SPRING CITY, Tenn. — Tom Wallace started working at the Watts Bar nuclear plant as a young man in 1979, hoping he could eventually become a reactor operator.

Mr. Wallace, 55, is still awaiting the plant’s opening 36 years later, one of the longest building projects in the country’s history.

In the time it has taken to build it, Mr. Wallace raised two daughters and became a grandfather. Meanwhile, the nuclear industry designed a generation of entirely new plants that are now rising in Georgia and South Carolina.

If nothing else, the second reactor at the Tennessee River site has become a cautionary tale for the power industry. When it is finished, it will provide enough electricity to power about 650,000 homes in the Tennessee Valley. The cost of running a nuclear plant is relatively steady, and doing so does not produce greenhouse gases and other air pollutants. But the facilities are enormously expensive and complicated to build.