Low Neutron Fluence Research Articles

A novel approach to enhance the ionic conductivity of silver nanoparticles incorporated PVA:NaBr polymer electrolyte films via fast neutron irradiation

This study investigates the unprecedented impact of fast neutron irradiation on the structural, electrical, and morphological properties of solid polymer electrolytes (SPEs). The SPE, a poly (vinylalcohol) (PVA)-sodium bromide (NaBr) based matrix with the incorporation of silver nanoparticles (AgNPs), was prepared via a solution casting method and subjected to various time intervals of fast neutron irradiation. X-ray diffraction (XRD) analysis revealed a distinct trend in the polymer electrolyte sample's amorphous phase, with decreased levels at lower neutron fluence and augmented levels at higher fluences. Fourier transform infrared (FTIR) spectroscopy highlighted plausible interactions leading to chain scission and cross-linking. Optical investgations below 300 nm exhibited increased absorbance and a shifted position of the surface plasmon resonance peak correlated with AgNPs. Validation of structural changes in the Ag-incorporated PVA-NaBr system was supported by reduced bandgap and elevation in Urbach energy, corroborating FTIR findings. Enhanced thermal stability was confirmed using thermogravimetric (TGA) analysis. Morphological modification upon irradiation were evident via Field emission scanning electron microscope (FESEM). Transport properties evaluated by Nyquist plot fitting showcased escalating room temperature conductivity with neutron fluence, and the highest conductivity obtained was 4.38 × 10−3 S/cm, an order higher than the pristine sample. Transference number measurements (TNM) indicated that the primary charge carriers are ions rather than electrons. This novel approach confirms that neutron irradiation can be used as a potential way to obtain highly conductive polymer electrolyte films.

Defect-curing effects of fast neutrons on n-type GaN

The defect-curing effects of fast neutrons have been studied using single-crystalline GaN samples irradiated under four different neutron fluences. The conditions of each neutron irradiation were quantified by gamma-ray spectroscopy. We found that some defects generated during GaN crystal growth were cured at a low neutron fluence, and at a certain irradiation condition, the number of cured defects were larger than that of newly produced defects. Electrical parameters of Schottky barrier diodes fabricated by using samples from every batch demonstrated different levels of defect-curing effects. In terms of optical characteristics, the defect-curing effects were checked by photoluminescence spectroscopy measurements. Through synchrotron X-ray topography characterization, the number of cured, newly generated, and altered defects were quantified and directly confirmed the curing effects of fast neutrons.

Synergistic Effect of Electrical Stress and Neutron Irradiation on Silicon Carbide Power MOSFETs

The synergistic effect of radiation and bias voltage for the commercial silicon carbide (SiC) power MOSFETs was investigated by 14-MeV neutron irradiation in this article. The types of C3M0065090D and C2M0080120D commercial SiC power MOSFETs from CREE studied in this article show consistent changes after the 14-MeV neutron irradiation. The results show that the devices biased during irradiation experienced more degradation than that only irradiated and biased devices. Compared with the devices only irradiated and biased, the devices biased during irradiation experienced an additional decrease in saturation current and a further decrease in peak transconductance. The defects in the devices were characterized by the low-frequency noise (LFN) and deep-level transient spectrum (DLTS) methods. The results show that the synergistic effect increased the defect concentration of the device, resulting in further degradation of the electrical performance of the device compared to the control group without a bias voltage. The defect characterization shows that the drain–source voltage leads to the accumulation of carbon and silicon vacancies induced by irradiation at the interface, which results in intensified radiation damage and significant changes in the performance of the device at low neutron fluence.

Investigation of the recovery process in low-dose neutron-irradiated 6H-SiC by lattice parameter and FWHM of diffraction peak measurements

Nitrogen-doped 6H-SiC single crystals irradiated with a low neutron fluence of 2.0 × 1020 n/cm2 (E > 0.1 MeV) at 200°C were investigated. To evaluate the crystalline defects induced by neutron irradiation and analyse their recovery behaviour, the specimens were isochronally annealed from room temperature to 1500°C. The lattice parameters and FWHM changes of (0006) plane diffraction peaks were recorded by X-ray diffraction. The XRD results exhibit that the FWHM was broadened and then narrowed linearly when isochronal annealed over 600°C. The lattice parameter recovery curves were analysed by the first-order model, and then the rate coefficient and activation energies at each annealing step were obtained. Based on the activation energies, the recovery process was divided into four stages and the recovery mechanism was discussed.

Fast neutron irradiation effects on Si- and GaN-based avalanche photodiodes

We report the behaviour of the dark current and gain characteristics of Si- and GaN-based avalanche photodiodes (APDs) irradiated by fast neutrons. For Si-based APDs, the dark current increases with the increase of neutron fluence, indicating that the avalanche property has been seriously affected. The gain values of Si-based APD slightly increase after irradiation by a low neutron fluence of 1.0 × 1012 cm−2, while the device exhibits gain degradation as the fluence increases to 1.0 × 1013 cm−2 or even above at high reverse bias voltage, unlike the previous studies where only degradation was observed. The steep increase of dark current and gain when approaching the breakdown voltage after neutron irradiation indicate that the avalanche property is almost unaffected for GaN-based APDs. Furthermore, we infer that the optical absorption between acceptor state and conduction band in the p-type layer plays an important role in influencing the change of dark current and gain at high reverse bias voltage. The findings not only enrich the understanding of neutron irradiation effect on Si- and GaN-based APDs, but also experimentally prove that GaN-based APDs hold better radiation resistance than Si-based devices.

Optical transmission and dimensional stability of single-crystal sapphire after high-dose neutron irradiation at various temperatures up to 688 °C

The use of single-crystal sapphire optical fibers has been considered to extend fiber-optic sensing to the extreme temperature (>1000 °C) environments encountered in nuclear applications. However, before these sapphire fiber–based sensors can be deployed, their optical transmission and dimensional stability (which impacts drift of some sensors) must be characterized under representative testing conditions. Data regarding the optical transmission of sapphire following high-dose neutron irradiation at temperatures >100 °C is extremely limited. This work provides measurements of optical density (i.e., attenuation) and directional dimensional changes in bulk single-crystal sapphire materials irradiated to a fast neutron fluence of 2.4 × 10 21 n/cm 2 (3.5 displacements per atom) at temperatures ranging from 95 to 688 °C. Optical density measured after irradiation at 95 and 298 °C showed ultraviolet and visible absorption bands corresponding to known defect centers and temperature trends that were generally consistent with previous ex situ and in situ measurements made at much lower neutron fluence. However, optical density measured after irradiation at 688 °C was as much as two orders of magnitude higher, indicating that the fundamental mechanism for radiation-induced attenuation changes at this irradiation temperature. Additional analysis and comparison with previous works suggest that the attenuation may result from void formation, leading to increased Rayleigh scattering losses in the material and increased swelling that would also result in drift of Bragg grating-based sensors in sapphire fibers. These results pose serious questions regarding the feasibility of sapphire fiber–based sensors for high-temperature nuclear applications.

Testing the NASA BioSentinel Pixel Dosimeter Using Gamma-ray and Neutron Sources at the LLNL Calibration Lab.

The objective of this paper is to evaluate the accuracy of the NASA BioSentinel Pixel Dosimeter (BPD) using gamma-ray and neutron sources in a standard calibration lab. The dosimeter tested here is the ground-based version of the BPD that will be onboard the BioSentinel mission. The BPD was exposed to radiation from 60Co, 137Cs, and 252Cf at selected distances (dose rates) at the Lawrence Livermore National Laboratory (LLNL) Radiation Calibration Laboratory (RCL), and the results were compared with NIST traceable benchmark values. It is recognized that these sources are not analogs for the space environment but do provide direct comparisons between BPD response and well characterized calibration lab values. For gamma rays, the BPD measured absorbed dose agrees to ≤ 3.8% compared with RCL benchmark values. For neutrons, the results show that the BPD is insensitive, i.e., the BPD detected only the gamma-ray dose component from 252Cf. The LET spectra obtained for gamma rays from 60Co and 252Cf are consistent with expectations for these gamma-ray energies, but the LET spectrum from the 137Cs gamma rays differs substantially. The potential causes for this difference are the high dose rate from 137Cs and the lower secondary electron energy produced by 137Cs gamma rays. However, neither of these results in errors in the absorbed dose. Based on comparisons with NIST-traceable standards, it is evident that the BPD can measure absorbed dose accurately from low LET charged particles. The sensor's insensitivity to neutrons is unlikely to be a limitation for the BioSentinel mission due to the expected low secondary neutron fluence.

Experience of various materials for design and manufacture of bellows for nuclear industry

Bellows find wide applications in reactor systems such as in bellows sealed valves as primary leak tight barriers and in piping systems to absorb differential thermal expansions. Reliable operation of bellows is strongly dependent of proper material selection. Various candidates for material of the bellows are austenitic stainless steels (such as SS316, SS304, SS316Ti, SS304L etc.), precipitation hardened stainless steels (such as AM350) and Nickel base alloys such as Inconel-718 and Inconel-625. This paper presents the review of the operating experience, mechanical & neutronic properties of various materials for bellows in nuclear industry. In this work, it is observed that Inconel alloys have superior mechanical properties than austenitic stainless steels but exhibit neutron embrittlement. Hence, use of Inconel bellows is limited to low neutron fluence applications. Precipitation hardened steels such as AM350 have high mechanical strength but lesser ductility. Though AM350 is not suitable for formed bellows, it has excellent operating experience for welded disc bellows in nuclear applications. Austenitic stainless steels have large operating experience. Variants of SS316 are used for high temperature and variants of SS304 are used for temperatures below creep range. ‘L’ grades or stabilized grades are used for resistance to sensitization during welding. Based on the present review work, guidelines for selection of material for bellows are drawn, satisfying the selection criterion. Copyright © 2017 VBRI Press.

Revealing the chemical environment of Cr, Fe, and Ni in high temperature-ultrafine precipitate strengthened steel subjected to low fluence neutron irradiation

• Neutron irradiated HT-UPS steel was characterized via synchrotron techniques. • Cr 23 C 6 precipitate evolution occurred at lower fluences than previously determined. • Nucleation, growth, and/or ballistic dissolution of Cr 23 C 6 at <0.3 dpa. • Atoms surrounding Fe and Ni demonstrated neutron resistance up to 0.3 dpa. High temperature-ultrafine precipitate strengthened (HT-UPS) steel has potential applications in advanced nuclear reactors as a structural material. However, little is currently known about its response to neutron irradiation. This research provides insight into the neutron irradiation-induced physicochemical changes of the major constituents in HT-UPS steels, including Fe, Cr, and Ni, using synchrotron X-ray absorption near edge structure (XANES) and diffraction. This study is the first known investigation using XANES to characterize HT-UPS steel to analyze the evolution of the atomic level chemistry. It was found that following neutron exposure, radiation-induced nucleation, growth, and/or ballistic dispersion of Cr 23 C 6 precipitates occurred at very low neutron irradiation fluences, from 0.003 displacements per atom (dpa) up to 0.3 dpa, at 600 °C, which were at least an order of magnitude lower than previous studies. Calculations of the angular momentum projected partial density of states and the XANES spectra confirmed the experimental findings of the Cr 23 C 6 precipitate evolution. In contrast, the local atomic structure around Fe and Ni atoms demonstrated irradiation resistance.

X-ray characterization of anisotropic defect formation in SiC under irradiation with applied stress

High-energy x-ray diffraction is employed to gain insight into the irradiation-induced creep behavior of silicon carbide (SiC). Polycrystalline β-SiC specimens were simultaneously exposed to elevated temperature neutron-irradiation and mechanically applied stresses. The structural disordering was subsequently examined using two-dimensional x-ray diffraction. The intensity of the (111) shoulder peak, an indication of stacking disorder, increased when the specimens were irradiated under tensile stress. This is the first observation of nanoscale stress-induced stacking disorder in SiC at low neutron fluences. These findings suggest stress-induced preferential nucleation and/or growth of defect clusters as a key creep mechanism in neutron irradiated SiC.

Reducing the reverse leakage current of AlGaN/GaN heterostructures via low-fluence neutron irradiation

We demonstrate that low-fluence neutron irradiation can be a promising way to reduce the reverse leakage current of AlGaN/GaN heterostructures grown by MOCVD on sapphire substrates while maintaining other electronic properties almost unchanged.

Recovery behavior of neutron-irradiated aluminum nitride with and without containing interstitial dislocation loops

Recovery behavior of the same AlN but neutron-irradiated with different conditions was studied by using XRD and dilatometry. Length recovery rates at each isothermal annealing step were analyzed by first to second-order kinetics. The specimen irradiated at lower neutron fluence at low irradiation temperature responded isotopic unit-cell expansion, which indicated random dispersion of point defects of vacancies and/or very small defect clusters. From irradiation temperature up to 523 K, the results of Arrhenius’ plot applied for second-order kinetics gives an activation energy value of 2.9 eV. An intermediate region, 573 to 1023 K, was represented by a slightly lower activation energy of 2.0 eV. For the first lower temperature range, migration of VN may be the mechanism and that of VAl for the second stage. At high-temperature region, 1073 to 1423 K, represents final constant recovery before the saturation of length recovery. In this region, the high activation energy of 5.8 eV was observed. It is supposed that dissociation of the vacancy-antisite cluster, interstitial nitrogen cluster, or formation of a complex containing vacancy and antisite can be candidate mechanisms. The specimen containing interstitial loops after the medium fluence at higher temperatures did not recover up to 723 K, indicating almost free of independent interstitials. Between 773 and 1123 K, recovery required a similar amount of energy to the high-temperature region of the lower fluence specimen (5.8 eV), and then the same mechanism could be applied. In the second region, 1173 to 1423 K, with a very high activation energy of ~18 eV, remarkable large and quick shrinkage was observed. Large volume shrinkage may be the result of diminishing anisotropic lattice swelling, i.e., unit-cell volume expansion by internal strain is relieved.

Effects of thermal aging and low-fluence neutron irradiation on the mechanical property and microstructure of ferrite in cast austenitic stainless steels

The effects of low-fluence neutron irradiation on hardening and microstructure evolution in ferrite of solution annealed or thermally aged CF3, CF3M, CF8 and CF8M cast austenitic stainless steels (CASSs) have been investigated by means of nanoindentation tests and atom probe tomography (APT). Thermal aging was performed at 400 °C for 500 h. Neutron irradiation was carried out to a fluence of 4.84 × 1018 n/cm2 (E > 1 MeV) at the temperature ranging from 289 to 292 °C in the LVR-15 research reactor. Irradiation hardening in thermally-aged specimens was found to be similar with or smaller than that in the corresponding solution annealed specimens. Phase decomposition and formation of solute clusters acted two major factors for the hardening in ferrite with thermal aging and/or neutron irradiation. The phase decomposition of ferrite increased with either the thermal aging or the neutron irradiation for the solution annealed materials; however, the change in the phase decomposition of ferrite was neither significant nor apparent with the low-fluence neutron irradiation for the thermally-aged materials. Ni–Si–Mn enriched solute clusters were observed in the matrix of ferrite in the aged specimens, and the irradiated specimens with/without thermal aging. Mo in the CASSs appeared to inhibit the formation of solute clusters under the neutron irradiations. In the thermally-aged specimen with low-C and without Mo, neutron irradiation enhanced the formation of solute clusters significantly. For the first time we discussed the relationship between hardening and microstructure evolution in ferrite of CASSs with consideration of both thermal aging and neutron irradiation.

Development of Advanced Instrumentation for Transient Testing

A fuel safety research program centered on in-pile transient testing experiments is being developed to support assessment and qualification of advanced nuclear fuel systems using the recently restarted Transient Reactor Test (TREAT) facility at the Idaho National Laboratory. While resumption of transient testing at TREAT is crucial to enable these programs, full recovery and cutting-edge transient testing capability also require a well-coordinated and innovative instrumentation development and qualification program to support near-term and future objectives. This paper summarizes the experimental approach of transient testing to focus on measuring the response of nuclear fuel to off-normal (or power-cooling mismatch) conditions for modern and advanced reactor environments requiring capabilities extending over wide measurement and environment conditions. It also highlights unique attributes of transient testing of importance to in-pile instruments including relatively low total neutron fluence, high gamma heating, and the need for a well-defined and possibly short time response. Historical approaches to instrumentation for transient testing are also reviewed to provide context to the modern instrument strategy. The paper details the instrumentation needs of modern transient testing. It also summarizes several ongoing research and development (R&D) activities that support the development of state-of-the-art and advanced measurement technologies that will provide a baseline capability for light water reactor and sodium-cooled fast reactor (SFR) experiment objectives. This R&D will extend to other advanced reactor needs and advanced sensing technology opportunities. Examples of specific sensors planned for near-term deployment with ongoing development include prompt response self-powered neutron detectors, miniature fission chambers, optical fiber–coupled infrared pyrometers, cladding surface thermocouples, electrical impedance–based boiling detectors, and linear variable differential transformer–based sensors for fuel elongation and pressure measurement.

A Numerical Solution of Low-Energy Neutron Boltzmann Equation

A multigroup method using a straight ahead approximation is created to calculate low energy neutron fluence due to the elastic scattering of evaporation neutrons produced in interactions of high energy particles with target nuclei. This multigroup method is added to NASA Langley Research Center's HZETRN particle transport code. This new code is used to calculate the energy spectra of the neutron fluence in several different materials. The multigroup method is found to be an efficient way of calculating low energy neutron fluence in multiple atom materials as well as single atom materials. Comparisons to results produced by Monte Carlo methods show that the straight ahead multigroup method is accurate for larger depths but less accurate for small depths due to leakage at the boundary. For this reason, an improved multigroup method is created which propagates neutrons in two directions, forward and backward approximately accounting for the isotropic distribution of the evaporation source. This new multigroup method compares well with the Monte Carlo method at all depths. For this reason, the multigroup method is considered an accurate method which is highly computationally efficient for calculating low energy neutron fluence.

Contribution of irradiation-induced defects to hardening of a low-copper reactor pressure vessel steel

We investigated the fluence dependence of irradiation-induced solute cluster, dislocation loop, and very small defect to reveal the hardening mechanism in surveillance test specimens from a reactor pressure vessel steel with low-Cu content (0.04 wt%) using atom probe tomography (APT), weak-beam scanning transmission electron microscopy (WB-STEM), and positron annihilation spectroscopy. A high number density (>1023 m−3) of solute clusters mainly composed of Ni, Mn, and Si atoms were found in highly neutron irradiated specimens (∼1024 neutrons m−2 (E > 1 MeV)) by APT. These solute clusters were one of the main sources of hardening as reported previously. On the other hand, it was also revealed that dislocation loops were formed with a number density of ∼1022 m−3 in the high-fluence specimens by WB-STEM. The estimated hardening due to dislocation loops was more than half of the actual hardening, showing that dislocation loops are also main source of irradiation hardening at high neutron fluence with the solid experimental evidences. Regarding specimens subjected to a low neutron fluence (∼1023 neutrons m−2), very small defects, not detected by either WB-STEM or APT, were formed by positron annihilation spectroscopy. This result suggested that, at a low neutron fluence, the defects were the initial hardening source and they may grow the dislocation loops observed by WB-STEM at high fluence range.

A study of the pressure vessel steel of the WWER-440 unit 1 of the Kozloduy nuclear power plant

A comparison between highly neutron irradiated samples from the region of weld № 4 and low irradiated samples from weld № 1 taken from the pressure vessel of the WWER-440 Unit № 1 of the Kozloduy NPP has been performed. Measurements of the residual activity of samples from the outer surface of the reactor pressure vessel bottom corpus reveal very low activity of 60Co. Insofar as there the base and weld metal appear to be exposed to a very low neutron fluence, the samples from these locations can be considered as practically not affected and may serve as a reference basis for comparison with highly irradiated pressure vessel regions. The Mossbauer parameters isomer shift (IS) and quadrupole splitting (QS) were found to be absolutely irradiation insensitive. A stepwise reduction of the internal hyperfine magnetic field Bhf, each by about 2.6 T, was observed. This can be attributed to the replacement of one or two surrounding iron atoms as first nearest neighbors by non-iron alloying atoms. The Mossbauer experimental line widths for irradiated and non-irradiated samples are practically the same, which is a quite unexpected result. The area fraction ratio for the three main Zeeman sextet subspectra S1:S2:S3 shows very high irradiation sensitivity. For the bottom low irradiated region of the reactor vessel the values are S1:S2:S3 = 50.1:40.0:9.4. After seven years of operation between the pressure vessel annealing in 1989 and the autumn of 1996 when the samples from weld № 4 were taken the ratio changes strongly to S1:S2:S3 = 56.4:34.7:8.5. A possible explanation of this result is that neutron irradiation gives rise to a precipitation process involving predominantly alloying atoms as Ni, Mn, Cr, Mo and V which become mobile and precipitate in the form of carbides and/or P-rich phases and alloying atom aggregates. This “refinement” process lowers the partial area of subspectra S2 and S3 where alloying atoms are involved and leads to a higher area fraction of the pure iron component S1, which is the major experimental result. For a more complete Mossbauer investigation on the processes of generation of structure defects caused by the neutron fluence, a new series of measurements will be performed by using a set of so-called surveillance specimens with different irradiation histories which are available only for the WWER-1000 reactors of the Kozloduy NPP.

The Development of a High Sensitivity Neutron Displacement Damage Sensor

The capability to characterize the neutron energy spectrum and fluence received by a test object is crucial to understanding the damage effects observed in electronic components. For nuclear research reactors and high energy density physics facilities this can pose exceptional challenges, especially with low level neutron fluences. An ASTM test method for characterizing neutron environments utilizes the 2N2222A transistor as a 1-MeV equivalent neutron fluence sensor and is applicable for environments with $1 \times 10^{12} - 1 \times 10^{14}\,\,1$ -MeV(Si)-Eqv.-n/cm2. In this work we seek to extend the range of this test method to lower fluence environments utilizing the 2N1486 transistor. The 2N1486 is shown to be an effective neutron displacement damage sensor as low as $1 \times 10^{10}\,\,1$ -MeV(Si)-Eqv.-n/cm2.

Influence of neutron irradiation on electron traps induced by NGB stress in AlInN/GaN HEMTs

This paper shows the impact of low fluence neutron irradiation on AlInN/GaN HEMTs before and after a negative gate bias stress done at a gate-to-source voltage ( $V_{\text {GS}})$ of −20 V and at a drain-to-source voltage ( $V_{\text {DS}})$ of 0 V during 216 h. We have shown that the neutron irradiation induces a decrease in the drain current of 6% and an increase in access resistance of 8% for the unstressed component in opposite to the stressed component. In fact, a rise in drain current of 9% and no evolution of the access resistance have been observed for a stressed AlInN/GaN HEMT. This phenomenon is related to the formation of complex cluster between the electron traps induced by the aging test and those involved by the neutron irradiation.

Influence of Neutron Irradiation on Electron Traps Existing in GaN-Based Transistors

This paper shows the impact of low fluence neutron irradiation on AlGaN/GaN HEMTs, AlInN/GaN HEMTs and AlInN/GaN MOS-HEMTs. We have shown that the technological structure of devices and the presence of electron traps in the devices seems to play a major role on the evolution of dc electrical characteristics of irradiated GaN based transistors. After having degrading the electrical performances of AlGaN/GaN HEMTs with an electrical stress, we have shown that it is possible to improve the electrical characteristics of the stressed devices by a neutron irradiation. Moreover, we have also reported that the degradation induced by neutron irradiation is more important for the AlInN/GaN HEMTs with a gate interfacial oxide than for the AlInN/GaN HEMTs without a gate interfacial oxide.