The codename of the first Soviet atomic bomb test in 1949 was RDS-1.
The device exploded on August 29, 1949, at the Semipalatinsk Test Site in the Kazakh Soviet Socialist Republic. Its yield was approximately 22 kilotons of TNT, making it the Soviet Union’s first successful nuclear test and ending the United States’ atomic monopoly.
RDS-1 was developed under the direction of scientists including Igor Kurchatov and Yulii Khariton, with political oversight from Lavrentiy Beria. The design closely resembled the American Fat Man bomb, partly because Soviet intelligence had obtained information about the Manhattan Project. That resemblance does not make Fat Man the answer: Fat Man was the U.S. weapon used at Nagasaki in 1945.
The United States later assigned the test the nickname “Joe-1,” referring to Joseph Stalin. RDS-1 is the Soviet designation and the standard name for the test in historical references. The blast was detected by U.S. aircraft collecting radioactive debris, and President Harry Truman announced the discovery on September 23, 1949.