"Broken Arrow" is the military term for an accidental event involving a nuclear weapon, but which does not create the risk for a nuclear war. The 'accidental event' typically involves losing and/or damaging the weapon.
There have been 32 officially recognized "Broken Arrow" events in the United States since the dawn of nuclear weapons, as of Sept 2013. Some of these events have been relatively minor, but others have been massive radiological accidents. One accident with a large Hydrogen Bomb nearly made North Carolina uninhabitable... not that you would have seen THAT in the newspapers when it happened. Apparently the news media were just as compliant in the 1950's as
they are today.
I will go over a few of the more interesting Broken Arrow events. Not surprisingly, most of these involve military aircraft crashing with weapons on board, as well as the intentional or inadvertent jettisoning of nuclear weapons. All of these event descriptions are courtesy of Wikipedia.