Neutron Measurement Techniques for Verification of Closely Packed Spent Fuel Assemblies Stored in a Spent Fuel Pond

Development of an appropriate spent fuel verification methodology has been challenging for some time for a reactor that has a hard-to-access spent-fuel storage arrangement. The movement of spent fuel assemblies for verification purposes would be prohibitively expensive for the facility. In the past, other verification methods using collimated gamma techniques were tried, but found not to be adequate because of the spent-fuel characteristics and various limitations imposed in the measurement geometry. In this paper, we present an innovative method that does not require any movement of the assemblies to verify the presence of long-cooled spent fuels. Our approach assumes that the sum of neutron signals from the low burn-up, long-cooled assemblies is linearly correlated to the sum of burn-ups of the measured assemblies. Field test measurements have been performed using an underwater neutron counter, SFNC (Spent Fuel Neutron Counter), specially developed for application at the heavy water reactor, to collect neutron signals in the gap between assemblies. Measurement results show that the calibration curve was indeed sufficiently linear in the burn-up range of interest (5000 ~ 8000 MWD/TU); a confirmation that this method can be effective to verify the presence of long-cooled spent fuels in the hard-to-access storage arrangement.