In New Mexico, fire and smoke has shut down the Los Alamos National Lab, and in Nebraska, floodwaters are closing in on the Fort Calhoun nuclear power plant.
The Las Conchas fire in Arizona has shut down the Los Alamos National Laboratory.
Floodwaters are threatening a nuclear power plant in Nebraska, after a protective barrier collapsed.
All hazardous and radioactive materials remain accounted for.
The U.S. government's Los Alamos National Laboratory was closed Monday due to the threat from approaching wildfires, officials said, stressing that all nuclear and other materials were safe.
An emergency operations unit remained active at the laboratory in New Mexico, while authorities were watching closely for a change in wind direction for the Las Conchas fire, nearing Los Alamos from the southwest.
"It's been a very long night for the fire crews," said Los Alamos National Laboratory (LANL) director Charles McMillan.
The main problem was smoke. "Winds generally from the northwest overnight have helped keep the fire from entering lab property, but forecasts call for a change by mid-day," the lab said in a statement.
"All hazardous and radioactive materials remain accounted for and are appropriately protected, as are key lab facilities such as its proton accelerator and supercomputing centers," it added.
Separately, U.S. nuclear authorities were watching floodwaters threatening a nuclear power plant in Nebraska, after a protective barrier collapsed.
The 2,000-foot long barrier, holding back flood waters from the Missouri river, collapsed early Sunday, threatening the Fort Calhoun nuclear power plant.
"This allowed floodwaters to surround the auxiliary and containment buildings, which are protected by design to a floodwater level of 1014 mean sea level," said the U.S. Nuclear Regulatory Commission.
The Missouri River was at 1,006.3 feet (306.7 meters), but river levels were not expected to exceed 1,008 feet, it added in a statement.
The collapse "also allowed floodwaters to surround the main electrical transformers," it said, adding that operators transferred power from off site sources "as a precautionary measure."
The plant has been shut down since April 7 for refueling, it said.