This is the historical X-10 Graphite Reactor at Oak Ridge National Laboratory in Oak Ridge, Tennessee. Weirdly enough, it now has a National Park Service sign—but why?

In fact, the U.S. Secretary of the Interior Sally Jewell and U.S. Secretary of Energy Ernest Moniz signed a memorandum of agreement on November 10th, establishing the Manhattan Project National Historical Park. As the official statement says:

The agreement directs how the National Park Service (NPS) and the Department of Energy will work together to preserve, protect, and provide access to the historic resources associated with the Manhattan Project at locations in Oak Ridge, Tennessee; Los Alamos, New Mexico; and the Hanford Site in Washington state. [...] The 2015 National Defense Authorization Act established the Manhattan Project National Historical Park, which tells the story of people, events, science and engineering that led to the creation of the atomic bomb, the role these weapons played in World War II and how the role of the United States in global affairs has evolved in the nuclear age.

The X-10 Graphite Reactor above (also known as the Clinton Pile or simply X-10 Pile), was the world’s second man-made nuclear reactor and was the first reactor designed and built for continuous operation in 1943. The X-10 pile was the first source of Plutonium-239, the primary fissile isotope used in nuclear weapons, thus paving the way to the first US atomic bombs.