"The prototype KRUSTY 1 kWe Kilopower reactor weighs 134 kg and contains 28 kg of 235U. The space rated 10 kWe Kilopower for Mars is expected to mass 1500 kg in total (with a 226 kg core) and contain 43.7 kg of 235U."
https://ntrs.nasa.gov/api/citations/20150011642/downloads/20150011642.pdf
"Figure 1 illustrates the flight concept of a 4-kWt reactor core coupled to a 1-kWe Stirling power conversion system and a 40-kWt core coupled to a 10-kWe conversion assembly."
Definitely a heat engine with 25% efficiency. So, if the reactor generates 1 kWe electrical power, it is generating 4-times as much thermal power, or 4 kWt.
"The solid core design provides sufficient thermal power while reducing the fuel, radial reflector, and shadow shield geometry and mass, giving the total system a higher specific power (W/kg) than other fuel forms. The baseline material for the core has been chosen to be 93 percent highly enriched U235 alloyed with 7 percent Mo by weight"
I was wrong, KRUSTY uses nearly pure U235, not just slightly enriched "reactor grade U235".
"The solid core works well with lower power reactors because of negligible fuel burnup and volume swelling issues that can challenge higher power reactors"
"Conservative thermal degradation of the 4-kWt core using this method are estimated to be 3 K/year with 0.1 percent fuel burnup over 15 years. An alternative approach is to use an active control rod that adds reactivity as needed throughout the mission to keep the reactor temperature and power constant. Using active control, the reactor can provide constant power for several hundred years at the 4 kWt level due to very little fuel burnup and needed reactivity insertion."
Pure U235 has a specific energy density of 144,000,000 MJ/kg. This is much higher potential energy than natural uranium, but it is released very slowly. A fuel core with 28 kg of highly enriched 93% U235 would have 4,032,000,000 MJ of potential energy. If it generated a constant 4 Kwt, it would take 16000 years to deplete half the fuel, assuming 50% burn is achievable, or 32 years to deplete 0.1% of the total energy. This design does not allow for fuel reprocessing to remove impurities, and so either the core temperature would decrease, or the control rod would have to be re-positioned to maintain the reaction rate, over time.
In contrast, a fast breeder reactor does not need any enriched uranium at all, and instead uses natural uranium. A core blanket of natural 0.7% U235 surrounds a plutonium core. Up to 60% of the potential energy can be used by re-processing to remove plutonium from the core blanket, and reuse it as fuel in the core. If only 0.1% of the energy of a 28 kg pure U235 core is used, or 4,032,000 MJ, and the same amount of energy can be extracted with 60% efficiency from natural uranium in a fast breeder reactor, then only about 78 grams of natural uranium would be needed. If it were possible, this would be a very small, lightweight power plant indeed, however this might not be enough mass of natural uranium to achieve critical mass, so the advantage of fast breeder reactors might only be seen in much larger power plants (MW or GW power levels) than are envisioned for the kilowatt KRUSTY reactor design.
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