Simultaneous reconstruction of emission and attenuation in passive gamma emission tomography of spent nuclear fuel

Abstract

The International Atomic Energy Agency (IAEA) has recently approved passive gamma emission tomography (PGET) as a method for inspecting spent nuclear fuel assemblies (SFAs), an important aspect of international nuclear safeguards which aim at preventing the proliferation of nuclear weapons. The PGET instrument is essentially a single photon emission computed tomography (SPECT) system that allows the reconstruction of axial cross-sections of the emission map of the SFA. The fuel material heavily self-attenuates its gamma-ray emissions, so that correctly accounting for the attenuation is a critical factor in producing accurate images. Due to the nature of the inspections, it is desirable to use as little a priori information as possible about the fuel, including the attenuation map, in the reconstruction process. Current reconstruction methods either do not correct for attenuation, assume a uniform attenuation throughout the fuel assembly, or assume an attenuation map based on an initial filtered back projection (FBP) reconstruction. Here, we propose a method to simultaneously reconstruct the emission and attenuation maps by formulating the reconstruction as a constrained minimization problem with a least squares data fidelity term and regularization terms. Using simulated data, we compare the proposed method to FBP, showing that our approach produces significantly better reconstructions by various numerical metrics and a much better classification of spent, missing, and fresh fuel rods. † Currently with STUK, Finland ‡ Currently with TRIUMF, Canada

Idea of an experimental technique for quantitative passive gamma emission tomography on irradiated nuclear fuel assemblies

“…A quantitative evaluation of the activity distribution using these devices would require detailed, and sometimes difficult-to-asses, knowledge of the spatial attenuation distribution of the full measurement geometry as well as a characterised detector system where intrinsic detection efficiencies need to be well understood. Backholm et al (2020) have tested simultaneous reconstruction of the spatial distributions of both attenuation and activity, however still producing images with relative data used for classification of the presence or absence of a nuclear fuel rod.…”

Quantitative imaging and automated fuel pin identification for passive gamma emission tomography

“…While the former can be mitigated using collimated detectors, the latter needs to be addressed by using specific imaging algorithms. Previous work has shown that the reconstruction quality could be significantly improved by applying the attenuation correction 8 . In this work, we further corrected for the gamma ray down-scattering in the energy window of interest.…”

Bayesian Activity Estimation and Uncertainty Quantification of Spent Nuclear Fuel Using Passive Gamma Emission Tomography

“…For any safeguards investigation of SFAs, it is important to use a minimum amount of a priori information on the SFA under study, in order to avoid biasing and potentially misleading the investigation. Eventually, IAEA approved the use of the passive gamma emission tomography (PGET) instrument [2][3][4][5][6][7] in inspections.…”

“…For instance, in [7], the authors proposed a method to simultaneously reconstruct the activity and attenuation maps by formulating the reconstruction as a constrained minimization problem with a least squares data fidelity term and quadratic regularization terms with different smoothness matrices. However, the forward model proposed in [7] considered only monoenergetic gamma rays and did not account for the scattering effects of gamma rays in the PGET system that make non-neglible contributions to the total counts, which added additional uncertainty to the activity estimation results. Similarly, in [22], the authors formulated the problem of pin activity estimation as an optimization problem where a sparse regularization function is used.…”