PGET Monte Carlo simulations using Serpent

Since 2017, over 100 spent nuclear fuel assemblies at the Finnish nuclear power plants have been imaged with the Passive Gamma Emission Tomography (PGET) device in preparation of the implementation of PGET in the safeguards infrastructure of the Finnish geological repository. In order to increase understanding of the PGET method and guide its further development, we have recently implemented PGET in Serpent, a widely-used neutron and photon transport Monte Carlo simulation code. We will discuss the major aspects of this implementation and illustrate the usefulness of the simulations with a few examples. The PGET device as used in the measurements (which was developed under the guidance of IAEA and is approved for safeguards inspections) was implemented in a very realistic way based on its technical drawings. The simulation produces sinograms in user-defined energy windows as well as the uncertainty on these sinograms. Tomographic images are then reconstructed using the exact same algorithm as used for the measured data. A dedicated variance reduction scheme was implemented, increasing the computational efficiency by about a factor of 30. The simulation of the PGET response at one angular measurement position for 1 billion primary photons takes a few hours on a single 40-core node. The 1-sigma uncertainty in the highest intensity sinogram pixels is about a few percent. Aiming at improving the imaging of VVER-440 assemblies, we have simulated assemblies containing one or a few missing fuel rods or having only one emitting rod (the other rods being present but not emitting) in various well-chosen places, configurations that are not accessible in practice. The single-emitting rod results show in great detail those parts of the sinogram that contain most of the information for the particular rod position. How this information might be used for obtaining better images, especially of the central region of a fuel assembly, will be discussed

γ‐ray diagnostics of α slowing in inertial confinement fusion targets

For large inertial confinement fusion deuterium-tritium targets, a way to diagnose alpha slowing might be via capture reaction gamma rays. Calculations are presented for two such methods: one uses the alpha+T direct capture gamma rays, the other is based on a series of resonant alpha-capture reactions. For small targets (rhoR less-than-or-equal-to 0.02 g/cm2), the total alpha+T gamma-ray yield relative to the DT neutron yield is temperature independent and proportional to the rhoR value. For large targets (rhoR greater-than-or-equal-to 0.2 g/cm2), this quantity becomes temperature dependent and rhoR independent. Some experimental aspects are discussed

PGET Monte Carlo simulations using Serpent

Since 2017, over 100 spent nuclear fuel assemblies at the Finnish nuclear power plants have been imaged with the Passive Gamma Emission Tomography (PGET) device in preparation of the implementation of PGET in the safeguards infrastructure of the Finnish geological repository. In order to increase understanding of the PGET method and guide its further development, we have recently implemented PGET in Serpent, a widely-used neutron and photon transport Monte Carlo simulation code. We will discuss the major aspects of this implementation and illustrate the usefulness of the simulations with a few examples. The PGET device as used in the measurements (which was developed under the guidance of IAEA and is approved for safeguards inspections) was implemented in a very realistic way based on its technical drawings. The simulation produces sinograms in user-defined energy windows as well as the uncertainty on these sinograms. Tomographic images are then reconstructed using the exact same algorithm as used for the measured data. A dedicated variance reduction scheme was implemented, increasing the computational efficiency by about a factor of 30. The simulation of the PGET response at one angular measurement position for 1 billion primary photons takes a few hours on a single 40-core node. The 1-sigma uncertainty in the highest intensity sinogram pixels is about a few percent. Aiming at improving the imaging of VVER-440 assemblies, we have simulated assemblies containing one or a few missing fuel rods or having only one emitting rod (the other rods being present but not emitting) in various well-chosen places, configurations that are not accessible in practice. The single-emitting rod results show in great detail those parts of the sinogram that contain most of the information for the particular rod position. How this information might be used for obtaining better images, especially of the central region of a fuel assembly, will be discussed

In-air and in-water performance comparison of Passive Gamma Emission Tomography with activated Co-60 rods

Abstract A first-of-a-kind geological repository for spent nuclear fuel is being built in Finland and will soon start operations. To make sure all nuclear material stays in peaceful use, the fuel is measured with two complementary non-destructive methods to verify the integrity and the fissile content of the fuel prior to disposal. For pin-wise identification of active fuel material, a Passive Gamma Emission Tomography (PGET) device is used. Gamma radiation emitted by the fuel is assayed from 360 angles around the assembly with highly collimated CdZnTe detectors, and a 2D cross-sectional image is reconstructed from the data. At the encapsulation plant in Finland, there will be the possibility to measure in air. Since the performance of the method has only been studied in water, measurements with mock-up fuel were conducted at the Atominstitut in Vienna, Austria. Four different arrangements of activated Co-60 rods, steel rods and empty positions were investigated both in air and in water to confirm the functionality of the method. The measurement medium was not observed to affect the ability of the method to distinguish modified rod positions from filled rod positions. More extended conclusions about the method performance with real spent nuclear fuel cannot be drawn from the mock-up studies, since the gamma energies, activities, material attenuations and assembly dimensions are different, but full-scale measurements with spent nuclear fuel are planned for 2023

Design of a novel instrument for active neutron interrogation of artillery shells

The most common explosives can be uniquely identified by measuring the elemental H/N ratio with a precision better than 10%. Monte Carlo simulations were used to design two variants of a new prompt gamma neutron activation instrument that can achieve this precision. The instrument features an intense pulsed neutron generator with precise timing. Measuring the hydrogen peak from the target explosive is especially challenging because the instrument itself contains hydrogen, which is needed for neutron moderation and shielding. By iterative design optimization, the fraction of the hydrogen peak counts coming from the explosive under interrogation increased from 53(-7)(-7)% to 74(-10)(+8)% (statistical only) for the benchmark design. In the optimized design variants, the hydrogen signal from a high-explosive shell can be measured to a statistics-only precision better than 1% in less than 30 minutes for an average neutron production yield of 10(9) n/s.Peer reviewe

Improved Passive Gamma Emission Tomography image quality in the central region of spent nuclear fuel

Reliable non-destructive methods for verifying spent nuclear fuel are essential to draw credible nuclear safeguards conclusions from spent fuel. In Finland, spent fuel items are verified prior to the soon starting disposal in a geological repository with Passive Gamma Emission Tomography (PGET), a uniquely accurate method capable of rod-level detection of missing active material. The PGET device consists of two highly collimated detector banks, collecting gamma emission data from a 360 degrees rotation around a fuel assembly. 2D cross-sectional activity and attenuation images are simultaneously computed. We present methods for improving reconstructed image quality in the central parts of the fuel. The results are based on data collected from 2017 to 2021 at the Finnish nuclear power plants with 10 fuel assembly types of varying characteristics, for example burnups from 5.7 to 55 GWd/tU and cooling times from 1.9 to 37 years. Data is acquired in different gamma energy windows, capturing the peaks of Cs-137 (at 662 keV) and Eu-154 (at 1274 keV), abundant isotopes in long-cooled spent nuclear fuel. Data from these gamma energy windows at well-chosen angles are used for higher-quality images, resulting in more accurate detection of empty rod positions. The method is shown to detect partial diversion of nuclear material also in the axial direction, demonstrated with a novel measurement series scanning over the edge of partial-length rods.Peer reviewe

Measuring spent fuel assembly multiplication in borated water with a passive neutron albedo reactivity instrument

Abstract The performance of a passive neutron albedo reactivity (PNAR) instrument to measure neutron multiplication of spent nuclear fuel in borated water is investigated as part of an integrated non-destructive assay safeguards system. To measure the PNAR Ratio, which is proportional to the neutron multiplication, the total neutron count rate is measured in high- and low-multiplying environments by the PNAR instrument. The integrated system also contains a load cell and a passive gamma emission tomograph, and as such meets all the recommendations of the IAEA’s recent ASTOR Experts Group report. A virtual spent fuel library for VVER-440 fuel was used in conjunction with MCNP simulations of the PNAR instrument to estimate the measurement uncertainties from (1) variation in the water boron content, (2) assembly positioning in the detector and (3) counting statistics. The estimated aggregate measurement uncertainty on the PNAR Ratio measurement is 0.008, to put this uncertainty in context, the difference in the PNAR Ratio between a fully irradiated assembly and this same assembly when fissile isotopes only absorb neutrons, but do not emit neutrons, is 0.106, a 13-sigma effect. The 1-sigma variation of 0.008 in the PNAR Ratio is estimated to correspond to a 3.2 GWd/tU change in assembly burnup.Peer reviewe

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

In the context of international nuclear safeguards, the International Atomic Energy Agency (IAEA) has recently approved passive gamma emission tomography (PGET) as a method for inspecting spent nuclear fuel assemblies (SFAs). 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 an 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 reconstruction. 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 show that our approach produces clear reconstructions which allow for a highly reliable classification of spent, missing, and fresh fuel rods.Peer reviewe