2 were internally initiated while Mods 3 to 6 were externally initiated. The initial Mod 0 warhead lacked an environmental sensing device until concerns about accidental or deliberate (sabotage) detonation were raised. Accidents and incidents 1966 Palomares B-52 crash - Four B28FI weapons with either Mod 2 or Mod 4 warheads with Y1 yield option. 1968 Thule Air Base B-52 crash RAF Greenham Common Survivors Four Mark 28 training variants (BDU-16/E) on their transporter (MHU-7/M) are on display in the Cold War Gallery at the National Museum of the United States Air Force in Dayton, Ohio. The Canadian War Museum, in Ottawa, holds a Mark 28RE training variant in its Cold War gallery. The Mark 28 armed CF-104 Starfighters in Germany, 1963–72, under the "Dual Key" protocol (both the US and Canada had to agree to use, with the weapons in US custody on Canadian bases). See also Red Snow (a British copy of the B28 warhead) B83 nuclear bomb List of nuclear weapons Python (nuclear primary) Command and Control References External links Complete List of All U.S. Nuclear Weapons, The Nuclear Weapon Archive Cold War aerial bombs of the United States Nuclear bombs of the United States Military equipment introduced in the 1950s