The concrete cooling towers that rise from a sliver of land south of Pennsylvania’s capital became symbols nearly a half-century ago of the risks of nuclear energy.

Now, a plan to restart one of the two reactors at Three Mile Island is at the leading edge of efforts to greatly expand the country’s reliance on atomic fission to meet the growing power demands of homes, businesses and data centers.

Refurbishing this aging plant — a fenced-off maze of pipes, valves, pumps and turbines in the middle of the Susquehanna River — depends on the financial backing of Microsoft. The technology giant has agreed to buy all of the electricity the plant generates, most likely starting in 2028, for 20 years.

Three Mile Island’s proposed revival reflects how vastly the perspectives on nuclear power in the United States have shifted since a cooling failure led to the partial meltdown of one of the island’s reactors in 1979.