 For natural uranium fuel, fissile component starts at 0.7% 235U concentration in natural uranium.At discharge, total fissile component is still 0.5% (0.2% 235U, 0.3% fissile 239Pu, 241Pu).Fuel is discharged not because fissile material is fully used-up, but because the neutron-absorbing fission products have built up and the fuel becomes significantly less able to sustain a nuclear reaction.