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

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

Effect of Gamma-Ray Energy on Image Quality in Passive Gamma Emission Tomography of Spent Nuclear Fuel

Gamma-ray images of VVER-440 and SVEA-96 spent nuclear fuel assemblies were reconstructed using the filtered backprojection algorithm from measurements with a passive gamma emission tomography prototype instrument at Finnish nuclear power plants. Image quality evaluation criteria based on line profiles through the reconstructed image are used to evaluate image quality for spent fuel assemblies with different cooling times, and thus different mixtures of gamma-ray emitting isotopes. Image characteristics at the locations of water channels and central fuel pins are compared in two gamma-ray energy windows, 600-700 and >700keV, for cooling times up to 10 years for SVEA-96 fuel and 24.5 years for VVER-440 fuel. For SVEA-96 fuel, images in the >700-keV gamma-ray energy window present better water-to-fuel contrast for all investigated cooling times. For VVER-440, images in the >700-keV gamma-ray energy window have higher water-to-fuel contrast up to and including a cooling time of 18.5 years, whereas the water-to-fuel contrast of the images taken in the two gamma-ray energy windows is equivalent for a cooling time of 24.5 years. Images reconstructed from higher energy gamma rays such as those in the >700-keV energy window present better water-to-fuel contrast in fuel cooled for up to 20 years and thus have the most potential for missing fuel pin detection.Peer reviewe

Passive Neutron Albedo Reactivity in the Finnish encapsulation context

1 Introduction The purpose of this report is to document the Monte Carlo simulation and analytic results for the implementation of the Passive Neutron Albedo Reactivity (PNAR) nondestructive assay (NDA) technique in the context of Finnish spent fuel encapsulation needs. This document is offered as partial fulfillment of the contract between Encapsulation NDA Services and Radiation and Nuclear Safety Authority (STUK) of Finland. The end goal of this research effort is two conceptual designs of the PNAR technique for both BWR and PWR fuel in Finland

Octupole correlations in the structure of O2 bands in the N=88 nuclei150Sm Gd

Knowledge of the exact microscopic structure of the 01 + ground state and first excited 02 + state in 150Sm is required to understand the branching of double β decay to these states from 150Nd. The detailed spectroscopy of 150Sm and 152Gd has been studied using (α,xn) reactions and the γ -ray arrays AFRODITE and JUROGAM II. Consistently strong E1 transitions are observed between the excited Kπ = 02 + bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited Kπ = 02 + band and also in terms of the “tidal wave” model of Frauendorf.Web of Scienc

Thin NaI(Tl) crystals to enhance the detection sensitivity for molten Am-241 sources

A thin 5-min NaI(Tl) scintillator detector was tested with the goal of enhancing the detection efficiency of( 241)Am gamma and X rays for steelworks operations. The performance of a thin (5 mm) NaI(Tl) detector was compared with a standard 76.2-mm thick NaI(Tl) detector. The 5-mm thick detector crystal results in a 55% smaller background rate at 60 keV compared with the thicker detector, translating into the ability to detect 30% weaker Am-241 sources. For a 5 mm thick and 76.2 mm diameter NaI detector in the ladle car tunnel at Outokumpu Tornio Works, the minimum activity of a molten Am-241 source that can be detected in 5 s with 95% probability is 9 MBq.Peer reviewe

Effect of Water Gap and Fuel Assembly Positioning in Passive Neutron Albedo Reactivity Measurements for Spent Fuel Encapsulation Safeguards

The Passive Neutron Albedo Reactivity (PNAR) Ratio is proportional to the net neutron multiplication of a spent fuel assembly. In the planned integrated non-destructive assay instrument for Finnish encapsulation safeguards, a PNAR instrument is used to confirm the presence of fissile material. In this study, the sensitivity of fuel-type-specific PNAR Ratio measurements to the size of the water channel of the instrument is determined using MCNP5 Monte Carlo simulations. Based on the study results, use of the smallest possible water channel is recommended to maximize the dynamic range of the instrument. In the Finnish fuel encapsulation context, this means using water gap sizes of 5 mm and 3 mm for measurements of boiling water reactor (BWR) and water-water energetic reactor (VVER-440) fuel, respectively. Based on the neutron emission rates of the Finnish spent fuel inventory, we recommend maximizing count rates by having detectors all around the fuel assembly, i.e., 4 detectors for BWR fuel and 6 detectors for VVER-440 fuel. With these water gap sizes, and neutron detectors all around the fuel assembly, the variation of the PNAR Ratio measurement caused by the uncertainty on the position of the fuel in the instrument is estimated to be 0.06% for BWR fuel and 0.13% for VVER-440 fuel.Peer reviewe

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 [Formula: see text]% to [Formula: see text]% (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 109 n/s

Energy spectrum of gamma rays incident on the detector for the time windows <i>t</i> <1 μs, <i>t</i> <30 μs and <i>t</i> >50 μs.

<p>The fully optimized instrument design in the shell-side configuration is used. The total gamma ray spectrum is shown in (a), while gamma rays from the artillery shell and TNT payload are shown in (b).</p