According to reports 5 of the 6 safeties tripped on the one.....and these were thermonuclear hydrogen bombs ( so almost 4 megatons) not atomic bombs....
"Two Mark 39 nuclear bombs were carried by a
B-52 Stratofortress that broke up in the air and
crashed near Goldsboro, North Carolina on January 24, 1961. According to Parker F. Jones, a supervisor of nuclear safety at
Sandia National Laboratories, in a 1969 report that was declassified in 2013, the Mark 39 bomb had four safety mechanisms, one of which was not effective in the air.
On one of the bombs involved, two more safety mechanisms were "rendered ineffective by aircraft breakup." As a result, that bomb nearly detonated, and a catastrophic nuclear burst was prevented only by the fourth mechanism, a simple "ready-safe" electric switch"