Simulating the Passive Neutron and Gamma Signatures of Containerized Nuclear Warheads for Disarmament Verification

Abstract

For nuclear disarmament verification, measuring passive neutron and gamma signatures is discussed for confirming the presence of weapons-grade plutonium. Using the Geant4 code, the effects of neutron and photon interactions with the various materials of containerized items are explored for (i) notional fission and thermonuclear warheads waiting for dismantlement, (ii) intentionally shielded plutonium in a scrap container. Due to strong neutronic linking of the various warhead materials neutron multiplicity measurements can not be expected to give correct results. Gamma emissions of the plutonium may even be completely shielded by a tamper. Gamma spectrometry could verify the presence of explosives from (n,γ) activation of hydrogen and nitrogen as well as of fission processes from their prompt fission gamma emissions. Limiting diameters of scrap containers together with long-time gamma measurements of the absence of photons produced by (n,γ) activation of shielding materials will provide an effective approach for detecting an intentional diversion of plutonium.

Acknowledgements

This work was initiated by and benefited much from the many inspiring and fruitful discussions the corresponding author had during the last years within the International Partnership for Nuclear Disarmament Verification. Thanks to all of them, but in particular to Nico van Xanten (Authority for Nuclear Safety and Radiation Protection, The Netherlands) for his continuous interest in the reliability of fissile material absence measurements.

Notes

1 These include S. Drell, P. Banks, C. Callan, K. Case, J. Cornwall, F. Dyson, D. Eardley, N. Fortson, M. Freedman, R. Garwin, et al., “Verification Technology: Unclassified Version,” JASON, The MITRE Corporation, McLean, VA, USA, 1990; U.S. Department of Energy, Office of Arms Control and Nonproliferation, “Transparency and Verification Options: An Initial Analysis of Approaches for Monitoring Warhead Dismantlement,” Washington, DC, USA, 1997; Christine Comley, Mike Comley, Peter Eggins, Garry George, Steve Holloway, Martin Ley, Paul Thompson, Keith Warburton, “Confidence, Security and Verification: The Challenges of Global Nuclear Weapons Arms Control,” Atomic Weapons Establishment, AWE/TR/2000/01, Aldermaston, UK, 2000; National Academy of Sciences, “Monitoring Nuclear Weapons and Nuclear-Explosive Materials: An Assessment of Methods and Capabilities,” The National Academies Press, Washington, DC, USA, 2005; David Cliff, Hassan Elbathimy, and Andreas Persbo, “Verifying Warhead Dismantlement - Past, Present, Future,” Verification Research, Training and Information Centre (VERTIC), London, UK, https://www.vertic.org/media/assets/Publications/VM9.pdf; Nuclear Threat Initiative, “Innovating Verification: New Tools & New Actors to Reduce Nuclear Risks: Verifying Baseline Declarations of Nuclear Warheads and Materials,” Washington, DC, USA, 2014, https://www.nti.org/analysis/articles/innovating-verification-new-tools-new-actors-reduce-nuclear-risks/. Building on this work, the International Partnership on Nuclear Disarmament Verification combines expertise and perspectives of more than 25 countries for developing concepts and procedures, identifying potentially attractive technologies and supporting practical exercises, Corey Hinderstein, “International Partnership for Nuclear Disarmament Verification: Laying a foundation for future arms reductions,” Bulletin of the Atomic Scientist 74 (2018): 305–11.

2 According to the U.S. Department of Energy it requires 5–21 days depending on the warhead type, https://www.youtube.com/watch?v=SOajFJIzQew.

3 This contribution focuses on plutonium as fissile material, since the neutron source term of HEU is marginal (approx. 1 s⁻¹ kg⁻¹).

4 “Transparency and Verification Options,” 76; “Confidence, Security and Verification,” 5, 25, 31; “Monitoring Nuclear Weapons and Nuclear-Explosive Materials,” 97–108; “Innovating Verification,” 31–2, 33–4.

5 These were reviewed by R. C. Runkle, A. Bernstein, and P. E. Vanier, “Securing special nuclear material: Recent advances in neutron detection and their role in nonproliferation,” Journal of Applied Physics 108 (2010): 111101; Malte Göttsche and Gerald Kirchner, “Measurement Techniques for Warhead Authentication with Attributes: Advantages and Limitations,” Science & Global Security 22 (2014): 83–110; Jie Yan and Alexander Glaser, “Nuclear Warhead Verification: A Review of Attribute and Template Systems,” Science & Global Security 23 (2015): 157–70; K. P. Ziock, “Principles and Applications of Gamma-Ray Imaging for Arms Control,” Nuclear Instruments and Methods in Physics Research A 878 (2018): 191–9.

6 A prominent example is the Trusted Radiation Identification System, Michael Hamel, “The Trusted Radiation Information System (TRIS): Designed to Encourage Trust,” INMM Just Trust Me Workshop, Albuquerque, NM, USA, 12–13 March 2019; TRIS discriminated between various warhead and component types in the 1997 radiation signature measurement campaign at the U.S. Pantex plant, see “Monitoring Nuclear Weapons and Nuclear-Explosive Materials,” 101.

7 Thomas E. Shea, “Report on the Trilateral Initiative,” IAEA Bulletin 43 (2001): 49–53; “Confidence, Security and Verification,” 18–19, 46–8; “Innovating Verification,” 31–32; “Measurement Techniques for Warhead Authentication with Attributes,” 87.

8 “Verification Technology,” 94, 96; “Transparency and Verification Options,” 140; “Confidence, Security and Verification,” 18; “Innovating Verification,” 32–3, 42.

9 This holds for the results addressed in the reviews cited in note 1 above and for the attribute measurement systems developed cooperatively by Russian and U.S. laboratories, https://www.lanl.gov/orgs/n/n1/FMTTD/presentations/pdf_docs/exec_sum.pdf; Sergey Kondratov, M. Bulatov, D. Decman, M. Leplyavkina, A. Livke, S. J. Luke, D. MacArthur, S. Razinkov, D. Sivachev, J. Thron, S. et al., “AVNG System Demonstration,” 51^st INMM Annual Meeting, Baltimore, MD, USA, 11–15 July 2010. It also holds for recent publications, see J. M. Mueller and J. Mattingly, “Passive One-dimensional Self-transmission Imaging of Subcritical Metallic Plutonium Assemblies,” Nuclear Instruments and Methods in Physics Research A 903 (2018): 277–86; Pete Chapman, Jonathan Mueller, Jason Newby, and John Mattingly, “Exploiting Fission Chain Reaction Dynamics to Image Fissile Materials,” Nuclear Instruments and Methods in Physics Research A 935 (2019): 198–206; and Eric Lepowsky, Jihje Jeon, and Alexander Glaser, “Confirming the Absence of Nuclear Warheads via Passive Gamma-Ray Measurements,” Nuclear Instruments and Methods in Physics Research A 990 (2021): 164983.

10 The effect of HDPE on neutron multiplicity measurements was studied by E. C. Miller, B. Dennis, S.D. Clarke, S. A. Pozzi, and J. K. Mattingly, “Simulation of Polyethylene-Moderated Plutonium Neutron Multiplicity Measurements,” Nuclear Instruments and Methods in Physics Research A 652 (2011): 540–43; its impact on detector efficiency by Mateusz Monterial, Peter Marleau, Marc Paff, Shaun Clarke, and Sara Pozzi, “Multiplication and Presence of Shielding Material from Time-Correlated Pulse-Height Measurements of Subcritical Plutonium Assemblies,” Nuclear Instruments and Methods in Physics Research A 851 (2017): 50–6; and the effect of HDPE on fission chain distributions by G. Heger, C. Dubi, A. Ocherashvili, B. Petersen, and E. Gilad, “Identifying Neutron Shielding in Neutron Multiplicity Counting,” Nuclear Instruments and Methods in Physics Research A 901 (2018): 40–5. Gamma signals produced by neutron activation in HDPE, which was placed in front of a detector, were addressed by Malte Göttsche, Janet Schirm, and Alexander Glaser, “Low-Resolution Gamma Ray Spectrometry for an Information Barrier Based on a Multi-Criteria Template-Matching Approach,” Nuclear Instruments and Methods in Physics Research A 840 (2016): 139–44.

11 K. Böhnel, “The Effect of Multiplication on the Quantitative Determination of Spontaneously Fissioning Isotopes by Neutron Correlation Analysis,” Nuclear Science and Engineering 90 (1985): 75–82; N. Ensslin, W. C. Harker, M. S. Krick, D. G. Langner, M. M. Pickrell, and J. E. Stewart, “Application Guide to Neutron Multiplicity Counting,” LA-13422-M, 50–51, Los Alamos, NM, 1998.

12 Marc Looman, Paolo Peerani, and Hamid Tagziria, “Monte Carlo Simulations of Neutron Counters in Safeguards Applications,” Nuclear Instruments and Methods in Physics Research A598 (2009): 542–50; P. Peerani, M. Swinhoe, A.L. Weber, and L. G. Evans, “ESARDA Multiplicity Benchmark Exercise,” ESARDA Bulletin 42 (2009): 2–25.

13 Since attribute measurements monitor whether defined threshold values are met by the analyzed item, a limited bias could be taken into account by adjusting the thresholds accordingly.

14 Steve Fetter, Valery A. Frolov, Marvin Miller, Robert Mozley, Oleg F. Prilutsky, Stanislav N. Rodionov, and Roald Z. Sagdeev, “Detecting Nuclear Warheads,” Science & Global Security 1 (1990): 225–302; G. Kessler, “Plutonium Denaturing by ²³⁸Pu,” Nuclear Science and Engineering 155 (2007): 53–73; Yoshiki Kamura, Masaki Saito, and Hiroshi Sagara, “Evaluation of Proliferation Resistance of Plutonium Based on Decay Heat,” Journal of Nuclear Science and Technology 48 (2011): 715–23.

15 In “Detecting Nuclear Warheads,” 232–50, passive and neutron-induced gamma and neutron flux densities were estimated; Huang Meng, Zhu Jianyu, Wu Jun, and Li Rui, “A Passive Method for the Detection of Explosives and Weapon-Grade Plutonium in Nuclear Warheads,” Science & Global Security 26 (2018): 57–69 explored the potential of verifying the presence of high explosives by (n,γ) activation processes; Pierre-Luc Drouin, “Geant4 Simulation Study of Nuclear Weapon Detection Using a CLYC Detector,” Defence Research and Development Canada, DRDC-RDDC-2018-R012 (2018) calculated photon spectra; and Moritz Kütt, Jan Elfes, and Christopher Fichtlscherer, “Fetter Model Revisited: Detecting Nuclear Weapons 30 Years Later,” INMM & ESARDA Joint Virtual Annual Meeting, 23–26 August, 30 August–1 September 2021, simulated neutron fluxes within the configuration.

16 Another potential deliberate deception method could be to replace the core by other fissile material. Since such a hoax may not include explosives due to difficulties of adopting these to a differing geometry, as discussed in “A Passive Method for the Detection of Explosives and Weapon-Grade Plutonium in Nuclear Warheads,” 58, this option has not been considered.

17 Partial exceptions are analyses of the AT-400R container on gamma and neutron correlation count rates from bare plutonium metal, Sara A. Pozzi, and John T. Mihalczo, “Monte Carlo Evaluation of Passive Correlation Measurements on Containerized Plutonium Shells,” 44^th INMM Annual Meeting, 13 July 2003, Phoenix, AZ, USA, and “Fetter Model Revisited” presenting neutron fluxes over component interfaces of the Fetter et al. model and of the energy spectrum of emitted neutrons.

18 When discussing the use of passive radiation measurements within IPNDV, representatives of a nuclear weapon state explained that according to their domestic safety regulations keeping 1 m distance from a fully assembled warhead is mandatory.

19 Irmgard Niemeyer, Jan Geisel-Brinck, Simon Hebel, Philip Kegler, Gerald Kirchner, Manuel Kreutle, and Stefan Neumeier, “Moving From Paper To Practice In Nuclear Disarmament Verification: NuDiVe–The Nuclear Disarmament Verification Exercise,” 61. Annual Meeting Proceedings, Institute of Nuclear Materials Management, 12–16 July 2020, virtual.

20 “Detecting Nuclear Warheads,” Appendix A, 255–63.

21 Nicholas J. Whitworth, “Modelling Detonation in Ultrafine TATB Hemispherical Boosters Using CREST,” 17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter, 26 June - 1 July 2011, Chicago, IL, USA, American Institute of Physics Conference Proceedings 1426 (2012): 213–16.

22 Pavel Podvig and Ryan Snyder, “Watch Them Go: Simplifying the Elimination of Fissile Materials and Nuclear Weapons,” United Nations Institute for Disarmament Research (UNIDIR), Geneva, 2019, 28, https://doi.org/10.37559/WMD/19/NuclearVer01.

23 International Panel on Fissile Materials, “Global Fissile Material Report 2015: Nuclear Weapon and Fissile Material Stockpiles and Production,” 2015, Appendix 1, 41–3, https://fissilematerials.org/publications/2015/12/global_fissile_material_report_7.html; Information on US W87 and W88 warheads in “The Nuclear Weapon Archive,” https://nuclearweaponarchive.org/ have been used also.

24 “Detecting Nuclear Warheads,” 238.

25 Except for the slightly larger dimensions it corresponds to the ALR8 container, which in the United States has been used for storage of ex-weapon plutonium pits, see Yevgeni V. Terekhine, Theodore A. Parish, and William S. Charlton, “Shielding and Criticality Characterization of ALR8(SI) Plutonium Storage Containers,” Journal of Nuclear Science and Technology 37, sup 1 (2000): 347–51; we disregarded the steel insert present in the ALR8(SI) version for corrosion prevention.

26 S. Agostinelli, J. Allison, K. Amako et al., “GEANT4–A Simulation Toolkit,” Nuclear Instruments and Methods in Physics Research A 506 (2003): 250–303; J. Allison, K. Amako, J. Apostolakis, P. Arce, M. Asai, T. Aso, E. Bagli, A. Bagulya, S. Banerjee, G. Barrand, et al., “Recent Developments in GEANT4,” Nuclear Instruments and Methods in Physics Research A 835 (2016): 186–225.

27 Examples illustrating the range of topics addressed with Geant4 are: A. Sh. Georgadze, “Monte Carlo Simulation of Active Neutron Interrogation System Developed for Detection of Illicit Materials,” Acta Physica Polonica B 48 (2017): 1683–91; Lin Zhuang, Quanhu Zhang, Wenming Zuo, and Chen Chen, “Computer Simulation of Fast Neutron Multiplicity Analysis,” Advances in Engineering Research 129 (2017): 225–30; “Geant4 Simulation Study of Nuclear Weapon Detection Using a CLYC Detector”; Daniel C. Poulson, “Interrogation of Spent Nuclear Fuel Casks Using Cosmic-Ray Muon Computed Tomography,” PhD Dissertation (2019), University of New Mexico, https://digitalrepository.unm.edu/ne_etds/90.

28 Jérôme M. Verbeke, Chris Hagmann, and Doug Wright, “Simulation of Neutron and Gamma Ray Emission from Fission and Photofission,” UCRL-TR-228518-REV-1, Livermore, California, 2016, https://nuclear.llnl.gov/simulation/fission.pdf.

29 Peter J. Karpius, William Clay, Katherine C. Frame, Duncan MacArthur, Peter Santi, Morag K. Smith, and Jonathan Thron, “Monte Carlo Simulations and Data Comparison for Liquid-Scintillator Detectors to be Used in a Multiplicity Counter,” 48^th Annual INMM Meeting, 8–12 July 2007, Tuczon, AZ, USA; B. M. van der Ende, J. Atanackovic, A. Erlandson, and G. Bentoumi, “Use of Geant4 vs. MCNPX for the Characterization of a Boron-Lined Neutron Detector,” Nuclear Instruments and Methods in Physics Research A 820 (2016): 40–7; Jiawei Tan and Joseph Bendahan, “Geant4 Modifications for Accurate Fission Simulations,” Physics Procedia 90 (2017): 256–65; H. N. Tran, A. Marchix, A. Letourneau, J. Darpentigny, A. Menelle, F. Ott, J. Schwindling, and N. Chauvin, “Comparison of the Thermal Neutron Scattering Treatment in MCNP6 and GEANT4 Codes,” Nuclear Instruments and Methods in Physics Research A 893 (2018): 84–94; R. Sarwar, V. Astromskas, C. H. Zimmerman, G. Nutter, A. T. Simone, S. Croft, and M. J. Joyce, “An Event-Triggered Coincidence Algorithm for Fast-Neutron Multiplicity Assay Corrected for Cross-Talk and Photon Breakthrough,” Nuclear Instruments and Methods in Physics Research A 903 (2018): 152–61.

30 These include the resolved and unresolved resonance regions as well as thermal scattering, see L. Thulliez, C. Jouanne, and E. Dumonteil, “Improvement of Geant4 Neutron-HP package: From Methodology to Evaluated Nuclear Data Library,” Nuclear Instruments and Methods in Physics Research A 1027 (2022): 166187 and references therein.

31 International Criticality Safety Benchmark Evaluation Project, “International Handbook of Evaluated Criticality Safety Benchmark Experiments,” NEA 7520, OECD Nuclear Energy Agency, Paris, 2020.

32 For this purpose we modified Geant4 to track and store the numbers of fission neutrons produced per simulated generation.

33 Christopher. J. Werner, editor, “MCNP User’s Manual, Code Version 6.2,” LA-UR-17-29981, Los Alamos, NM, USA, 2017.

34 B. T. Rearden and M. A. Jessee, editors, “SCALE Code System,” ORNL/TM-2005/39, version 6.2.3, Oak Ridge, TN, USA, 2018.

35 Manuel Kreutle, Alessandro Borella, Riccardo Rossa, Celine Scholten, Gerald Kirchner, and Claas van der Meer, “Benchmarking Monte Carlo Simulations in the Context of Nuclear Disarmament Verification via Monte Carlo Simulations with GEANT4,” INMM & ESARDA Joint Virtual Annual Meeting, 23–26 August & 30 August-1 September 2021.

36 United Nations Scientific Committee on the Effects of Atomic Radiation, “UNSCEAR 2000 Report to the General Assembly,” Volume I, Annex B, 86 gives a value of 0.013 cm⁻² s⁻¹ at sea level, which will be smaller in buildings, but may show an increased level in a nuclear weapons maintenance facility, https://www.unscear.org/unscear/en/publications/2000_1.html.

37 See note 8 above.

38 See note 11 above.

39 A similar effect is caused within bare plutonium by hydrogeneous material present in a storage container, Malte Göttsche, Paolo Peerani, and Gerald Kirchner, “Concepts for Dismantlement Verification and Neutron Multiplicity Measurements for Plutonium Mass Attribute Determination,” 35^th ESARDA Annual Meeting, 27–30 May 2013, Bruges, Belgium, May Proceedings, EUR-26127 (2013): 477–85.

40 See note 8 above.

41 Nicholas Reed, Anders Axelsson, Gerald Kirchner, Sakari Ihantola, Manuel Kreutle, Kari Peräjärvi, and Antonin Vacheret, “nFacet 3D: Fission Neutron Measurements With A Segmented Scintillation Detector,” Proceedings of the INMM & ESARDA Joint Virtual Annual Meeting, 23–26 August, 30 August–1 September 2021.

42 A concept for realizing this is developed by Svenja Sonder, Simon Hebel, Carina Prünte, and Gerald Kirchner, “Impact of Concrete Building Structures on Neutron Radiation,” ESARDA Bulletin 64 (2022): 2–9.

43 These include calorimetry and for warheads without tamper gamma spectrometry.

44 This value follows from the neutron die-away times within the helium-3 detectors of about 50 µs, “Application Guide to Neutron Multiplicity Counting,” 17.

45 With 8.89 ⋅10⁴ spontaneous fissions per second and a typical detector efficiency of 40 percent signals of the bare plutonium are separated by about 30 µs on average.

46 “Transparency and Verification Options,” 62–71.

47 “Confirming the Absence of Nuclear Warheads via Passive Gamma-Ray Measurements,” 2–3.

48 “Monitoring Nuclear Weapons and Nuclear-Explosive Materials,” 101–2.

49 The exception would be the case of a single tamper including type, since its template will significantly differ from the others.

50 T. Douglas Reilly, “Passive Nondestructive Assay of Nuclear Materials. 2007 Addendum,” 11–14, Los Alamos, New Mexico, 2007, https://www.lanl.gov/org/ddste/aldgs/sst-training/_assets/docs/PANDA %202007%20Addendum/0.%20PANDA%20Preface%20v1-3.pdf.

51 With an effective detection area of 25 cm² and an absolute efficiency of 10% as typical for HPGe detectors at the energies of interest, the accumulation of 100 counts for the 642 keV plutonium-240 line will take roughly 2.4 hours; taking into account the background of 142 counts caused by Compton scattering of uranium decay gammas with energies above 642 keV, the measurement will result in a statistical uncertainty of 18.5%.

52 For the United States, a mean radium-226 concentration in soil of 40 Bq kg⁻¹ is given with a range of 8–160 Bq kg⁻¹, “UNSCEAR 2000 Report to the General Assembly,” Volume I, Annex B, 115.

53 Steve Fetter, Thomas B. Cochran, Lee Grodzins, Harvey L. Lynch, and Martin S. Zucker, “Gamma-Ray Measurements of a Soviet Cruise-Missile Warhead,” Science 248 (1990): 828–34.

54 “Confidence, Security and Verification,” 19; “Monitoring Nuclear Weapons and Nuclear-Explosive Materials,” 105; (n,γ) lines present in the gamma spectra recorded during the Black Sea experiment could originate from activation of high explosives, but also of cruise missile fuel, “Gamma-Ray Measurements of a Soviet Cruise-Missile Warhead,” 829.

55 “A Passive Method for the Detection of Explosives and Weapon-Grade Plutonium in Nuclear Warheads,” 64.

56 See note 36 above.

57 This will allow to verify the absence of the shielded plutonium with commercially available multiplicity counters such as the High-Efficiency Neutron Counter, see H. O. Menlove, J. Baca, J. M. Pecos, D. R. Davidson, R. D. McElroy, and D. B. Brochu, “HENC Performance Evaluation and Plutonium Calibration,” LA-13362-MS, Los Alamos, NM, USA, 1997.