What is the smallest nuclear weapon possible?

I’d guess there’s a limit to how small you can make an A bomb due to the high explosive used as the primary detonator. You need a certain amount of bang to start the fission reaction so even if the actual nuclear explosion wasn’t that big there’d still be a significant conventional component.

For comparison a hydrogen bomb uses an atomic bomb as the primary detonator.

A-maze-ing

I would think the ones used in Japan were definitely two of the smallest. The ones now are exponentially more powerful. I believe Russia currently has the largest unclassified warhead…

There was the US M-28/29 which was a man portable rocket with a .01 kilo ton(ish) yield.

For comparison
T = equivalent of 1 ton of TNT:

M-28/29…...................................10 T
Fat Man (Hiroshima bomb)...............21,000 T
W78 (current US ICBM warhead).........350,000 T
Tsar bomba (largest bomb tested)...57,000,000 T

It’s limited by the size of critical mass unless you are including dirty bombs too. In that case they can be firecracker size.
Critical mass depending on the fissionable material is a sphere with diameter in the range of 10–20 cm so a mini bomb could eadily be made to fit in a suitcase or small box.

Probably worth mentioning that for Tsar Bomba 57 megatons was the test version. The live version that was pointed at the US was thought to be about 100megatons as it had more uranium in it.

Since it is pretty well accepted that the Russians executed / assassinated a former defector in England several years ago with a lethal dose of polonium, it would appear that “the smallest nuclear weapon possible” (if we’re talking human kills) is “a single dose, delivered personally”.

Presumably, even smaller doses could be administered if the intent was “only” to cause suffering without killing, but that’s not the normal function for “weapons”. At that point it would be more of a “torture device”.

Just for fun…the most recent Jack Reacher novel NIGHT SCHOOL uses the Davy Crockett as one of the major plot devices.

I would hazard a guess that somewhere, somehow, there was something smaller than the Davy Crockett. It just never saw the light of day. There are, supposedly, instructions for using nuclear medical waste for dirty bombs in the Dark Web…that could be hokum. If the stories have substance, then something along those lines would certainly be classed as “atomic” bombs in the sense that such a device would render an area unusable for a time…