Neutron Irradiation Environment Research Articles

Defect evolution induced by low-dose neutron irradiation and elastoplastic deformation mechanism of indium-doped GaN materials

The low-dose neutron irradiation defect evolution and nanoindentation response of indium (In)-doped gallium nitride (GaN) materials are investigated in this work, which is of great significance for improving the reliability and safety of nitride semiconductors in neutron irradiation environments, as well as for clarifying their mechanical properties. A numerical simulation approach based on molecular dynamics (MD) is utilized to construct wurtzite GaN models comprising Indium doping concentrations up to 2%. Nanoindentation simulations are performed to analyze the elastoplastic deformation mechanisms of the c-plane and m-plane In-doped GaN materials. The low-dose neutron irradiation process of In-doped GaN is further simulated, and its structural changes and defects evolution are analyzed. The indentation hardness of the c-plane GaN decreases and its Young’s modulus increases as the doping concentration increases. Increases in both the indentation hardness and Young’s modulus are seen for the m-plane GaN with the increasing doping concentration. Moreover, the doping of indium suppresses the plastic deformation both in the c-plane and m-plane GaNs. Two types of prismatic loops formation mechanisms are unraveled, i.e. the “nested shear loops pinch-off” mechanism in the m-plane perfect GaN, and the “lasso-shape shear loop pinch-off” mechanism in the m-plane In-doped GaN. When subject to low-dose neutron irradiation, In-doped GaN materials exhibit higher resistance to the generation of irradiation defects. As the doping concentration increases, the amounts of phase transformation, Frenkel pairs, and amorphization decrease.

Simulation Study on Defect Damage Behavior of Fe under Irradiation Environment

Abstract As a commonly used structural material in nuclear power plants, Reactor pressure vessel (RPV) steel is subjected to high-energy neutron irradiation from the reactor core for a long time, and the service environment is harsh. The accumulation of point defects (interstitial atoms and vacancies) generated by irradiation over a long period can seriously affect the microstructure of the material. The macroscopic properties of the material changed until the material failed, which threatened the safety and operation stability of nuclear power plants. This paper used the method of molecular dynamics to simulate the cascade collision process of Fe in the irradiation environment. The relationship between different factors and radiation damage defects was studied. Firstly, the quantity of the Frenkel pairs increases rapidly in the irradiation environment, and then recombination occurs after reaching the peak, which makes the quantity of the Frenkel pairs decrease rapidly. Finally, the quantity of Frenkel pairs is in a stable trend. When PKA energy and temperature increase, the higher the quantity of Frenkel pairs at the peak is, the higher the recombination rate of defects is. Meanwhile, the larger the cluster size of interstitial atoms and vacancies is, the greater the corresponding quantity of clusters is. As PKA energy increases, the quantity of residual Frenkel pairs at stability increases, while the number of residual Frenkel pairs at stability decreases with temperature. This is attributed to the intense thermal motion of molecules at high temperatures, resulting in the quantity of Frenkel pairs decreasing at the stable stage. Therefore, by simulating the irradiation damage process of Fe, the prediction of material irradiation damage under a neutron irradiation environment is provided, and the service life of materials can be provided with theoretical guidance.

Design and performance evaluation of a compact thermal and fast neutron spectrometer

Measurement of the space neutron radiation environment has put forward new requirements for the miniaturization and lightweight design of neutron spectrometers. In this paper, a compact broad-spectrum neutron spectrometer was designed, in which the fast neutron detector was based on the EJ-254 scintillator-coupled with SiPMs array, and the thermal and epithermal neutron detectors were based on the NaI(Tl + Li) (NaIL) scintillator coupled with SiPMs array encapsulated in Al and Cd shells, respectively. The overall weight of the neutron spectrometer was only 2.4 kg, and its dimensions were 154 mm × 123 mm × 115 mm. The fast neutron energy spectrum was obtained based on the capture-gated method and the Maximum Likelihood Expectation Maximization (MLEM) unfolding algorithm; the counts of thermal and epithermal neutrons were obtained based on the optimized frequency-domain Pulse Shape Discrimination (PSD) method and cadmium difference method. The proposed neutron spectrometer with all applied methods was tested as a whole system with a252Cf and a241Am–Be source. The spectrum result was compared to an ISO standard source. The accuracy of fast part of 0.5–10 MeV was 5.9% in definition of Relative Root Mean Squared Error (RRMSE). The thermal and epithermal neutron counts were far higher than standard spectrum, and we proved it being effected by laboratory introduced neutron scattering through a simulation, thus the thermal and epithermal flux measurements was believed to be decent. The Figure of Merit (FoM) of the NaIL's PSD neutron gamma discrimination were 2.57 and 2.90 respectively.

First-principles study on the effects of fast neutron irradiation and fast neutron irradiation under the external electric field on carbon-based material AB bilayer graphene

The irradiation damage mechanism of fast neutron irradiation and the fast neutron irradiation under the practical application of an additional electric field on carbon-based material AB bilayer graphene is not yet clear. This paper systematically investigates the impact of fast neutron irradiation and fast neutron irradiation under the practical application of the additional electric field (−0.2 V/Å) on carbon-based material AB bilayer graphene through density functional theory and molecular dynamics simulations. The research results indicate that the changes in the configuration of AB bilayer graphene and the appearance of surface ripples induced by fast neutron irradiation are the two main reasons for the alterations in the system's microstructure, band structure, Young's modulus, and optical properties, which may lead to performance degradation and damage of carbon-based devices. The further variation in the average interlayer spacing of the system under external electric field irradiation leads to the diversity of the system configuration, which ultimately leads to the diversity of the band structure of the system. The uncertainty in the type and width of the bandgap in AB bilayer graphene under fast neutron irradiation with an external electric field may be one of the reasons for the performance degradation and permanent damage of carbon based devices in practical applications. In the fast neutron irradiation environment, the precise control of the bandgap of AB bilayer graphene by an applied electric field will fail.

Synergistic effects of polymaleic acid and di(dioctylpyrophosphato) ethylene titanate on B4C modification in highly filled polymer fibers for improved neutron protection safety and wear comfort of articles

Developing wearable articles with radiation protection safety and wear comfort is one of the urgent issues in that flourishing nuclear technologies are accompanied by increasing radiation threats. However, current wearable articles either display insufficient shielding performance or exhibit poor air and water vapor permeability, which are difficult to meet the growing demand of personal protection in nuclear environments. In this paper, polymaleic acid (PMA) and di(dioctylpyrophosphato) ethylene titanate (DPET) have been used to superficially modify boron carbide (B4C) particles for constructing sufficient hydrogen bonds with polyvinyl alcohol (PVA) matrix. Synergistic effects of PMA and DPET lead to enhanced interfacial interaction. As high as 55 wt% B4C/PVA fibers have been obtained with smooth wet spinning. Their multi-layered fabrics display excellent air and water vapor permeability meanwhile thermal neutron shielding. Three-layer B4C/PVA fabrics have 89.2 % thermal neutron (0.025 eV) shielding rate. In addition, 321.1 mm/s air and 2236.2 g/m2·24 h water vapor permeability have been measured. Besides, the fabrics still show great flexibility in spite of the fibers with extreme filling of inorganic particles. This work provides alternative wear articles to cope with the dilemma of radiation protection safety and wear comfort in neutron radiation environments.

Irradiation-induced age-related degradation mechanisms effect on integrity of baffle former bolts

For long-term operation of nuclear power plants, integrity of structures, systems and components should be ensured rigorously. In particular, since the complicated reactor vessel internals (RVIs) have been exposed to high temperature and neutron irradiation environment, materials reliability programs on the RVIs to resolve anticipated issues were conducted by several research institutes. The objectives of this research are to examine evaluation procedures derived from the previous works and to apply them for RVIs in two operating reactors with baffle former assembly from different electricity powers. Finite element (FE) analyses were performed employing five user-subroutines developed by the authors for reflection of multiple age-related degradation mechanisms (ARDMs) as well as pre- and post-processing. Temperature distributions due to gamma heatings, fuel cycles, typical normal operating temperature and pressure histories were considered with relevantly varying material properties. As results, effects of irradiation were quantified in terms of stresses, strains and irradiation assisted crack sensitivity parameters. Further assessment was carried out through not only investigating contribution of each ARDM but also comparing the present FE analysis results with those obtained from a different reactor type in detail.

Suppression of Rhenium and Osmium Production in Tungsten by Selective Isotopic Enrichment

Tungsten (W) based materials are primary candidates for the plasma facing components (PFCs) in fusion power stations. A side effect of the use of W in neutron irradiation environments such as fusion is the transmutation to Rhenium (Re) and Osmium (Os), which are known to degrade the mechanical and thermal properties. In this study, neutron transport and nuclear inventory simulations were used to investigate a strategy of selective isotopic enrichment and/or depletion of W isotopes to supress the formation of Re and Os in a representative first-wall monoblock design. It was found that Re and Os production can be reduced by up to three orders of magnitude compared to natural W by depleting W-186 isotope to 100%, as 186W(n,γ) 187W followed by beta decay is the main pathway for Re production. Two design methodologies were investigated that considered 1) an approach with two discrete, depleted in W-186 regions, with a range of depletion degree investigated and 2) a graded approach with layers of different levels of depletion to emulate a functionally graded material.

Measurement of 6Li burn-up reaction rate using a single crystal CVD diamond detector under fast neutron irradiation environment

The study aimed to develop a technique for measuring the 6Li burn-up rate using a single crystal diamond (SCD) detector (SDD) made by chemical vapor deposition (CVD) with a thin lithium fluoride foil. The method relied on pulse shape discrimination (PSD) to separate pulses by nuclear reaction products of 6Li from pulses due to fast neutrons and gamma-rays.By applying the PSD method, the rectangular shape pulses induced by events occurred at the specific depth positions of the SCD, i.e., the incident surface, the ballistic center region (BCR), and the rear surface, were extracted. Thereafter, pulses occurred each specific depth position of the SCD were discriminated according to the pulse width.Due to a short range of nuclear reaction products in the SCD, pulses at the incident surface should be induced by both fast neutrons and nuclear reaction products, although pulses at the BCR are caused by fast neutron only. The study demonstrated that quantitative measurements of nuclear reaction products in the 6Li burn-up reaction under fast neutron irradiation can be achieved using only one SDD by analyzing event rates at the incident surface and in the BCR region.

Neutron Radiation Testing of RISC-V TMR Soft Processors on SRAM-Based FPGAs

Soft, configurable processors within FPGA designs are susceptible to SEUs. SEU mitigation techniques such as TMR and configuration memory scrubbing can be used to improve the reliability of soft processor designs. This paper presents the improvements in reliability of five different TMR soft processors within a neutron radiation environment. The TMR processors achieved up to a 75× improvement in reliability at the cost of potentially 4.8× resource utilization and an average 12.4% decrease in maximum frequency compared to the unmitigated designs. This work compares the metrics of reliability, power consumption, and performance among the default unmitigated processors and their TMR variations.

Recent progress in experimental investigation of neutron irradiation response of tungsten

In order to facilitate its application as plasma facing material, neutron irradiation response of tungsten (W) has been extensively studied by employing fission reactors. In this article, the major findings of the microstructural evolution and thermo-mechanical property degradation of W following neutron irradiation in various reactors are summarized by reviewing the papers published in 2019–2021 from the US-Japan joint projects on fusion materials research and the EUROfusion program. Microstructural development of W following irradiation has been well documented as functions of irradiation temperature, dose, and neutron spectrum. Detailed characterization of the primary defect features, i.e., dislocations, voids, and precipitates, enables a better understanding of the interactions between displacement damage and solid transmutant elements. Irradiation hardening and embrittlement are the primary degradation phenomena of W exposed to neutrons, manifested by hardness and yield strength increase, ductility loss at high doses, and shift of the ductile to brittle transition temperature (DBTT). It is believed that the segregation and precipitation of the solid transmutant elements dominantly contributes to the property deterioration. Predicting W performance in real fusion neutron irradiation environments based on the current understanding of W response to fission neutrons remains an open question.

Segregation and diffusion behaviours of helium at grain boundaries in silicon carbide ceramics: first-principles calculations and experimental investigations

Under a severe neutron irradiation environment, helium behaviour has become a significant concern due to helium-induced degradation of mechanical properties in SiC structural materials. Experimental observations showed that helium prefers to aggregate at grain boundaries (GBs) in SiC ceramics. However, the interaction mechanisms between helium and GBs have remained unclear. By performing first-principles calculations, we studied helium segregation at six symmetric tilt GBs and helium diffusion along and across the GBs. The solution energies of helium at the GBs are much lower than that of helium in grains, indicating preferential helium segregation at the GBs. The negative segregation energies of helium decrease linearly with the excess volume of GBs and the relative charge densities of the interstitial sites, which reveals that helium easily segregates at the GBs having open structures and low charge density interstitial sites. The interstitial helium atoms at GBs are difficult to form clusters due to weak interactions. The diffusion energy barriers of helium along the GBs Σ9(221)[11¯0] and Σ27(552)[11¯0] in the direction of tilt axis [11¯0] are much lower than that of helium in the grains. The two GBs can significantly enhance the helium diffusivity, serving as fast diffusion channels for helium. The calculation results are analysed and compared with our experimental data.

Geant4 simulation of neutron displacement damage effect in InP

As the second-generation compound semiconductor material, indium phosphide (InP) has strong irradiation resistance and high photoelectric conversion efficiency. It has advantages in the field of photonics and radio frequency. In atmospheric space, high-energy cosmic rays enter into the earth’s atmosphere and interact with nitrogen (N), oxygen (O) and other elements to produce secondary cosmic rays. The irradiation particles in the atmosphere are mainly neutrons because the penetration of charged particles is weak. The InP semiconductor devices are affected by atmospheric neutron irradiation of various energy from all directions, which results in the internal defects in InP crystals, the degradation of device performance and the reduction of device lifetime. In this paper, Monte Carlo simulation software Geant4 is used to simulate the neutron irradiation effect, and the initial state distribution of displacement damage caused by neutrons with different energy is obtained, including the distribution of non-ionized energy loss (NIEL) with depth, the relationship between NIEL and the energy of incident neutrons, and the type, number and energy of primary knock-on atoms (PKA). The results show that 1) the NIEL is uniformly distributed when material thickness is on the order of μm and for the material thickness on the order of cm and more, the NIEL decreases as the depth increases and can be reduced to zero when the target material is thick enough; 2) by analyzing the NIEL produced by 1–20 MeV neutrons incident on 3-μm InP and their distribution with depth, it is found that the NIEL first increases and then decreases with incident neutron energy increasing. This trend is caused mainly by PKA produced through the inelastic scattering reaction; 3) by analyzing the type and the energy of PKA produced by 1–20 MeV neutrons incident on 3 μm InP, it is found that the PKA of In/P accounts for a large proportion, which causes displacement damage mainly, and the higher the neutron energy, the richer the variety of PKA is and the greater the maximum kinetic energy of PKA, but the PKAs mainly distribute in the low energy part. The present research has theoretical and guiding value for the long-term application of InP-based 5G devices in atmospheric neutron irradiation environment.

Synergistic effects in MOS capacitors with an Au/HfO2–SiO2/Si structure irradiated with neutron and gamma ray

The properties of oxide trapped charges and interface state density in the metal oxide semiconductor (MOS) capacitors with an Au/HfO2–SiO2/Si structure were investigated under irradiation of 14 MeV neutron and 60Co gamma-ray. In the mixed neutron and gamma irradiation environment, the formation of the oxide trapped charges in the HfO2–SiO2 layer is determined by the total deposited ionization energy, i.e. the sum of ionization energy deposition of the neutrons and the accompanying gamma rays, while the influence of the displacement damage caused by 14 MeV neutrons can be ignored. The interface state density depends not only on the ionizing energy loss (IEL) but also the non-IEL (NIEL), and NIEL plays a major role below the critical neutron fluence of 4.5 × 1012 n cm−2. The synergistic effect of the interface state is observed increases with energy deposition in the oxide at lower fluences, while decreasing above the critical fluence. These results confirm the existence of the synergistic effect of neutron and gamma irradiation in damaging HfO2 MOS devices.

Radiation tolerance analysis of 4H-SiC PIN diode detectors for neutron irradiation

Silicon carbide (SiC) detector shows sufficient merits for the application of radiation measurement in harsh neutron radiation fields due to the strong radiation tolerance, good environment adaptability, and excellent electrical properties. In order to meet the demand for higher precision measurements, the reliability of SiC detectors under harsh neutron irradiation environments must be characterized. In this work, electrical characteristics, α-particle response, and pulse response speed of 4H-SiC PIN diode detectors were carefully investigated under 1 MeV equivalent neutron irradiation fluence from 5 × 1012 cm−2 to 1 × 1014 cm−2. Before neutron irradiation, SiC detector showed dark current as low as 10 pA at 200 V bias, with superior charge collection efficiency of more than 98%, perfect energy resolution within 0.9%, and ultrafast response speed with a rise time and decay time of 2.1 ns and 5.9 ns, respectively. After a neutron irradiation at fluence of 1 × 1014 cm−2, noticeable changes were found in the forward IV characteristics of SiC detector, but the dark current was stable below 1 nA, with only 5.7% and 0.26% degradation in charge collection efficiency and energy resolution respectively, and the pulse response speed remained unchanged. These results demonstrate 4H-SiC PIN diode detector has superior neutron radiation tolerance, and the obtained evolution characteristics are very valuable for improving the measuring accuracy of SiC detectors in the intense neutron field.

Fiber/matrix debonding evaluation of SiCf/SiC composites using micropillar compression technique

Third-generation silicon carbide (SiC) composites reinforced by SiC fibers (Hi-Nicalon S [HNS] and Tyranno SA3 [SA3]) are attractive for use in next generation reactors owing to their high strength and chemical inertness at high temperatures, as well as enhanced radiation tolerance under neutron irradiation environments. To optimize composite performance, the interfacial mechanical properties of chemical vapor–infiltrated (CVI) SiCf/SiC composites are investigated in this effort by using a slant interface micropillar compression testing procedure. The micropillar test specimens, containing an inclined pyrolytic carbon (PyC) interphase, are prepared using a focused ion beam. The novel microcompression testing successfully quantifies the debond shear strength and internal friction coefficient of micropillar test samples by using the Mohr-Coulomb formulation. According to four types of SiCf/SiC composite microcompression test results, interfacial properties and debond mechanisms are significantly affected by the PyC layer thickness, the local bonding mechanism of the PyC interphase on the SiC fiber surface, and the surface roughness of fibers. Regardless of PyC thicknesses, SA3-reinforced CVI SiCf/SiC composites are found to have much higher debond shear strengths than HNS-reinforced SiCf/SiC CVI composites. By using this micropillar compression technique alongside analytical methods, we uncover new understandings of PyC interface properties. Additionally, the micropillar test results obtained are correlated with macroscopic mechanical properties of neutron-irradiated CVI SiCf/SiC composites.

Low dose rate γ-ray detection using a MAPS camera under a neutron radiation environment.

We present γ-ray radiation detection in a neutron radiation environment using a monolithic active pixel sensor (MAPS) camera without conversion or shielding layers. The measured output signal is the sum of the pedestal value, noise, and real radiation response signal. The sensor response shows that the MAPS camera is sensitive to neutrons and can capture a single photon. The number of pixels with a signal exceeding 100 exhibits a strong dependence on the dose rate and is the best indicator of this value. Therefore, a MAPS camera can be efficiently used as a radiation detection sensor in a robotic system, further limiting human errors in performing radiation detection in complex nuclear radiation environments.

Neutron irradiation tolerance of potassium-doped and rhenium-alloyed tungsten

Tungsten (W) based materials for fusion reactor applications have been developed in Japan for the last decade to improve thermo-mechanical properties and tolerance with respect to neutron-irradiation effects. Potassium (K) doping for dispersion strengthening and alloying by rhenium (Re) for solid solute softening and strengthening were applied to W fabricated by powder metallurgy. Thereby modified W materials, e.g. K-doped W and K-doped W-3%Re, demonstrated a higher recrystallization temperature threshold, an increase in strength and ductility, and a reduction in ductile-to-brittle transition temperature (DBTT) compared to pure W in the non-irradiated state. Embrittlement caused by displacement damage and its amplification due to solid transmutation products is expected to be an intrinsic issue of W based materials under neutron irradiation environment inherent to future fusion reactors. In this paper, the effect of K-doping and alloying by Re on the neutron-irradiation-induced embrittlement of W was investigated at neutron fluence causing a considerable amount of solid transmutation products.

Thermal Barrier Coatings Overview: Design, Manufacturing, and Applications in High-Temperature Industries

Today’s competitive world economy is creating an indispensable demand for increased efficiency of engineering components that operate in harsh environments (i.e., very high-temperature, corrosive, or neutron irradiation environments), for applications in the energy, automotive, aerospace, electronics, and power industries. Increased research is being done on thermal barrier coatings (TBCs) for protecting such components, since the versatility of manufacturing techniques and the scale of deployment result in increased life, economics, performance, and durability. This review focuses on the advances that led to using TBCs for component life extension and, more recently, as an integral part of advanced component design for high-temperature and other types of harsh environments, such as those found in nuclear-related applications. Factors that led to state-of-the-art advanced coating-fabrication techniques [e.g., electron-beam physical vapor deposition (EB-PVD), plasma spray deposition, and electrophoretically deposited TBCs, as well as functionally graded material (FGM) manufacturing] have also been emphasized in current coating R&D. This review explores the current state of TBCs, i.e., the latest advances regarding their fabrication and performance, associated challenges, and recommendations for their future use in aerospace, nuclear, high-temperature, or otherwise harsh environments.

Observations of neutron radiation environment during Odyssey cruise to Mars

In April 2001, Mars Odyssey spacecraft with the High Energy Neutron Detector (HEND) onboard was launched to Mars. HEND/Odyssey was switched on measurement mode for most of transit to Mars to monitor variations of spacecraft background and solar activity. Although HEND/Odyssey was originally designed to measure Martian neutron albedo and to search for Martian subsurface water/water ice, its measurements during cruise phase to Mars are applicable to evaluate spacecraft ambient radiation background. The biological impact of the neutron component of this radiation background should be understood, as it must be taken into account in planning future human missions to Mars. We have modeled the spacecraft neutron spectral density and compared it with HEND measurements to estimate neutron dose equivalent rates during Odyssey cruise phase, which occurred during the maximum period of solar cycle 23. We find that the Odyssey ambient neutron environment during May – September 2001 yields 10.6 ± 2.0 μSv per day in the energy range from 0 to 15 MeV, and about 29 μSv per day when extrapolated to the 0–1000 MeV energy range during solar quiet time (intervals without Solar Particle Events, SPEs). We have also extrapolated HEND/Odyssey measurements to different periods of solar cycle and find that during solar minimum (maximum of GCR flux), the neutron dose equivalent rate during cruise to Mars could be as high as 52 μSv per day with the same shielding. These values are in good agreement with results reported for a similar measurement made with an instrument aboard the Mars Science Laboratory during its cruise to Mars in 2011–2012.

New designs of target and cooling scheme for water cooled divertor in DEMO

Developing a robust and reliable divertor with an adequate heat removal capability is one of the most crucial issues for the realization of DEMO. Although lots of designs and technologies have been accumulated for the water cooled divertor and a full tungsten water cooled divertor for ITER has been successfully developed, there is still a gap to catch up the requirements of DEMO mainly because of the neutron irradiation. For the purpose of putting forward a feasible proposal of the divertor for DEMO concept in a short to medium term, new designs for the target and cooling circuit have been explored in this article. According to the distribution characteristics of heat load and neutron irradiation damage on the divertor and the unique properties of materials, CuCrZr and RAFM are chosen as the heat sink material for the plasma facing unit (PFU) at strike zone and other zone respectively, which is named ‘dual PFUs’. A single coolant circuit is applied to both PFUs and a new design ideology of the cooling scheme has been developed. To assess the performance of the designs, corresponding thermal and structural response for the PFUs and thermal-hydraulics analysis for the cooling circuit had been performed. The property of CuCrZr in neutron irradiation environment is the main limitation for the performance of the target. And the new design for cooling circuit can effectively reduce the pressure drop without any negative impact on the divertor by rationally allocating the coolant through outer vertical target to inner vertical target and dome.