Year
2018
Abstract
In late December 2017, passive gamma emission tomography (PGET) was approved by the International Atomic Energy Agency as a new system for safeguards verification. In the upgraded PGET instrument prototype, two heads, each containing 87 CdZnTe detector pixels (2x5x5 mm3) behind a 10 cm tungsten slit collimator, collect tomographic projection data while they rotate 360 degrees around a spent nuclear fuel assembly. The data is collected while the fuel is underwater in a fuel storage pool. The tomographic data is used to reconstruct a gamma-ray emission image of the spent fuel. PGET images allow pin-level partial defect testing, which is a critical requirement before encapsulation for storage into deep geological repositories. Finland will be the first country to begin operation of such a repository, in the mid-2020s. A PGET instrument will be included in the integrated nondestructive assay station at encapsulation plants. In this paper, we present a full processing chain for PGET data, from data corrections to image evaluation. The performance of different image reconstruction algorithms is shown for the fuel types present in Finnish nuclear power plants (VVER-440, BWR). Different image analysis metrics and methods are presented and discussed. Based on the performance of these methods, we arrive at a recommended procedure for data collection, image reconstruction and image analysis for the Finnish deep geological repository case.