From Wikipedia:
Broken Arrow
Pinnacle - Broken Arrow refers to an accidental event that involves nuclear weapons, warheads or components, but which does not create the risk of nuclear war. These include:
Accidental or unexplained nuclear detonation.
Non-nuclear detonation or burning of a nuclear weapon.
Radioactive contamination.
Loss in transit of nuclear asset with or without its carrying vehicle.
Jettisoning of a nuclear weapon or nuclear component.
Public hazard, actual or implied.
Examples of Broken Arrow events are:
1950 British Columbia B-36 crash
1956 B-47 disappearance
1958 Mars Bluff, South Carolina, when an unloaded Mark 6 bomb was accidentally dropped from a US Air Force B-47 Stratojet (the bomb's fissile core was stored in a containment area on the plane, but the bomb still contained 7600 pounds of chemical explosive trigger).
1958 Tybee Island mid-air collision
1961 Yuba City B-52 crash
1965. LTJG Douglas M. Webster, a United States Navy aviator, was the sole victim of a 1965 Broken Arrow in the Pacific Ocean that went unacknowledged by the Pentagon until 1981. His A-4 Skyhawk was lost over the side of the USS Ticonderoga (CVA-14) on 5 December 1965 while the attack jet, armed with a B43 nuclear bomb, was being rolled from a hangar bay onto an elevator during a training exercise off the coast of Japan. Webster, the A-4E Skyhawk, BuNo 151022, of Attack Squadron VA-56, and the nuclear weapon were lost when the jet rolled off an elevator,[4] of the aircraft carrier in 16,000 feet of water in the Pacific Ocean, 80 miles from Okinawa.[5][6] The Skyhawk was being rolled from the number 2 hangar bay to the number 2 elevator when it was lost.[7] Airframe, pilot, and the bomb were never found.[8] No public mention was made of the incident at the time and it would not come to light until a 1981 Pentagon report revealed that a one-megaton bomb had been lost.[9] Japan then asked for details of the incident.[10]
1966 Palomares B-52 crash[11]
1968 Thule Air Base B-52 crash