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The work presented in this paper focuses on the first 10 years (2011–2020) of radiation test object (RTO) operations at the National Criticality Experiments Research Center. RTOs are subcritical co...

Many radionuclide identification algorithms use statistical inference to collect a variety of features from gamma-ray spectra to deduce the presence of particular radionuclides. More modern algorithms require large amounts of data to learn and use latent features from spectra for classification. Both approaches are computationally expensive, which is reflected in their power consumption, and require large amounts of user intervention to prepare. In this article, we introduce a low-power, neuromorphic algorithm for the real-time identification of radionuclides which simultaneously considers the entire shape of a gamma-ray spectrum. Utilizing the output of a traditional gamma-ray detector, our spiking, locally competitive algorithm uses sparse coding optimization to compare global patterns in a gamma-ray spectrum with a dictionary of radionuclide templates. This approach allows us to model informative global features resulting from both photoelectric absorption and Compton scattering. For the purpose of radiation threat reduction, the dictionary consists of data from the Nuclear Wallet Cards, a list of radionuclides and their properties compiled by the National Nuclear Data Center. To test our algorithm, we use a variety of gamma-ray spectra created using radionuclides measured under laboratory conditions with varying durations, distances, activity levels, and backgrounds, resulting in a wide range of signal-to-noise ratios. We have created test sets for three different gamma-ray detector types, with 57Co, 137Cs, 152Eu, 60Co, 239Pu, and 235U sources, to quantify the effect of resolution, efficiency, and background on the accuracy of the algorithm. We demonstrate a true positive accuracy of 91% with a high-resolution detector and 89% with a low-resolution detector on the corresponding test sets. Experimenting with the same radionuclides included in the test sets in a variety of special nuclear material (SNM) masking configurations, we show that our algorithm is capable of correctly identifying both SNM and mask even when the activity level of the mask is several times higher than that of the SNM. We also determine that our algorithm achieves over a 99% reduction in power consumption over other radionuclide identification software applications, which is critical for long-term, independent monitoring and is the goal of this research.

A compact radiation imaging system capable of detecting, localizing, and characterizing special nuclear material (e.g. highly-enriched uranium, plutonium…) would be useful for national security missions involving inspection, emergency response, or war-fighters. Previously-designed radiation imaging systems have been large and bulky with significant portions of volume occupied by photomultiplier tubes (PMTs). The prototype imaging system presented here uses silicon photomultipliers (SiPMs) in place of PMTs because SiPMs are much more compact and operate at low power and voltage. The SiPMs are coupled to the ends of eight stilbene organic scintillators, which have an overall volume of 5.74 × 5.74 × 7.11 cm3. The prototype dual-particle imager’s capabilities were evaluated by performing measurements with a 252Cf source, a sphere of 4.5 kg of alpha-phase weapons-grade plutonium known as the BeRP ball, a 6 kg sphere of neptunium, and a canister of 3.4 kg of plutonium oxide (7% 240Pu and 93% 239Pu). These measurements demonstrate neutron spectroscopic capabilities, a neutron image resolution for a Watt spectrum of 9.65 ± 0.94° in the azimuthal direction and 22.59 ± 5.81° in the altitude direction, imaging of gamma rays using organic scintillators, and imaging of multiple sources in the same field of view.

International audience; Many developments are currently in progress towards new threat detection technologies for security-related applications. In particular, the photofission technique can be applied to detect Special Nuclear Material (SNM) hidden inside cargo containers, which can potentially be used to make nuclear weapons or dirty bombs. This technique relies on detecting prompt and delayed neutrons and gamma rays emitted by the photofission reaction for which the energy threshold is around 6 MeV for plutonium and uranium isotopes. In the frame of the H2020 ENTRANCE project and following the H2020 C-BORD project, CEA List is investigating the performance of the photofission technique using a linear electron accelerator (linac) operated at 7 MeV, i.e. delivering photons with energies not far above the threshold of the reaction of interest. One of the aim is to couple the photofission technique with compact linacs embeded in mobile X-ray scanning systems used by customs for control at borders. Moreover, we investigate the potentialities of triple-discriminating plastic scintillators for the detection of SNM with such systems. These detectors are manufactured by the laboratory and are based on $^6$Li-doped plastic scintillators which allows them to discriminate between gamma-rays, thermal neutrons and fast neutrons by Pulsed Shape Discrimination (PSD). Thus, they could potentially measure simultaneously both the neutron and gamma signatures from SNM and enable to overcome the challenges met when measuring the low signals expected due to the fact that the photofission cross-section values reach a few millibarns between 6 and 7 MeV. First, the PSD capabilities and scintillation yields of these new detectors are characterized in the laboratory environment using a $^{252}$Cf source. Then, we perform measurements at the SAPHIR platform located at CEA Paris-Saclay with the 9 MeV linac available and several SNM samples. Neutron sensitivity as well as detection limits are then compared to reference measurements carried out with a high-density polyethylene detection block housing five 150NH100 $^3$He detectors. In parallel, we implemented an MCNP6 Monte Carlo simulation model of the experimental setup at 9 MeV. Once validated, this model is used to assess the performance of triple-discriminating plastic scintillators for photofission measurements carried out at 7 MeV. In collaboration with Smiths Detection, field tests are planned in 2023 at the seaport of Rijeka, Croatia, with different cargo matrices and a 7 MeV mobile X-ray scanner equipped with triple-discriminating plastic scintillators.

Radiation imaging is one potential method to verify nuclear weapons dismantlement. We present a method to discriminate between non-multiplying and multiplying neutron sources using a neutron coded aperture imaging system. This method applies time-correlated pulse-height (TCPH) analysis to identify neutrons in fission chain reactions and recreate images using only these events. This analysis was applied to measurements of weapons-grade plutonium (WGPu) metal and 252Cf performed at the Nevada National Security Site (NNSS) Device Assembly Facility in July 2015. The results demonstrate it is possible to eliminate the non-fissile 252Cf source from the image while preserving the fissile WGPu source. TCPH analysis was also applied to additional scenes in which the WGPu and 252Cf sources were measured individually. The results of these separate measurements further demonstrate the ability to remove the non-fissile 252Cf source and retain the fissile WGPu source. Simulations performed using MCNPX-PoliMi indicate that in a one hour measurement, hollow WGPu spheres are retained at a 1 σ level for neutron multiplications M ≃ 2 . 7 and above. This work has potential application to dismantlement verification where it may be valuable to verify that all neutron sources in an image are multiplying.

Pyroprocessing treats nuclear fuel using electrochemical techniques for a variety of applications, whether to process used uranium oxide fuel for back-end management, or to reprocess used metallic fuel for an advanced fuel concept. Since there are proliferation risks associated with special nuclear material (SNM) processed in a pyroprocessing facility, the high reliability safeguards (HRS) methodology has been developed to implement safeguards, safety, and physical security (3S) together with operations from a design-driven perspective. This paper suggests the design strategies by integrating a hazard and operability (HAZOP) analysis. It is a process hazard analysis, essential for identifying hazards, operability issues, severe accident scenarios, and for mitigating consequences with the corresponding protection methods. It is a preliminary step for an eventual quantitative, probabilistic risk analysis, which may be needed for obtaining an operating license for a commercial pyroprocessing facility. The focus of this HAZOP analysis was on major pyroprocessing subsystems; voloxidation, electroreduction, electrorefining, electrowinning, and the argon atmosphere control system. Deviations were analyzed from normal operating conditions, and methods were thoroughly prepared to prohibit or mitigate off-normal situations. Importantly, pressure buildup or high temperature in the systems can increase the risk of initiating fire/explosion, which may eventually release radiation to the outside. Safety relief valve installations, valve actuation and heater automatic shutdown systems, etc., would be helpful to alleviate operational problems. This enhanced HAZOP also suggests a strong safeguards design for the facility by proposing nuclear material accounting (NMA) locations for the purpose of reducing potential proliferation risks.

This paper discusses the current practice of assessing the Mark-V electrorefiner inventory and a proposed new way of tracking this inventory. The current practice involves the estimation/calculation of the salt phase mass in the transferred material mass and the calculation of the holdup. The proposed method involves the mass measurement of the transferred materials and the total inventory volume measurement along with the consideration of metallic uranium phase in the inventory. Without estimating the involved salt inventory in the incoming and outgoing material streams, the proposed method yields the salt inventory agreeing with the current mass tracking practice. The estimated holdup in the current mass tracking practice is entirely explained with the introduced metallic uranium phase in the proposed model. The current practice of closing out the special nuclear material inventory is revisited with the insight gained from the proposed model. Suggestions are given to achieve the bounded uncertainty of the inventory estimates with the emerging in situ measurement technique on the molten salt density.

The risk of revealing sensitive information of nuclear weapon is an obstacle for comprehensively applying the identification technology in nuclear verification and nuclear security. In order to reduce the risk, low-resolution radiation spectra are suggested to be used in the activities of identifying special nuclear material (SNM) items’ types. In this article, we proposed an effective algorithm that extracts characteristic information from low-resolution gamma-ray spectra of SNMs and identifies the types of SNMs through backpropagation (BP) neural network and template matching method. We established the algorithm by numerical simulations, and then conducted series of experiments to verify and validate this algorithm. The identification results of applying this algorithm to real plutonium (PU) and high enriched uranium (HEU) pits showed that the proposed algorithm is an eligible option for both nuclear verification and nuclear security.

The role of active-interrogation systems for nuclear security is to detect the presence of special nuclear material inside an object by observing the radiation emitted by that object when it has been exposed to known sources of external radiation. Because of the cost, complexity, and the need to avoid irradiating occupants, active-interrogation systems are intended for cargo applications where shielding can prevent detection by passive radiation detection systems. To ensure that active-interrogation systems for detection of special nuclear material are designed and tested to a consistent level of performance, technical standards are needed for evaluating such systems. This paper addresses the testing standards needed for active-interrogation systems to detect high atomic number materials, fissionable materials, and specific special nuclear materials. Because the use of special nuclear material for a testing standard is not practical, this work focuses on the determination of materials that could be used as surrogates in that they provide a similar response as targets of special nuclear materials. The results of this paper, determined through analytic calculations and radiation transport modeling, are based upon scenarios constructed and applied to specific active-interrogation modalities.

The overall research objective has been to develop a high reliability safeguards model for a full, commercial-scale pyroprocessing facility based on the fundamental principle of safeguards-by-design. To this end, a quantitative safeguards model with the metric of false alarm probability has successfully been developed for the fuel fabrication process to the point of being useful and flexible in practical nuclear material accounting (NMA), particularly with weight measurements. Fuel fabrication was selected for study because a large amount of special nuclear material is included in this process. The current model allows for adding key measurement points (KMPs) or sub-processes, and it updates an inventory difference (ID) calculation at each KMP. A facility design (the baseline design), which contains a KMP between the melting and trimming processes, reduced the product throughput compared to the equipment design without the KMP, given the same false alarm probability of 0.05 (It is based on the IAEA recommended target value). Therefore, the operator will prefer the equipment design to process more material while the baseline design can tell the exact location at which a diversion was attempted. In this paper, a sensitivity analysis has been developed for various facility parameters, such as false alarm probability, melter failure rate, alarm threshold at each KMP, etc., to observe the results in the productivity of operation. With a false alarm probability of 0.1, about ∼8% less material had been processed compared to that of 0.05. In addition, if the melter failed more often than once per month, less material was processed, and vice versa. Changes related to the heel amount accumulated in the melter did not affect the number of campaigns because the model correspondingly calculates the alarm thresholds to achieve the same desired false alarm probabilities. If a different false alarm probability was given for each sub-process unit; i.e., an alarm threshold was set differently at each KMP, safeguards on sub-processes could effectively be applied. For instance, a sub-process, which includes more amount of special nuclear material than other processes, can use a high false alarm probability to detect the diversion attempts following the higher detection probability. In addition, the functionality of random sampling for physical or interim inventory verification was tested. This concluded that a smaller container of Pu compared to 20 kg would be necessary for reducing the time for physical or interim inventory verification, given the relatively high detection probability.

Subcritical neutron noise measurements are often used to determine multiplication of a system with Special Nuclear Material. This work presents an uncertainty approach which incorporates the singles and doubles counting rates determined by the Hage-Cifarelli formalism. This is a moments-based approach which utilizes time correlations between prompt neutrons in order to assess the system multiplication. After the method is described, a validation is presented which utilizes simulated data generated using a 0-D point-kinetics Monte Carlo code.

A Wearable Detector Device (WDD) has been outfitted with Microstructured Semiconductor Neutron Detectors (MSNDs) to aid in the search and localization of special nuclear materials (SNMs). Many SNMs decay by spontaneous fission and emit free neutrons. The WDD detects these neutrons and stores interaction rate information to alert the operator to the presence of special nuclear material. The WDD is composed of 16 Modular Neutron Detectors (MNDs), each populated with a 4 × 6 array of 1-cm2 active area, 500-μm thick MSNDs. The individual MSNDs each have an intrinsic thermal-neutron detection efficiency of approximately 30%. Each MND connects to a communications dongle, and the MND and dongle were encased in a 3-in. wide by 5-in. tall by 0.6-in. thick high-density polyethylene moderator (HDPE) case. The 16 MNDs, connected to Controller Area Network dongles, communicate with a master control board, which also contains the battery bank and power conditioning electronics. The operational lifetime of the battery-powered WDD is greater than 12 h per single charge. The WDD, mounted on an ANSI 42.53 phantom, reported 8.12 ± 0.07 cps and 12.93 ± 0.07 cps for a bare and moderated 21.9-ng 252Cf source at a distance of 1 m, respectively. The background count rate was 0.446 ± 0.002 cps. The gamma-ray rejection ratio of the WDD for 137Cs measured at a dose rate of 10 mR/h was 1.8 × 10−8.

Surrogate radiation sources are used to test detection equipment and train security personnel in the detection and interdiction of special nuclear material (SNM). Some current surrogate sources do not accurately match SNM energy spectra, including for HEU. In this research, we propose and investigate novel source configurations for higher fidelity surrogate SNM sources using thermal neutron capture reactions that result in prompt gammas. The objective is to create a surrogate source that has a more representative spectrum, low cost, and requires minimal security, special handling, and safety precautions.

The constant concerns in global nuclear safety, aimed at deterring and combating the illicit trafficking of Material Out of Regulatory Control (MORC) and its possible use in criminal acts has raised the necessity of new detection solutions with higher efficiency and resolution to provide a high level of accuracy in the report to the authorities. Today’s radioactive isotope identifiers perform gamma spectroscopy identification and, sometimes, neutron counting. This paper presents the results of measurements performed in real-scenario conditions with a new type of portable radioactive isotope identifier for the detection and identification of both gamma and neutron sources. Its singular features are: 1) the capability to identify sources through the detection of neutrons, discriminating spontaneous fission sources (Cf-252), α-n sources (Am/Be, Am/Li) and nuclear material containing mix of isotopes of plutonium or uranium 2) the capability to make cross correlation between gamma and neutron measurements to achieve a higher level of accuracy in the identification of SNM that emits both neutrons and characteristics gammas. The test results are compared with international standards. The device exceeds the standard performance by triggering a neutron alarm for a 20.000 n/s Cf-252 source at a five times greater distance than the ANSI N42.34 one.