Neutrons available for fission in uranium fuels
It is easier to run a reactor loaded with enriched uranium than with natural uranium. The number of secondary neutrons produced on average by slow neutron capture in uranium nuclei increases with the ratio of the fissile uranium-235 isotope. For the sustainability of chain reaction, this number should be significantly higher than 1 to compensate for the various losses: this condition holds comfortably with fuel enriched to 3.5% as that of PWRs but marginally with natural uranium which contains only 0.7% uranium-235.