Soviet Breakthrough: "Joe I" Soviet Nuclear Test
The Soviet effort was led by Igor Kurchatov at a secret site known as Arzamas-16. Early efforts were greatly aided by spies inside the Manhattan Project, most notably by Klaus Fuchs. After the bombings of Hiroshima and Nagasaki, the program accelerated into high gear. The Soviets began construction of a near copy of the Fat Man bomb, using the detailed design descriptions provided by Fuchs. This replica, named Joe-1 by the West, was detonated at the Semipalatinsk Test Site in Kazakhstan on August 29, 1949. Its estimated yield was about 22 kilotons.
A few weeks later, a specially-equipped U.S. weather plane flying off the coast of Siberia collected radioactive debris suggesting a recent atomic explosion. President Truman was slow to accept the news. Although this event shocked most of the world, many scientists had pointed out that given sufficient time, any industrial nation could construct an atomic bomb.
The Soviets did not test another atomic bomb for more than two years.