Hmm, I think you have a miscalculation about neutronics here, and fission/capture ratios. Cf neutrons are in their majority come nicely in 1MeV-2MeV, so you have much more fission going per neutron, and much less thermal neutron captures.
If you have an intense neutron emitter dispersed in fuel + moderation + reflectors you should be able to go with much lower k to generate a meaningful amount of power.
You should me able to go down to single grams of Cf per megawatt. Yes, not that economical, but at least possible for things like reactors for space use and such. At that price, that will be cheaper than current RTG material.
Correct me if I am wrong.
Correcting away!
The energy of the Cf neutrons doesn't matter much, since after that first generation the neutrons will be ordinary U fission neutrons.
The comment about moderators/reflector is silly, since the purpose of those is to make k higher. One you know what k is, they are irrelevant.
The ratio of Cf burned to (Cf + U) is 1 - k, so unless k is very close to 1 you are going to use a hell of a lot of Cf. But if Cf is very close to 1, you need to control it anyway as fuel burns up, so you might as well just go critical. Cf buys you nothing.
Er, "if k is very close to 1"
Hi, I checked my math and found it completely unsound. It will still have to be very, very close to criticality, though with a bit lower critical mass.
It will also require regulation over the years as Cf burns out, and u238 fast fission products themselves product less neutrons than fission products of thermal neutron reactors.