A feasibility study of the burnup determination based on the neutron interrogation of a spent fuel assembly

A neutron interrogation has been performed to verify the burnup of the C15 Assembly which has an initial enrichment of 3.2 wt% and a NPP-declared burnup of 31.9 GWd/tU and has cooled for 21 years. The nuclide compositions were calculated with the ORIGEN2 code on the basis of the estimated burnup at the axial position of the assembly, which were then used in the MCNP calculation to simulate the spent fuel assembly and the interrogation process in order to obtain the background and induced fission neutrons, respectively. The 16 control rod guide tubes, a instrument thimble and 22 rod-empty positions as well as 157 spent fuel rods were described in the MCNP model. The ratios of the induced fission neutron to the background neutron were obtained from the measurement and the ORIGEN2-MCNP calculation, respectively, which were then used for the burnup determination of the C15 assembly. The results show that the measured burnups are in the range from 31.6 to 32.2 GWd/tU which are quite close to the declared burnup, 31.9 GWd/tU. (authors)