Ceci N’est Pas Une Bombe: Lessons from a Field Experiment Using Neutron and Gamma Measurements to Confirm the Absence of Nuclear Weapons

Abstract

AbstractIn March 2023, the UN Institute for Disarmament Research held a verification experiment that included a mockup onsite inspection at a former military facility in the municipality of Menzingen, Switzerland. The experiment included a visit to the site by an inspection team, accompanied by the host team. Among other activities, radiation measurements were used to confirm the non-nuclear nature of selected items stored onsite. In this paper, we discuss the neutron and gamma measurement systems used during the experiment and the inspection protocols followed to confirm the absence of nuclear weapons. Results from the experiment and a laboratory reproduction are presented, before concluding with lessons learned for how absence-confirmation measurements can help support verification of future arms control agreements. Disclosure statementNo potential conflict of interest was reported by the author(s).AcknowledgementsThe experimental analog presented in this work would not have been possible without the support of numerous researchers, staff, and the Health Physics team from Princeton Plasma Physics Laboratory. We also thank Gawoon Shim for assistance with the production of the ACX2. We acknowledge the Spiez Laboratory, UNIDIR, and the Swiss Armed Forces for their coordination in making these measurements possible. Special thanks go to David Chichester, Steve Fetter, Moritz Kütt, Pavel Podvig, and all other participants in the Menzingen Verification Experiment. The authors thank two anonymous reviewers for their thoughtful feedback, which significantly improved the published version of this manuscript; two new endnotes are based on specific suggestions made by the reviewers. Eric Lepowsky’s contributions to this project have been supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-2039656. This work was partly supported by the Consortium for Monitoring, Technology, and Verification under the Department of Energy National Nuclear Security Administration award number DE-NA0003920.Notes1 M. Göttsche and A. Glaser (eds.), Toward Nuclear Disarmament: Building Up Transparency and Verification (Berlin: German Federal Foreign Office, 2021); P. Podvig and J. Rodgers, Deferred Verification: Verifiable Declarations of Fissile Material Stocks (Geneva: UNIDIR, 2017).2 Treaty Between the United States of America and the Russian Federation on Measures for the Further Reduction and Limitation of Strategic Offensive Arms (“New START”), April 2010; Radiation Detection Equipment: An Arms Control Verification Tool, Product No. 211P, Defense Threat Reduction Agency, Fort Belvoir, VA, October 2011.3 S. Fetter, V. A. Frolov, A. Miller, R. Mozley, O. F. Prilutsky, S. N. Rodionov, and R. Z. Sagdeev, “Detecting Nuclear Warheads,” Science & Global Security 1, no. 3–4 (1990): 225–253.4 Deferred verification is a proposed arrangement, in which an initial declaration is verified only at the time when the materials or items that originally contained these materials are eliminated. See P. Podvig and J. Rodgers, 2017, op. cit.5 The base was operational until 1999 and now hosts a museum, www.mhsz.ch/bloodhound.6 Uranium-235 only emits low-energy gamma radiation. Despite the small uranium-238 content, highly enriched uranium and weapon-grade uranium (more than 90% U-235) are best detected using gamma radiation from uranium-238, namely, via a prominent gamma line at 1.001 MeV. With appropriate scaling of results, depleted uranium can therefore be used as a stand-in for weapon-grade material.7 E. Lepowsky, J. Jeon, and A. Glaser, “Confirming the Absence of Nuclear Warheads via Passive Gamma-Ray Measurements,” Nuclear Instruments and Methods in Physics Research A 990 (2021).8 Mirion Technologies, 802 Scintillation Detectors, Datasheet, 2017; Mirion Technologies, Osprey: Universal Digital MCA Tube Base for Scintillation Spectrometry, Datasheet, 2017.9 The protocol, as followed during the experiment, assumes that the transmission measurement with the reference source was aligned with the center of the inspected object. In practice, to make the measurement more robust against positioning, multiple measurements would be preferable, and the inspector should be allowed to choose the locations of those measurements.10 Selection of the regions of interest is described in detail in E. Lepowsky et al., 2021, op. cit. As lower-energy gammas from uranium-235 are easily shielded, gamma emissions from weapon-grade uranium (90% uranium-235 and higher) are still dominated by the 1.001-MeV line associated with the decay of uranium-238.11 Another solution (suggested by a reviewer) is a curved shield that spans 180° and can be rotated behind the detector when measuring the container and rotated in front of the detector when collecting background; this will allow the detector to remain stationary for both measurements.12 For the purposes of this self-shielding approximation, pure uranium-238 was used for the isotopic composition with mono-energetic 1.001 MeV photons spawned uniformly throughout the solid geometry, and the fraction of emitted gammas was tallied within each region of interest. The simulation was performed using MCNP6.2: C. J. Werner, et al., MCNP6.2 Release Notes, LA-UR-18-20808, New Mexico: Los Alamos National Laboratory, February 2018.13 One way for the host to build confidence with inspectors is to allow sweeping of the facility with gamma and neutron detectors to establish the background rate at multiple points in the room and identify possible anomalies. Hosts should not find this intrusive if the location is truly empty. This should be done in concert with an inspection of the walls and floor to look for gaps or cavities where sources could be hidden to manipulate the background. Inspectors should request that measurements be made at the center of the room instead of near the walls, making it more difficult for hosts to manipulate the background using sources in an adjacent area.14 F. Hiatt, “Insertable Nuclear Warheads Could Convert Arms,” The Washington Post, June 15, 1986; for a related discussion, see A. Glaser, “Toward Verifiable Definitions of a Nuclear Weapon,” Arms Control Today 53, no. 6 (July/August 2023).