Fusion Neutron Research Articles

Measurement of leakage neutron spectra for aluminium with D-T fusion neutrons and validation of evaluated nuclear data

In the integral benchmark experiment presented here, an aluminium (Al) slab sample (300 mm × 300 mm × 100 mm) was irradiated with Deuterium-Tritium (D-T) fusion neutrons. The neutrons leaking from the sample were detected by a BC501A organic liquid scintillator through the time-of-flight (TOF) method; the neutron energy spectra were derived from the as-obtained TOF curves by considering the detector efficiency. For comparison, some leakage neutron spectra were also simulated by combining a Monte Carlo neutron transport code with evaluated nuclear data from the ENDF/B-VIII.0, JENDL-4.0 and CENDL-3.1 libraries. All the calculated values agreed with the experimental data within 10%, except for a few energy ranges. The values predicted using the ENDF/B-VIII.0 library were lower and higher than the measured ones at below and higher above 4 MeV, respectively; the data based on the CENDL-3.1 and JEDNL-4.0 libraries exhibited some larger differences in the 9.5–12 MeV range at 120∘. The possible causes of these discrepancies are also discussed.

Helium-hydrogen synergistic effects on swelling in in-situ multiple-ion beams irradiated steels* *Project supported by the National Natural Science Foundation of China (Grant No. 11935004).

The development of reliable fusion energy is one of the most important challenges in this century. The accelerated degradation of structural materials in fusion reactors caused by neutron irradiation would cause severe problems. Due to the lack of suitable fusion neutron testing facilities, we have to rely on ion irradiation experiments to test candidate materials in fusion reactors. Moreover, fusion neutron irradiation effects are accompanied by the simultaneous transmutation production of helium and hydrogen. One important method to study the He–H synergistic effects in materials is multiple simultaneous ion beams (MSIB) irradiation that has been studied for decades. To date, there is no convincing conclusion on these He–H synergistic effects among these experiments. Recently, a multiple ion beam in-situ transmission electron microscopy (TEM) analysis facility was developed in Xiamen University (XIAMEN facility), which is the first triple beam system and the only in-running in-situ irradiation facility with TEM in China. In this work, we conducted the first high-temperature triple simultaneous ion beams irradiation experiment with TEM observation using the XIAMEN facility. The responses to in-situ triple-ion beams irradiation in austenitic steel 304L SS and ferritic/martensitic steel CLF-1 were studied and compared with the results in dual- and single-ion beam(s) irradiated steels. Synergistic effects were observed in MSIB irradiated steels. Helium was found to be critical for cavity formation, while hydrogen has strong synergistic effect on increasing swelling.

Metastable acceptors in boron doped silicon: evidence of the defects contributing to carrier induced degradation

This work focuses on the recombination-enhanced reactions of boron related defects in compensated silicon and their impact on the electrical performance of silicon devices. Using deep level transient spectroscopy, we measured the defect spectra in the collector of 14 MeV fusion neutron irradiated p-n-p silicon transistors during forward current injection, as well as the corresponding degradation kinetics of both current gain and leakage current on the same device. Two hole traps at + 0.43 eV and + 0.53 eV are proved to be acceptors in the lower half of the gap with significant recombination enhanced effects. These metastabe acceptors degrade the current gain by changing the space charge distribution in the p-n-p structure and are linked with carrier-induced boron-oxygen complexes responsible for the minority-carrier lifetime degradation of silicon photovoltaics.

Uranium integral fission product yields for a spectrally-shaped 14.1 MeV neutron source at the National Ignition Facility

This paper describes the experimental results for an energy tuning assembly created to modify the National Ignition Facility deuterium–tritium fusion neutron source into a notional thermonuclear and prompt fission neutron spectrum, which has applications in integral measurements, nuclear data benchmarks, and radiation effects on microelectronics. The Monte Carlo neutron transport utilized MCNP5 to estimate the ETA-modified fluence using the ENDF-B/VIII.0 and IRDFF-II continuous energy nuclear data libraries, and SCALE Sampler was used to estimate the systematic nuclear data covariance using ENDF-B/VII.1 and IRDFF-II in a 252-group structure. The experiment fielded eight activation foils and a highly enriched uranium sample. This provided fifteen reaction channels that were used in a forward-fit comparison to the modeled results and to unfold the neutron spectrum using STAYSL. Gamma-ray spectrometry was performed on the activation and highly enriched uranium foils, and the reduced χ2 between the modeled and experimental values was 1.21. The results from the STAYSL unfold, reduced χ2=1.62, indicated that the modeled neutron spectrum was achieved and the systematic nuclear data uncertainty associated with the neutron transport and activation product cross sections was representative of the experiment. Integral cumulative fission product yield data were collected for 37 mass chains with a combination of gamma-ray spectrometry and radiochemical analysis. Fission product analysis was generally in agreement with two models using a semi-empirical fit and the General Observables of Fission code, with the exception of mass chains 88, 109, 111, 112, 113, 129, 139, 142, 144, 151, and 156.

Mitigation of mode-one asymmetry in laser-direct-drive inertial confinement fusion implosions

Nonuniformities present in the laser illumination and target in laser-driven inertial confinement fusion experiments lead to an asymmetric compression of the target, resulting in an inefficient conversion of shell kinetic energy to thermal energy of the hot-spot plasma. In this paper, the effects of asymmetric compression of cryogenic deuterium tritium laser-direct-drive implosions are examined using a suite of nuclear and x-ray diagnostics on the OMEGA laser. The neutron-averaged hot-spot velocity (u→hs) and apparent ion temperature (Ti) asymmetry are determined from neutron time-of-flight measurements of the primary deuterium tritium fusion neutron energy spectrum, while the areal density (ρR) of the compressed fuel surrounding the hot spot is inferred from measurements of the scattered neutron energy spectrum. The low-mode perturbations of the hot-spot shape are characterized from x-ray self-emission images recorded along three quasi-orthogonal lines of sight. Implosions with significant mode-1 laser-drive asymmetries show large hot-spot velocities (>100 km/s) in a direction consistent with the hot-spot elongation observed in x-ray images, measured Ti asymmetry, and ρR asymmetry. Laser-drive corrections have been applied through shifting the initial target location in order to mitigate the observed asymmetry. With the asymmetry corrected, a more-symmetric hot spot is observed with reduced u→hs, Ti asymmetry, ρR asymmetry, and a 30% increase in the fusion yield.

High yield polar direct drive fusion neutron sources at the National Ignition Facility

Polar direct drive neutron source experiments were performed at the National Ignition Facility showing substantial improvement in total neutron yield and efficiency of conversion of laser energy to fusion output. Plastic capsules 3–4 mm in diameter were filled with 1.5 mg/cc of deuterium–tritium (DT) fuel and imploded with laser beam pointing and defocus designed to compensate for polar asymmetry introduced by the facility beam entrance angles. Radiation-hydrodynamics simulations were employed to optimize the multi-dimensional laser and target parameter space, within facility and target fabrication constraints. Ensembles of 1D simulations tuned to match the outputs of early shots in the series were used to design subsequent shots in the series. This allowed the later shots to be designed based on empirically motivated sensitivities to laser and target input parameters, while eliminating the need to explicitly model phenomena such as hydrodynamic instabilities and nonlinear laser–plasma interactions. One experiment with a 3.0 mm diameter CH capsule produced 13.6 kJ (4.81 × 1015 DT neutrons) from a laser input below the NIF optics damage threshold at 585 kJ, 328 TW. Two experiments with 4.0 mm capsules produced 31.3 and 33.6 kJ of fusion output (1.11 × 1016 and 1.19 × 1016 DT neutrons) with 1.10 MJ, 390 TW and 1.26 MJ, 425 TW of laser input, respectively.

Experiments and modelling of multiple sequential MeV ion irradiations and deuterium exposures in tungsten

Bulk tungsten samples were irradiated sequentially with 20 MeV tungsten ions and exposed to deuterium plasma. The experiments were performed in order to simulate the displacement damage that fusion neutrons will cause in a tungsten plasma-facing component of a future fusion device. To study the influence of the presence of hydrogen isotopes during the creation of displacement damage on the final defect density, tungsten irradiation and deuterium decoration cycles were performed up to three times. Deuterium depth profiling with 3He Nuclear Reaction Analysis and Thermal Desorption Spectroscopy showed that the deuterium concentration increased after each additional tungsten irradiation and deuterium exposure. After the third cycle, the deuterium concentration reached a maximum of 3.6 at.% at the given plasma exposure temperature of 370 K. We attribute this increase in retention to the stabilization of the displacement damage during the tungsten irradiation by the presence of deuterium. The experimental results were simulated using the MHIMS-R macroscopic rate-equation code, which was recently upgraded with a damage stabilization term to describe experiments where tungsten was irradiated with MeV tungsten ions and simultaneously exposed to low-energy deuterium ions. Using this novel model, it was possible to quantitatively describe also the present results for the sequential irradiation/exposure scheme, with model parameters that were congruent with parameters derived from the simultaneous experiment. Modelling shows that kinetic de-trapping of trapped deuterium takes place during irradiation However, it is not the dominant process that explains defect stabilization. In addition, the model facilitates the extrapolation of present experimental results to an even larger number of sequential tungsten irradiation and deuterium exposure cycles. The model predicts that after about five sequential irradiation and plasma exposure cycles, a stationary state is reached with an associated maximum trapped D concentration of 4.2 at.% for the given exposure temperature of 370 K.

Evaluation of tritium production rate in a blanket mock-up using a compact fusion neutron source

We report a neutronics study of a blanket mock-up using a discharge-type compact fusion neutron source. Deuterium–deuterium fusion neutrons were irradiated to the mock-ups composed of tritium breeder and neutron reflector/moderator. The tritium production rate (TPR) per source neutron was measured by a single-crystal diamond detector with a 6Li-enriched lithium fluoride film convertor after the calibration process. Despite the low neutron yield, energetic alpha and triton particles via 6Li(n, t)α neutron capture as well as 12C via elastic scattering were successfully detected by the SDD with high signal to noise ratios. The TPRs were experimentally evaluated with errors of 8.4%–8.5% at the 1σ level at the positions with high thermal neutron fluxes where the errors were dominantly introduced by uncertainties in the monitoring of the neutron production rate. The calculated to experimental (C/E) values of TPR were evaluated to be 0.91–1.27 (FENDL-2.1) and 0.94–1.28 (FENDL-3.1). As the neutron source can generate 14 MeV neutrons using a mixed gas of deuterium and tritium, this approach provides more opportunities for blanket neutronics experiments.

Time resolved ablator areal density during peak fusion burn on inertial confinement fusion implosions

Near peak compression, inertial confinement fusion implosions release both deuterium–tritium (DT) fusion gamma rays and neutron induced gamma rays from carbon from the areal density of the remaining ablator shell. The gamma reaction history diagnostic makes a time resolved measurement of both. Across many recent implosions, the carbon gamma ray peak arrives systematically 11 ± 10 ps later compared to DT fusion burn. The timing shift is consistent with the carbon areal density increasing throughout the peak of the fusion burn, implying that the carbon portion of the capsule continues to converge. A model finds that the observed timing shift is consistent with a 4π averaged carbon ablator inward velocity of 80 μm/ns for the contemporary National Ignition Facility implosions. The timing shift is possibly related to the energy balance of the implosion, with the expectation that a high performing, igniting capsule would see the carbon gamma rays arrive before the DT fusion peak.

New method for time alignment and time calibration of the TOFOR time-of-flight neutron spectrometer at JET.

The TOFOR time-of-flight (TOF) neutron spectrometer at the Joint European Torus (JET) is composed of 5 start (S1) and 32 stop (S2) scintillation detectors. Recently, the data acquisition system (DAQ) of TOFOR was upgraded to equip each of the 37 detectors with its own waveform digitizer to allow for correlated time and pulse height analysis of the acquired data. Due to varying cable lengths and different pulse processing pathways in the new DAQ system, the 160 (5 · 32) different TOF pairs of start-stop detectors must be time-aligned to enable the proper construction of a summed TOF spectrum. Given the time (energy) resolution required by the entire spectrometer system to measure different plasma neutron emission components, it is of importance to align the detector pairs to each other with sub-nanosecond precision. Previously, the alignment partially depended on using fusion neutron data from Ohmic heating phases of JET experimental pulses. The dependence on fusion neutron data in the time alignment process is, however, unsatisfactory as it involves data one would wish to include in an independent analysis for physics results. In this work, we describe a method of time-aligning the detector pairs by using gamma rays. Given the known geometry and response of TOFOR to gamma rays, the time alignment of the detector pairs is found by examining gamma events interacting in coincidence in both S1-S1 and S1-S2 detector combinations. Furthermore, a technique for separating neutron and gamma events in the different detector sets is presented. Finally, the time-aligned system is used to analyze neutron data from Ohmic phases for different plasma conditions and to estimate the Ohmic fuel ion temperature.

Experimental Study on Fusion Neutron Activation of Metal Materials in EAST

Fusion neutron activation experiments have been carried out to investigate the activation of metal materials in Experimental Advanced Superconducting Tokamak (EAST). Some cylindrical samples made of stainless steel 316L, carbon steel (A3), high purity cadmium (Cd), oxygen-free copper (Cu), and high purity aluminum (Al) were transported to Port G in the EAST hall for neutron irradiation. The characteristic gamma rays emitted by activated samples were measured with a spectrometer consisting of a digital multichannel analyzer (MCA) and a high purity germanium (HPGe) detector. Qualitative identification of the dominant radionuclides and quantitative measurement of the specific activity have been performed. The experimental results indicated that the high radioactive nuclides with short half-life were produced, such as 56Mn in 316L and A3, 28Al in Al, 66Cu in Cu, and 111m Cd in Cd. Moreover, the long half-life radionuclides were generated in 316L, A3, Cu and Cd samples, which will become the major factors on environment and personnel. In addition, the normalized dose rate at 10 cm in the air of 316L, A3, Al, Cd, and Cu was $1.53\times 10^{-23}$ , $4.83\times 10^{-24}$ , $5.57\times 10^{-23}$ , $4.90\times 10^{-\mathrm {24}}$ and $1.10\times 10^{-\mathrm {23}}$ Sv/h/n, respectively. Finally, the comparison between the experimental neutron activation results and the predicted quantities calculated with FISPACT-2007 was discussed.

Study on turbulent heat transfer mechanism inside high-speed lithium jet irradiated by ion beam for fusion neutron sources

Liquid lithium (Li) jet is planned as a beam target in fusion neutron sources developed in Japan and EU. We have been studying the hydrodynamic characteristics of the Li jet experimentally and numerically. For the safety and the efficiency of fusion neutron sources, it is necessary to understand the heat transfer inside the Li target irradiated by ion beams. In this paper, in order to evaluate the heat transfer under the turbulent flow with complex vortex structure inside the Li jet, 3-dimensional simulation of it with beam irradiation is conducted using LES. As the first step of this study, the Li jet is modeled as single-phase for reducing the computational load, in which the Li surface is treated as the free-slip boundary condition. Simulation model is large-scale with over thirty million meshes for resolving the detailed vortex structure. In this simulation, it was found that the temperature peak was disturbed by wall turbulence near the bottom wall of the flow channel.

Comparison of spallation and fusion neutron sources in fuel transmutation and regeneration

In this work, a comparison between the capabilities of a spallation source and a fusion source in the burnup of reprocessed fuels and in the regeneration of thorium was carried out. We have performed Monte Carlo simulations of a sub-critical core loaded with thorium and reprocessed fuel under irradiation of two different neutron sources. Two different fuels reprocessed by the GANEX technique were evaluated, one spiked with depleted uranium and the other one spiked with thorium. The aim is to investigate the neutronic behavior of thorium and reprocessed fuels under different neutron sources and analyze the fuel evolution during 10 years of burnup.

Fabrication of solid CH-CD multilayer microspheres for inertial confinement fusion

Deuterated polymer microspheres can be used as a neutron source in conjunction with lasers because thermonuclear fusion neutrons can be produced efficiently by collisions of the resulting energetic deuterium ions. A new type of solid deuterated polymer microsphere with a carbon hydrogen–carbon deuterium (CH-CD) multilayer has been designed for preparing the target for inertial confinement fusion (ICF) experiments. To fabricate these solid CH-CD multilayer microspheres, CH beads are first fabricated by a microfluidic technique, and the CD coating layer is prepared by a plasma polymerization method. Both polystyrene (PS) and poly(α-methylstyrene) (PAMS) are used as the material sources for the CH beads. The effects of the PS and PAMS materials on the quality of the solid CH beads and the resulting CH-CD multilayer polymer microspheres are investigated. The solid PS beads have better sphericity and a smoother surface, but large vacuoles are observed in solid PS-CD multilayer microspheres owing to the presence of residual fluorobenzene in the beads and a glass transition temperature of the solid PS beads that is lower than the temperature of plasma polymerization. Therefore, solid PAMS beads are more suitable as a mandrel for fabricating solid CH-CD multilayer polymer microspheres. Solid CH-CD multilayer microspheres with specified size have been successfully prepared by controlling the droplet size and the CD deposition rate and deposition time. Compared with the design value, the diameter deviation of the inner CH beads and the thickness deviation of the CD layer can be controlled within 20 µm and 2 µm, respectively. Thus, an approach has been developed to fabricate solid CH-CD multilayer microspheres that meet the physical design requirements for ICF.

The effect of areal density asymmetries on scattered neutron spectra in ICF implosions

Scattered neutron spectroscopy is a diagnostic technique commonly used to measure areal density in inertial confinement fusion experiments. Deleterious areal density asymmetries modify the shape of the scattered neutron spectrum. In this work, a novel analysis is developed, which can be used to fit the shape change. This will allow experimental scattered neutron spectroscopy to directly infer the amplitude and mode of the areal density asymmetries, with little sensitivity to confounding factors that affect other diagnostics for areal density. The model is tested on spectra produced by a neutron transport calculation with both isotropic and anisotropic primary fusion neutron sources. Multiple lines of sight are required to infer the areal density distribution over the whole sphere—we investigate the error propagation and optimal detector arrangement associated with the inference of mode 1 asymmetries.

Preliminary study on radiation safety management of Gas Dynamic Trap based D-T fusion volumetric neutron source

The fusion neutron source based on Gas Dynamic Trap (GDT) device has the advantages of high neutron flux, large testing space and low construction cost, so it is an ideal solution for high-flux fusion volumetric neutron source. As a new type of radioactive facility, how to carry out radiation safety management under the current nuclear safety regulatory laws and regulations in China is an important issue that must be addressed before the construction of this facility. In this paper, the radiation safety characteristics of GDT fusion neutron source was analyzed and the main radiation terms of GDT fusion neutron source were expounded. Combined with the existing nuclear safety regulatory laws and regulations in China, the key points of radiation protection and safety management in the process of construction and application of GDT fusion neutron source were put forward. It is recommended that the GDT neutron source should be managed as a class I radiological device. Prior to operation, radiation safety permit and nuclear material permit should be obtained, and special attention should be paid to the safety protection of radioactive tritium. This study will pay the way for the radiation safety management of the international high flux fusion neutron source ALIANCE project, and also provide the reference for the related nuclear and radiation safety management departments in China. 摘要： 随着聚变能研宄进入工程化阶段, 迫切需要高通量聚变体中子源开展材料和部件的辐照测试。基于气动磁镜 (Gas Dynamic Trap, GDT) 装置的聚变中子源具有中子通量高、测试空间大、建造成本低等优点, 是高通量聚变体中子 源的理想方案。作为一种新型的放射性装置, 在我国现行核安全监管法律法规体系下, 如何对其开展辐射安全管理是 在该装置建造之前必须要理清的问题。本文分析了 GDT 聚变体中子源的辐射安全特性, 阐明了 GDT 聚变体中子源 的主要辐射源项, 并结合我国现有核安全监管法律法规要求, 提出 GDT 聚变体中子源在建造及应用过程中的辐射防 护与安全管理要点, 建议 GDT 中子源作为I类射线装置管理, 运行之前应取得辐射安全许可证和核材料许可证, 特别 需要注意对放射性氚的安全防护。本文为国际高通量聚变体中子源 ALIANCE 的辐射安全管理工作提供指导, 同时可 供我国的核与辐射安全管理部门参考。

A multi-channel x-ray temporal diagnostic for measurement of time-resolved electron temperature in cryogenic deuterium-tritium implosions at OMEGA.

Electron-temperature (Te) measurements in implosions provide valuable diagnostic information, as Te is unaffected by residual flows and other non-thermal effects unlike ion temperature inferred from a fusion product spectrum. In OMEGA cryogenic implosions, measurement of Te(t) can be used to investigate effects related to time-resolved hot-spot energy balance. The proposed diagnostic utilizes five fast-rise (∼15 ps) scintillator channels with distinct x-ray filtering. Titanium and stepped aluminum filtering were chosen to maximize detector sensitivity in the 10 keV-20 keV range, as it has been shown that these x rays have similar density and temperature weighting to the emitted deuterium-tritium fusion neutrons. Initial data collected using a prototype nosecone on the existing neutron temporal diagnostic demonstrate the validity of this diagnostic technique. The proposed system will be capable of measuring spatially integrated Te(t) with 20 ps time resolution and <10% uncertainty at peak emission in cryogenic DT implosions.

Detector Response to D-D Neutrons and Stability Measurements with 4H Silicon Carbide Detectors.

The use of wide-band-gap solid-state neutron detectors is expanding in environments where a compact size and high radiation hardness are needed, such as spallation neutron sources and next-generation fusion machines. Silicon carbide is a very promising material for use as a neutron detector in these fields because of its high resistance to radiation, fast response time, stability and good energy resolution. In this paper, measurements were performed with neutrons from the ISIS spallation source with two different silicon carbide detectors together with stability measurements performed in a laboratory under alpha-particle irradiation for one week. Some consideration to the impact of the casing of the detector on the detector’s counting rate is given. In addition, the detector response to Deuterium-Deuterium (D-D) fusion neutrons is described by comparing neutron measurements at the Frascati Neutron Generator with a GEANT4 simulation. The good stability measurements and the assessment of the detector response function indicate that such a detector can be used as both a neutron counter and spectrometer for 2–4 MeV neutrons. Furthermore, the absence of polarization effects during neutron and alpha irradiation makes silicon carbide an interesting alternative to diamond detectors for fast neutron detection.

Shielding design and neutronics calculation of the GDT based fusion neutron source ALIANCE

This paper presents the high flux neutron shielding design and extensive neutronics calculations of GDT based fusion neutron source ALIANCE. Neutron distribution of ALIANCE is strongly inhomogeneous along the axis: significant portion of the neutron flux is generated near the two mirrors, while the rest of it is spread over the remaining central volume of plasma. The shielding design includes 40 cm stainless steel as the main shielding layer and an additional 5 cm tungsten carbide shielding layer at mirror plugs to protect superconducting coils from neutron damage and reduce nuclear heating. The simulations have been carried out by using Monte Carlo transport code SuperMC with nuclear data library FENDL 3.1. Results show that the nuclear heating on the mirror coils can be reduced by more than two thirds with additional tungsten carbide shield, and fast neutron fluence by 30 %. The highest nuclear heating and the highest fast neutron fluence zones are located at the mirror coils, and the values are about 300 W/m3 and 9 × 1018 n/cm2 respectively, which meets the threshold of ITER superconducting coils. The specific activities of shielding layers are of order of 1012 Bq/kg. The structural materials’ specific activities will decrease to 4 × 1011 Bq/kg in one year after shutdown, and their decay heat will quickly drop below 2 kW/m3 after one day. Besides, all the structural materials of ALIANCE can be recycled by different recycling technologies. The modeling and calculations reported in this paper will be beneficial for the pre-conceptual engineering design of ALIANCE.

Deuterated polyethylene nanowire arrays for high-energy density physics

Abstract The interaction of intense, ultrashort laser pulses with ordered nanostructure arrays offers a path to the efficient creation of ultra-high-energy density (UHED) matter and the generation of high-energy particles with compact lasers. Irradiation of deuterated nanowires arrays results in a near-solid density environment with extremely high temperatures and large electromagnetic fields in which deuterons are accelerated to multi-megaelectronvolt energies, resulting in deuterium–deuterium (D–D) fusion. Here we focus on the method of fabrication and the characteristics of ordered arrays of deuterated polyethylene nanowires. The irradiation of these array targets with femtosecond pulses of relativistic intensity and joule-level energy creates a micro-scale fusion environment that produced $2\times {10}^6$ neutrons per joule, an increase of about 500 times with respect to flat solid CD2 targets irradiated with the same laser pulses. Irradiation with 8 J laser pulses was measured to generate up to 1.2 × 107 D–D fusion neutrons per shot.