>>13343448>industry insiderkek. I just did a nuclear minor and worked in the lab for 6 months.
Anyway, most, if not all very heavy nuclei undergo some level of spontaneous fission, even ones with multi billion year half lives, like 235-U and 238-U. It's actually how special nuclear material is tracked. I suspect the function of this reactor effectively uses the small amount of neutrons generated from spontaneous fission events to start the reaction. The control rod absorbs enough of these neutrons to prevent the reactor from going critical. Once removed, enough neutrons from spontaneous fission are able to strike fuel nuclei and start a chain reaction. A single boron carbide rod is insufficient to absorb enough free neutrons to truly stop the reaction. There is simply too many fissions going on for the rod to be able absorb enough neutrons to keep the reaction from sustaining itself.
Diagrams say the fuel is highly enriched. Previous space reactors have used 90+% 235-U. Kilopower probably uses similar enrichment to these and US naval fuel, which is also 90% or greater 235-U. Higher enrichment enables much simpler design and operation. Fast fission factor calculations are especially simplified, since the amount of 238-U and the ability of the reactor to breed transuranics are drastically reduced. Plutonium production in reactors can especially change reactivity and thus operation. Kilopower also appears to no moderator, meaning fast fission factor is the most important part of its reactor physics calculations.
tl;dr it's optimized to be something they know will work and be relatively simple to design and operate and uses concepts from previous space reactors
