Accidents involving nuclear weapons systems could result from some kind of mechanical failure, or from the miscalculation or insubordinate behaviour of members of the military forces who operate the weapons delivery systems. Such accidents might start a nuclear war if one country detonated a bomb by accident on the territory of a nuclear power or a nuclear power's ally; it might also happen if it dropped a bomb on its own territory and another country was suspected. Accidental detonation, even if it did not start a nuclear war, could do great damage if it were detonated over a populated area. There is also the possibility of radioactive contamination from weapons which are damaged or destroyed but not detonated. A nuclear attack could be provoked in the time taken to detect and rectify the effects of the accidents.
The USA definition of nuclear weapon accident is any unexpected event involving nuclear weapons or nuclear components which results in any of the following: (a) accidental or unauthorized launching, firing or use, by USA forces or US-supported allied forces, of a nuclear-capable weapon system which could create the risk of outbreak of war; (b) nuclear detonation; (c) non-nuclear detonation/burning of a nuclear weapon; (d) radioactive contamination; (e) seizure, theft or loss of a nuclear weapon or nuclear component, including jettisoning; and (f) public hazard, actual or implied. A nuclear weapon incident is any unexpected event involving nuclear weapons or nuclear components which does not fall in the nuclear weapon accident category but which: (a) results in evident damage to a nuclear weapon or nuclear component to the extent that major rework, complete replacement or examination or recertification by the Energy Research and Development Administration (ERDA) is required; or (b) requires immediate action in the interest of safety or which may result in adverse public reaction (national or international) or premature release of information.
There have been an unspecified number of accidents when intercontinental ballistic missiles, presumably fitted with nuclear warheads, have been destroyed by fire or explosion. Anti-aircraft missiles have misfired or been accidentally launched on several occasions. The risk of accident per nuclear weapon system deployed may be diminishing owing to the shift to missiles and improved safety systems, but the number of weapon systems deployed is increasing rapidly. From 1945 through March 1968, there had been at least 32 'major' accidents: those involving the complete destruction of a nuclear weapon delivery system (aircraft, missile, ship and so on) containing a nuclear weapon, and with the destruction, loss or other involvement of the nuclear warhead itself. In addition, a semi-official study completed in 1960, carried out with some access to classified information, indicated that there had further been 'about 50 lesser accidents' involved in the maintenance, transport or modernization of USA nuclear weapons between 1945 and 1960.
A less definitive source reported that President Kennedy had been told subsequent to a 1961 investigation (following the Goldsboro, North Carolina, B-52 accident) that there had been 'more than 60' accidents involving USA nuclear weapons as of that date. As of mid-1976, the official USA figures include 27 major USA nuclear weapon accidents and 70 incidents - with no specific identification of its items in either category except for the 13 major accidents identified in 1968. There is one group of accidents in which nuclear weapons were believed to have been destroyed or seriously damaged. The 13 nuclear weapon accidents specifically identified by the Pentagon are included in this list and they are identified by source. The total number of accidents listed in this group is 32. Another group lists the incidents in which nuclear weapons were present or involved and in which they may have been placed in danger of destruction or serious damage. The total number of USA incidents in this group is 59. Yet another group lists a number of additional accidents or incidents which could fall into either category. In each of these events, nuclear weapons may have been present. Sufficient information is not available to confirm their presence or involvement. The total number of events listed in this group is 17. The final group lists a total of six Soviet nuclear weapon accidents, and 16 Soviet incidents, eight British nuclear weapon incidents, and four French incidents.
In a study of naval accidents from 1945, Greenpeace notes that the world's five nuclear navies have been involved in over 1200 accidents at sea and in ports. As a result, 50 nuclear weapons and 9 nuclear reactors lie abandoned on the seabed, presenting a serious long-term threat to the marine environment. The study documents 2,000 incidents including sinking, grounding, fire and collision but does not include incidents on or over land. In 1994, following an accident two a French nuclear submarine in which 10 people were killed, renewed fears were expressed concerning the safety of French submarines.