Chicago Pile-3 (CP-3) was the world's first heavy water reactor. One of the first research reactors, it was constructed in 1943 at Site A, a research facility around ten miles from the University of Chicago campus in the city of Chicago. Joining CP-1/CP-2, it first went critical on 15 May 1944, and was at first used in the experimental physics work of the Metallurgical Laboratory for the Manhattan Project. After a rebuilding in 1950, its useful research-life ended when it was deactivated in 1954.