Novel Particle Probes in Nuclear Verification

Since radioactivity is a defining feature of nuclear materials, the verification of nuclear inventory and facilities often relies on radiation detection devices. Simultaneously, radiation protection requires these materials to be biologically shielded to ensure safe operations, making reliable measurements—and hence verification—more challenging. In addition to the dangers that radioactive materials pose to living organisms and the environment, they can also be used for the—prohibited—construction of nuclear weapons, ranging from dirty bombs to nuclear warheads. Consequently, the whereabouts of nuclear material must be meticulously documented and verified. This is partially due to the need for long-term management of increasing quantities of spent nuclear fuel (SNF) in the absence of a clear pathway toward its final disposal. Simultaneously, nuclear energy is experiencing a global resurgence as a potential means of reconciling demands for energy sovereignty and low-carbon electricity. New nuclear energy programs have been launched by several countries, including Poland,¹ Türkiye,² and Egypt,³ while existing programs are being expanded or revived following earlier phase-out decisions. One major development is the emergence of new deployment concepts,⁴ including fleets of small modular reactors (SMRs) and microreactors for decentralized, industrial, transportable, or mobile nuclear power—potentially even for the maritime propulsion of cargo vessels. In addition to this, new novel advanced reactors (NARs or Gen IV reactors) using different fuel forms and coolants are also being developed, particularly by nuclear power start-ups in the U.S.⁵ These new concepts challenge existing safeguards measures designed to ensure non-proliferation: larger, more widely dispersed fleets of reactors make inspection logistics more challenging, longer refueling cycles or sealed cores prevent access to fuel elements, and new coolants that cannot be detected by existing measurement techniques further hinder verification efforts. Accommodating these new concepts, designs, and deployment approaches would require major modifications to current safeguards methods as well as additional personnel. New nuclear verification techniques are therefore needed to ensure that current safeguards remain effective, and research and academic institutions are investigating such approaches. These efforts depend on improvements in signal detection and data-processing techniques resulting from ongoing R&D. nda approaches for the passive or active detection of nuclear materials are of particular interest because they offer nonintrusive procedures that can minimize the burden and risks to operators. Therefore, the development of novel techniques capable of enhancing existing surveillance systems through nondestructive, continuous, or remote monitoring is particularly promising in terms of future verification tools for nuclear facilities and material.

Current Probes: Existing Safeguards Tools

Established safeguards rely on the verification of operator-declared nuclear material accountancy. This is achieved through inspections that combine independent qualitative and quantitative measurements to verify the presence, composition, and quantity of nuclear material with containment and surveillance (C/S) techniques to maintain an unbroken continuity of knowledge (CoK) between inspections. The radioactive emissions of the material, usually in the form of neutrons, α-, β-, or γ-rays, are used by passive detectors to characterize the inventory during verification. As nuclear materials also interact with particles in characteristic, isotope-specific ways, active detection methods exploit their responses to different types of radiation to gather even more information. Inspectors often rely on NDA methods to perform measurements in the field without needing to take samples. Current methods employed for NDA rely primarily, but not exclusively, on the detection of three main probes: γ-rays, neutrons, and Cherenkov radiation.

• γ-rays are emitted by most nuclear material, and their energy spectra contain characteristic information about the sample. γ-ray spectrometry determines the presence of specific isotopes. Depending on their energy, they can penetrate lower density materials, allowing limited measurements to be made through thin shielding. However, dense materials—including nuclear materials such as uranium or plutonium—can block a significant fraction of these emissions, as the material’s high density effectively shields against its own radiation. High γ-ray emission levels from the fission products of these two elements make it even more difficult to measure their quantities directly using traditional γ-ray spectrometry.

• Neutrons are emitted primarily through spontaneous or induced fission and α-ray-induced reactions. Coincidence and multiplicity counting techniques can be used passively or actively to determine the mass of fissile material. Neutrons are effectively shielded only by materials with a high neutron-capture cross-section, such as water or borated polyethylene, making measurements possible through certain types of shielding.

• Cherenkov radiation (photons) is emitted when β-rays, produced directly or indirectly by spent nuclear fuel, travel faster than the speed of light in water in storage ponds—the optical equivalent of a sonic boom. By detecting this UV light, the presence of spent fuel in the assemblies can be verified from above the ponds. Many materials—including water—are transparent to Cherenkov light.

Recent Developments: New Instruments and Approaches

While the potential increase in the number of nuclear facilities resulting from the deployment of SMRs and NARs was first anticipated in the 2010s, with more concrete plans emerging in the 2020s, the challenges posed by nuclear waste have long been. To address these challenges, new technologies such as PGET scanners (see Table 1) are currently being developed. Other approaches under consideration include enhancing existing methods, such as remote detector control and data transfer, and modifying algorithms to address reactor remoteness and new fuel compositions, as well as using new particle types:

Passive neutron albedo reactivity (PNAR) scanners: This technique is a modifies existing neutron detection techniques by using the neutrons emitted by the nuclear material itself to characterize its response to neutrons and reconstruct a tomographic image of spent fuel rods.⁶ While not yet in use, there are plans to combine PNAR and PGET into a single device capable of producing highly detailed characterizations of fuel rods prior to their encapsulation in long-term storage casks.

Muography: Cosmic-ray muography uses high-energy particles in the Earth’s upper atmosphere to produce density maps of the interior of objects or structures, resulting in X-ray-like imaging. As a naturally occurring form of highly penetrating radiation, it enables imaging without additional radiation exposure. There are two muography approaches: 1) absorption radiography using a single detector positioned below the object of interest to produce 2D images and 2) scattering tomography that enables a 3D reconstruction but requires detectors to be positioned both above and below the interrogated object.⁷ Muography excels at imaging and positioning within large static or even geological structures⁸ (e.g., underground mine shafts) and smaller objects composed of materials with different densities, such as spent-fuel casks containing nuclear material.⁹ However, deployment scenarios are limited by the incoming muon flux, angular distribution, and possible solid-angle coverage of a detector station. Muography is thus already being used on a small scale to image nuclear waste containers, although the process currently requires days to weeks. Future applications are imaging of nuclear facilities, such as reactors or disposal facilities.

Novel Particle Probes: Frontier Developments for 2030 and Beyond

Research and academic institutions are uniquely positioned to assess the feasibility of employing novel particle probes for nuclear non-proliferation and to develop initial concepts for new verification techniques. Four of the most promising novel particle probes currently under investigation are discussed below.

Antineutrino monitoring: Antineutrinos are subatomic particles emitted primarily during radioactive decay, particularly of fission products in reactors, making them a clear signature of nuclear facilities. They also pass through all materials unhindered, making them impossible to shield or falsify—but also very hard to detect. Antineutrino detectors have advanced significantly over the last few decades. Potential applications include serving as safeguards for SMRs and advanced reactors by remotely monitoring the power level and isotopic inventory of operating reactors without the need for on-site inspections; verifying the absence of operating reactors within a certain area; and conducting nondestructive assays of interim storage casks.¹¹ Several applied research projects are underway to develop demonstrator devices,¹² while basic research is exploring new detection approaches with higher sensitivity.¹³

Neutron beam interrogation: Neutron beams are being proposed to probe the structure of nuclear material by measuring their attenuation with spatially resolving detectors to create neutron radiographs. The energy-resolved detection of neutron transmission through a sample also enables the spatial reconstruction of its composition owing to the unique attenuation features of individual isotopes. Several approaches to this technique are being developed, with the most ambitious exploring the feasibility of obtaining tomographic images showing the 3D distribution of different isotopes.¹⁴ Potential applications include characterizing nuclear material, such as fuel, within heavily shielded enclosures, such as storage casks.

Optical/laser spectroscopy: There has also been growing interest in the use of laser-based optical probes as an alternative to particle probes employing radioactive beams. Lasers can excite the electronic energy levels in samples, which subsequently relax through the emission of photon signals. This allows for nondestructive, real-time, in situ analysis of trace quantities of nuclear material, while requiring significantly less sample preparation for laser ionization spectroscopy.¹⁵ This has also enabled the use of Light Detection and Ranging (LiDAR) imaging instead of Radio Detection and Ranging (RADAR) imaging, offering greater detail and accuracy, particularly for the kind of short-range static targets typically encountered by nuclear inspectors.¹⁶ One possible verification application is an NDA tool allowing identification of trace amounts of nuclear material in the field.

Nuclear resonance fluorescence (NRF): NRF is a process in which a nucleus absorbs and resonantly reemits a γ-ray. The energy of the emitted γ-ray depends on the nuclear structure and can therefore be used as a signature to identify specific isotopes. A γ-ray excitation beam, typically with an energy larger than 1 MeV, can thus be used to determine the composition of a sample or, for verification purposes, to directly measure the isotopic quantity of fissile material in a nonfissile environment. The γ-ray energies involved are high enough for them to travel through thick shielding materials, making active NRF probing a promising alternative to passive techniques that are more sensitive to background and shielding. This enables the characterization of nuclear material in heavily shielded, opaque environments, such as NARs using non-water coolants or SNF stored in shielded casks.¹⁷

Integration of Novel Verification ­Techniques

If integrated into nuclear verification procedures, novel particle probes could contribute to the overarching objective of obtaining more information more efficiently. Their remote operation and automatization would reduce inspector burden by requiring less supervision time and fewer personnel, while also reducing radiation exposure and at the same time enabling more continuous and informative measurements. Specific challenges arising from the potential large-scale deployment of SMRs and microreactors could also be addressed with novel particle probes. Operations such as the refueling of remotely located small reactor arrays could be monitored using in situ antineutrino detectors with data-transmission capabilities. Fuel-loaded reactors being transported on land or water could also be inspected at specified locations, such as production facilities or designated ports, using passive methods such as antineutrino monitoring or active ones such as NRF or neutron-beam interrogation. Active methods, particularly NRF, are expected to be well suited to inspecting sealed reactor designs with high levels of radiation opacity, which may, in some cases, be further increased by high-density coolant or fuel materials. This approach is currently constrained by the high cost and large footprint of the necessary equipment and is therefore expected, in the near term, to be deployed only at specific locations, such as reactor production facilities or spent-fuel storage sites. With further advances in laser, accelerator, and detector technology, however, these methods could conceivably become portable in the longer term. Bilateral tailoring of future reactors and verification techniques, as encouraged by the concept of safeguards by design, should also play a central role in the ongoing development of methods based on novel particle probes.

Technological Drivers and Sustained ­Innovation

Beyond their intended use for nuclear safeguards, the recent developments and projected future technologies examined in this chapter share several common innovation drivers:

On the technological side, one major milestone has been the upgrade from pmt, first developed in 1930s, to sipm. The widespread adoption of SiPMs, first in research¹⁸, then for applications,¹⁹ has overcome the drawbacks of PMTs while leveraging the massive economies of scale of the modern semiconductor industry, since SiPMs are produced in the same foundry used to produce chips. Verification scenarios typically present low-signal situations where SiPMs provide superior performance owing to their improved spatial and timing resolutions, higher photoefficiency, and immunity to magnetic-field interference, albeit at the cost of a slightly lower energy resolution. Additionally, their smaller size, lower operating voltages (< 1 kV), reduced power consumption, physical ruggedness, and tolerance to vibrations make SiPM-based detectors more portable, adaptable and reliable for on-site use. Almost all the detectors listed here have benefited directly from the maturation of this technology.

Advances in scintillator development have driven a shift from liquid to plastic scintillators, which offer more durability, lower toxicity and flammability, increased portability and scalability, and the potential for bespoke geometries. These properties are all highly desirable for detectors intended for deployment across a range of field verification environments.²⁰ Handheld detectors could benefit from this in particular.

The adoption of transition-edge sensors has also enabled developments in microcalorimetry, which identifies tiny shifts in temperature that are indirectly produced when a particle is detected. This could potentially enable the identification of rare decays that would be obscured by traditional detectors, which measure light flashes or electrical currents directly. This is particularly advantageous for nuclear forensics or in scenarios requiring the analysis of complex mixtures of nuclear material.

Significant progress in robotics tools and applications has also facilitated the safer deployment of nuclear monitors, with growing potential for remote operation, as demonstrated by the RCVD. Continuous advances in signal processing, mainly driven by communication technology, has resulted in a variety of viable off-the-shelf components, reducing the need for expensive custom application-specific integrated circuits for fast processing. Advances in data-handling technologies, including artificial intelligence (AI) and machine learning (ML)-assisted methods, enable the analysis of more complex and noisy datasets with many measurement channels, while simultaneously improving the efficiency of both data transfer and storage. Muon tomography, PGET, and PNAR have particularly benefited from these advances, leading to high-fidelity image reconstruction of measured objects.

With regard to innovations in techniques, one major milestone has been the incorporation of multimodal imaging. By integrating sensors specifically tailored to different particle probes, the resulting system becomes sensitive to a broader range of radioactive by-products, as envisaged in the planned combination of PGET and PNAR. Nevertheless, there is still scope for stronger integration of complementary techniques in this area.

Adapting widely used industrial technologies to physics-related detection applications has enabled this wave of new detector advancements. Beyond general collaboration between industry and research, medical physics stands out as one of the major drivers.²¹ Here, imaging and detection technologies demonstrate high societal impact, strong R&D, and continued training opportunities. At the same time, the application of technological advances with high industrial R&D investment in areas not served by these industries created a fertile environment for maturation. To sustain innovation for verification purposes, supporting interdisciplinary environments that foster maturation is likely to be the most effective way of leveraging existing scientific and R&D efforts. Explicitly funding collaborations and projects at the intersection of security applications and high-investment R&D could create similar maturation dynamics to those seen in medical physics. While there has already been an increase in security- and dual-use-focused R&D programs, it is important to ensure that nuclear verification is included in these efforts. At the same time, the civil security aspect must be maintained. Since verification relies on trust and transparency, new trusted verification technology results are unlikely unless these security research initiatives are also firmly anchored in civilian and public research.