B Reactor, built in just 11 months from 1943 to 1944, was the world’s first full-scale nuclear reactor, designed to produce plutonium as part of the Manhattan Project. Plutonium from B Reactor and its two sisters, D and F Reactors, were used in the Trinity test bomb and the Nagasaki bomb.

The tour season runs from April through mid-November and currently is free of charge. There are no age or citizenship restrictions. In the future the site will be run by the National Park Service and there may be fees associated with visiting, but for now the tours are free. A 45-minute bus ride to and from the reactor is required (no private vehicles allowed at this time).  Docents are on the bus and at the reactor to provide information.