A “Broken Arrow” is defined as an unexpected event involving nuclear weapons that results in the accidental launching, firing, detonating, theft or loss of the weapon.
A US B52 bomber, carrying four hydrogen bombs collided with a KC-135 aerial tanker during a routine refuelling flight near Palomares, Spain. Three bombs fell to the ground, leading to two non-nuclear explosions on impact. Some radioactive material was released contaminating 1400 tons of soil. The fourth weapon was recovered from the sea.
Further materials:
https://www.atomicarchive.com/almanac/broken-arrows/index.html#1960
