There's a reason nuclear plants are built close to water.

Harnessing the enormous power of nuclear fission, plants generate steam, which shoots through pipes to spin a turbine that generates massive amounts of electricity. To keep from getting dangerously hot, the plants suck up surrounding water from the nearby rivers, lakes or oceans around which they're built to cool the steam.

Now, increasingly, more frequent heat waves and hotter average temperatures are making those waters so warm that engineers are concerned that it can't do the job. Analysts say climate change is to blame.

In little-noticed but publicly available reports to regulators, nuclear plant owners revealed that unusually hot temperatures last year forced them to reduce the plants' electricity output more than 30 times – most often in the summer, when demand from nuclear plants is at its highest. In 2012, such incidents occurred at least 60 times. At one plant in Connecticut a reactor was taken offline for nearly two weeks when temperatures in the Long Island Sound surged past 75 degrees.

The incidents, submitted to the Nuclear Regulatory Commission, reflect a sharp uptick from even a decade ago, when plants appear to have submitted only nine such reports in 2009. In 1988, 1989 and 1991, there was just one such report. The dramatic increase mirrors the rise in average U.S. and global temperatures spurred by climate change.

"I've heard many nuclear proponents say that nuclear power is part of the solution to global warming," says David Lochbaum, a retired nuclear engineer who compiled the reports based on data submitted to the NRC, and former director of the Nuclear Safety Project at the Union for Concerned Scientists. "It needs to be reversed: You need to solve global warming for nuclear plants to survive."

Regulations set strict temperature limits for the water around each plant: 75 degrees for Millstone Generating Station in Connecticut, 85 degrees for Braidwood Generating Station outside Chicago, as high as 90 degrees for Turkey Point Generating Station south of Miami. Nuclear plants are now more regularly bumping up against those limits.

And even when water temperatures only approach those thresholds, plants can still be forced to dial down their output if the water used to cool their reactors will cause the temperatures in surrounding waterways to rise so much that it will endanger the habitats of fish and plants.

Limerick Generating Station outside Philadelphia, for example, reported turning down its output 79 times between 2008 and 2016.

Paul Adams, a spokesman for Exelon, which owns the nuclear plant and more than a dozen others, said in a statement that the company's nuclear plants are "more reliable than ever before," adding that last summer the company's 14 plants produced the maximum amount of power 96.7% of the time and generated a record amount of electricity for the year.

A handful of plants in recent years have received temporary waivers from regulators to continue operating with warmer water. The NRC, in a statement, emphasized that such plants, when they calculated their original heat limits, based the figures "on a number of pessimistic assumptions," allowing a wide margin to operate at higher temperatures.

"No matter what avenue a plant takes in attempting to deal with water discharge heat limits … the NRC will only allow such actions if the evidence presented shows the plant will continue to operate safely and that people and the environment are adequately protected," NRC spokesman Scott Burnell says.

Climbing temperatures are not the first climate impact to strike nuclear power plants: The sector has also faced challenges from periodic but increasingly frequent droughts that can cause local water sources to run low.

And it's not just water temperatures that plants have to contend with. Air temperatures can also cause conditions inside the plant to get too hot to operate. So desperate was a power plant in France during last year's heat wave that it began spraying water on the outside of the building to keep the interior from overheating. Plants in the U.S., meanwhile, have regularly slashed their output by anywhere from 3% to 60%.

Such dynamics could cause output from nuclear plants to fall by as much as 16% in the coming decades, according to a 2012 analysis. Moreover, climate concerns threaten to pierce nuclear's carefully constructed – and increasingly embattled – narrative that it's the only reliable source of zero-emissions power. Already, the country's aging fleet of nuclear plants is facing stiff competition from cheap natural gas and, more recently, falling prices for solar and wind paired with battery storage.

Rising temperatures, by forcing nuclear plants to reduce their output, have "made this already problematic resource even less viable in the current energy environment and economy, says Dan Kammen, a professor of nuclear engineering at the University of California-Berkeley.

Unlike other power sources such as gas, oil and coal, increasing and decreasing the output from a nuclear plant puts considerable strain on the facility – forcing earlier and more frequent maintenance expenses in a cash-strapped industry.

"Ramping plants is terrible for them, because nuclear plants don't respond well to the changes in stresses. It's a big deal to ramp up and down," Kammen says. "It's definitely not ideal for the long-term: The only way a nuclear plant ever makes money is if they operate for the long-term with low maintenance costs."

Matthew Wald, spokesman for the Nuclear Energy Institute, the industry's chief trade association, says dialing back the power output hasn't yet had a serious effect on the industry. While it "obviously lowers your revenue marginally" to not operate at full power, "I don't think the wear-and-tear factor is significant at the levels of variable operation that we're seeing," he says.

Experts generally agree that the new normal of rising temperatures, and even allowing plants to operate at hotter temperatures, does not pose an immediate safety risk, especially given the conservative margins built into plants' original calculations. Most plants in the U.S. were built between the 1970s and 1990s, meaning that modeling the impacts of hotter temperatures – and what plants can safely endure – has only improved.

However, the concern is not zero.

"For every 10 degrees that the temperature goes up, the lifetime of the electrical equipment is reduced quite a bit. Some of your safety equipment may then just fry," Lochbaum says.

