Design of a Detector System to Measure the Axis Profile of Spent Fuel Burnup

We designed and fabricated detector system to measure the axis profile of spent fuel burnup. The spent fuel burnup should be determined accurately for the burnup credit (BUC) and safeguards of nuclear materials. The spent fuel burnup is varied along the fuel axis, and it’s effect has to be considered. Detector system, which includes the gamma and neutron detector, was designed to measure the axis profile of gamma-ray and neutron counts. The measurement was done at Post-Irradiation Examination Facility (PIEF) of KAERI (Korea Atomic Energy Research Institute). The gamma ray and neutron count profile inside the spent fuel assembly were measured with an ionization chamber and a fission detector, which were inserted into the control rod of the spent fuel assembly. A detector system, which could measure the burnup profile outside the spent fuel assembly, was also designed. The gamma ray would be measured with an ionization chamber and a room temperature semiconductor detector (CZT), and the neutron would be measured with a fission chamber. Monte Carlo simulation was done to estimate the operation characteristics of the detector system. From the simulation, we obtained the dependency of the gamma-ray count profile along the spent-fuel axis on the collimator design, and the energy resolution degradation due to the absorber design.