Experimental Validation of the Methodology for Partial Defect Verification of thePressurized Water Reactor Spent Fuel Assemblies

A set of controlled experiments and Monte Carlo simulation studies with actual commercial pressurized water reactor (PWR) spent fuel assemblies were performed in order to validate an earlier proposal for partial defect testing of the PWR spent fuel assemblies. The proposed methodology involved insertion of tiny neutron and gamma detectors into the guide tubes of PWR assemblies, measurements and data evaluation. One of the key features of the data evaluation method was the concept of the base signature obtained by normalizing the ratio of gamma to neutron signals at each measurement position. As the base signature is relatively invariant to the characteristic variations of spent fuel assemblies such as initial fuel enrichment, cooling time, and burn-up, the methodology could be a powerful verification method which does not require operator declared information on the spent fuel. The benchmarking experiments indeed demonstrated that the methodology can be used for partial defect verification of the PWR spent fuel assemblies without operator declared data. The results from the experiments were compared with the simulations and the agreement between the two was well within ten percent. Thus, based on the simulation studies and benchmarking measurements, the methodology developed promises to be a powerful and practical way to detect partial defects that constitute 10% or more of the total active fuel pins. This far exceeds the detection threshold of 50% missing pins from a spent fuel assembly, a threshold defined by the IAEA Safeguards Criteria.