Application of Passive Gamma Emission Tomography (PGET) to the Inspection of Spent Nuclear Fuel

In collaboration with several Member-State Support Programs, the IAEA has developed a passive gamma emission tomography (PGET ) capability for the underwater verification of spent nuclear fuel. In a single five-minute measurement, the PGET system integrates three verification methods: gross neutron and gamma-ray counting; medium-resolution gamma-ray spectroscopy; and imaging of the gamma-ray emission from a two dimensional cross section of the fuel assembly. The tomographic images created by PGET enable a partial-defect detection capability for spent fuel verification which in many cases provides single-missing-pin detection, and introduces a new capability - verification of the number and relative activity of pins in closed containers. The PGET system consists of a torus shaped detector system and a system controller. Inside the torus, a rotating platform holds 174 highly collimated CdZnTe gamma-ray detectors and two 10B neutron detectors. As the inner assembly rotates during operation, sinogram data from all 174 detectors are collected in four broad energy windows, spectral data are accumulated with a subset of the detectors, and neutron counts are recorded. For an inspection, the PGET is placed in the pool, either on top of the fuel rack or on the floor of the pond, a process that typically requires 30 minutes. To inspect an item, the operator’s fuel-handling device is used to take the item from its storage position in the rack and lower it into the central region of the PGET torus. The item is held stationary while data are collected and then is moved back to its storage position. Data processing and automated evaluation requires an additional minute. In the first quarter of 2017, PGET was sent to three nuclear power plants in two countries and tested on WWER-440, BWR, and PWR fuel assemblies with burnups in the range 5.7 - 57.8 GWd/tU and cooling times from 1.9 - 26.6 years. For all assembly types, the capability to detect single missing or replaced pins was demonstrated and burnup declarations were confirmed using neutron data. Imaging of irradiated items (model fuel elements) and closed containers demonstrated the utility of the integrated spectroscopy capability.