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Title: Analysis of an Indirect Neutron Signature for Enhanced UF6 Cylinder Verification

Abstract

The International Atomic Energy Agency (IAEA) currently uses handheld gamma-ray spectrometers combined with ultrasonic wall-thickness gauges to verify the declared enrichment of uranium hexafluoride (UF6) cylinders. The current method provides relatively low accuracy for the assay of 235U enrichment, especially for natural and depleted UF6. Furthermore, the current method provides no capability to assay the absolute mass of 235U in the cylinder due to the localized instrument geometry and limited penetration of the 186-keV gamma-ray signature from 235U. Also, the current verification process is a time-consuming component of on-site inspections at uranium enrichment plants. Toward the goal of a more-capable cylinder assay method, the Pacific Northwest National Laboratory has developed the hybrid enrichment verification array (HEVA). HEVA measures both the traditional 186-keV direct signature and a non-traditional, high-energy neutron-induced signature (HEVANT). HEVANT enables full-volume assay of UF6 cylinders by exploiting the relatively larger mean free paths of the neutrons emitted from the UF6. In this work, Monte Carlo modeling is used as the basis for characterizing HEVANT in terms of the individual contributions to HEVANT from nuclides and hardware components. Monte Carlo modeling is also used to quantify the intrinsic efficiency of HEVA for neutron detection in a cylinder-assay geometry.more » Modeling predictions are validated against neutron-induced gamma-ray spectra from laboratory measurements and a relatively large population of Type 30B cylinders spanning a range of enrichments. Implications of the analysis and findings on the viability of HEVA for cylinder verification are discussed, such as the resistance of the HEVANT signature to manipulation by the nearby placement of neutron-conversion materials.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1339798
Report Number(s):
PNNL-SA-121713
Journal ID: ISSN 0168-9002; DN4001030
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment; Journal Volume: 846
Country of Publication:
United States
Language:
English
Subject:
uranium hexafluoride; uranium enrichment; nondestructive assay; Monte Carlo; neutron measurements; nuclear safeguards

Citation Formats

Kulisek, Jonathan A., McDonald, Benjamin S., Smith, Leon E., Zalavadia, Mital A., and Webster, Jennifer B. Analysis of an Indirect Neutron Signature for Enhanced UF6 Cylinder Verification. United States: N. p., 2017. Web. doi:10.1016/j.nima.2016.12.003.
Kulisek, Jonathan A., McDonald, Benjamin S., Smith, Leon E., Zalavadia, Mital A., & Webster, Jennifer B. Analysis of an Indirect Neutron Signature for Enhanced UF6 Cylinder Verification. United States. doi:10.1016/j.nima.2016.12.003.
Kulisek, Jonathan A., McDonald, Benjamin S., Smith, Leon E., Zalavadia, Mital A., and Webster, Jennifer B. Tue . "Analysis of an Indirect Neutron Signature for Enhanced UF6 Cylinder Verification". United States. doi:10.1016/j.nima.2016.12.003.
@article{osti_1339798,
title = {Analysis of an Indirect Neutron Signature for Enhanced UF6 Cylinder Verification},
author = {Kulisek, Jonathan A. and McDonald, Benjamin S. and Smith, Leon E. and Zalavadia, Mital A. and Webster, Jennifer B.},
abstractNote = {The International Atomic Energy Agency (IAEA) currently uses handheld gamma-ray spectrometers combined with ultrasonic wall-thickness gauges to verify the declared enrichment of uranium hexafluoride (UF6) cylinders. The current method provides relatively low accuracy for the assay of 235U enrichment, especially for natural and depleted UF6. Furthermore, the current method provides no capability to assay the absolute mass of 235U in the cylinder due to the localized instrument geometry and limited penetration of the 186-keV gamma-ray signature from 235U. Also, the current verification process is a time-consuming component of on-site inspections at uranium enrichment plants. Toward the goal of a more-capable cylinder assay method, the Pacific Northwest National Laboratory has developed the hybrid enrichment verification array (HEVA). HEVA measures both the traditional 186-keV direct signature and a non-traditional, high-energy neutron-induced signature (HEVANT). HEVANT enables full-volume assay of UF6 cylinders by exploiting the relatively larger mean free paths of the neutrons emitted from the UF6. In this work, Monte Carlo modeling is used as the basis for characterizing HEVANT in terms of the individual contributions to HEVANT from nuclides and hardware components. Monte Carlo modeling is also used to quantify the intrinsic efficiency of HEVA for neutron detection in a cylinder-assay geometry. Modeling predictions are validated against neutron-induced gamma-ray spectra from laboratory measurements and a relatively large population of Type 30B cylinders spanning a range of enrichments. Implications of the analysis and findings on the viability of HEVA for cylinder verification are discussed, such as the resistance of the HEVANT signature to manipulation by the nearby placement of neutron-conversion materials.},
doi = {10.1016/j.nima.2016.12.003},
journal = {Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment},
number = ,
volume = 846,
place = {United States},
year = {Tue Feb 21 00:00:00 EST 2017},
month = {Tue Feb 21 00:00:00 EST 2017}
}
  • Here, the Ricochet experiment seeks to measure Coherent (neutral-current) Elastic Neutrino-Nucleus Scattering (CEνNS) using dark-matter-style detectors with sub-keV thresholds placed near a neutrino source, such as the MIT (research) Reactor (MITR), which operates at 5.5 MW generating approximately 2.2 × 10 18 ν/second in its core. Currently, Ricochet is characterizing the backgrounds at MITR, the main component of which comes in the form of neutrons emitted from the core simultaneous with the neutrino signal. To characterize this background, we wrapped Bonner cylinders around a 3 2He thermal neutron detector, whose data was then unfolded via a Markov Chain Monte Carlo (MCMC) to producemore » a neutron energy spectrum across several orders of magnitude. We discuss the resulting spectrum and its implications for deploying Ricochet at the MITR site as well as the feasibility of reducing this background level via the addition of polyethylene shielding around the detector setup.« less
  • International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF6 cylinder enrichment declarations. Measurements are performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facility’s operations. As additional enrichment plans come online to support the expansion of nuclear power, reducing person-days of inspection will take on greater importance. Pacific Northwest National Laboratory (PNNL) is developing a concept to automate the verification of enrichment plant cylinders to enable 100% product-cylinder verification and potentially, mass-balance calculations on the facility as a whole (by also measuring feed and tailsmore » cylinders). The Automated Cylinder Enrichment Verification System (ACEVS) would be located at key measurement points and will positively identify each cylinder, measure its mass and enrichment, store the data along with operator inputs in a secure database, and maintain continuity of knowledge on measured cylinders until IAEA inspector arrival. Given the potential for reduced inspector presence, the operational and manpower-reduction benefits of the portal concept are clear. However, it is necessary to assess whether the cylinder portal concept can meet, or potentially improve upon, today’s U-235 enrichment assay performance. PNNL’s ACEVS concept utilizes sensors that could be operated in an unattended mode: moderated He-3 neutron detectors and large NaI(Tl) scintillators for gamma-ray spectroscopy. The medium-resolution NaI(Tl) scintillators are a sacrifice in energy resolution but do provide high collection efficiency for signatures above 1 MeV. The He-3/NaI sensor combination allows the exploitation of additional, more-penetrating signatures than those currently utilized: Neutrons produced from F-19(μ,n) reactions (spawned primarily from U-234 alpha emission) and high-energy gamma rays (extending up to 10 MeV) induced by neutrons interacting in the steel cylinder. These signatures are indirect measures of U-235 that require a relatively stable U-234/U-235 ratio in the product material in order to be useful. The hypothesis of this work is that the U-234/U-235 ratio is sufficiently constant, for the specific facility where the automated system is installed, to rely on neutron and high-energy gamma-ray signatures for indirect measurement of U-235. Further, these highly penetrating signatures can be combined with a modified form of NaI-based 185-keV enrichment measurements to meet target uncertainties for the verification of product cylinders, with the additional benefits of full-volume assay of the cylinder and 100% product-cylinder verification (as opposed to today’s sampling-based approach). This paper focuses on the enrichment measurement aspects of the ACEVS concept: neutron and high-energy gamma-ray signatures, the radiation sensors designed to collect those signatures, and proof-of-principle cylinder measurements and analysis. Preliminary analysis indicates that an automated cylinder verification approach has the potential to meet target uncertainty values for 30B products cylinders (5%), assuming ore-based enrichment feed and a facility-specific calibration. Also described is the additional work needed to more definitively assess the concept’s viability, particularly through a better understanding of the U-234/U-235 ratio variability in modern enrichment plants.« less
  • Hybrid Enrichment Assay Methods for a UF6 Cylinder Verification Station
  • Pacific Northwest National Laboratory (PNNL) is developing the concept of an automated UF6 cylinder verification station that would be located at key measurement points to positively identify each cylinder, measure its mass and enrichment, store the collected data in a secure database, and maintain continuity of knowledge on measured cylinders until the arrival of International Atomic Energy Agency (IAEA) inspectors. At the center of this unattended system is a hybrid enrichment assay technique that combines the traditional enrichment-meter method (based on the 186 keV peak from 235U) with non-traditional neutron-induced high-energy gamma-ray signatures (spawned primarily by 234U alpha emissions andmore » 19F(alpha, neutron) reactions). Previous work by PNNL provided proof-of-principle for the non-traditional signatures to support accurate, full-volume interrogation of the cylinder enrichment, thereby reducing the systematic uncertainties in enrichment assay due to UF6 heterogeneity and providing greater sensitivity to material substitution scenarios. The work described here builds on that preliminary evaluation of the non-traditional signatures, but focuses on a prototype field system utilizing NaI(Tl) and LaBr3(Ce) spectrometers, and enrichment analysis algorithms that integrate the traditional and non-traditional signatures. Results for the assay of Type-30B cylinders ranging from 0.2 to 4.95 wt% 235U, at an AREVA fuel fabrication plant in Richland, WA, are described for the following enrichment analysis methods: 1) traditional enrichment meter signature (186 keV peak) as calculated using a square-wave convolute (SWC) algorithm; 2) non-traditional high-energy gamma-ray signature that provides neutron detection without neutron detectors and 3) hybrid algorithm that merges the traditional and non-traditional signatures. Uncertainties for each method, relative to the declared enrichment for each cylinder, are calculated and compared to the uncertainties from an attended HPGe verification station at AREVA, and the IAEA’s uncertainty target values for feed, tail and product cylinders. A summary of the major findings from the field measurements and subsequent analysis follows: • Traditional enrichment-meter assay using specially collimated NaI spectrometers and a Square-Wave-Convolute algorithm can achieve uncertainties comparable to HPGe and LaBr for product, natural and depleted cylinders. • Non-traditional signatures measured using NaI spectrometers enable interrogation of the entire cylinder volume and accurate measurement of absolute 235U mass in product, natural and depleted cylinders. • A hybrid enrichment assay method can achieve lower uncertainties than either the traditional or non-traditional methods acting independently because there is a low degree of correlation in the systematic errors of the two individual methods (wall thickness variation and 234U/235U variation, respectively). This work has indicated that the hybrid NDA method has the potential to serve as the foundation for an unattended cylinder verification station. When compared to today’s handheld cylinder-verification approach, such a station would have the following advantages: 1) improved enrichment assay accuracy for product, tail and feed cylinders; 2) full-volume assay of absolute 235U mass; 3) assay of minor isotopes (234U and 232U) important to verification of feedstock origin; single instrumentation design for both Type 30B and Type 48 cylinders; and 4) substantial reduction in the inspector manpower associated with cylinder verification.« less