is there a lower limit on the size of a nuclear bomb?

Question

And if so, what factors determine it?

I know that the scale of the explosion is an insane amount larger than the teeny molecules causing it, but I’m wondering if there could ever be nuclear explosions small enough to take out a single house or block, rather than a whole city from high above.

In: Physics

Answer 1

Kind of.

There’s a minimum size for the device, but not the yield.

Ultimately, a nuclear bomb is just a lot of nuclear reactions happening in a very small span of time. The more reactions, the bigger the yield. We can build a device that will trigger anywhere between 1 reaction (enough to move a grain of sand slightly) to a few trillion reactions (enough to cause a megaton explosion).

This is usually done by taking the neutrons from one nuclear reaction, and using them to trigger other nuclear reactions. In it’s most simplest form, a nuclear bomb just brings enough nuclear material together that natural decay reactions triggers enough secondary reactions that everything goes very rapidly exponential and you get a significant energy release before the bomb blows itself back apart.

If you do this badly however, the bomb blows itself apart before it’s reacted very much material, this is known as a fizzle.

Fizzles can go to any yield small enough to destroy the bomb, so down to a few kg of TNT equivalent. Less than a fizzle, and the bomb won’t even break, and you just fired a nuclear reactor at the enemy.

Now real nuclear weapons are more complicated, but I don’t think boosted fission or tampers or the classified role of polystyrene, or lithium deutride are ELI5 level.

Key point is, modern nuclear weapons are variable yield, by varying how exponential the reaction gets, and therefore how big the yield gets before the bomb itself is destroyed. The smallest nuclear weapon developed was the W54, with a yield of 10-1000 tonnes. Which is a bit more than house destroying, but could certainly limit itself to one house. You could modify it to be only 1 tonne if you really wanted. But i don’t think that would make it much smaller than it’s 26.5kg.