Reactor Architectures

There are many ways to design a nuclear reactor to extract and utilize the nuclear heat. What are the principal design choices and trades for a nuclear system? Nuclear systems have many interacting subsystems and there are many choices and trades to consider with a multitude of effects on safety and cost, both subtle and drastic. And many of the choices are not straightforward to understand or predict.

For introductory purposes, we can distill the many technology choices to just a couple: fuel, coolant, temperature, size, and power rating as listed in the table below. Each usually has some partisan zealot, championing a particular choice and design path. As designers, we are forced to down-select technologies and we cannot forever be open to all the thousand combinations of reactor technologies. The diversity of design possibilities suggests many ways to the same end, but the fact is that one architecture can be best for a given set of goals.

Parameter Options
Fuel Form Oxide, carbide, nitride, metallic, molten salt, liquid metal.
Fuel Wrapper and Geometry TRISO-matrix, metallic clad, ceramic clad, no clad, (geometry: cylinders, annuli, pebbles), fuel wrapped moderator
Fuel Type and Cycle / Enrichment LEU converter, Natural U converter, HALEU converter, HEU burner, Pu burner, U-Pu breeder, Th-U breeder
Coolant and pressure Helium, water, CO2, sodium, sodium heat pipe, molten salt (fluoride or chloride), other liquid metals, organic, other gases (H2, N2, air)
Moderator / Reflector Graphite, water, heavy water, hydrides, Beryllium (no clad, ceramic, or metallic clad, composite / entrained)
Size Physical dimension of the reactor [~1-6m diameter, 1-20m height]
Power Rating Power rating per reactor core [0,inf] MWth
Temperatures [150-1050 °C], 2000 °C+ for Nuclear Thermal Propulsion
1-15 MPa, typically 3-7 MPa for He