For the first time in almost 20 years, federal regulators have given a new nuclear power plant a license to begin generating electricity.

The Nuclear Regulatory Commission gave the go-ahead to the Tennessee Valley Authority on Thursday to load uranium fuel into the Watts Bar Unit 2 reactor in Spring City, Tenn., about 50 miles southwest of Knoxville. The government-run power agency can put the plant into commercial operation once it successfully completes a series of tests and inspections.

The regulatory move ends one of the lengthiest construction sagas in the history of the nuclear-power industry. The TVA began building the Watts Bar Unit 2 reactor in the early 1970s but broke off work in 1985 in part because of a construction scandal at the agency. It revived the project in 2007.

That tangled history helps explain why experts aren't predicting a big wave of new nuclear power projects, which today produce about 19% of the electricity in the U.S. Two other utilities in the Southeast— Southern Co. and Scana Corp. —are building four reactors in Georgia and South Carolina that are expected to enter service by the end of the decade.

But few utilities are likely to follow suit because of the steep investments nuclear plants require and the difficulty they are having competing with low-cost natural-gas plants. Several energy companies have announced plans to close down existing reactors, which are costly to operate. Just last week, Entergy Corp. said it would shut the aging Pilgrim plant in Massachusetts by mid-2019.