A fire Sunday afternoon on Rattlesnake Mountain burned an estimated 50 to 100 acres, said Rae Weil, spokeswoman for Department of Energy contractor Mission Support Alliance.
The cause has not been determined.
The Hanford Fire Department had the fire contained about 3:45 p.m. It was moving slowly, Weil said.
The fire was on the Arid Lands Ecology Reserve of the Hanford Reach National Monument.
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The relatively pristine grassland is used primarily for research and is closed to the public.