Increasing the percentage of U-235 in the material enhances the fuel’s ability to sustain fission reactions which means the reactor can be smaller, use less moderator, and less total Uranium. That said, reactors can be made with today’s natural uranium ore, but they are often quite large with lots of moderator material and fuel as in the heavy water CANDUs.

Isotopic enrichment methods take advantage of the slightly different properties of the two isotopes, such as slight mass differences, slightly different light absorption, or different reaction rates. Isotopic separation is also used for many other materials in the reactor where only certain isotopes are desired, for example in boron control rods which use the neutron absorbing B-10 instead of the less absorbing B-11.

The energy required to enrich the uranium is significant, though process and method improvements have steadily decreased the energy and infrastructure investment. Early methods for the Y-12 Manhattan project used a devices called calutrons to ionize uranium isotopes and separate them based on their mass difference – essentially a gigantic mass spectrometer. Curiously, they borrowed 13,300 metric tons of silver ($10.3B in 2024) from the US Treasury for their magnets. I wonder how effective calutrons would be with modern super conductors. Post-war enrichment at K-12, also at Oak Ridge, used gaseous diffusion taking advantage of the slightly different diffusion rates of the two isotopes in highly corrosive Uranium Hexafluoride gas. These were mile long, power hungry facilities. Current methods also take advantage of mass differences but use gas centrifuges spinning at around 50,000 rpm. These are more compact and energy efficient than previous methods. Other methods in development use lasers with a tight bandwidth to take advantage of slight differences in the energy absorption of different isotopes at particular wavelengths of light. This method can allow a laser to deposit the isotope of choice from a gas in a kind of chemical vapor deposition process.

Parameter Unit Value
Enrichment (% 235)
Category -
SWU SWU / kg
Ore Cost $/kg
Conversion $/kg
Enrichment $/kg
Fabrication $/kg
Total Cost $/kg
Burn Up MWd/kg
Burn Up % of fuel
Fuel Cost ¢/kWh
Fuel Cost Diesel ¢/kWh [2, 25]