[throwaway_yy2Di]

It means 96% of the uranium fuel undergoes fission (gets its nuclear potential energy converted to heat). When they separate out fission products (waste) by chemical processing, about 4% of the fuel gets dragged into the waste stream. It looks like a cost/benefit decision not to recover it:

    Of the 200 kilogram lanthanide mass removed by liquid metal
    extraction, we estimate that approximately 20 kilograms will
    be actinide contaminant with a longer half-life similar to
    SNF. It may be most practical to leave such a small quantity
    embedded in the ceramic granules, as it would be well
    distributed and would not materially extend the time for the
    overall waste form to reach background levels. If desired,
    however, the actinides can be further separated offsite with
    additional post-processing techniques.

[pdf] http://www.transatomicpower.com/wp-content/uploads/2015/04/t...

The mass flow is 500 kg/year fuel in, and 500 kg/year waste out -- of which, 20 kg (4%) is unrecovered actinide fuel.