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 dependenceon 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. Three Mile Island Nuclear Generating Station. An accident at Three Mile Island harmed the reputation of nuclear power in the US. Todd MacDonald

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. Demand for electricity in the US is fairly stable. Bruno Vincent/Getty Images