Jeremy Bernstein in the New York Review of Books:
In 1944, when the atomic project was well underway, if I went to the bar in the La Fonda Hotel in Santa Fe I might have seen some physicists I would recognize. For example, Robert Serber, one of Oppenheimer’s closest associates, and his wife were sent to Santa Fe to plant the rumor that what was going on at Los Alamos had to do with submarines. I am not sure how much one would have learned by monitoring their conversations with other physicists, since they used a code when talking about the nuclear bomb project. Uranium-235 was called “25” and plutonium-239 was known as “49.” These were the fissionable isotopes. Niels Bohr was known as “Nicholas Baker” and Enrico Fermi was known as “Henry Farmer.” Los Alamos was known as “Project Y” and the plutonium bomb was known as the “gadget.”
By the fall of 1945, such deceptions would have been futile. All the details involved in making the plutonium bomb, the gadget, were known to the Russians. They were revealed basically by a single agent on the ground— Klaus Fuchs.
More here.