High Neutron Fluence Research Articles

Dopant, coating, and grating effects in silica optical fibers under extreme neutron irradiation

Radiation effects on fiber Bragg gratings (FBGs) have been studied but data gaps related to the effects of displacement damage resulting from high fast neutron fluence remain. In this work, Type-I and Type-II FBGs inscribed in optical fibers with various core dopants (Ge and F) and fiber coatings (acrylate, polyimide) were monitored in situ during 75 days of neutron irradiation to a peak fast (>0.1 MeV) neutron fluence of 3×1021 nfast/cm2. The reflected intensity of the Type-I FBGs inscribed in a Ge-doped core fiber decreased by >30 dB within 7 h of irradiation (1019 nfast/cm2), whereas Type-II FBGs inscribed in a pure silica core fiber eventually approached >40 dB attenuation after accumulating a fast neutron fluence on the order of 1020 nfast/cm2. Type-II FBGs inscribed in F-doped core fiber improved stability: the attenuation approached an equilibrium value in the range of 10 to 20 dB.

Influence of neutron decay on nuclear reactor materials

While many of the leading factors for material degradation in irradiated reactor components have been identified through past studies, spontaneous neutron decay within components is a factor absent in the literature. The byproducts of neutron decay, largely composed of a proton, could lead to excessive hydrogen content, irradiation assisted stress corrosion cracking, epsilon martensite formation, hydrides, ion induced damage, voids, bubbles, and dislocation loops. Neutrons are the most abundant relevant fission product, and neutrons are continuously decaying at a rate proportional to their production. This paper hypothesizes that at high neutron fluence, a quantifiable contribution to material degradation can be associated with the proton byproducts of neutron decay. Literature-based degradation mechanisms are presented along with initial calculations of the neutron decay contribution. The paper will demonstrate a potential new factor for consideration in the material degradation of nuclear reactor components.

Induction brazing of DEMO's large bore cooling pipes

The DEMO first wall blanket components will be exposed to high neutron fluence and high levels of thermal energy, the latter being removed by a coolant fluid transmitted out of the reactor via service piping. Due to the thermal and nuclear loading on the first wall, the components will need to be periodically removed and replaced. Installation of new blankets requires the joining of their service pipes by remote handling.Brazing is one of the alternatives that are considered for joining the service pipes of the DEMO breeding blankets. This technique has great potential and has several major advantages over welding.The Fusion Technology Department of HUN-REN Centre for Energy Research (CER) has been working on the development of a viable brazing concept of thick, large bore pipes inside the ports of DEMO. Induction has been chosen as the heat source for the brazing operation, which is a well-studied technology and has many uses in the industry already. However, brazing in this size is a completely new horizon for induction heating so CER's work includes a significant amount of development. Firstly, the coil is naturally a bespoke component, which has been optimized by electromagnetic analysis. Secondly, a test rig has been designed and manufactured in-house by CER, to test the performance of the induction heater in a real scenario.This paper outlines the development strategy of the induction brazing of DEMO's large bore pipes.

HICU PIE results of neutron-irradiated lithium metatitanate pebbles

Lithium metatitanate (Li2TiO3) pebbles were irradiated with neutrons within the HICU (High neutron fluence Irradiation of pebble staCks for fUsion) experiment to investigate their material properties and tritium release behaviour in a post-irradiation examination (PIE). The irradiation temperature is the most significant influence on the material. Besides a higher irradiation temperature, a higher initial Li–6 content tends to lead to an increased tritium generation resulting in the formation of the secondary lithium depleted phase Li4Ti5O12. A pre-compaction of the pebble bed does not have an apparent influence on the material properties. Tritium is released as semi-tritiated water and semi-tritiated hydrogen (HTO and HT) and its release is enhanced at elevated irradiation temperatures.

Radiation effects of high-fluence reactor neutron on Ni/ β -Ga2O3 Schottky barrier diodes

This study investigates the broad-energy-spectrum reactor-neutron irradiation effects on the electrical characteristics of Ni/β-Ga2O3 Schottky barrier diodes (SBDs), where the irradiated neutron fluence was up to 1 × 1016 cm−2. On the one hand, the high neutron fluence of 1016 cm−2 resulted in a reduction in forward current density by two orders of magnitude and an extremely high on-resistance property due to the radiation-generated considerable series resistance in the SBD. On the other hand, the irradiation brought little influence on the Ni/β-Ga2O3 Schottky contact, since the extracted ideality factor and barrier height from temperature-dependent current–voltage (I–V–T) characteristics showed no significant changes after the radiation. Moreover, the capacitance–voltage (C–V) characterization revealed that the net carrier density in the β-Ga2O3 material was only reduced by 25% at the neutron fluence of 1015 cm−2 but a significant reduction by 2–3 orders at 1016 cm−2. Within the neutron fluence range of 2 × 1014 cm−2 up to 1016 cm−2, the carrier removal rates trended to be saturated with the increased fluences, following an exponential regular. In addition, the C–V measurement on the 1016 cm−2 irradiated sample exhibited an obvious frequency dispersion, and the extracted carrier distribution was not uniform.

Oxygen vacancies and local amorphization introduced by high fluence neutron irradiation in β -Ga2O3 power diodes

Beta phase gallium oxide (β-Ga2O3) is emerging as a promising material for space applications due to its unique properties and potential high performance in extreme environments. In this work, we systematically study the impact of β-Ga2O3 Schottky barrier diodes (SBDs) under a high fluence neutron irradiation to explore the degradation mechanism of the devices. After irradiated by neutrons with an average energy of 1–2 MeV and a dose rate of 1.3 × 1012 cm−2 s−1, SBDs with a homoepitaxial layer suffered serious performance degradation. The main manifestation of this degradation was a substantial increase in on-resistance, which rose from 3.9 to 3.5 × 108 mΩ·cm2 under the aforementioned irradiation conditions. The appearance of amorphous/polycrystalline striped lattice damage in the epitaxial layer as well as the presence of deep-level defects caused by oxygen vacancies are factors related to this phenomenon. The simulation revealed that the capture reaction of neutrons and Ga elements is the primary cause of neutron irradiation. This reaction generates high-energy beta- particles (β-particles) resulting in the formation of defects. This paper reveals the degradation mechanism of β-Ga2O3 SBDs under neutron irradiation and provides a possible design roadmap for radiation-resistant β-Ga2O3 power devices. Moreover, a high-temperature oxygen annealing process was implemented, which proved to be in restoring the device performance.

Integration of thermo-electric coolers into the CMS MTD SiPM arrays for operation under high neutron fluence

Abstract The barrel section of the novel MIP Timing Detector (MTD) will be constructed as part of the upgrade of the CMS experiment to provide a time resolution for single charged tracks in the range of 30–60 ps using LYSO:Ce crystal arrays read out with Silicon Photomultipliers (SiPMs). A major challenge for the operation of such a detector is the extremely high radiation level, of about 2 × 1014 1 MeV(Si) Eqv. n/cm2, that will be integrated over a decade of operation of the High Luminosity Large Hadron Collider (HL-LHC). Silicon Photomultipliers exposed to this level of radiation have shown a strong increase in dark count rate and radiation damage effects that also impact their gain and photon detection efficiency. For this reason during operations the whole detector is cooled down to about -35°C. In this paper we illustrate an innovative and cost-effective solution to mitigate the impact of radiation damage on the timing performance of the detector, by integrating small thermo-electric coolers (TECs) on the back of the SiPM package. This additional feature, fully integrated as part of the SiPM array, enables a further decrease in operating temperature down to about -45°C. This leads to a reduction by a factor of about two in the dark count rate without requiring additional power budget, since the power required by the TEC is almost entirely offset by a decrease in the power required for the SiPM operation due to leakage current. In addition, the operation of the TECs with reversed polarity during technical stops of the accelerator can raise the temperature of the SiPMs up to 60°C (about 50°C higher than the rest of the detector), thus accelerating the annealing of radiation damage effects and partly recovering the SiPM performance.

Minority Carrier Traps Induced by Neutron Reactions with 4H-SiC

This work presents the characterization of minority carrier traps in epitaxial n-type 4H-SiC after high fluence neutron irradiation using minority carrier transient spectroscopy (MCTS) in a temperature range of 20 K to 660 K. Three minority carrier trap levels are reported, labelled as X, B and Y, whose activation energies were estimated by Arrhenius analysis and where the B level is assigned to substitutional boron (BSi). The dynamic behaviour of the trap levels was studied by consecutive temperature scans.

Increase of the surface recombination velocity at high bias voltage in silicon irradiated by neutrons to extremely high fluences

The upgrading of ionizing radiation detectors is an actual problem especially related to the high energy physics and space research experiments. The simplest way to restore the signal of the irradiation degraded detector is the increase of the detector bias voltage. This method is widely used worldwide, including high energy physics experiments in ATLAS and CMS. This work presents an effect, which was caused by increased bias voltage in detectors irradiated to extreme high neutron fluence at low temperature. The effect could be related to the increase of surface recombination velocity.The intrinsic photoconductivity spectra were exploited in order to investigate the properties of highly irradiated silicon as this effect depends on parameters that are important in radiation detectors. Two characteristic effects were observed in such highly irradiated samples: the increase of photoconductivity quantum yield and the enhancement of surface recombination at higher bias voltages. The increase of the quantum yield was analyzed in Ref. [1]. The increase of the surface recombination with bias electric field was analyzed in this work as an extension of the performed investigation in the same samples as in Ref. [1]. The investigated silicon samples were irradiated by neutrons to wide range fluence up to 1017 n/cm2.The origin of this effect was analyzed and related to the radiation clusters, which decrease the free carrier lifetime.

Radiation-induced alteration of sandstone concrete aggregate

This study investigates the alteration of felsic sandstone-type rock, which is used as a coarse aggregate in concrete, subject to the effects of gamma-ray and neutron irradiation. The effects of three gamma-ray doses (27, 55, and 108 MGy) and four neutron fluence levels (1.22, 2.19, 6.99, and 14.30 × 1019 n/cm2, E ≥ 0.01 MeV) were investigated. Quartz and albite were found to be the major rock-forming minerals, with microcline intermediates, chlorite, and muscovite as the minors. Gamma rays caused no significant changes to the physical properties of the sandstone aggregates, even at high doses (108 MGy). In contrast, neutron irradiation caused alterations that became more pronounced at higher neutron fluences. The solid was confirmed to expand through metamictization of the rock-forming minerals. Quartz and muscovite were the most affected phases, whereas albite and microcline intermediates were only slightly affected, and chlorite was almost unaffected. The decrease in density was measured by He and water pycnometry, and this value was almost reproduced by calculations using the rock-forming mineral composition of the pristine sample measured using X-ray powder diffraction/Rietveld analysis and the cell volume change of the major forming minerals. In addition, light optical microscopy and scanning electron microscopy images confirmed the presence of intergranular and intragranular cracks. Intergranular cracks appeared to have initiated from the quartz grains, which expanded significantly. The intragranular cracks were frequently observed in the albite and microcline intermediates. These cracks can be described as radial cracks starting from the expanding quartz, caused by enforced displacement for deformation consistency with quartz expansion. The crack area ratio quantified by SEM image analysis corresponds to the discrepancy of the volume expansion difference calculated by He or water pycnometry and dimensional change measurements. An evaluation of solid expansion and crack openings in aggregates is important to estimate concrete degradation.

Evaluation of Neutron‐Radiation Tolerance of Lithium Indium Diselenide Semiconductors

Lithium indium diselenide (LISe) is a semiconductor that holds promise for neutron imaging sensor technologies because of its high neutron absorption efficiency and its corresponding ability to discriminate between gamma rays and neutrons. However, being a semiconductor, LISe may not be sufficiently radiation hard for practical application in radiation hard environments. Therefore, a systematic evaluation of the changes in material and electronic properties of LISe after high neutron fluence exposures is investigated. Herein, the characterization methods are utilized which included UV–vis, X‐ray diffraction, radioluminescence, Raman, fourier transform infrared spectroscopy (FTIR), current–voltage, and neutron sensing. Characteristics of LISe material that appeared in the literature are identified herein along with several that are expected to appear based on theoretical analyses. The results obtained show clear changes in the material properties of LISe after neutron exposure up to a fluence of 1016 n cm−2. However, LISe is still able to sense neutrons above the background at 1016 n cm−2, suggesting that LISe may be suitable for use in neutron imaging sensors at neutron imaging facilities.

Tritium Breeding Performance Analysis of HCLL Blanket Fusion Reactor Employing Vanadium Alloy (V-5Cr-5Ti) as First Wall Material

Neutronic analysis in the HCLL blanket module has been established, and the calculation was performed by the ITER team, including the first wall (FW). In this study, seven materials have been investigated for FW material by considering characteristics such as high neutron fluence capability, low degradation, under irradiation, and high compatibility for blanket material. A three-dimensional configuration simulated in MCNP5 program codes was performed to investigate the neutronic performance and radiation damage effect. Employing seven candidates are vanadium carbide (VC), titanium carbide (TiC), vanadium alloy (V-5Cr-5Ti), graphite (C), tungsten alloy (W-CuCrZr), ceramic alloy (SiC), and HT-9 to study optimization of FW materials configurated in the HCLL blanket module. This novelty study concludes that vanadium alloy (V-5Cr-5Ti) is becoming a promising material candidate. This alloy has the highest number of neutronic performing for 1.27 TBR and 1.26 in multiplication energy factor in all investigations. Meanwhile, the amount of atomic displacement, hydrogen, and helium production are around 22.31 appm, 765.55 appm, and 281.57 appm, respectively. Even though vanadium alloy has a reasonably high radiation damage effect, it is still tolerable compared to several thresholds of DPA. So, it is considered excellent material for FW. Nevertheless, this alloy can replace after 13.45 years for radiation damage.

Neutron irradiation effect on amorphous porous silica

AbstractPorous silica is widely used as antireflection films in inertial confinement fusion facilities and be irradiated by high fluence neutron. To date, the neutron irradiation effect of amorphous porous silica is still unclear. In this paper, we give a comprehensive insight into neutron irradiation effect on microstructure, mechanical, and optical properties of porous silica under 14 MeV neutron irradiation by using molecular dynamics and density‐functional theory‐based methods. We find that, for microstructure, neutron irradiation elongates Si–O bond distance, obviously reduces the max value of Si–O–Si bond angle, coordination number distribution, and increases Si3+ and non‐bridging oxygen defects. The bulk modulus, shear modulus, and Young's modulus are all obviously reduced due to neutron irradiation, but Poisson's ratio changes relatively small. The light transmittance is reduced, and the optical absorption coefficient is increased after neutron irradiation, especially in the photon energy range of 3–4 eV, which is detrimental to the resistance of laser‐induced damage. Moreover, we find that neutron irradiation has more influence on high‐porosity porous silica than low‐porosity porous silica, which is mainly due to the higher defect percentage in high porosity. Present work is expected to be valuable in the study of degradation mechanism of nuclear material under neutron radiation.

Cluster dynamics modeling of Mn-Ni-Si precipitates coupled with radiation-induced segregation in low-Cu reactor pressure vessel steels

Formation of precipitates enhanced or induced by irradiation causes hardening and embrittlement of nuclear structural materials. Post-irradiation microstructure characterization of reactor pressure vessel (RPV) steels has shown that precipitation can be strongly associated with radiation-induced segregation (RIS) of solutes on dislocations. However, the effect of RIS on the kinetics of Mn-Ni-Si precipitation has not been quantified and coupled in previous model predictions. In this study, a new hybrid and spatially-dependent precipitation model is developed that couples cluster dynamics with RIS with transport coefficients of diffusion mediated by vacancies and interstitials. The new approach provides a unique way to account for concurrent evolution of heterogeneous cluster densities as well as solute and point defect concentration profiles. The model is applied to study the segregation of Mn, Ni, and Si on dislocations and heterogeneous nucleation of Mn-Ni-Si rich precipitates (MNSPs) in a low-Cu RPV steel. The RIS result shows that the segregation tendency is dominated by the vacancy mechanism for Mn, Ni, and Si. The coupled cluster dynamics modeling result shows that the onset of MNSP nucleation on dislocations depends strongly on RIS and occurs at the fluence of 2×1023n..m−2. The number density and mean radius can reach ∼1024m−3 and 1−2 nm at high fluence, respectively. These observations suggest that the role of RIS and heterogeneous nucleation can be significant for RPV steels under high neutron fluence. Results of simulations adopting various dose rates and dislocation densities show an increased fraction of MNSPs on dislocations at the condition of low dose rate, high dislocation density, and high fluence.

Activated Corrosion Products Evaluations for Occupational Dose Mitigation in Nuclear Fusion Facilities

Activated corrosion products generation in primary heat transfer systems of nuclear fusion facilities is a relevant radiological source term for occupation dose assessments. The formation of the Chalk River Undefined Deposit, already well known in nuclear fission power plants, represents a significant safety issue in fusion applications due to the intense high energy neutron fluences (about 14 MeV in Deuterium-Tritium operation). The activated corrosion products formation is a multi-physical problem. The combined synergy of activation, corrosion, dissolution, erosion, ejection, precipitation, and transport phenomena induces the contamination of coolant loop regions located outside the bio-shield, where scheduled worker operation might take place. The following manuscript shows how activated corrosion products are evaluated for the nuclear fusion power plant design under investigation by the Safety and Environment Work Package (WPSAE) of the Eurofusion Consortium (i.e., the European Demonstration power plant, EU-DEMO). The major issues in activated corrosion products estimations are here exposed and the main results for mass and activity inventories are briefly shown for some main Primary Heat Transfer Systems of EU-DEMO.

Effectiveness of Thermal Annealing in Recovery of Tensile Properties of Compositionally Tailored PWR Model Steels Irradiated in LYRA-10

Understanding the mechanical behaviour of reactor pressure vessel (RPV) steels at high fluences has become an important topic in regard to Long-Term Operations (LTO) of existing nuclear power plants (NPP). The effectiveness of thermal annealing treatments to recover the mechanical properties of compositionally tailored pressurised water reactor (PWR) model steels irradiated to high neutron fluences, up to 1.22 × 1020 n·cm−2, is analysed in this study. Tensile testing of four different PWR RPV steels was performed after irradiation and subsequent recovery annealing treatment at 450 °C for 40 h. Irradiation-induced hardening and the effectiveness of recovery thermal annealing have been assessed by comparing the strength and ductility properties of irradiated and irradiated and subsequently annealed samples with unirradiated reference samples for all four model steel. The annealing treatment resulted in a significant recovery of the yield strength (~75–89%) and the ultimate tensile strength (~78–96%) of all four PWR model steels. This study proves that substantial irradiation-induced hardening (up to ~389 MPa) observed in steels containing high Ni and Mn contents can still be recovered using the thermal annealing treatment. No influence of annealing on ductility properties has been observed for all four model steels. Microscopy analyses of these steels to understand the underlying irradiation damage and recovery mechanisms are planned for the near future.

An atom probe study on the Fe distribution in Zr-based alloys with different Fe content under high fluence neutron irradiation

In the present study, atom probe tomography was used to investigate the relationship between the distribution of Fe and other alloying elements and irradiation defects in irradiated zirconium alloys. The alloying element Fe, which is effective in increasing the corrosion resistance of fuel cladding materials, has low solubility in the Zr matrix phase and forms intermetallic precipitates during fabrication, but the distribution of Fe changes due to its dissolution into the matrix phase by neutron irradiation. The Fe atom map revealed that the dissolved Fe concentrated in the 〈a〉 dislocation loops, which is one of the irradiation defects in the Zr lattice, and formed spheroidal and ring-shaped Fe nanoclusters in the matrix phase. The ring-shaped Fe nanoclusters were widely observed in the matrix of the highly irradiated sample with high Fe content, and the distribution of dissolved Fe was affected by the Fe content and the neutron fluence. A stripe pattern of Fe distribution with these Fe nanoclusters lined up on the basal plane appeared after high fluence irradiation, and the stripe pattern was clear in the sample with higher Fe content. The dissolved Fe was retained in the matrix phase as nanoclusters, which could contribute to the corrosion resistance as well as the mechanism by Fe in the precipitates. Regarding the distribution of other alloying elements, Ni was observed in the same position as the spheroidal and ring-shaped Fe nanoclusters in the matrix phase. On the other hand, Cr was only observed as spheroidal Fe-Cr nanoclusters formed around the precipitates.

Thermal properties of paramagnetic radiation-induced defects in lithium orthosilicate containing breeder material

Lithium orthosilicate (Li4SiO4) containing ceramics are currently being developed as potential solid-state candidate materials for tritium breeding in future thermonuclear fusion reactors. Under the expected operational conditions, the tritium breeding material will be exposed to the simultaneous influence of several strong energetic factors, for example, radiation, temperature, magnetic field, etc. In the present work, thermal properties of the formed and accumulated paramagnetic radiation-induced defects (containing unpaired electrons) in the Li4SiO4 pebbles with a 2.5 wt.% surplus of silicon dioxide (SiO2) were investigated after irradiation with photons of different types and energies: X-rays with an energy up to 45 keV, gamma rays with an average energy of 1.25 MeV, and bremsstrahlung with an energy up to 6 MeV. The photon-irradiated pebbles were analysed using two complementary spectroscopic methods: electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). Individual signals contributing to the acquired EPR spectra, TSL glow curves and spectra were distinguished, deconvoluted and simulated in order to obtain a more detailed understanding about the local structure, electron configuration, and thermal stability of the accumulated radiation-induced defects. The simulation and deconvolution data were also compared with the results, which have been acquired for the long-term neutron-irradiated pebbles from the HICU experiment (High neutron fluence Irradiation of pebble staCks for fUsion).

Toward Confocal Chromatic Sensing in Nuclear Reactors: In Situ Optical Refractive Index Measurements of Bulk Glass

To measure fuel rod swelling in a nuclear research reactor, our research has focused on the development of an optical confocal chromatic sensor, allowing contactless measurements of radial changes in fuel rods. This sensor must be able to operate in the harsh and hot environment of a reactor, especially under high neutron fluences up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{19}~{n}_{\mathrm{ fast}}/\text {cm}^{2}$ </tex-math></inline-formula> and gamma doses of a few GGy. When exposed to such radiation levels, the properties of bulk optical glasses are known to be affected by two main phenomena: radiation-induced attenuation (RIA) and radiation-induced refractive index change (RIRIC). With the objective of testing glasses as candidates for a future confocal chromatic sensor, we created a dedicated setup for the online monitoring of the glasses’ RIRIC in the core reactor with a measurement principle relying on interferometry. Since not only the refractive index (RI) but also the length of the samples changes under neutron radiation through compaction, we measured the variation of the optical path (OP) ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$nL$ </tex-math></inline-formula> ). To assess the performance of our setup, preliminary in-lab tests were performed to evaluate the feasibility of measuring the small RI changes caused by external temperature changes. Due to this study, we were able to obtain data that were not yet available in the literature regarding the temperature-induced RI variation of several bulk glasses, including radiation-hardened glasses, up to 350 °C. This information is crucial for the targeted application, as the confocal chromatic sensor will have to operate at such elevated temperatures.

Cut-Off Degradation of Output Current Induced by High Fluence Neutron Radiation in High-Voltage Silicon-on-Insulator Lateral Double-Diffused MOSFET

In this letter, a novel cut-off degradation of output current induced by high fluence neutron radiation is investigated for high-voltage silicon-on-insulator lateral double-diffused MOSFET (HV SOI LDMOS). Unlike low/middle fluence, severe current collapse occurs at low drain voltage. Combining experiments and energy band analysis, we reveal that a secondary charge removal effect leads to cut-off degradation, which is closely related to the field enhancement and high-density defects at high neutron fluence. Although the gate channel can be turned on after radiation, below drain threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\text {DT}}$ </tex-math></inline-formula> ), electron drift is blocked by a potential barrier caused by the local space charge region with low trap occupation rate, rather than the global impact at low fluence. In addition, the influences of structural parameters on the cut-off degradation and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\text {DT}}$ </tex-math></inline-formula> are studied to guide the rad-hardening design of HV SOI LDMOS.