Record-setting heat and drought gripping the western United States are exposing a potentially severe risk to the nation’s long-term power supply, and experts warn that grid operators lack sufficient tools to plan and carry out a defense.

A future of worsening water scarcity in heat-blistered parts of the United States could imperil fossil fuel power plants and nuclear reactors that depend on enormous quantities of fresh water in their operations, according to a report by a group of analysts from the Energy Department’s National Renewable Energy Laboratory and other researchers.