Today in 1958, a B-47 on ground alert at Chennault AFB, Louisiana, carrying a sealed-pit H-bomb containing no plutonium caught fire when the Jet-Assisted Take-Off (JATO) bottles accidentally discharged during the pilot’s acceptance check, pushing the plane into a towing vehicle. 
The aircraft and most of the bomb were destroyed in the fire, but the secondary remained Intact and the tritium reservoir was subsequently recovered. Contamination was reportedly limited to the weapon residue “slag” within the aircraft wreckage.
At the time of this accident, the B-47 bomber could be armed with B15, B28, B36, or B39 thermonuclear bombs. It could also carry the B18, the highest-yield US uranium fission bomb ever built (500 kilotons). Of these weapons, the B28 definitely utilized a sealed-pit design.
That is significant, because a design flaw in the B28 bomb meant that if exposed to prolonged heat, two wires too close to the casing could short circuit, arm the bomb, trigger an accidental detonation of the high explosives surrounding the core, and set off a nuclear explosion.
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