DENVER (AP) — Cyclists and hikers explored a newly opened wildlife refuge at the site of a former nuclear weapons plant in Colorado on Saturday, while a protester in a gas mask brought signs warning about the dangers of plutonium.

With no fanfare, the U.S. Fish and Wildlife Service opened the gates of Rocky Flats National Wildlife Refuge on the perimeter of a government factory that made plutonium triggers for nuclear bombs for nearly four decades.

Spread across a rolling, wind-swept plateau 16 miles (26 kilometers) northwest of downtown Denver, the refuge is a rare oasis of tallgrass prairie,...