France became the fourth nation to possess nuclear weapons on February 13, 1960, when the atomic bomb "Gerboise Bleue" was detonated in Algeria, then still a French colony (formally a part of Metropolitan France). France began making plans for a nuclear-weapons program shortly after the Second World War, but the program did not actually begin until the late 1950s. Eight years later, France conducted its first thermonuclear test above Fangatuafa Atoll. It had a yield of 2.6 megatons. This bomb significantly contaminated the atoll with radiation for six years, making it off-limits to humans. During the Cold War, the French nuclear deterrent was centered around the Force de frappe, a nuclear triad consisting of Dassault Mirage IV bombers carrying such nuclear weapons as the AN-22 gravity bomb and the ASMP stand-off attack missile, Pluton and Hades ballistic missiles, and the "Redoutable"-class submarine armed with strategic nuclear missiles.