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It's pretty good, but what you're talking about is a thermalized critical mass which is very different from a fast critical sphere.
This means that you can't really measure it in "density" my question is do you want minimum volume or minimum weight of fissile material?
For minimum volume an imploded amount of Am242 or Cm252 would be denser. But practically speaking a reflected plutonium critical mass is about the size of a tangerine and adding deuterium doesn't make it much smaller because the neutrons mean free path will be a differential equation with non leakage, of which density actually helps.
Now if we're talking minimum fissile material you'll want to thermalize the neutrons, but this becomes a question of geometry not mass. Ex, an infinite tank of liquid helium 4 and a single rod of even natural Uranium would be critical as over infinite distance and zero adsorption you'll be able to get thermal fission. Really what you're modeling is close to a homogenous reactor which has the worst resonance escape probability of any kind of reactor because it doesn't separate the fuel from the moderator so you tend to get neutrons in the epithermal range getting munched by parasitic 238 resonance or U236 production. This is why almost all reactors have rods or liquid fuel channels. But yeah, you're right that deuterium is a good moderator and slowing down neutrons dramatically reduces critical mass. What you want to look into is how critical mass is more an extrapolation of volume.
Side note, I wish there was more fission threads, it's like it's taboo or something but it's way cooler than a fusor or something and requires acutal physics to discuss so thanks anon.
