[elcritch]

It makes me wonder how small such systems could go. If you pack the fissile material just so then perhaps you could make the lifetime fairly short (not an RTG). Then could there be an optimal size which would both provide reasonable life while making it infeasible to stockpile enough to make a nuclear weapon?

I'm too lazy to even back-of-a-napkin calculation. Though it seems like you'd need either a way to highly compress the fissile material or a really good neutron reflector to reach any sorta criticality. Neither seem likely exist though.

[joshjob42]

Well we know that aqueous homogeneous reactors (AHRs) can be very small; we’ve operated them with < 1 meter diameter cores at powers of >5MWth before. A big plant with 380MWth was estimated to be an approximately 2.5m diameter cylinder and 5m long primary reactor and breeding/shielding blanket, power density ~16kWth/L. https://www.osti.gov/servlets/purl/4123899

Smallest AHRs were ~2 feet or so in diameter and put out ~5kWth, so it’s at least possible that you could deploy them buried in your backyard or something like that.