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Journal Article•DOI•

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

Stephen J. Tobin1, Pauli Peura2, Camille Bélanger-Champagne2, Mikael Moring3, Peter Dendooven2, Tapani Honkamaa3 •
Los Alamos National Laboratory1, University of Helsinki2, Radiation and Nuclear Safety Authority3
21 Jul 2018-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment (North-Holland)-Vol. 897, pp 32-37
TL;DR: In this article, 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.
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.

Summary (4 min read)

Jump to: [1 INTRODUCTION] – [2 PASSIVE NEUTRON ALBEDO REACTIVITY PHYSICS] – [3 PASSIVE NEUTRON ALBEDO REACTIVITY VVER-440 HARDWARE] – [4 SIMULATED PASSIVE NEUTRON ALBEDO REACTIVITY SIGNAL] – [5 DYNAMIC RANGE AND UNCERTAINTY] – [6 UNCERTAINTY DUE TO VARIATION IN BORON CONTENT] – [7 UNCERTAINTY DUE TO POSITIONING OF ASSEMBLY IN DETECTOR] – [8 UNCERTAINTY DUE TO COUNTING STATISTICS] – [9 CUMULATIVE UNCERTAINTY, SENSITIVITY AND SAFEGUARDS] – [10 UNCERTAINTY IN THE SAFEGUARDS VERIFICATION CONTEXT] and [11 CONCLUSION]

1 INTRODUCTION

  • Among those characteristics, PNAR has the unique role, in the integrated system, of measuring the assembly’s neutron multiplication.
  • Their recommendations are not IAEA policy; the inclusion of multiplication as a metric is novel.
  • In Finland there will be two measurement locations.
  • The BWR fuel will be measured in fresh water while the VVER-440 fuel, as with most pressurized water reactor spent fuel pools, will be measured in borated water.

2 PASSIVE NEUTRON ALBEDO REACTIVITY PHYSICS

  • The PNAR concept involves the comparison of the neutron count rate of an object when that object is measured in two different setups.
  • One setup is designed to enhance neutron multiplication while the other setup is designed to suppress it.
  • As the result of criticality safety regulations, the water in pool containing VVER-440 fuel is borated, while the pool containing BWR fuel is fresh.
  • The PNAR signature, the PNAR Ratio, is calculated by dividing the count rate measured in the high multiplying section by the count rate measured in the low multiplying section.
  • In isolation, these high-energy neutrons that are unaffected by the Cd-liner create a PNAR Ratio of 1.0; any deviation from 1.0 is due to counts produced by chain reactions initiated by neutrons that are absorbed by the Cd-liner.

3 PASSIVE NEUTRON ALBEDO REACTIVITY VVER-440 HARDWARE

  • The PNAR conceptual design is part of an integrated NDA system that needs to meet the safeguards and safety needs of Finland in the context of VVER-440 spent fuel encapsulation and geological disposal.
  • In Figure 2 a horizontal cross-cut illustrates that there are three detectors around the assembly at one axial location.
  • Several aspects of the PNAR design are listed here:.
  • The 3He tube and cylindrical PE are surrounded by a layer of cadmium so that, as a unit, the detector module detects primarily epithermal and fast neutrons incident upon it.
  • The Cd-liner located close to the fuel, the full 0.74 m length of which is indicated in Figure 3, is the core hardware part needed to implement the PNAR concept.

4 SIMULATED PASSIVE NEUTRON ALBEDO REACTIVITY SIGNAL

  • To assess the capability of the PNAR detector customized for VVER-440 fuel, the PNAR Ratio was simulated and calculated using 12 assemblies that span a range of initial enrichment (3, 4 and 5 wt.%) and burnup (15, 30, 45 and 60 GWd/tU) for a cooling time of 20-years.
  • This boundary was defined as a cuboid, 0.4 m on two sides that extended 1.6 meters in the vertical direction.
  • There is a large difference in the PNAR Ratio between any irradiated assembly and a non-multiplying assembly; for the three assemblies simulated, the average difference in the PNAR Ratio is 0.137.
  • These additional data points are for the three fully irradiated assemblies for which the isotopic content was altered to represent the expected isotopic content after 40 and 80-years of cooling.

5 DYNAMIC RANGE AND UNCERTAINTY

  • The conclusions drawn from the simulated data illustrated in Figure 4 and Figure 5 assume that the cumulative uncertainty inherent in a PNAR measurement is small enough such that the noted trends are not obscured.
  • In the subsequent sections, the major anticipated uncertainties are analyzed to obtain an estimate of the expected aggregate uncertainty.
  • For this study, the authors will primarily focus on the dynamic range between the following two cases: a fresh 4 wt.% assembly and a 4 wt.%, 45 GWd/tU, 20-year cooled assembly.
  • Another useful uncertainty metric of comparison is the difference in the PNAR Ratio between a fully irradiated assembly and a non-multiplying assembly.

6 UNCERTAINTY DUE TO VARIATION IN BORON CONTENT

  • The change in PNAR Ratio for a change in boron content caused by the 2 g of boric acid per kg of water variation was 0.013 for a fresh assembly and 0.008 for a fully irradiated assembly.
  • The greater sensitivity of a fresh assembly to a change in the water boron content is expected because a fresh assembly is significantly more multiplying; hence, a given change in boron content will have a greater impact on changing the neutron multiplication.
  • Given these assumptions and the simulations performed, a one-sigma uncertainty of 0.005 in the PNAR Ratio is suggested for the boron content variation when measuring VVER-440 assemblies.
  • The decision to use 0.005 instead of 0.004 is a decision weighting the fact that most assemblies are fully irradiated but not all are.

7 UNCERTAINTY DUE TO POSITIONING OF ASSEMBLY IN DETECTOR

  • For all the simulation results presented in this report, the simulated assemblies were positioned in the center of the detector opening.
  • When measuring actual assemblies, it is noted that they will be suspended from the end of a crane from which they are lowered into the detector and that they are likely to have varying degrees of irradiation-induced bending, resulting in a non-centered assembly position.
  • Comparison between the two showed that there was a slight systematic shift between the two sets of codes and cross sections.
  • The results for which the MCNP5 code is being used is a comparison of MCNP5 results with MCNP5 results, a relative change.
  • The uncertainty calculated for the MCNP5 statistics of each of the individual PNAR Ratios was again 0.0011.

8 UNCERTAINTY DUE TO COUNTING STATISTICS

  • The uncertainty due to counting statistics for such an assembly is expected to be 0.004 when both PNAR section measurements last 2 minutes.
  • Yet, if the count time is increased for the particularly weak emitting assemblies or if more detector tubes are included, the authors expect that the one-sigma uncertainty due to counting statistics can be kept below 0.005 in the PNAR Ratio.

9 CUMULATIVE UNCERTAINTY, SENSITIVITY AND SAFEGUARDS

  • In the previous 3 sections, the one-sigma uncertainty in the PNAR Ratio was estimated for the variation in the boron content in the water, the positioning uncertainty of the assembly in the detector and the statistical uncertainty; values of 0.005, 0.002 and 0.005 were obtained, respectively.
  • If the two largest uncertainties are halved then a one-sigma uncertainty of 0.005 is possible.
  • Both the estimation of a 13 sigma variation between a fully irradiated assembly and non-multiplying assembly, as well as the estimation that a 1 sigma variation in the PNAR Ratio corresponds to a 3.2 GWd/tU variation in the burnup inform the utility of the PNAR instrument.
  • He tube and gross gamma intensity with a nitrogen filled ion chamber that must agree with the declaration.
  • Two suggestions are made with respect to how an inspectorate might use multiplication as a metric: (1) the calculations performed by the inspectorate, currently envisioned to be a SCALE + MCNP6 TM calculation, could simulate both the two parts of the PNAR measurement, with and without Cd present.

10 UNCERTAINTY IN THE SAFEGUARDS VERIFICATION CONTEXT

  • To this point in the paper the authors have focused on uncertainties that are associated with the PNAR hardware or the measurement environment: boron content of the water, counting statistics and assembly location in the detector.
  • These records may be more or less detailed because the data required as part of a safeguards declaration are often less detailed than the records maintained by facilities.
  • Often pin-by-pin burnup data is available; yet, such detailed data does not need to be declared.
  • What level of detail is provided by the State to Euratom and the IAEA is outside of the scope of this work; the point being made here is that the State may want to provide more detail to increase the likelihood of agreement between the measured values and the values estimated from by simulations using the declared data as input.
  • The uncertainty inherent in the simulation of the PNAR Ratio and/or the net multiplication by the coupled SCALE and MCNP6 is a topic beyond the scope of the current research effort.

11 CONCLUSION

  • By combining PNAR, PGET and a load cell, STUK has created an integrated NDA system that satisfies all the characteristics suggested by the NDA Focus Group convened by the IAEA as part of the ASTOR Experts Group.
  • The performance of the PNAR instrument designed to measure VVER-440 fuel was examined.
  • The PNAR instrument was included in the integrated system to measure the assembly’s neutron multiplication; this capability is of particular interest in the context of the VVER-440 fuel in Finland because the instrument must work in a pool of borated water, which reduces the neutron multiplication.
  • The uncertainty caused by variation in the boron content, assembly positioning in the detector and counting statistics were all examined to estimate an aggregate uncertainty of 0.008 in the PNAR Ratio.
  • The anticipated sensitivity of the PNAR instrument for the VVER-440 case was quantified.

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Measuring spent fuel assembly multiplication in borated water
with a passive neutron albedo reactivity instrument
Tobin, Stephen J.
2018-07-21
Tobin , S J , Peura , P , Bélanger-Champagne , C , Moring , M , Dendooven , P & Honkamaa
, T 2018 , ' Measuring spent fuel assembly multiplication in borated water with a passive
neutron albedo reactivity instrument ' , Nuclear Instruments & Methods in Physics Research.
Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment , vol. 897 ,
pp. 32-37 . https://doi.org/10.1016/j.nima.2018.04.044
http://hdl.handle.net/10138/314628
https://doi.org/10.1016/j.nima.2018.04.044
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This is an electronic reprint of the original article.
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Accepted Manuscript
Measuring spent fuel assembly multiplication in borated water with a
passive neutron albedo reactivity instrument
Stephen J. Tobin, Pauli Peura, Camille Bélanger-Champagne, Mikael Moring,
Peter Dendooven, Tapani Honkamaa
PII: S0168-9002(18)30551-5
DOI: https://doi.org/10.1016/j.nima.2018.04.044
Reference: NIMA 60764
To appear in: Nuclear Inst. and Methods in Physics Research, A
Received date : 17 January 2018
Revised date : 15 April 2018
Accepted date : 22 April 2018
Please cite this article as: S.J. Tobin, P. Peura, C. Bélanger-Champagne, M. Moring, P. Dendooven,
T. Honkamaa, Measuring spent fuel assembly multiplication in borated water with a passive
neutron albedo reactivity instrument, Nuclear Inst. and Methods in Physics Research, A (2018),
https://doi.org/10.1016/j.nima.2018.04.044
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Measuring Spent Fuel Assembly Multiplication in Borated Water with a Passive
Neutron Albedo Reactivity Instrument
Stephen J. Tobin
1
, Pauli Peura
2
, Camille Bélanger-Champagne
2
, Mikael Moring
3
,
Peter Dendooven
2
, Tapani Honkamaa
3
1
Encapsulation Nondestructive Assay Services, Los Alamos, NM 87544, USA.
2
Helsinki Institute of Physics, FI-00014 University of Helsinki, Finland.
3
Radiation and Nuclear Safety Authority - STUK, 00881 Helsinki, Finland.
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.
KEY WORDS: Spent fuel encapsulation, Non-destructive Assay
1 INTRODUCTION
The Finnish Radiation and Safety Authority (STUK), in order to implement the
recommendation of the International Atomic Energy Agency (IAEA) assembled NDA
experts outlined in the “Application of Safeguards to Geological Repositories
(ASTOR) Report on Technologies Potentially Useful for Safeguarding Geological
Repositories,” [1] funded research to conceptually design two integrated
nondestructive assay (NDA) systems; one system to measure boiling water reactor
(BWR) fuel and one to measure VVER-440 fuel. The integrated instruments each
have three parts, a Passive Gamma Emission Tomography (PGET) instrument [1, 2,
3, 4], a Passive Neutron Albedo Reactivity (PNAR) instrument [1, 5, 6, 7] and a load
cell that will measure the assembly weight. This study will focus on the PNAR
instrument, which supports several of the recommended characteristics outlined for
*Manuscript
Click here to view linked References
2
the NDA system by the ASTOR experts. Among those characteristics, PNAR has the
unique role, in the integrated system, of measuring the assembly’s neutron
multiplication. Although the ASTOR participants were organized by the IAEA, their
recommendations are not IAEA policy; the inclusion of multiplication as a metric is
novel. Multiplication was included because it is a direct indication of the presence of
fissile material.
In Finland there will be two measurement locations. The BWR fuel will be measured
in fresh water while the VVER-440 fuel, as with most pressurized water reactor spent
fuel pools, will be measured in borated water. The task of measuring the assembly’s
neutron multiplication in borated water reduces the sensitivity of the instrument and
increases the uncertainty. The current study quantifies both the anticipated sensitivity
and uncertainty of a conceptual PNAR instrument designed to measure VVER-440
fuel in borated water.
2 PASSIVE NEUTRON ALBEDO REACTIVITY PHYSICS
The PNAR concept involves the comparison of the neutron count rate of an object
when that object is measured in two different setups. One setup is designed to
enhance neutron multiplication while the other setup is designed to suppress it. As
implemented in Finland, the high multiplying section is produced by the assembly in
water, while the low multiplying section is created by putting 1 mm of Cd as close as
possible to the fuel while it remains in the pool. As the result of criticality safety
regulations, the water in pool containing VVER-440 fuel is borated, while the pool
containing BWR fuel is fresh. Cd was selected for the low multiplying section due to
its extremely large absorption cross-section for all neutron energies below ~0.5 eV.
The PNAR signature, the PNAR Ratio, is calculated by dividing the count rate
measured in the high multiplying section by the count rate measured in the low
multiplying section.
The PNAR implementation in Finland, an implementation that combines (a) a
3
He
detector tube and polyethylene (PE) surrounded by Cd and (b) a low multiplying
section produced with a Cd-liner, lends itself to a conceptual discussion of the PNAR
physics. The only significant difference in the measured count rate for a section of
fuel measured in both the high and low multiplying sections, is the counts resulting
from the multiplication caused by the neutrons that are absorbed in the Cd-liner. The
contribution from neutrons not absorbed in the Cd-liner, are in both the numerator and
denominator of the PNAR Ratio. In isolation, these high-energy neutrons that are
unaffected by the Cd-liner create a PNAR Ratio of 1.0; any deviation from 1.0 is due
to counts produced by chain reactions initiated by neutrons that are absorbed by the
Cd-liner. Because the PNAR signal is produced by the neutrons returning into the fuel
with an energy below the Cd-cutoff energy of ~0.5 eV, the PNAR technique is
sometimes described as interrogating the fuel with low energy neutrons from the
location of the Cd-liner.
3
3 PASSIVE NEUTRON ALBEDO REACTIVITY VVER-440 HARDWARE
The PNAR conceptual design is part of an integrated NDA system that needs to meet
the safeguards and safety needs of Finland in the context of VVER-440 spent fuel
encapsulation and geological disposal. In Figure 1, a vertical cross-cut of the VVER-
440 PNAR detector module is illustrated. In Figure 2 a horizontal cross-cut illustrates
that there are three detectors around the assembly at one axial location. Below the
three detector modules illustrated at one axial level are three more detectors which are
rotated around the fuel assembly by 60 degrees. The two levels are separated by ~0.1
m. In Figure 3, the full 74 cm vertical extent of the detector is evident as well as the
vertical separation between the two detector layers as one detector from each layer is
shown. This number of detectors was selected to improve simulation statistics as well
as to enable research into how the number of detectors impacts the sensitivity of the
instrument to assembly location in the detector. The final deployment is expected to
have three detectors unless there is some need for redundant instruments.
Several aspects of the PNAR design are listed here:
The
3
He in the neutron detector has a 0.1 m active length, 17.4 mm or 3/4
th
inch diameter, 6 atm pressure, and is surrounded by a cylinder of PE that has a
diameter of 58 mm.
The
3
He tube and cylindrical PE are surrounded by a layer of cadmium so that,
as a unit, the detector module detects primarily epithermal and fast neutrons
incident upon it.
The layer of lead is 46 mm thick at the thickest point in Figure1.
Location of Figure 1
Location of Figure 2
Location of Figure 3
The Cd-liner located close to the fuel, the full 0.74 m length of which is indicated in
Figure 3, is the core hardware part needed to implement the PNAR concept. This Cd-
liner, in the Finnish implementation of PNAR, will be mobile. For the low
multiplication part of the PNAR measurement, it will be located as illustrated and for
the high multiplying part it will be moved below the PE slab.
The PE slab located outside the detector modules is there for two primary reasons: (a)
to raise the neutron multiplication of an assembly inside the detector when the Cd-
liner is not present and (b) to reduce the uncertainty in the neutron count rate resulting
from the variation in the boron content of the water
Citations
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Comparison of machine learning models for the detection of partial defects in spent nuclear fuel

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Passive neutron albedo reactivity measurements of spent nuclear fuel

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Topi Tupasela, Peter Dendooven1, Stephen J. Tobin2, Vladyslav Litichevskyi1, Pirkitta Koponen1, Asko Turunen, Mikael Moring, Tapani Honkamaa •
University of Helsinki1, Los Alamos National Laboratory2
11 Jan 2021-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, the authors presented a verification system for spent nuclear fuel in Finland using a combination of passive gamma emission tomography (PGET), passive neutron albedo reactivity (PNAR), and weight measuring NDA-instruments.
Abstract: The upcoming disposal of spent nuclear fuel in Finland creates new challenges for nuclear safeguards. Part of the national safeguards concept for geological repositories, developed by STUK — Radiation and Nuclear Safety Authority, is non-destructive assay (NDA) verification of all fuel items before disposal. The proposed verification system is a combination of PGET (Passive Gamma Emission Tomography), PNAR (Passive Neutron Albedo Reactivity) and weight measuring NDA-instruments. PGET takes a pin-level image of the fission products inside of a fuel assembly and PNAR verifies the multiplication of the assembly, a quantity that correlates with the fissile content. PGET is approved by IAEA (International Atomic Energy Agency) for safeguards measurements, but the feasibility of PNAR has not yet been established. A first of its kind PNAR prototype instrument was built in a collaboration coordinated by STUK. This paper concludes the results of the first measurements of spent BWR (Boiling Water Reactor) nuclear fuel with the prototype in July 2019. Based on the measurements, the ability of the PNAR instrument to detect the presence of fissile material in a repeatable manner in a reasonable amount of time was demonstrated. Furthermore, the instrument was able to detect differences in multiplication between partially and fully spent fuel assemblies, and axial differences in multiplication within a single assembly.

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

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TL;DR: In this article , the PGET device consists of two highly collimated detector banks, collecting gamma emission data from a 360° rotation around a fuel assembly, which is used to detect partial diversion of nuclear material also in the axial direction, demonstrated with a measurement series scanning over the edge of partial-length rods.
Abstract: Abstract 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° 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.

2 citations

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International Atomic Energy Agency1, Radiation and Nuclear Safety Authority2, Swedish Radiation Safety Authority3
01 Oct 2017
TL;DR: The Passive Gamma Emission Tomography (PGET) as discussed by the authors was developed for the IAEA Safeguards for verification of irradiated nuclear fuel assemblies (SFAs) in 2015-2016 and its performance has been tested on multiple SFA types.
Abstract: A Passive Gamma Emission Tomography system (PGET) [1] was developed for the IAEA Safeguards for verification of irradiated nuclear fuel assemblies (SFAs). In 2015-2016 PGET underwent significant re-design and its performance has been tested on multiple SFA types. The re-designed PGET features the functionality of traditional non-destructive assay systems commonly used for spent fuel verification: total neutron counting (Fork Detector, FDET), medium-resolution gamma spectrometry (Irradiated Item Attribute Tester, IRAT or Spent Fuel Attribute Tester, SFAT) and spent fuel assembly’s lattice image (Digital Cherenkov Viewing Device, DCVD). Two 10B neutron detectors and one-hundred-seventy-four collimated CdZnTe detectors are grouped in two arrays on a rotary baseplate inside a watertight stainless steel enclosure. A SFA is lowered through the center of the enclosure and held stationary to perform an underwater measurement. Detector arrays are then rotated on a baseplate in the horizontal plane around vertical axis of symmetry to obtain gamma sinogram and neutron count data simultaneously, typically in 3-5 min per assembly. Additionally, medium resolution spectra from all gamma detectors can be collected and recorded. Functional, technical and operational performance of the PGET was tested at four nuclear reactors on mockup, PWR, BWR and WWER-440 SFAs. Measurements have been performed on fuel with burnup in the range 5.7-58GWd/tU and cooling times from 1.9 to 27years. Lateral pin structure of the SFAs could be reconstructed for any tested fuel design in the above range of cooling times and burnups. Missing or replaced pins in all fuel types could be clearly visualized in the reconstructed images; spectrometric information (134Cs/137Cs peak ratio) and neutron counting rates were found to be consistent with declared fuel radiation history. This paper describes details of the PGET hardware and electronics and presents some results of performance evaluation.

20 citations

"Measuring spent fuel assembly multi..." refers methods in this paper

  • ...The integrated instruments each have three parts, a Passive Gamma Emission Tomography (PGET) instrument [1, 2, 3, 4], a Passive Neutron Albedo Reactivity (PNAR) instrument [1, 5, 6, 7] and a load cell that will measure the assembly weight....

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Technical Cross-Cutting Issues for the Next Generation Safeguards Initiative's Spent Fuel Nondestructive Assay Project

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S. J. Tobin, Howard O. Menlove, Martyn T. Swinhoe, Pauline Blanc, Tom Burr, Louise G Evans, Andrea Favalli, Michael L Fensin, C. R. Freeman, Jack D. Galloway, J. Gerhart, A. Rajasingam, E. Rauch, Nathan P. Sandoval, Holly R. Trellue, Timothy J. Ulrich, Jeremy Lloyd Conlin, Stephen Croft, John Hendricks, Vladimir Henzl, Daniela Henzlova, J. M. Eigenbrodt, William E Koehler, D. W. Lee, T. H. Lee, Adrienne M. LaFleur, Melissa A Schear, M. A. Humphrey, Leon E. Smith, Kevin K. Anderson, Luke W. Campbell, Andrew M. Casella, Christopher J. Gesh, Mark W. Shaver, Alex C. Misner, S. D. Amber, Bernhard A. Ludewigt, Brian J. Quiter, Alexander A Solodov, William S. Charlton, A. Stafford, Catherine E Romano, Jesse Cheatham, Michael H Ehinger, S. J. Thompson, David Chichester, James Sterbentz, Jianwei Hu, Alan W. Hunt, Vladimir Mozin, J. G. Richard 
01 Mar 2012
TL;DR: In this paper, the role of neutron absorbers with emphasis on how these absorbers vary in spent fuel (SF) as a function of initial enrichment, burnup (BU) and cooling time (CT).
Abstract: Ever since there has been spent fuel (SF), researchers have made nondestructive assay (NDA) measurements of that fuel to learn about its content. In general these measurements have focused on the simplest signatures (passive photon and total neutron emission) and the analysis has often focused on diversion detection and on determining properties such as burnup (BU) and cooling time (CT). Because of shortcomings in current analysis methods, inspectorates and policy makers are interested in improving the state-of-the-art in SF NDA. For this reason the U.S. Department of Energy, through the Next Generation Safeguards Initiative (NGSI), targeted the determination of elemental Pu mass in SF as a technical goal. As part of this research effort, 14 nondestructive assay techniques were studied . This wide range of techniques was selected to allow flexibility for the various needs of the safeguards inspectorates and to prepare for the likely integration of one or more techniques having complementary features. In the course of researching this broad range of NDA techniques, several cross-cutting issues were. This paper will describe some common issues and insights. In particular we will describe the following: (1) the role of neutron absorbers with emphasis on how these absorbers vary in SFmore » as a function of initial enrichment, BU and CT; (2) the need to partition the measured signal among different isotopic sources; and (3) the importance of the “first generation” concept which indicates the spatial location from which the signal originates as well as the isotopic origins.« less

20 citations

"Measuring spent fuel assembly multi..." refers methods in this paper

  • ...The isotopic mixture of the various assemblies was produced by the Monteburns code [10] as part of the Next Generation Safeguards Initiative [11, 12]....

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Frequently Asked Questions (8)
Q1. What have the authors contributed in "Measuring spent fuel assembly multiplication in borated water with a passive neutron albedo reactivity instrument tobin," ?

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. 

The task of measuring the assembly’s neutron multiplication in borated water reduces the sensitivity of the instrument and increases the uncertainty. 

For a typical VVER assembly to be measured at the Finnish encapsulation facility a burnup of ~32 GWd/tU and a cooling time of ~40 years is anticipated. 

The PE slab located outside the detector modules is there for two primary reasons: (a) to raise the neutron multiplication of an assembly inside the detector when the Cdliner is not present and (b) to reduce the uncertainty in the neutron count rate resulting from the variation in the boron content of the water4To assess the capability of the PNAR detector customized for VVER-440 fuel, the PNAR Ratio was simulated and calculated using 12 assemblies that span a range of initial enrichment (3, 4 and 5 wt.%) and burnup (15, 30, 45 and 60 GWd/tU) for a cooling time of 20-years. 

For the three assemblies that were irradiated to the level at which assemblies are generally removed from a commercial reactor (3 wt.% and 30 GWd/tU, 4 wt.% and 45 GWd/tU, 5 wt.% and 60 GWd/tU), which are three assemblies with PNAR Ratios of about 1.10, additional simulations were performed to calculate the PNAR Ratio for the case when no induced fission could take place. 

The uncertainty inherent in the simulation of the PNAR Ratio and/or the net multiplication by the coupled SCALEand MCNP6 is a topic beyond the scope of the current research effort. 

The following points are the reasons for including a PNAR instrument in the safeguards system: (1) PNAR indicates that fissile material is present in the assembly. 

It is interesting to note that the two largest uncertainties can reasonably be reduced by a factor of two in the following manner: (a) measure the boron content so that a correction to the PNAR Ratio calculation can be used, and (b) make the PNAR detector more efficient and/or count for longer.