Development of axial burnup measurement system for PWR spent fuel assembly

An axial burnup measurement system for a PWR spent fuel assembly based on inside gamma measurements has been developed. The measurement system consists of a spent fuel assembly measurement system, an ORIGEN-ARP and MCNPX simulation, and chemical assay reference burnup data. The spent fuel assembly is scanned axially using an ionization chamber inside each spent fuel guide tube, and an axial gamma dose rate profile from each guide tube is obtained from the measurement. The measured dose rate from each guide is then averaged and converted into burnup using a conversion factor calculated by the ORIGEN-ARP and MCNPX simulation. The measured burnup obtained from the conversion of measured dose rate is then corrected using the chemical assay burnup data, and the uncertainty of the measured burnup is calculated using an error propagation formula. Five assemblies have been applied, which are three 14×14 Kori Unit 1 assemblies, one 16×16 Kori Unit 2 assembly, and one 17×17 Uljin Unit 1 assembly, with a declared burnup ranging from 32 to 50 GWd/tU. The results show that the measured burnup has a good agreement with the chemical assay burnup with an average difference between the measured and chemical burnup of 6.1 %. The pure uncertainty resulting from the measurement and simulation is estimated to be 2 %.