Climate change—particularly intense heat—is advancing so rapidly that it poses physical as well as credit risks to America’s aging nuclear fleet, a new report from Moody’s Investors Service finds.

“Our plants are fairly hardened to severe weather,” said David Kamran, a projects and infrastructure analyst at Moody’s and the lead author of the report. “But climate change is moving quickly.”

Nuclear plants are cooled by water, and in times of intense heat and drought, water resources can become either too warm or too scarce, forcing shutdowns. This has already happened, and not just in the South: in 2012, Dominion Energy Inc.’s Millstone nuclear plant in Waterford, Conn. The report predicts that nuclear plants in the Rocky Mountain states, the Colorado River region, and California face the highest levels of water stress risk going forward.

Kamran said in an interview that this report wasn’t about estimating the risk of a meltdown. Currently, he said, most U.S. plants are sufficiently resilient not to face a catastrophe, even in the event of exceptionally severe weather.

Instead the report was meant to highlight the extent of the environmental pressures plants will have to adapt to withstand if they want to operate consistently in the coming decades. Resisting those stresses is potentially expensive— even more expensive than the plants have currently estimated, he said: “In certain cases they will need to make investments to further reinforce their plants and they need to have money in their cap-ex funds to do that.”