The reactor was the world's first industrial-scale nuclear reactor. Located at the Hanford site in south-central Washington state, the reactor was, among other things, used to produce plutonium for the atomic bomb that was dropped on Nagasaki, Japan, to end World War II.

The designation was announced by U.S. Department of Interior Deputy Secretary Lynn Scarlett and U.S. Department of Energy Acting Deputy Secretary Jeffrey Kupfer.

Kupfer said the Energy Department plans to expand a public access program that will enable more citizens to visit the reactor during the 2009 tourist season.

"B Reactor has a special feeling and association -- as a landmark should, for its role in the events that ended World War II, the B Reactor holds a powerful historic significance," said Scarlet. "Scientists, engineers and skilled workers showed the power of human ingenuity and enterprise in serving at this significant point in U.S. history."

The B Reactor went into operation in 1944 and was shut down in 1968.