An alternative: mix californium and natural uranium and you get a solid state subcritical reactor with no need for regulation.
Though, there will be no way to turn it off until Cf burns out.
Though, there will be no way to turn it off until Cf burns out.
Let's suppose we have a just barely subcritical reactor with k = .99. Then, you're going to need about 1% Cf in the core to sustain a chain reaction.
The core of a 1 GW reactor contains about 100 tonnes of fuel, and fissions maybe a tonne of fuel per GW(e)-year. So, at k = .99, it would consume 10 kg of Cf per year. At current market prices this would cost $270 B/year, assuming even that much Cf were available.
It would be BY FAR cheaper to provide the neutrons to drive a subcritical reactor by means of an accelerator, rather than with californium. 1 GeV protons on a uranium target will produce about 60 neutrons per proton, from spallation and fast fission. This sort of scheme has been repeatedly investigated, but has no sufficient advantage over conventional reactors to justify the extra cost and complexity.
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.
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.
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.