Low Neutron Research Articles

Binder jetting and melt infiltration for ceramic-metal fuel fabrication

This study explored the feasibility of fabricating a ZrO2-Zr2Cu ceramic-metal (CERMET) fuel through additive manufacturing (AM) followed by melt infiltration. ZrO2 was selected as a surrogate for UO2 owing to its similar properties, and Zr2Cu was selected owing to its low melting point and neutron absorption cross-section to explore the feasibility of forming a uniform distribution of ceramic particles within a metal matrix. The binder jetting (BJ) AM technique was employed to fabricate preforms with precise control over the particle distribution, addressing the challenges associated with particle agglomeration and ensuring preform shape integrity. Subsequently, spontaneous melt infiltration was employed to achieve a dense and homogeneous CERMET structure. This study emphasized the significance of controlling wettability and surface energies for successful melt infiltration, focusing on the interfacial reactions between Zr2Cu and ZrO2. These results indicate that this novel approach can significantly enhance the thermal and irradiation performance of CERMET fuels.

Activation cross section for 85Rb(n,2n)84mRb and 85Rb(n,p)85mKr reactions with uncertainty propagation and covariance analysis

The cross section of 85Rb(n,2n)84mRb and 85Rb(n,p)85mKr reactions were measured at neutron energy range 12–20 MeV using activation analysis followed by off-line γ-ray spectroscopic technique. The quasi mono-energetic neutrons were produced through 7Li(p,n)7Be reaction. The measurements were done relative to 27Al(n,α)24Na reference monitor reaction cross section. The detailed uncertainty propagation from the attributes present in measurements was performed using covariance analysis. The γ-ray self-attenuation and background low neutron energy corrections were performed in the measurement studies. Theoretical calculations were performed by TALYS-1.96 nuclear code. The comparison of measured values with the available data in EXFOR database and evaluated data from JENDL-5 and EAF-2010 shows the measured cross section values of 85Rb(n,p)85mKr reaction were slightly higher than the published values, evaluated data and theoretical data while for 85Rb(n,2n)84mRb, the measured values were consistent with the published data.

Deterministic and stochastic computational models for the analysis of Nuclear Power Plant (NPP) start-up operations

The aim of this paper is to present an overview of both the deterministic and stochastic aspects of nuclear power plant (NPP) start-up and start-up accident scenarios. The neutron kinetics and corresponding nuclear reactor control mechanisms associated with deterministic NPP start-up are explained. The stochastic behaviour of neutron kinetics is explained and the procedures required to safely start-up an NPP due to these stochastic phenomena are outlined. Specific attention is given to the stochastic behaviour of neutrons in a low neutron source regime and the corresponding mathematical methods required for modelling such phenomena during NPP start-up accidents. The Pál-Bell equations are compared against the forward generating function equations utilising the gamma approximation in their ability to model NPP start-up accidents. The Pál-Bell equations are shown to be necessary to accurately calculate the maturity time for systems with either a very low neutron source strength or a very rapid reactivity insertion profile. Both the Hurwitz calculation route and MacMillan calculation route for low neutron source systems are explained. Full deterministic and stochastic calculations are performed for multiple nuclear reactor systems to illustrate the necessary analyses required for low neutron source NPP start-up accident scenarios.

First-principles study of the stability and mechanical properties for binary Mo–Re alloys

Mo–Re alloys are highly suitable as core materials in nuclear reactor applications due to their excellent radiation resistance, high melting point, superior mechanical strength, and low neutron absorption characteristics. In this study, we investigate the stability and mechanical properties of eight Mo–Re alloys by first-principles calculations, including Re24Mo5, ReMo3-1, ReMo3-2, Re3Mo2, ReMo, Re3Mo-1, Re3Mo-2, Re3Mo-3. The results of formation enthalpy, elastic stability criteria and phonon dispersion curves indicate that all Mo–Re alloys are stable except for the ReMo3-2. Subsequently, the mechanical parameters of the remaining seven Mo–Re alloys, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, B/G ratio, and anisotropy index are investigated. The bulk modulus of Re24Mo5 is the highest, at 344.597 GPa, the Young's modulus and shear modulus of Re3Mo-3 are the highest, measuring 390.617 GPa and 149.294 GPa, respectively. Furthermore, all Mo–Re alloys exhibit ductile characteristics. This study is of great significance for the selection of suitable Mo–Re alloys with sufficient mechanical strength for use as core materials.

The hydrogen-helium-vacancy interaction and hydrogen-vacancy clusters formation mechanisms in chromium: A first-principles study

Attributing to excellent corrosion resistance and low neutron absorption cross-section, chromium (Cr) has been considered as a promising advanced structural nuclear material. Under extreme radiation conditions, the coexistence of vacancies (Vac), hydrogen (H), and helium (He) modifies the evolution trajectory of H-defect clusters, and significantly impacts the performance of materials, all of which are under extensive investigations. Here, the occupancy mechanism of H atoms and influencing factors in Vac1/He1Vac1 systems are comprehensively studied by first-principles calculations. Based on the exhaustive investigation of H-He-Vac interactions, the equilibrium distance between H-H pairs is delineated. Serving as an effective trap center, the Vac1/He1Vac1 cluster exhibits robust attractive forces capable of luring dissociative H atoms within the crystal lattice. The accumulation of H atoms from remote regions towards the defects results in the formation of stable Hn-Vac1/Hn-He1Vac1 complexes. Moreover, it is revealed that the hydrogen-enhanced vacancy mechanism and meticulously examines the factors affecting defect-trapping capabilities and the growth of H-He-Vac complexes, providing an insight into the behavior of hydrogen-defects clusters formation mechanism and the H-He-Vac interaction in Cr.

Development of fast 2.5MeV neutron detectors for high-intensity stray magnetic field environments.

Several small to medium-scale magnetic confinement fusion devices operate using deuterium as fuel. These low neutron rate (108-1010 n/s) devices rely on 2.45MeV neutron measurements to validate physical models and to assess their performance. Given the modest rate, neutron monitors have to be placed as close as possible to the machine to maximize data gathering. In these regions, intense stray magnetic fields could affect the detector's performance. In this work, the development of a neutron detector based on an EJ-276D scintillator crystal coupled with a SiPM and a custom-made readout system is presented. The detector has particle discrimination capability and is insensitive to magnetic fields.

A novel plasma-facing NdB6 particulate reinforced W1Ni matrix composite: Powder metallurgical fabrication, microstructural and mechanical characterization

Tungsten (W) is one of best candidate metal for plasma-facing materials (PFM), especially due to its high melting temperature and neutron absorption capability. However, converting W into bulk PFM is hard because of its high melting point. This problem can be solved by adding metallic sintering aids with low melting points. In this study, W matrix with 1 wt% Ni aid was reinforced by adding NdB6 particles (1, 5, and 10 wt%). It can be introduced as a novel potential PFM, thanks to its low volatility and high neutron absorbability. The ceramic and composite powders produced via mechanochemical synthesis and mechanical alloying were examined in terms of composition, particle size, crystallite size, and lattice strain. Samples sintered via pressureless sintering (PS) and spark plasma sintering (SPS) were microstructurally analyzed by using an X-ray diffractometer (XRD), a scanning electron microscope (SEM) attached with an energy dispersive spectroscope (EDS), and mechanically analyzed in terms of microhardness and wear behavior. Based on the results, W2B and WB phases emerged in the SPS'ed W1Ni-5NdB6 and PS'ed./SPS'ed W1Ni-10NdB6 composites. SPS'ed W1Ni-10NdB6 composite had the highest hardness value and the lowest specific wear rate. The SPS'ed W1Ni-5NdB6 composite showed fewer surface damages and higher irradiation resistance as compared with other samples after exposure of He+ irradiation.

A Method to Calculate the Maturity Time for Low Neutron Source Nuclear Reactor Startup Simulations

An approximate method for determining the maturity time is presented for applications in low neutron source nuclear reactor startup simulations. This new method relies only on the calculation of the mean neutron density and does not require the additional calculation of the variance in the neutron density as the traditional method does. The most accurate method for determining the safe neutron source strength, required to sufficiently mitigate the probability of a rogue transient during nuclear reactor startup, uses the Pál-Bell equations. However, as space and energy dependencies are included, the numerical computation become computationally demanding. Therefore, approximate methods that significantly reduce the computation time and improve the computational efficiency of the simulation while remaining very accurate are extremely useful. The approximate method for determining the maturity time presented in this study has shown excellent agreement with traditional methods while offering an order of magnitude reduction in computation time.

Radial reflector macroscopic cross sections preparation impact on the low neutron leakage core Beavrs

Full core nodal calculations are essential for safe nuclear power plant operation. The accuracy and compliance with the calculation and the experiment are based on the proper macroscopic data preparation model. While fuel assembly data preparation is usually based on the simple infinite lattice model simulation, the reflector data preparation is not standardised and the influence of the reflector macroscopic data on full core simulation is not quantified. The paper evaluates the large core with low neutron leakage behaviour with the different reflector cross section preparation models with respect to criticality and power distribution prediction. However, in opposition to the first insight, the different reflector model cannot increase the compliance with the power prediction near the fuel assembly reflector boundary. However, the usage of a special rehomogenization model can be helpful for pin power reconstruction compliance near the edge of the core.

Combined X-ray Diffraction Analysis and Quantum Chemical Interpretation of the Effect of Thermal Neutrons on the Geometry and Electronic Properties of CdTe

Objective: Substantiation of the result of the interaction of thermal neutrons with CdTe crystals by quantum-chemical methods and comparison of the experimental results with the results of quantum-chemical calculations. Methods: Single crystal samples of cadmium telluride (CdTe) were obtained by a modified Bridgman method. The plates cut from the single crystal were subjected to mechanical grinding with abrasive paper of the type P1,200-P4,000, mechanical polishing with silver paste. To clean the surface of the plate from contamination, it was washed in ethyl alcohol. In the experimental part of the work, the influence of low thermal neutron fluxes on the structural parameters of CdTe was studied. The samples were irradiated with a thermal neutron flux in the range from 2.64×107 neutron/cm2 to 1.85×109 neutron/cm2. To study the structure of CdTe crystals, the method of X-ray structural analysis was used, using a DRON-3 X-ray diffractometer. In the second part of the work, computer modeling of the process of interaction of thermal neutrons on CdTe crystals was carried out. Results: X-ray diffraction analysis of the crystals showed that as a result of irradiation with low thermal neutron fluxes, the structural parameters of CdTe improve, as evidenced by the ordering of the crystallites. In addition, in this region of the thermal neutron flux the resistance of the crystals decreases. Conclusion: Calculations of the crystal lattice parameters of CdTe are in good agreement with experimental data. The electronic and optical properties of CdTe have been studied. A decrease in the band gap after irradiation with thermal neutrons has been shown.

JET far-infrared interferometer/polarimeter diagnostic system—40 years of lessons learned

Originally designed for 5 years of plasma operations, the JET far infrared interferometer/polarimeter diagnostic system was still operating at full capability nearly 40 years later in ITER-relevant conditions (e.g. metal wall, tungsten divertor) for multiple D–T campaigns, albeit with significantly lower neutron fluences. The original design had to be adapted substantially over the years due to machine changes, leading to reduced signal and access to mirrors etc, and the diagnostics still worked due to the excellent dynamic range of the detectors. This paper will discuss invaluable lessons learned from designing, operating, optimising and enhancing such a complex system and how these can be used for developing a new class of laser-based diagnostics for next-generation reactor-grade machines.

Evolution of Microstructure During Stress Relieving Heat Treatment of 1S Aluminium

Aluminium and its alloys are extensively used in nuclear research reactors due to its low neutron absorption coefficient, good heat transfer properties, excellent corrosion and oxidation resistance in air and water environment combined with desirable mechanical properties along with considerable radiation stability. The thin walled tubes are normally manufactured through port hole die extrusion route and used in 'O' tempered condition. The thin tubes in 'O' tempered condition are undergone canning operation which may alter the microstructural features to some extent which may affect mechanical properties. The paper describes the extent to which microstructural and subsequently mechanical properties are altered due to the simulated canning process and further requirements of stress relieving heat treatment to restore the microstructural features and mechanical properties. The following studies are carried out with 1S aluminum tube in 'O' tempered state, simulated canning condition and stress relieved at different temperature, - hardness measurement, X ray line profile analysis for dislocation density measurement, tensile properties measurement using ring tensile testing, electron back scattered diffraction (EBSD) analysis for recrystallization fraction determination and changes in texture components. It is found that the canning operation changes dislocation density, micro-strain, coherent domain size which are well reflected in hardness and ring tensile testing. Subsequent stress relieving at 350°C for 2 hrs. leads to improvement of mechanical properties appreciably.

Gold content determination in small core-samples by instrumental neutron activation analysis

Application of instrumental neutron activation analysis (INAA) based on the relative method of concentration standardization was considered to determine Au content in small solid core-samples of the gold-bearing ores and rocks up to 10 g of the mass. The small core-samples about 10 mm in diameter and 22-23 mm in length were cut from the previously collected ore lumps or in-situ using a handheld drilling rig. The studied small core-samples and the polyethylene capsules of the same dimensions filled up with the corresponding certified reference material (CRM) were irradiated for 2 min by a lower density neutron flux with the help of the automated pneumatic transport system (PTS). Maximum mass and dimensions of the core-samples corresponded to the PTS design and to the conditions of its safe operation. A special device was made to fix the transport capsules in the stable counting geometry making possible to eliminate the influence of the neutron flux gradient during irradiation. Due to the substantial differences in CRM and solid sample densities, corrections for neutron self-shielding and for gamma-ray self-absorption by the core-samples were applied. The method was tried to analyze Au content in 170 small core-samples presenting different gold-bearing ores and country rocks from the Kazakhstan’s gold-barite-polymetallic deposit Maikain. This approach, rather simple methodically and requiring no unique equipment, can be used to assess gold resources together with the methods of geostatistics.

Advancing Neutron Detection: Fabrication, Characterization, and Performance Evaluation of Self‐Powered PIN BGaN/GaN Superlattice‐Based Neutron Detectors

Solid state semiconductor based neutron detectors have the potential to be energy efficient and compact, making them suitable for applications where low power consumption and size constraints are important considerations. Herein, neutron detection devices based on PIN structures consisting of BGaN/GaN superlattice (SL) are demonstrated. These SL structures enable to incorporate significant boron (B) content and achieve good crystalline quality epilayers crucial for better neutron detection. Further, by leveraging the built‐in electric field generated by the PIN structure, these devices can be operated without any applied bias, simplifying overall operation and enabling a more compact size system for detection. Their performance is evaluated by measuring real‐time current response under neutron irradiation (IN) and without it (ID). The neutron induced current density (ΔJ = JN − JD) is determined, reaching an impressive value of 0.67 pA cm−2 (two times JD) under thermal neutron flux of 1.2 × 104 n cm−2 s−1 without biasing, demonstrating their self‐powered capability. They exhibit a linear response to varying thermal neutron flux levels. Additionally, the detectors successfully detect low thermal neutron fluxes down to 300 n cm−2 s−1, showcasing their potential for diverse applications, including in low neutron environments, screening nuclear warheads, and preventing illegal trafficking of radiological materials.

Image up-sampling method for dual-radiation nondestructive cargo inspection system

In this paper, an image up-sampling method for a dual-radiation nondestructive cargo inspection system, that uses x-ray and neutron, is proposed. The inspection system is assumed to have a larger neutron detector pixel size than that of x-ray detector pixel, which is usually the case to address the lower neutron source flux and the lower detector efficiency than those in the x-ray imaging system. The proposed technique adopts a cluster map of an x-ray projection image with high spatial resolution to up-sample the corresponding neutron projection image with low spatial resolution. The cluster map is acquired by use of a modified k-means clustering algorithm. The up-sampling process is formulated by a linear least-square problem of mapping the neutron projection image onto the cluster map. Consequently, the neutron projection image would have the same dimension with the x-ray projection image array allowing R-value based material decomposition. We have conducted a numerical study using GATE simulation to demonstrate the efficacy of the proposed method. The proposed method was compared with a linear interpolation method and also with an up-sampling technique that does not use the cluster map. The outperformance of the proposed method over those reference techniques has been successfully presented.

Environmentally resilient data collecting system optimized for measuring low density neutron flux

In this work we detailed present the data collecting system (DCS), designed and optimized for use with a helium neutron counter even under very challenging environmental conditions. The DCS, in addition to its simplicity, allows for the flexible configuration of multiple DCS units into a single measurement system and can also interface with various types of detectors and different data acquisition systems as per the experimental requirements. The main advantages of the presented DCS include its autonomy, remote control capability, and modularity. The pulse shape analysis procedure employed within the DCS enhances measurement sensitivity, which is crucial for detecting extremely low neutron fluxes, such as those in underground laboratories.

Conceptual design of a macromolecular diffractometer for the Jülich high brilliance source.

In this work, a concept for a neutron diffractometer for high-resolution macromolecular structures has been developed within the Jülich High Brilliance Neutron Source (HBS) project. The SELENE optics are adapted to the requirements of the instrument to achieve a tunable low background neutron beam at mm2 scale sample area. With the optimized guide geometry, a low background neutron beam can be achieved at the small sample area with tunable divergence and size. For the 1 × 1mm2 sample, a flux of 1.10 × 107 n/s/cm2 for 0.38° divergence is calculated in the 2-4Å wavelength range, which is about 84.6% of the flux at MaNDi of the high-power spallation source SNS at ORNL. Virtual neutron scattering experiments have been performed to demonstrate the instrument's capabilities for studies of mm scale samples with large unit cells. Results of Vitesse simulations indicate that unit cell sizes of up to 200Å are possible to be resolved with the proposed instrument.

Microstructure and Properties of Pressureless-Sintered Zirconium Carbide Ceramics with MoSi2 Addition.

Zirconium carbide (ZrC) ceramics have a high melting point, low neutron absorption cross section, and excellent resistance to the impact of fission products and are considered to be one of the best candidate materials for fourth-generation nuclear energy systems. ZrC ceramics with a high relative density of 99.1% were successfully prepared via pressureless sintering using a small amount of MoSi2 as an additive. The influence of the MoSi2 content on the densification behavior, microstructure, mechanical properties, and thermal properties of ZrC ceramics was systematically investigated. The results show that the densification of ZrC was significantly enhanced by the introduction of MoSi2 due to the formation of a liquid phase during sintering. In addition, the ZrC grains were refined due to the pinning effect of the generated silicon carbide. The flexural strength and Vickers hardness of ZrC ceramics with 2.5 vol% MoSi2 sintered at 1850 °C were 408 ± 12 MPa and 17.1 GPa, respectively, which were approximately 30% and 10% higher compared to the samples without the addition of MoSi2. The improved mechanical properties were mainly attributed to the high relative density (99.1%) and refined microstructure.

Hot deformation behaviour of Swaged Zircaloy-4 at varying strain rate and its microstructural evolution

Zirconium alloy demonstrates a favourable combination of strength, ductility, low neutron absorption cross-section, as well as excellent corrosion and oxidation resistance. In the context of its application as a reactor component, the critical hot deformation behaviour of isothermally swaged Zr-4 alloy has been extensively studied. Different combinations of elevated temperatures (873 K to 1023 K) and strain rates (0.01 s−1 to 10/s) have shown enhancement in flow stress. Analysis of stress flow, microstructural changes, and activation energy behaviour was performed using a state-of-the-art thermomechanical simulator with 50% reduction in sample thickness. The modified Arrhenius equation was employed to correlate theoretical and experimental flow stress behaviour. EBSD was utilized to understand texture evolution in hot deformed samples, revealing a transformation from pure basal to basal plus pyramidal poles concerning the compression direction. Texture evolution played a significant role in microstructural development, further characterized through TEM, which showed influential characteristics like subgrain boundaries, dislocations, and precipitate behaviour. Dynamic recrystallization mechanisms in swaged Zr-4 hot deformed samples were analysed using bright and dark field TEM modes, which were subsequently correlated with other microstructural characteristics such as grain morphology and structural defects.

An optimization design for fiber-optic neutron detector based on 6LiF/ZnO:Ga and wavelength shifting fibers

The fiber-optic neutron detector consists principally of a neutron-sensitive scintillator, optical fiber, and photomultiplier tube. It has features such as small size, real-time online measurement capability, and high resistance to electromagnetic interference. This detector is excellent for neutron detection in areas with limited space and strong electromagnetic interference. However, its small size results in a comparatively low neutron sensitivity. The goal of this study is to look into the relationship between detector parameters and performance in order to improve the detector design. The research begins with the development of a detector model using Monte Carlo simulation programs to investigate the relationship between the 6LiF/ZnO:Ga mass ratio, thickness, wavelength-shifting fiber length, and detector performance. The 6LiF/ZnO:Ga mass ratio was then used as the test parameter to create equivalent detector samples for experimental validation. The results show that the detector has the highest neutron sensitivity when the mass ratio of 6LiF/ZnO:Ga is 1:1. This pattern is consistent with theoretical simulation results, indicating that the optimization strategy for detector parameters is feasible. The results of this work give a theoretical foundation for the development and practical implementation of the fiber-optic neutron detector.