Effects of a higher burn-up
The diagram shows the effects of reactor residence time for a constant quantity of energy produced. It compares the composition upon removal from the reactor of a fuel used to generate 60 GWd/tonne and a standard fuel delivering 33 GWd/tonne. These contents are restated for an equivalent energy output of 1 GWd (GigaWatt-day). At equivalent energy production, the quantities of fissile uranium 235 burned and fission products generated are essentially the same. However, there is 30% less plutonium and the unburned residual fissile material (U-235 and Pu-239) content is 44% lower.
Source: CEA