Why does this reactor need a higher percentage of U-235 in the fuel than other fissionable isotopes ? I thought the idea of a fast reactor was that it could fi

Why does this reactor need a higher percentage of U-235 in the fuel than other fissionable isotopes ? I thought the idea of a fast reactor was that it could fission U-238, Thorium, etc ? i.e. why can’t you feed the Natrium reactor the usual 5% enriched fuel you’d use in an LWR ?