Detection of Partial Defects in irradiated BWR fuel assemblies. A Preliminary Study.

The possibility of detecting partial defects in spent BWR fuel assemblies by means of measure- ments of the emitted neutron and gamma-ray radiation has been the subject in a preliminary study. Computer simulations have been made both for the case of removed rods and the case of replacement by dummy rods. The focus was put on the replacement scenario in which verica- tion by using weighing techniques is not possible. Using the Origen-S code, the source strength for a typical 8x8 BWR fuel assembly was calculated. For various geometrical patterns of the replaced rods, the radiationeld outside the fuel assembly was calculated with the Monte Carlo code MCNP-4C. The results suggest that neutron measurements alone, signicantly can reveal if 3 % or more of the totalssile material has been replaced. In a case where operator-declared data are unavailable or assumed incorrect, an anomalous neutron signal will indicate erroneously stated burnup and/or replaced fuel rods. An inspector is then either forced to use an indirect method such as gamma scanning to verify the burnup or tomography in order to directly verify a possible partial defect.