Development of High-Efficiency Multisensor Detection System for Gamma Emission Tomography in Spent Nuclear Fuel Inspection

Background: Gamma emission tomography (GET) is one of the most reliable methods for detection of partial-defects within spent nuclear fuel (SNF). In our previous study, we developed and proposed the scintillation crystal-based GET instrument named Yonsei single-photon emission computed tomography (YSECT). However, this conventional YSECT instrument has consequential limitations related to the low inspection accuracy in the central region, which fact is due to the high density of nuclear fuel rods. This study aimed to derive, as proposed in this paper and using Monte Carlo simulation, a multisensor-based YSECT for enhancement of partialdefect detection accuracy. Materials and Methods: The gamma-ray energy spectra for gadolinium aluminum gallium garnet (GAGG), CsI(Tl), CdWO₄, and PbWO₄ were obtained to determine the appropriate material for image quality improvement in the central region. The shapes of the spectra and the detection efficiencies were compared among the scintillation crystal materials. The tomographic images were acquired with both conventional YSECT and the newly proposed YSECT, and their quality was compared based on the signal-to-noise ratio (SNR). Results and Discussion: CdWO₄ was found to have a detection efficiency twice as high as that of GAGG, owing to its high-density despite poor light yield. Based on these results, the optimal scintillation crystal material for enhancement of detection efficiency for high-energy (>662 keV) gamma rays was determined to be CdWO₄. In accordance with the optimization study, the SNR of the CdWO₄ image was calculated as 7.95, which was higher than that of the GAGG image by a factor of 2. Furthermore, the SNR of the synthesis image was determined to be 9.85, which was higher than that of the single-sensor-based YSECT image by a factor of 1.59. Conclusion: Based on these results, we believe that the multisensor-based YSECT can enhance inspection accuracy for partial defects arising in pressurized water reactor-type SNF. In further studies, the effect and influence of neutrons will be evaluated and noise-reduction methods will be developed.