This piece explains why breeders are not a weapons risk.
The plutonium that remains in the spent pellets is a mix of several plutonium isotopes, still useful as reactor fuel but not useful in a nuclear weapon.
A little more than half of the plutonium is Pu239, which needs to be at 93 percent for the plutonium to be weapons-grade. If there is more than 20 percent Pu240, which tends to fission spontaneously, all the plutonium is reactor grade and “entirely unsuitable for use in a bomb,” per the World Nuclear Association.
The plutonium in spent fuel pellets from commercial reactors contains 24 percent Pu240, as well as other plutonium isotopes that inhibit bombmaking. That plutonium is reactor-grade, not weapons-grade.
Kahn’s carefully drawn amendment (she is a physics graduate) will not limit new commercial reactors, since they don’t produce weapons-grade plutonium.