RICHLAND, Wash. -- The world's first full-scale nuclear reactor, built in 13 months to produce plutonium for an atomic bomb during World War II, is now a National Historic Landmark, the federal government announced Monday.

In this photo provided by the U.S. Department of Energy, the Hanford nuclear reservation's B Reactor is shown in this aerial file photo taken in the mid-1940s, near Richland, Wash. The federal government has designated the world's first full-scale nuclear reactor, built in 13 months to produce plutonium for an atomic bomb during World War II, a National Historic Landmark. [Agencies]

The Hanford nuclear reservation's B Reactor produced plutonium for the first man-made atomic blast, the Trinity test in New Mexico on July 16, 1945, and for the bomb that was dropped on Nagasaki, Japan, on Aug. 9, 1945. The designation doesn't guarantee that the reactor will never be torn down but very likely opens the door for more public tours and moves it closer to becoming a museum.

"Building the B Reactor was a feat of engineering genius. So, too, was the construction a testament to the excellence of working Americans," said Lynn Scarlett, deputy secretary of the Department of Interior. "There was no wiggle room for error."

History buffs, former weapons workers and local officials have been seeking recognition for the plant for six years to help save it from being dismantled or permanently cocooned as part of the cleanup of the highly contaminated complex in south-central Washington state.

Hanford and B Reactor were the centerpiece of the Manhattan Project, a top-secret effort to build an atomic bomb in the 1940s. More than 50,000 workers moved to the Tri-Cities of Richland, Kennewick and Pasco for the massive project on the banks of the Columbia River.

Construction began on June 7, 1943, six months after physicist Enrico Fermi turned the theory of nuclear power into the reality of the Atomic Age. Eight more reactors were built at Hanford to produce plutonium for nuclear weapons, leaving a legacy of pollution that has made Hanford the nation's most contaminated nuclear site, with cleanup costs expected to top $50 billion.

Five reactors have been dismantled and cocooned, a process in which buildings around the reactors are removed, all but the shield walls surrounding the reactor cores are leveled and the cores are sealed in concrete.