  In spring 2019 the international atomic energy agency (IAEA) has run a public
competition to look at tomographic reconstruction algorithms for its PGET
detector, used to determine activity levels in nuclear reactors spent fuel
rods. The algorithm is also supposed to identify missing fuel rods. The task is
particularly challenging because uranium rods have a very high cross section
for gamma photons in the energy range of interest. Therefore, signal from pins
at the center of the assembly is heavily attenuated. Furthermore, most photons
reaching the detector will have undergone Compton scattering, originating a
high background deprived of spatial information. In this paper, we describe our
competition entry. Image reconstruction is based on robust, unbiased, emission
tomography algorithms that have been routinely used for decades in nuclear
medicine. Attenuation correction is performed by subsequent approximations. An
intermediate reconstruction is used to detect missing pins. This information is
used to generate the final attenuation map, which is used to generate the image
used for quantification. The compton scatter background is estimated by
comparing the counts in the different energy windows. Results of
reconstructions of training datasets in presence of 60Co mock assemblies show a
100% accuracy in the determination of missing pins and high quantitative
accuracy.
