Optimised non-invasive method to determine 238U-captures-to-total-fissions in reactor fuel

High-resolution gamma spectroscopy was performed on individual fuel rods of a fresh, highly heterogeneous Boiling Water Reactor (BWR) fuel assembly, after irradiation at low power in the PROTEUS reactor at PSI, to determine the ratio of neutron captures in 238U ( C 8 ) to total fissions ( F tot ) . The methods for correcting the measured gamma count-rates for attenuation within the fuel rods and the collimator shielding are now so advanced that one can obtain several independent estimates of F tot for each fuel rod by using gamma lines from different fission products with a wide range of energies. The recording of a detailed irradiation history further allowed the gamma activities to be precisely corrected for radioactive decay between the time of irradiation and measurement. Due to these facts, and the good statistical quality of the present data, the main limitation on the accuracy of C 8 / F tot comes from uncertainties on the basic nuclear quantities: fission yields and beta–gamma branching ratios. The various steps involved in determining C 8 / F tot from recorded gamma spectra, and their contributions to the final error, are analysed. The results for 80 fuel rods of varying material composition exposed to different neutron moderation conditions in the reactor core are compared with the results of a Monte Carlo full-assembly calculation.