What is the smallest possible yield for a nuclear explosive? In other words, what’s the smallest possible mass required to become critical? Could you build a nuclear bomb with the explosive force of a grenade, for example?

I understand generally how a nuclear fission reaction occurs, and I’m guessing that the answer has to do mainly with the fissile material used; it takes a minimum X amount of plutonium to generate fission, etc. etc.

But does anybody know what the actual equation for this would be?

  1. Michael R. Bernstein

    I assume you mean a grenade-like yield in a smaller-than-grenade package, because it’s pretty easy to dial down the yield to a fizzle by doing various things subtly wrong.

    In theory, there is no real lower bound, if you are clever enough, but you might have to expend quite a bit of energy to get criticality for small yields.

    There are some fascinating possibilities though if you don’t mind getting out to areas of wild speculation. For example, there is some evidence that sonoluminescence is actually a nano-fusion phenomenon.

    From a practical standpoint, with current tech, the smallest I’ve heard of are briefcase nukes in the kiloton range. As far as I know no one has expended research effort on smaller packages/yields because kinetic and chemical explosives are so much easier to deal with and manufacture, it’s just not worth the effort.

  2. I think off the top of my pointy head that there is a lower practical bound. For a small mass to reach criticality, you have to have a neutron reflector, there are temperature related issues to criticality for a small mass, and the radiation produced would take the grenade launcher anyway. the yield would still be in the MOAB range and higher, so a man-thrown device is essentially a suicide bomb. The critical mass for weapons grade Pu is around 9 kg as a practical lower bound and the smallest launchable weapon was still a heafty 35 kg, I think, the Davy Crocket shell. The yield was in the 5o ton range and fairly dirty, launched from a recoilless rifle to a couple of kilometers, the radiation was a serious issue as it was part of the purpose of the device to keep an area unusable for a couple of days. That leaves the suitcase size weapons as the smallest practical device of the low yield kind. I’d think you couldn’t get a fissile trigger to set off a fusion device much smaller than this, so the sutcase range is it.

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