Nuclear Weapon Effects A nuclear detonation creates a severe environment including blast, thermal pulse, neutrons, x- and gamma-rays, radiation, electromagnetic pulse (EMP), and ionization of the upper atmosphere. Depending upon the environment in which the nuclear de-vice is detonated, blast effects are manifested as ground shock, water shock, “blueout,” cratering, and large amounts of dust and radioactive fallout. http://www.fas.org/nuke/intro/nuke/effects.htm
U.S. Nuclear Testing Program in the Marshall Islands During the period from June 30, 1946, to August 18, 1958, the United States conducted 67 nuclear tests in the Marshall Islands, all of which were considered atmospheric. The most powerful of those tests was the "Bravo" shot, a 15 megaton device detonated on March 1, 1954, at Bikini atoll. That test alone was equivalent to 1,000 Hiroshima bombs. http://www.nuclearclaimstribunal.com/testing.htm