WikipediaExtracts:RDS-1

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The RDS-1 (Russian: РДС-1), also known as Izdeliye 501 (Device 501) and First Lightning (Russian: Пе́рвая мо́лния, romanized: Pyérvaya mólniya, IPA: [ˈpʲervəjə ˈmolnʲɪjə]), was the first nuclear weapons test that was conducted by the Soviet Union. It was detonated on 29 August 1949 at 7:00 a.m. Kazakhstan Time (decree time) (UTC+06:00), at the Semipalatinsk Test Site in the Kazakh Soviet Socialist Republic, and yielded 22 kilotons of TNT.

It was roughly based on the American design, Fat Man, and the United States assigned it the code-name Joe-1, in reference to Joseph Stalin. RDS-1 also refers to the model of nuclear gravity bomb put into weapons production, with 29 bombs stockpiled by 1951.

The United States initially detected the test's nuclear fallout four days later via its anticipatory program for aerial sampling regions near the Soviet Union. On 23 September, US President Harry S. Truman publicly announced evidence that "an atomic explosion occurred in the U.S.S.R." The Soviet government initially denied the report, suggesting a misidentification of its excavation works for hydroelectric power, however it made cryptic statements before and after the test about its nuclear capability. In response the United States government ordered a crash program to develop the "Super" i.e. hydrogen bomb, accelerating the nuclear arms race.