Technology companies are racing to connect data centers directly to nuclear power plants to secure clean energy and power artificial intelligence, drawing resistance from some utilities over the potential impact on the electric grid.

Data centers, which house thousands of computers to run the internet, now require, in some cases, a gigawatt or more of power—on par with the average capacity of a U.S. nuclear reactor.

Joe Dominguez, chief executive of Constellation Energy, which operates the largest U.S. nuclear fleet, said data centers are essential to U.S. economic competitiveness and national security as the country competes with adversaries such as China for supremacy in the race to develop artificial intelligence.

“When you’re talking about a load this big, and you’re also talking about using emission-free energy, you’re going to locate very close to nuclear power plants,” Dominguez said on Constellation’s second-quarter earnings call.

Baltimore-based Constellation runs 21 of the 93 reactors in the United States. Its shares have surged 62% this year, making it the sixth-best performer in the S&P 500 index, as investors attach a premium to the company’s ability to generate nuclear power to meet growth in data centers.

Shares of Vistra Corp., based outside Dallas and the owner of six reactors, have doubled this year, making it the second-best performer in the S&P 500 behind chipmaker Nvidia.

The data center buildout by tech companies comes as the retirement of coal-fired power plants and rising demand from reshoring manufacturing and vehicle electrification tighten energy supplies.

PJM Interconnection, the largest U.S. grid operator, warned in late July that power supply and demand are tightening, with new generation trailing demand.

PJM covers 13 states, primarily in the U.S. midsection, including the world’s largest data center hub in Northern Virginia.