Fusion Reactor Research Articles

Directed energy deposition with a graded multi-material compatibility interface enables deposition of W on Cu

Multi-material Directed Energy Deposition of new high-performance components is limited by phase incompatibilities between metals and by the composition variations which complicate parameter optimization. In this work, a methodology of incorporation of a graded Multi-Material Compatibility Interface (MMCI) was employed to allow deposition of tungsten layers (melting at 3410 °C) on a much less refractory metal, copper (boiling at 2562 °C). The manufacturing of parts combining tungsten and copper is of interest for components designed to dissipate heat in extreme environments, such as the divertor for tokamak fusion reactors. A succession of miscible filler metals rationally selected from thermodynamics were printed by laser and powder injection, with optimization of energy and material inputs for each layer, supported by material characterization and physical numerical analysis of the process phenomenology. 90 %wt. tungsten layer deposition on a copper initial substrate is achieved using an MMCI with 6 intermediate layers and 1.6 mm total thickness. The results reveal the importance of controlling the dilution ratio to achieve stable depositions of multi-material with compositional gradients, with a progressive increase in melt pool temperatures. The numerical analysis highlighted the important role of the dissolution and mixing of partially melted refractory powders, and the selective vaporization of volatile elements.

Effect of boron on the microstructure and performance of HIP-prepared high boron steels

High boron steels containing 2 wt% and 3.3 wt% boron were fabricated using hot isostatic pressing (HIP). The enhanced shielding performance of the steel with higher boron content was quantitatively evaluated through Monte Carlo simulations using software Fluka. The steels were studied with XRD and the phase ratio were calculated using Rietveld refinement method. The microstructure of the materials was investigated using electron backscatter diffraction (EBSD). Comprehensive thermal property measurements, complemented by Hamilton thermal conductivity model analysis, were conducted on high boron steels. The results revealed that the decreased overall thermal conductivity in these materials is primarily attributed to two factors: the increased volume fraction of the boride phase and the enhanced interconnectivity among boride particles. These findings provide crucial insights into the thermal behavior of high boron steels. Room temperature tensile tests indicate that high boron content can cause the fracture mode of the material to transition from ductile to brittle. This study provides a detailed analysis of the effects of boron content on various properties of high boron steels, which can serve as shielding materials in fusion reactors. Advances were proposed for optimizing the thermal performance of high boron steels in this study.

Improvement in thermal stability of ODS-W alloy through formation of complex oxide dispersoids

Thermal stability is a crucial issue for engineering application of tungsten, especially as plasma facing material in future fusion reactor. Two kinds of ODS-W alloys of W-0.5Y2O3 (WY) and W-0.5Y2O3-0.25Ti (WYT) were fabricated by ball milling and spark plasma sintering aiming at formation of oxide dispersoids with different crystallographic structures. Thermal stability of both ODS-W alloys was evaluated in terms of microhardness and microstructure evolution during isothermal aging at 1100 °C. Dispersoids of sesquioxide Y2O3 were identified in sintered WY, while pyrochlore Y2Ti2O7 with finer size and higher number density were developed in sintered WYT, resulting in higher bending strength of 1020 MPa and microhardness of 880 HV. Compared with sesquioxide Y2O3, complex dispersoids of pyrochlore Y2Ti2O7 exhibited much better coarsening resistance during isothermal aging at 1100 °C up to 100 h, where the intracrystalline and GB dispersoids maintained around 10 nm and 30 nm, respectively. The excellent coarsening resistance of complex Y2Ti2O7 with pyrochlore structure contributed to grain refinement and improved microstructure stability of WYT due to the effect of Zener drag.

Synthesis and radiation damage tolerance of Mo0.75W0.25AlB solid solution for nuclear fusion reactor applications

MAB phases (M = transition metal, A = aluminium (Al), B = boron (B)), a new family of nano-laminated ternary transition metal borides with an excellent combination of functional and structural properties, are currently being investigated as potential radiation shielding materials in nuclear fusion reactors. Here, we report the fabrication of a solid solution MAB phase Mo0.75W0.25AlB (Mo - molybdenum, W - tungsten), in the form of dense and high purity pellets, synthesised for the first time using a reactive hot-pressing method. The shielding and irradiation damage properties of the material were also evaluated. Monte Carlo N-Particle (MCNP) calculations showed that W doping in the MoAlB phase is likely to enhance the shielding capacity against neutrons and gamma rays of different energies. The experimental damage tolerance of Mo0.75W0.25AlB to silicon ion (Si+) irradiation indicated that this solid solution compound has a high resistance to crack formation and other radiation damage effects compared to that of pure MoAlB. Thermal annealing of the amorphous Mo0.75W0.25AlB layer formed by irradiation showed that the damaged structure easily recrystallizes at high temperatures, completely returning to the virgin state. The results show that this solid solution should be considered as a promising candidate material for radiation shielding components.

Introduction on tritium transport analysis model for HCCP breeding blanket system

In the context of implementing the He-cooled breeding blanket system for a fusion reactor, several crucial aspects need consideration. Among these, the development of a tritium transport model plays a pivotal role in ensuring safety and effective design. Given that tritium release into the environment from the breeding blanket system poses a radioactive risk, accurate calculation and prediction area essential to prevent potential incidents in a fusion reactor. To address this challenge, a collaborative effort between the Korea Institute of Fusion Energy (KFE) and the University of California, Los Angeles (UCLA) resulted in the creation of THETA-FR (Tritium/Hydrogen Enhanced dynamic Transport Analysis Tool for Fusion Reactor). THETA-FR is an integrated analysis tool that combines Matlab Simulink and COMSOL Multiphysics. Its purpose is to model and predict the dynamic transport phenomena of H isotopes (H/D/T) within the breeding blanket system. Specifically, THETA-FR focuses on transient tritium retention, permeation, and release amounts from the breeding blanket system into the environment. In this paper, we introduce the integrated system and components of THETA-FR, shedding light on various tritium behaviors within the HCCP breeding blanket FW/BU component. By leveraging THETA-FR, researchers and engineers can make informed decisions regarding tritium control and safety in fusion reactors.

Numerical Calculations of Liquid-Metal Magnetohydrodynamic Flows in Rectangular Ducts with Sudden Contractions

For the design of the liquid-metal blanket in a fusion reactor, numerical calculations have been carried out on liquid-metal magnetohydrodynamic flows in rectangular ducts with sudden contractions. Conservation equations of fluid mass and fluid momentum and the Poisson equation for electrical potential have been solved numerically. The numerical calculations have been conducted for a Hartmann number of ~10 000; a Reynolds number of ~10 000; and contraction ratios (CRs) of 2, 3, and 4. The pressure loss through the contraction has been estimated by the loss coefficient ζ divided by the interaction parameter N, i.e. ζ/N. The loss coefficient ζ/N through the contraction parallel to the magnetic field is much larger than that through the corresponding contraction perpendicular to the magnetic field. The loss coefficient ζ/N increases consistently with the CR and does not change very much with N. While ζ/N also does not change very much with the wall conductance ratio for the contraction parallel to the magnetic field, ζ/N increases gradually with the wall conductance ratio for the contraction perpendicular to the magnetic field.

Adding a Vorticity Source to the Hasegawa-Wakatani Equations

Instabilities in fusion reactor plasma are an area of ongoing research as they represent the primary hurdle in producing an ongoing fusion reaction for fusion energy production. To study one possible source of these instabilities, computational simulations to solve the modified Hasegawa-Wakatani Equations and the evolution of edge plasmas through time are required. This work shows that the addition of a source of vortices leads to edge instabilities using the TOKAM2D turbulence code. This is an important step into developing our understanding of how instabilities form in plasmas and maintaining a fusion reaction.

Helium bubble evolution in Zr alloys: Effects of sinks and temperature

Implanted helium resulting from fusion reactions can pose a significant threat to zirconium alloys used as fusion reactor materials. Intrinsic structures and temperatures in nuclear materials can significantly influence the behavior and distribution of these helium bubbles under irradiation. Therefore, the behavior of helium bubbles in zirconium alloys was investigated by in-situ He+ irradiation at 573 K, 623 K, 673 K, 723 K and 773 K. Results indicated that the smallest and densest bubbles are found at GBs, while the largest helium bubbles are observed near precipitates. The size of the helium bubbles first increased and then decreased with an increase in temperature, while the density of bubbles changed in the opposite trend. The swelling induced by irradiation was maximum at 673 K when the irradiation dose was constant, while at this temperature, the geometry of the helium bubble changed from spherical to polygonal with a larger size than those at other temperatures.

DBTT and recrystallization behavior analyses for rolled and forged potassium-doped tungsten alloys

Tungsten-potassium (WK) alloys are regarded as one of the candidates for plasma-facing materials (PFMs) intended for the divertor and first wall of future fusion reactors. Determining the range of the ductile-to-brittle transition temperature (DBTT) and the recrystallization temperature (RCT) for the WK alloy is critical as it is closely related to the safe operational temperature window of the fusion reactor. Variable temperature tensile tests were conducted on rolled and high energy rate forging (HERF) WK alloy specimens to elucidate their DBTT. Additionally, gradient high-temperature isochronal annealing was performed on the HERF samples, coupled with Vickers hardness characterization, to ascertain their distinct RCT for various orientation planes within the same alloy. Subsequently, the DBTT of the as-rolled WK alloy is determined to be approximately between 100 °C and 150 °C, while that of the HERF WK alloy is found to be around lower than 200 °C. Furthermore, the RCT of the ND plane of the HERF WK alloy is approximately 1600 °C, whereas that of the RD counterpart exceeded 1700 °C. The internal mechanisms underlying these phenomena are comprehensively analyzed in this study. This work has identified the safe service temperature window for future fusion reactors with this kind of advanced WK alloy and provided a new perspective for determining the RCT of many other W-based materials.

Optimization of lithium vapor box divertor evaporator location on NSTX-U using SOLPS-ITER

Commercial fusion reactors will be faced with extremely high divertor target heat fluxes that will require mitigation. Simulations of detachment in an NSTX-U scenario projected to have 92 MW m−2 unmitigated peak target heat flux are presented, which reaches sub-10 MW m−2 target heat flux using a highly dissipating lithium vapor box divertor design. The lithium vapor box is a detached divertor design which employs lithium vapor evaporation and condensation to contain lithium below the X-point. Previous SOLPS modeling has indicated a lithium vapor box can reduce the heat flux down to 10 MW m−2 via simultaneous evaporation from the Private Flux Region (PFR) and the Common Flux Region (CFR) sides of the vapor box. It is found here that PFR evaporation has improved access to the separatrix leading to significantly more efficient power dissipation than CFR evaporation. Simulations of target evaporation with an evaporation distribution that is self-consistent with the temperature of a Capillary Porous System with Fast flowing liquid lithium could reach nLi / ne∼ 0.025–0.030 at the Last Closed Flux Surface (LCFS) depending on the liquid metal flow speeds and lithium sputtering yield, while PFR-side evaporation can reach acceptable heat fluxes with nLi / ne∼ 0.038 at the LCFS. However, PFR evaporator performance can be improved if the target is allowed to be hot enough such that it reflects lithium, reaching nLi / ne∼ 0.028 and reducing required lithium evaporation. Ultimately PFR evaporation and target evaporation are found to have similar ability to produce acceptable heat flux solutions with minimal upstream concentration.

Thermodynamic and economic analyses of the retrofit of existing electric power plants with fusion reactors

Electricity generation will need to reach net zero emissions globally in 2050. This will require an increase in share of renewable energy and the implementation of a controllable carbon-free base-load source. Nuclear fusion is a promising option to decarbonize base-load electricity production but its capital cost still doubles the one of technologically more mature alternatives such as large photovoltaic fields or off-shore wind installations. Within this framework, the retrofit of a dismissed power-plant could allow significant cost savings, thus facilitating the realization of a fusion electricity demonstrator. Among fusion reactors, stellarators are a valid alternative to tokamaks thanks to the higher blanket temperature and inherent continuous operation. In this scenario, we posit the challenge to use a nuclear fusion stellarator-based reactor to retrofit conventional power plants (PPs). Specifically, we select a nuclear fission plant in France and a supercritical coal fired site in Italy, by constructing 4 different retrofit scenarios as a function of the re-used components. We compare each option with a greenfield and optimized plant with the same reactor thermal power. through a thermodynamic, economic, and investment analysis.The results proves significant savings by retrofitting an existing plant, with a CapEx reduction up to 50% compared to the greenfield plants. Specifically, the most convenient retrofit strategy is to select a site that already implements cutting edge thermodynamic parameters while reusing the most existing systems (i.e. buildings, steam cycle, electricity generation, and heat rejection). This is the case of the 2 x 660 MWe supercritical coal-fired plant in Italy. Therein, the LCOEs are 39 $/MWh and 51 $/MWh, calculated with an interest rate of 2.7% and 6%, respectively, and compare with the conventional energy technologies. Moreover, such costs are competitive in the current European energy markets and yield significant net present values at the plant end of life.

Photoelectric charging and lofting of dust particles on a conducting surface with external electric fields

We present a laboratory study of photoelectric charging of dust particles and their lofting on a conducting surface in the presence of external electric fields. Insulating particles with diameter <45 μm are dispersed on a conducting surface exposed to ultraviolet (UV) light. In addition to the UV exposure, a positive or negative external electric field is applied. Independent of the orientation of the external electric field, the dust particles are found to be positively charged but with different mechanisms. It is shown that the orientation of the external electric field controls the dynamics of photoelectrons emitted from the dust particles and the conducting substrate surface. Distinctly different lofting results are shown between these two electric field cases. The results provide insight for understanding dust charging and release and helping develop mitigation solutions in particle accelerators, semiconductor manufacturing, fusion reactors, and space exploration to planetary bodies.

The initial measurement of a compact D-T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for fusion plasma diagnostic.

For developing and characterizing a novel compact D-T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for plasma diagnostics toward future D-T fusion reactors, the initial measurement was performed using the accelerator-based D-T neutron sources OKTAVIAN at Osaka University. This neutron spectrometer was designed for the detection of 3-17MeV neutrons and operated in the proton recoil telescope configuration by installing a polyethylene converter in front of the diamond stack. The measured neutron energy spectra were obtained by summing the energy of the recoil protons deposited in the diamond stack after the coincidence of the recoil protons identified by the time coincidence analysis. The neutron energy peaks measured by the compact D-T neutron spectrometer were almost in agreement with those obtained by the Monte Carlo N-Particle transport (MCNP) simulation. The energy resolution of the compact D-T neutron spectrometer was emulated to be about 4%-5% in D-T neutron measurement. In future work, the design of the compact D-T neutron spectrometer would be optimized to measure the fusion neutron for plasma diagnostics.

Effects of magnetic fields and nanoparticle concentration on hydro-thermal performance of a surface enhanced microchannel

This article numerically investigates fluid flow and heat transfer in plain wall (PWMC) and oblique fin (OFMC) microchannels under varying magnetic fields using water- Al2O3 nanofluid as a working fluid. Effects of magnetic flux (Hartmann number, Ha = 0–30), fluid velocity (Reynolds number, Re = 100–500), and nanoparticle volume fraction (ϕ = 0%–2%) on the overall thermo-fluid performance are analyzed. Magnetic fields can significantly improve the heat transfer in OFMC in a higher rate compared to the PWMC. Also, there is a less increment on the pressure drop in the OFMC in comparison to PWMC. In addition, individual, as well as combined effect of Joule and viscous heating on the thermo-fluid performance is investigated. Due to the combined effect of Joule and viscous heating, the Performance Evaluation Criterion (PEC) in OFMC can be improved by 21%. However, the PEC in OFMC can be enhanced by 55% without considering the combined effect. The detailed characteristics flow and heat transfer are also studied by analyzing the velocity contours, isotherms, streamlines, friction factors, and Nusselt numbers. The present study concludes the importance of consideration of Joule and viscous heating for an accurate modeling, where the viscous heating plays the dominant role. The present findings can be instrumental in enhancing the performance of micro heat sinks to optimize hydrothermal performance in high powered motors, nuclear fusion reactors etc., where magnetic field is present.

Design, characterization, and modeling of the Charge eXchange Recombination Spectroscopy (CXRS) suite at the SMall Aspect Ratio Tokamak (SMART).

Ion temperature, rotation, and density are key parameters to evaluate the performance of present and future fusion reactors. These parameters are critical for understanding ion heat, momentum, and particle transport, making it mandatory to properly diagnose them. A common technique to measure these properties is charge exchange recombination spectroscopy (CXRS). For characterizing positive and negative triangularity plasmas at the small aspect ratio tokamak, a poloidal array of gas puff based CXRS diagnostics will be measuring the ion properties in different poloidal positions. In this work, the modeling of the expected signal and spatial coverage using the FIDASIM code is presented. Furthermore, the design and characterization of the low field side midplane CXRS diagnostic are described. Each diagnostic is composed of a gas injection system, an optical system that collects the light emitted by the plasma, and a spectrometer. These systems will provide ion temperature, rotation, and density with a radial resolution of 3.75mm and a temporal resolution of 2.2ms.

Facile Synthesis of U2Ti Intermetallic by Direct Electrochemical Reduction of UO2-TiO2 Composite in LiCl-Li2O Melt

Alloys of U with Ti are of importance in nuclear industry as fuel for Gen IV fast reactors, hydrogen isotope storage medium for fusion reactors, super conductors, and also as corrosion resistant material for use in various applications. Here, preparation of U2Ti intermetallic compound was investigated by the direct electrochemical reduction of mixed oxide of UO2-TiO2 in LiCl-0.5% Li2O molten salt at 650 °C. The mixed oxide pellet of UO2-TiO2 sintered at 1500 °C was found to be a mixture of UTi2O6 and UO2 as evidenced by X-ray diffraction (XRD) analysis. Direct electro-lithiothermic reduction of UO2, TiO2, and a mixture of sintered UO2-TiO2 and UTi2O6 coupled with cyclic voltammetry of these oxides in the melt was performed to understand the electro-reduction mechanism. Potentials of reduction of these oxides in the melt, w.r.t Ni/NiO reference electrode, obtained by analysis of CV data of these oxides contained in metallic cavity electrodes and XRD analysis of partially electro-reduced oxides were used to arrive at the electro-reduction mechanism. Results indicate that U2Ti can be prepared conveniently by the electro-lithiothermic reduction of sintered pellet of UO2-TiO2 cathode by constant current electrolysis in a two-electrode cell.

Compatibility of divertor detachment and ELM suppression in DIII-D high- βp plasmas with ITER-similar shape

Integration of transient and steady-state divertor heat fluxes control with a high-performance core is necessary for future fusion reactors. In recent DIII-D high- βp experiments, divertor detachment and simultaneous edge localized mode (ELM) suppression are demonstrated while the plasma confinement quality is maintained high in ITER-similar shape. By optimizing the neon injection in high- βp scenario with ITER-similar shape, deep detachment and ELM suppression are achieved with a high-performance core ( βN ∼ 2.8, βp ∼ 2.3) at q95 ∼ 7.5. Partial divertor detachment and suppression of large ELMs are achieved at q95 ∼ 6. The stability analyses suggest that with low neon injection, the density pedestal becomes higher and steeper and the Ti profile also increases, therefore the increased edge pressure and higher current density destabilize the Peeling-Ballooning mode (PBM), which would lead to a large ELM collapse. With strong neon gas puffing, the significantly reduced pedestal pressure and current density, due to the degraded Te pedestal, lead to the stabilization of PBM and ELMs are suppressed. For both cases, the coupling between the large radius internal transport barrier (ITB) and edge pedestal is the key reason for maintaining high global performance. The formation of large radius ITB compensates for pedestal degradation. Such results could provide an attractive scenario to well control the transient and steady-state heat flux onto the divertor plates while maintaining good plasma performance, which is an important step toward the steady-state operation of future fusion reactors.

Hydrogen extraction from 0.1 % H2–He mixture: The interplay phenomena of electrode, temperature, and voltage in BZYN & BZCYYb

Hydrogen pumps, crafted from proton-conductive ceramic electrolyte and paired with either oxide or metallic electrodes, have been designed for the extraction of hydrogen isotopes from helium gas mixtures containing 0.1 % hydrogen isotopes, particularly within the context of TES for nuclear fusion reactors. This study presents the electrochemical hydrogen permeation research conducted on the perovskite proton conductor materials BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) and BaZr0.8Y0.16Ni0.04O3-δ (BZYN) under these operational conditions. It was observed that nickel electrodes provided superior performance over SrFe0.8Mo0.2O3-δ (SFM) electrodes in terms of hydrogen extraction efficiency. Hydrogen pumps that integrated BZCYYb as the electrolyte with nickel electrodes showed enhanced efficiency, while those utilizing BZYN as the electrolyte coupled with nickel electrodes demonstrated greater stability. Furthermore, the study explored the voltammetric nonlinearity at low hydrogen concentrations and the dependency of concentration polarization efficiency on both voltage and temperature, aiming to establish optimal conditions that balance stability and efficiency for both types of hydrogen pumps.

MECHANICAL PROPERTIES OF POROUS SILICON CARBIDE CERAMICS: A REVIEW

Porous silicon carbide (SiC)-based ceramics exhibit exceptional structural and functional properties, such as excellent mechanical, chemical, and thermal stability, and controlled electrical resistivity. Owing to their superior properties, porous SiC ceramics are suitable for various industrial applications, including heatable filters, heating elements, thermoelectric energy converters, fusion reactors, thermal insulators, water purifiers, molten metal and hot gas filters, diesel particulate filters, membrane supports, and catalyst supports. A deeper understanding of the mechanical properties of porous SiC ceramics, coupled with the development of new strategies for tuning these properties, will enable the realization of numerous new applications. In this review, important factors known to determine the mechanical strength of porous SiC ceramics, such as microstructures (necking area) and pore characteristics (porosity, pore size), have been analyzed. With increasing porosity and pore size of porous SiC ceramics, the flexural strength tends to decrease. The flexural strength increases with decreasing pore size at a constant porosity, whereas the flexural strength decreases with increasing porosity at a constant pore size. In addition, the flexural strength of porous SiC ceramics is primarily influenced by the developed necking area between SiC grains, which can be obtained through the doping of soluble atoms into the SiC lattice. Based on these critical factors affecting the mechanical properties, a novel strategy for tuning the flexural strength of porous SiC ceramics is proposed.

Influence of Oxide Inclusions on the Creep Properties of Electron Beam Welds in Ti-added Reduced Activation Ferritic/Martensitic Steel

In this study, the effects of reducing oxide inclusions on the microstructure, tensile properties at 550 ℃, and creep resistance of electron beam welded (EBW) joints in Ti-added reduced activation ferritic/martensitic (RAFM) steel were investigated. Two RAFM steels, designated as Steel A and Steel B, were prepared by adjusting the Al and Si contents to contain different fractions of oxide inclusions. The microstructures of the base metal and heat-affected zone (HAZ) were analyzed, and the mechanical properties of the welds were evaluated. Reducing the Al and Si content in Steel B resulted in a decrease in both the fraction and size of oxide inclusions compared to Steel A. Both steels exhibited tempered martensitic microstructures with M23C6 and MX precipitates. Hardness tests revealed that the over-tempered HAZ (OTHAZ) was the weakest region. However, both tensile test and creep tests at 550 ℃ showed fractures occurring in the base metal, indicating that the low heat input of EBW effectively minimized the weak HAZ regions. Steel B demonstrated improved tensile properties and creep resistance due to the reduced oxide inclusions. It was found that Steel B had a significantly longer creep life than Steel A at 550 ℃. SEM analysis revealed ductile fracture characterized by dimples, where oxide inclusions containing Al, Si, Ta, and Ti were predominant. The presence of these inclusions reduced the effectiveness of solid solution and precipitation strengthening by consuming Ta and Ti. Additionally, microvoids could easily nucleate at the interface between the hard oxide inclusions and the soft metal matrix during creep. Therefore, the reduction of oxide inclusions improved deformation resistance and high-temperature stability, resulting in better creep resistance in Steel B. In conclusion, the reduction of oxide inclusions in Ti-added RAFM steel enhances the high-temperature mechanical properties and creep resistance of EBW joints, underscoring the importance of oxide control in the development of RAFM steels for fusion reactor applications.