Gamma Emission Tomography for the Inspection of Spent Nuclear Fuel

Abstract

A Passive Gamma Emission Tomography system (PGET) [1] was developed for the IAEA Safeguards for verification of irradiated nuclear fuel assemblies (SFAs). In 2015-2016 PGET underwent significant re-design and its performance has been tested on multiple SFA types. The re-designed PGET features the functionality of traditional non-destructive assay systems commonly used for spent fuel verification: total neutron counting (Fork Detector, FDET), medium-resolution gamma spectrometry (Irradiated Item Attribute Tester, IRAT or Spent Fuel Attribute Tester, SFAT) and spent fuel assembly’s lattice image (Digital Cherenkov Viewing Device, DCVD). Two 10B neutron detectors and one-hundred-seventy-four collimated CdZnTe detectors are grouped in two arrays on a rotary baseplate inside a watertight stainless steel enclosure. A SFA is lowered through the center of the enclosure and held stationary to perform an underwater measurement. Detector arrays are then rotated on a baseplate in the horizontal plane around vertical axis of symmetry to obtain gamma sinogram and neutron count data simultaneously, typically in 3-5 min per assembly. Additionally, medium resolution spectra from all gamma detectors can be collected and recorded. Functional, technical and operational performance of the PGET was tested at four nuclear reactors on mockup, PWR, BWR and WWER-440 SFAs. Measurements have been performed on fuel with burnup in the range 5.7-58GWd/tU and cooling times from 1.9 to 27years. Lateral pin structure of the SFAs could be reconstructed for any tested fuel design in the above range of cooling times and burnups. Missing or replaced pins in all fuel types could be clearly visualized in the reconstructed images; spectrometric information (134Cs/137Cs peak ratio) and neutron counting rates were found to be consistent with declared fuel radiation history. This paper describes details of the PGET hardware and electronics and presents some results of performance evaluation.