You could always try industrial lasers, which are sort-of “atomic”. And you can hum the Goldfinger theme while you do it.
Ants: You expect us to talk?
You: No, I expect you to die!
It’s not just a question of the amount needed for critical mass, it is the casing required to direct a very precise charge of conventional explosives to set off the chain reaction.
A search for ‘suitcase bombs’ here will bring up discussions where it was described in more detail.
Well, if we’re using James Bond analogs, you could just go back to the tried-and-true method of using a magnifying glass, and say it’s the big reflective laser thingy from either Diamonds are Forever or Die Another Day. Just depends on whether you want to sing a John Barry tune or one by Madonna.