Nuclear weapons tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the 20th century, most nations that have developed nuclear weapons have tested them. Testing nuclear weapons can yield information about how the weapons work, as well as how the weapons behave under various conditions and how structures behave when subjected to nuclear explosions. Additionally, nuclear testing has often been used as an indicator of scientific and military strength, and many tests have been overtly political in their intention; most nuclear weapons states publicly declared their nuclear status by means of a nuclear test.
The first nuclear weapon was detonated as a test by the United States at the Trinity site on July 16, 1945, with a yield approximately equivalent to 20 kilotons. The first hydrogen bomb, codenamed „Mike”, was tested at the Enewetak atoll in the Marshall Islands on November 1 (local date) in 1952, also by the United States. The largest nuclear weapon ever tested was the „Tsar Bomba” of the Soviet Union at Novaya Zemlya on October 30, 1961, with an estimated yield of around 50 megatons.
In 1963, many (but not all) nuclear and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground nuclear testing. France continued atmospheric testing until 1974, China continued up until 1980. Neither has ever signed the treaty.
Underground tests in the United States continued until 1992 (its last nuclear testing), the Soviet Union in 1990, the United Kingdom in 1991, and both China and France in 1996. After signing the Comprehensive Test Ban Treaty in 1996 (which has as of 2012 not yet entered into force), all of these states have pledged to discontinue all nuclear testing. Non-signatories India and Pakistan last tested nuclear weapons in 1998.
The most recent nuclear test was announced by North Korea on May 25, 2009.