+1 to all of this, and thanks for the other excellent comments.
There were, however, several accidents where the conventional explosives (that would trigger a nuclear detonation in intended use cases) in a nuclear weapon detonated (but where safety features prevented a nuclear detonation)
It’s probably worse than that—there is at least one incident where critical safety features failed, and it was luck that prevented a nuclear explosion
From a declassified report on a 1961 incident, in which a bomber carrying two 4MT warheads broke up over North Carolina [1]:
Weapon 1, which landed essentially intact, was in the “safe” position when it dropped, preventing detonation. The T-249 Arm/Safe switch worked exactly as it was supposed to, preventing a nuclear explosion.
...
[Weapon 2] landed in a free-fall. Without the parachute operating, the timer did not initiate the bomb’s high voltage battery (“trajectory arming”), a step in the arming sequence. While the Arm/Safe switch was in the “safe” position, it had become virtually armed because the impact of the crash had rotated the indicator drum to the “armed” position. But the shock also damaged the switch contacts, which had to be intact for the weapon to detonate. While Weapon 2 was not close to detonation, the fact that the physical impact of a crash could activate the same arming mechanism that had kept Weapon 1 safe showed the danger of such accidents.
In other words—the critical safety mechanism that prevented one bomb from detonating failed on the other bomb (and detonation of this bomb was avoided due to contingent features of the crash).
Thanks for mentioning this. I had meant to refer to this accident, but after spending 2 more minutes looking into got the impression that there is less consensus on what happened than I thought.
However, Michael H. Maggelet and James C. Oskins, authors of Broken Arrow: The Declassified History of U.S. Nuclear Weapons Accidents, dispute this claim, citing a declassified report. They point out that the arm-ready switch was in the safe position, the high-voltage battery was not activated (which would preclude the charging of the firing circuit and neutron generator necessary for detonation), and the Rotary Safing Switch was destroyed, preventing energisation of the X-Unit (which controlled the firing capacitors). The tritium reservoir used for fusion boosting was also full and had not been injected into the weapon primary. This would have resulted in a significantly reduced primary yield and would not have ignited the weapon’s fusion secondary stage.
Bomb 2, the object of the Goldsboro controversy, was not “one step” away from detonation. [...]
In Bomb 2, the High Voltage Thermal Battery was not activated, so no electrical power could reach any components necessary to fire the weapon and produce a nuclear explosion. [...]
While the Ready/Safe Switch in Bomb 2 showed “armed” after recovery, it was actually safe [...]. Most importantly, the high voltage necessary to fire bomb components was not present for bomb 2.
[...]
How close was the Goldsboro bomb to producing a nuclear explosion? Not at all.
I didn’t attempt to understand the specific technical claims (not even if there is a dispute about technical facts, or just a different interpretation of how to describe the same facts in terms of how far away the bombs was from detonating), and so can’t form my own view.
Do you have any sense what source to trust here?
In any case, my understanding is that nuclear weapons usually had many safety features, and that it’s definitely true that one or a few of them failed in several instances.
+1 to all of this, and thanks for the other excellent comments.
It’s probably worse than that—there is at least one incident where critical safety features failed, and it was luck that prevented a nuclear explosion
From a declassified report on a 1961 incident, in which a bomber carrying two 4MT warheads broke up over North Carolina [1]:
In other words—the critical safety mechanism that prevented one bomb from detonating failed on the other bomb (and detonation of this bomb was avoided due to contingent features of the crash).
[1] https://nsarchive2.gwu.edu/nukevault/ebb475/docs/doc%205%20AEC%20report%20Goldsboro%20accident.pdf
[2] More info on the incident: https://nsarchive2.gwu.edu/nukevault/ebb475/
Thanks for mentioning this. I had meant to refer to this accident, but after spending 2 more minutes looking into got the impression that there is less consensus on what happened than I thought.
Specifically, the Wikipedia article says:
One of the Wikipedia references is a blog post by one of the authors mentioned above, with the title Goldsboro- 19 Steps Away from Detonation. Some quotes:
I didn’t attempt to understand the specific technical claims (not even if there is a dispute about technical facts, or just a different interpretation of how to describe the same facts in terms of how far away the bombs was from detonating), and so can’t form my own view.
Do you have any sense what source to trust here?
In any case, my understanding is that nuclear weapons usually had many safety features, and that it’s definitely true that one or a few of them failed in several instances.