoun Nuclear Station. And in Illinois, Exelon is threatening to close three reactors at two power plants.

It turns out that the best the U.S. nuclear industry can hope for is precisely what every manager of a nuclear power plant hopes for: that the existing core remains stable.