Nuclear fusion experiments performed at the National Ignition Facility produce radioactive debris, arising in reactions of fast neutrons with the target assembly. We have found that postshot debris collections are fractionated, such that isotope ratios in an individual debris sample may not be representative of the radionuclide inventory produced by the experiment. We discuss the potential sources of this fractionation and apply isotope-correlation techniques to calculate unfractionated isotope ratios that are used in measurements of nuclear reaction cross sections. •Debris from fusion experiments at the National Ignition Facility is inhomogeneous.•Fractionation techniques allow calculation of the average radionuclide inventory.•Corrected radionuclide concentrations support the calculation of nuclear cross sections.