Nuclear food cycle

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The nuclear food cycle is a hypothetical food cycle based upon methanotrophs which are fed on methanol produced in nuclear powered sabatier reactors, which are in turn fed on syngas produced from nuclear powered Zinc/Sulfur/Iodine[1] reactors. Another product produced by the Zn/S/I reactor is breathable O2. This then forms a loop, where people eat the methanotrophs, producing CO2 and H2O through their metabolism, which are extracted via atmospheric processing and water recycling, processed to produce methanol which is then fed back to the methanotrophs to grow more food.

Energy analysis

This analysis assumes that nutrients (nitrogen, sulfur, phosphorus, etc.) are entirely recycled, in practice due to imperfect recycling or colony growth, small amounts of additional nutrients would need to be added periodically from mining, atmospheric processing, etc.

The growth yields of methanotrophs varies considerably[2][3][4], but somewhere around 10-40% of the methanol used ends up as cellular mass. Methanol has an energy density of 15.6 MJ/L and a density of 792g/L[5]. That works out to be 20 KJ per gram of methanol. Taking 20% yield as an approximation, that leads to 100KJ/g of cell mass assuming the methanol production process is 100% efficient.

As methanol production is at best 70% efficient in the current state of things, the yield might correspond to about 140 MJ/kg (to be verified).

From animal studies, the nutritional value of Methylococcus capsulatus is 8.96 MJ/kg[6], making the cycle 8.9% efficient at converting thermal energy into nutritional energy. (about 7% with the methanol conversion efficiency).

The bacteria would probably be used as animal feed, reducing somewhat the energy efficiency of the process.

1kg of 235U contains 8.64×1013 joules of energy. The average adult needs approximately 8700kj/day. That means that 1kg uranium could be converted to approximately 850,000 person days of food. Assuming that a molten salt reactor that can almost completely consume its nuclear fuel is utilized. In other words, feeding a person using the nuclear food cycle requires approximately an extra 1kwth per person.

Further analysis

  • The actual growth media is going to be recycled biomass, which may contain undigested food or more complex proteins that require additional energy for catabolism.
  • The actual efficiency of small sabatier or Zn/S/I reactors is currently unknown.
  • Radiotrophic fungi have been observed[7], which may be able to more directly exploit nuclear energy.