Migration Of Impurities Research Articles

Wall conditions in WEST during operations with a new ITER grade, actively cooled divertor

Future fusion reactors like ITER and DEMO will have all-tungsten (W) walls and long pulses. These features will make wall conditioning more difficult than in most of the existing devices. The W Environment Steady-state Tokamak (WEST) is one of the few long pulse (364 s) fusion devices with actively cooled W plasma-facing components in the world. WEST is a unique test bed to study impurity migration and plasma density control via reactor relevant wall conditioning techniques. The phase II of WEST operations began in 2022, after the installation of a new lower divertor, now entirely equipped with actively cooled, ITER grade, W monoblocks. After pump down, we baked WEST between 90 °C and 170 °C for ∼2 weeks. After 82.5 h at 90 °C and 33 h at 170 °C, vacuum conditions were stable with a vessel pressure of 6x10-5 Pa and mass spectra dominated by H2 molecules. While at 170 °C, we performed ∼40 h of D2 glow discharge cleaning (GDC) and ∼5 h of glow discharge boronization (GDB), using a 15 %-85 % B2D6-He mix and a total boron mass of ∼12 g. This was the very first GDB at such high temperature for WEST. The whole wall conditioning sequence led to a ∼10 times reduction of the H2O signal as well as to a ∼3 times reduction of the O2 signal, according to mass spectra. Once back to 70 °C, the vessel pressure was 5.5x10-6 Pa and plasma restart was seamless with ∼30 s cumulated over the very first 5 pulses and an Ohmic radiated power fraction Frad = 0.6, showing successful conditioning of the new ITER grade divertor. The effect of the first, ‘hot’ GDB faded with a characteristic cumulative injected energy of 2.45 GJ and saturation towards Frad ∼0.8. After 1.4 h and 7.5 GJ of cumulative plasma time and injected energy, we carried out a second GDB, this time at 70 °C. This ‘cold’ GDB initially led to a much lower Ohmic Frad = 0.3–0.4 but the effect lasted ∼7 times less, with a characteristic cumulative injected energy of 0.37 GJ. At the end of the campaign, we cumulated ∼3h and ∼30 GJ through repetitive, minute long pulses without any boronization. Throughout this 4-weeks-long experiment, Frad in the 4 MW heating phase evolved only marginally (from 0.5 to 0.55). This increase is mostly due to the build-up of re/co-deposited layers on both lower divertor targets.

Modeling of plasma facing component erosion, impurity migration, dust transport and melting processes at JET-ILW

An overview of the modeling approaches, validation methods and recent main results of analysis and modeling activities related to the plasma-surface interaction (PSI) in JET-ILW experiments, including the recent H/D/T campaigns, is presented in this paper. Code applications to JET experiments improve general erosion/migration/retention prediction capabilities as well as various physics extensions, for instance a treatment of dust particles transport and a detailed description of melting and splashing of PFC induced by transient events at JET. 2D plasma edge transport codes like the SOLPS-ITER code as well as PSI codes are key to realistic description of relevant physical processes in power and particle exhaust. Validation of the PSI and edge transport models across JET experiments considering various effects (isotope effects, first wall geometry, including detailed 3D shaping of plasma-facing components, self-sputtering, thermo-forces, physical and chemically assisted physical sputtering formation of W and Be hydrides) is very important for predictive simulations of W and Be erosion and migration in ITER as well as for increasing quantitative credibility of the models. JET also presents a perfect test-bed for the investigation and modeling of melt material dynamics and its splashing and droplet ejection mechanisms. We attribute the second group of processes rather to transient events as for the steady state and, thus, treat those as independent additions outside the interplay with the first group.

Evaluation and Analysis of Trace Impurity Migration Pathways on the Design of Highly Efficient Solution Crystallization-Based Purification Process

Evaluation and Analysis of Trace Impurity Migration Pathways on the Design of Highly Efficient Solution Crystallization-Based Purification Process

Performance and modeling of freezing crystallization in desalination: Heat transfer and impurities flow

This work applies the freezing crystallization in the desalination and establishes the theoretical models with experiments, to correlate the technical parameters with product properties. We first establish the heat flow models to simulate the 3-dimensional spatiotemporal temperature distribution and deduce the two heat flows through the ice layer, in which the latent heat from the phase transition plays a dominant role in heat transfer. Next, mass transfer and boundary layer models are proposed to evaluate the desalination efficiency and demonstrate the roles of kinetic and thermodynamic factors on the impurity migration. Later, three process optimization strategies, including cooling rate regulation, air bubble-assisted, and crystal seed-induced techniques, are imported to enhance the separation efficiency. At last, sweating is incorporated to reach the secondary purification effect, in which the desalination ratio under the optimal trajectory exceeds 83 %. The established sweating models accurately predict the product properties with time and can trace the impurities flow from the ice layer and adherent inclusion. The inclusion occupies a major discharged melt at the initial sweating stage, but more ice layer begins to melt and enter into the sweating liquid, confirming that a slight increase in product purity accompanies the huge yield loss.

High efficiency solar-grade silicon producing by directional solidification through controlling diffusion of impurity

The precise distribution of impurity of the whole silicon ingot is investigated through a theoretical model and the experiments. This model solves the problem of inaccurate prediction of impurity distribution in the final solidification region. The model was used to calculate the distribution of iron impurity in polycrystalline silicon ingots after directional solidification and compared with experiments. The calculation accuracy was significantly improved compared to the traditional Scheil equation. This paper investigates the influence of cooling rate, silicon ingot height and solidification rate on the yield of purified products after directional solidification. For the experimental equipment and raw materials in this paper, the optimal cooling rate of silicon ingots after directional solidification is 0.33 °C/s, the optimal height of silicon ingots is 0.34 m, and the yield of silicon purification increases with the decrease of directional solidification rate. Based on the research results, a standard for removing the top skin of silicon ingots after directional solidification and a method for improving the purification yield by suppressing impurity diffusion during the cooling stage were obtained. This model can provide theoretical guidance for the development of low-cost and high-efficiency methods for removing impurities from polycrystalline silicon, and is of great significance for the development of the photovoltaic industry. In addition, the theoretical model proposed in this paper can also be applied to the directional solidification purification process of other materials, such as the preparation of high-purity aluminum and high-purity titanium, and can predict the impurity migration behavior and distribution pattern throughout the purification process.

Migration and balance of 13C in the stellarator Wendelstein 7-X after 13CH[formula omitted] local injection into the island divertor

A 13C tracer experiment was carried out to investigate impurity transport and migration in the stellarator Wendelstein 7-X (W7-X). About 4.0 × 1022 molecules of 13C marked methane were injected through the divertor gas injection system during plasma operation. After the experiment, the divertor target elements were extracted to be analyzed by post-mortem methods. In particular, the results of the Nuclear Reaction Analysis are presented, and the 13C balance and predominating contributors to migration are discussed.A two-dimensional 13C deposition pattern on 12C graphite modules was obtained with over 6000 single measurements. The post-mortem analysis results extrapolations show a high local deposition of around 25% of the 13C within a 5 cm radius around the injection holes embedded in a graphite target module of one of the ten divertor units. Additionally, a striation is detected in the toroidal direction along the outer strike line and the net carbon deposition zone along the intersecting magnetic island containing about 23% of the 13C within the same divertor unit where injections were made. The residual 52% of 13C are globally migrated across the machine. In total, ∼ 90% of the injected 13C could be extrapolated by post-mortem analysis. Thus, the 13C balance is closed. The distribution indicates a non-negligible amount of global transport and the strong influence of the magnetic topology on migration. In contrast to the strike line pathway, the transport along the magnetic island only takes place counterclockwise in toroidal direction. These results are used to provide benchmark data for ERO2.0 impurity tracing simulations and to be a reference for impurity migration processes in a stellarator.

How cooling surface affect nucleation and impurity migration behavior during layer melt crystallization: A case study of 2′,4′-dichloro-5′-fluoroacetophenone

As a green and efficient purification technology, layer melt crystallization is widely used in chemical separation industry. However, during the melt crystallization process, how surface properties and cooling rate affect the metastable zone width (MSZW) and how MSZW affects the impurity migration in the cooling stage and sweating stage are rare explored. In this paper, 2′,4′-dichloro-5′-fluoroacetophenone is taken as a case study to explore this problem. Firstly, our results indicate that MSZW increases with increasing surface energy of the cooling surface. Importantly, we propose a new model to predict that ΔT/T0/T1 has a linear relationship with ln (R/T0/ΔT), which is validated by MSZW experiments. Secondly, the separation effect of the cooling surface with low surface energy is better than that of the surface with high surface energy. The purity of the crystal layer during sweating is limited by the cooling stage's separation effect. Finally, by employing mass balance, we predict that the impurity distribution coefficient of the crystal layer before and after sweating is linear, and this relationship is unaffected by the cooling surface and cooling rate, which is successfully verified by experiments.

Design and analysis of actively-cooled, edge-transport diagnostic for long-pulsed operation in WEST

Next step fusion devices that will operate in steady-state will require complex plasma-facing components (PFCs) that can survive the harsh environment over long timescales not common in current devices. This will require robust plasma facing surfaces that are integrated with active cooling systems. In a collaboration between CEA and ORNL, a plasma-interacting diagnostic is being designed for the W Environment in Steady-state Tokamak (WEST) in Cadarache, France which requires plasma-facing protection like those needed for steady-state PFCs. This integrated diagnostic studies edge transport and impurity migration within WEST, and is planned to include imbedded temperature Langmuir probe sensors, as well as removeable sample slots for ex-situ surface analysis of plasma-material interactions. The entire assembly is expected to move into the plasma edge for periods up to 1000 s having an energy removeable capability of ∼6 kW and seeing a peak heat flux of more than 7 MW/m2. The assembly compliments WEST high fluence campaigns that plan for multiple 1000 s pulses. Within these specifications, the assembly will require a refractory metal plasma facing surface and integral cooling in order to function within the limited space allotted for such diagnostics. Because of the limited space and linear actuator needs, additive manufacturing of the high heat flux working end of the assembly is being considered which could allow for precision cooling-channels and lighter weight designs. Conceptual design along with simulation and analysis results will be presented for this complex diagnostic with novel PFCs.

Highly efficient crystallization for sustainable azeotrope separation of formic acid-Water

The cost-effective separation of azeotropes poses a shared challenge. In this work, we utilized a green technique: layer melt crystallization (LMC), to separate the formic acid and water. Characterized by the lower energy consumption and high selectivity, LMC is welcome in high-purity production preparation. Firstly, we discussed the effects of temperature, cooling and heat rates, and time on the separation performance. We discovered that temperatures were important in the solute distribution and impurity migration. Then, the stirring technique is coupled to optimize the separation performance. Under the optimal trajectory, the product purity exceeds 99.7%. Next, to better understand the crystallization process, several theoretical models are established to clarify the intrinsic correlations of process variables and target parameters. Kinetic models are established to clarify the crystal layer growth, and sweating melt flow with the temperature difference-based driving force. Interfacial solute distribution factor and mass transfer coefficient are introduced to give the solute distribution rule and impurity migration mechanisms. At last, a dimensionless number is used to evaluate the sweating intensity, and we also correlated this coefficient with the separation performance of LMC qualitatively. In conclusion, LMC provides a viable approach for separating azeotropes, and the proposed theoretical models can offer valuable support for crystallization design.

Highly efficient freezing desalination technology: Parameters optimization and model configuration

Seawater desalination has become an available route to reduce the shortage of freshwater resources. Lower energy consumption and suitable operational conditions make freezing crystallization a green technique in the desalination area. Herein, we coupled the sweating intensification with the freezing crystallization to reach a highly efficient desalination performance. Under the trajectory of the freezing temperature and time: 259.15 K and 150 min, the sweating temperature, heating rate, and sweating time: 272.15 K, 0.5 K/min, and 120 min, the desalination ratio exceeds 70 %. Meanwhile, we established the theoretical models to expound the intrinsic correlations of process variables and target parameters, to visualize the impurity migration. Two kinetic models are proposed to analyze temperature effects on the crystal layer growth, sweating melt flow, and inclusion entrapment. Next, the fractal slice model is proposed to describe the complex microstructure of the crystal pillar. The thermal coefficients are deduced based on the fractal theory, where the larger porosity is against thermal conduction. Finally, we discussed the kinetic and porosity effects on the impurity migration and desalination performance, and established a boundary layer model to correlate the separation efficiency with process kinetic and impurity distribution successfully.

New insight into the plastic deformation mechanisms during the SiO2 phase transition process.

The removal of lattice impurities is the key to the purification of high-purity quartz (HPQ), especially for the intracell lattice impurities. Generally, the intracell lattice impurities can be migrated to the quartz surface via roasting, then removed by acid leaching. In order to reveal the phase transition of quartz during the roasting process, the evolution of structure, bond length, volume, lattice parameter and lattice stress in original, Ti4+, Al3+/Li+ and 4H+ substituted SiO2 phases were employed to investigate the mechanisms of plastic deformation based on density functional theory calculations. Results showed that the evolution of bond lengths and volumes were mainly dominated by phase transition, and the interstitial volume in high temperature SiO2 phases was higher than that in low temperature, indicating that the phase transition from α-quartz to β-cristobalite was beneficial to the migration of interstitial impurities. In addition, the phase transition from α-quartz to β-cristobalite needs to overcome the energy barriers while the phase transition from α-cristobalite to β-cristobalite needs to overcome the lattice stress. This study therefore provides an excellent theoretical basis for the plastic deformation mechanism, for the first time, beneficial to understanding the removal mechanisms of lattice impurities.

Key Roles of Nucleation and Impurity Migration Behavior in Layer Melt Crystallization: Case Study of 4,5-Dimethyl-1,3-dioxol-2-one

Key Roles of Nucleation and Impurity Migration Behavior in Layer Melt Crystallization: Case Study of 4,5-Dimethyl-1,3-dioxol-2-one

Removal of Low-Content Impurities from Pure Al by Supergravity Combined with Semi-Solid Method

Recently, the purification technology for high-purity aluminum (Al) has become the focus and difficulty of the majority of researchers. In this study, a novel approach for removing iron (Fe) impurities from pure Al via combining the supergravity field and semi-solid refining was proposed. Various separation temperatures (T), holding times (th), and separation times (ts) were applied within a gravitational field to explore their impact on the purification process and its underlying mechanisms. The optimal conditions were achieved at T = 653 °C, th = 40 min, ts = 3 min, and a gravity coefficient G = 1000, with the loss rate of purified Al reaching up to 4.1% and the removal rate of Fe reaching 81.9%. The Fe content in pure Al was reduced from 0.32 wt.% to 0.06 wt.%. Moreover, the purified mechanism of supergravity in a semi-solid method was reported for the first time. It was concluded that supergravity could decrease the value of the effective distribution coefficient (ke), thereby promoting the continuous migration of Fe impurities at the solidification interface into the liquid phase. The Fe-rich phase in the Al melt was completely filtered to the lower part of the crucible in the supergravity field, completing the further purification of the pure Al.

Characterization of impurity distribution and composition on the shutter plate for an optical diagnosis in EAST tokamak using laser-induced breakdown spectroscopy

The quantification of impurities in tokamak devices is of great significance as it allows for a better understanding of the plasma performance and the lifetime of plasma-facing components (PFCs). This study focuses on the characterization of the impurity distribution and composition on the surface of a shutter plate for an optical diagnosis in the Experimental Advanced Superconducting Tokamak (EAST) by Laser-Induced Breakdown Spectroscopy (LIBS). The primary impurity elements, including copper (Cu), tungsten (W), molybdenum (Mo), lithium (Li), and Sodium(Na), were identified by analyzing the LIBS spectra obtained from the surface of the shutter plate. The spatial distribution of these impurities on both sides of the plate was investigated. The analysis results indicate an increase in impurity content on the outer surface of the shutter plate as proximity to Last Closed Flux Surface (LCFS), followed by a near-linear decrease with the distance from the LCFS. The results of the inner ring on the inner side facing the glass window show fewer metal impurities than the outer rings on the edge for the inner side of the plate. Moreover, the thickness of the deposition layer was determined by estimating the Average Ablation Rate (AAR) through the measurement by confocal microscopy. In addition, Calibration-Free LIBS method were performed to determine the relative content of impurities. The results of this study provide valuable references for studying the migration and transportation of impurities.

Migration Mechanisms of Al3+/Li+ Lattice Impurities during Phase Transition from α-Quartz to β-Quartz: An Implication for Purification of High-Purity Quartz

The quality of high-purity quartz (HPQ) that used in strategic industries is normally limited due to lattice impurities. In order to reveal the migration pathway of lattice impurities in quartz particles during phase transition from α-quartz to β-quartz, α-quartz and Al3+/Li+-substituted α-quartz (S-α-quartz) was investigated under 846.15 K and 101.325 kPa based on density functional theory. Results showed that β-quartz exhibited more interstitial volume, dominating the migration of lattice impurities. This further indicates that the phase transition process was beneficial for the migration of lattice impurity from a structural point of view. Moreover, Al3+ and Li+ mainly migrated from the intracell to lattice surface along the c axis. In addition, Li+ migrated more easily than Al3+ due to higher mean square displacement values. From a thermodynamic point of view, the lower energy barrier in the case of the Al3+ and Li+ lattice suggested that the presence of lattice impurities promoted phase transition. This study therefore provides an excellent theoretical basis for the removal of lattice impurities of HPQ from an atomic perspective, for the first time.

Recycling of kerf loss silicon: An optimized method to realize effective elimination of various impurities

Photovoltaic solid waste, particularly diamond-wire sawing silicon powder (DWSSP), has emerged as a significant concern in the industry. Consequently, the recycling and reuse of such waste have become a prominent research focus. In this study, focusing on achieving direct recycling and reuse of DWSSP, the key driving factors for effective purification were identified. It was found that simply increasing the melting temperature and time was insufficient to completely remove volatile impurities, and the migration process in the melt had to be taken into consideration. Additionally, this study focused on analyzing the instability model of inter-granular grain growth and its impact on the stable migration of impurities, with particular attention to the microstructure of inter-granular micro-regions. The thermal control directional solidification technique focused on controlling the temperature gradient during the melt solidification process. This approach helped stabilize the microstructure, enhance impurity migration, and ultimately led to a more effective purification of DWSSP.

Computational studies of impurity migration during induction stirring of molten uranium

Understanding and controlling impurity behavior in actinide metal casting processes are foundational for efficient part production yet remain major challenges for researchers and industry. To help provide insight regarding impurity distribution during actinide metal casting, we have developed computational fluid dynamics (CFD) models for a laboratory-scale system using commercial and open-source codes. This article describes multiple experiment-informed models designed to simulate a specific laboratory system in which uranium melt, containing a known starting concentration of carbon impurity, is electromagnetically stirred in an induction furnace. The goal of the simulations is to predict the motion of impurity particles in the form of uranium monocarbide in the velocity field of the melt. Prior to simulating the uranium–carbon system, numerical models were validated using a previously published nonradioactive experimental system. Effects of the size and shape of impurity particles in the models were investigated and agree with experimental findings. Simulation of smaller particles (less than 50 μm) shows more homogenous distribution throughout the stirred melt. With increased particle size (100 μm), the body forces, which include the buoyancy force, dominate over the drag force, causing larger particles to move toward the crucible walls and upward in the system.

Study on diffusion and migration characteristics of impurities in secondary side of simplified u-tube steam generator

During the operation of nuclear power systems, the occurrence of leakage accidents of primary and secondary condensers are a major concern due to the potential threat to safe operation. The entry of seawater into the secondary loop from the condenser, along with impurities such as Cl-, can accumulate near the heat transfer tube, causing wear and corrosion, which can further aggravate the situation. Therefore, it is essential to accurately understand the migration and diffusion process of impurities in the secondary side of the steam generator. This study aims to investigate the three-dimensional multi-tube physical model of fluid–structure coupling heat transfer in the primary and secondary loops of the steam generator, including the heat exchange tube, to gain insights into the migration and diffusion of Cl- and other impurities. The distribution of important thermal parameters, such as volume vapor fraction, temperature, and velocity, in a U-shaped tube bundle were analyzed by numerical simulation of heat transfer and fluid flow. The diffusion and migration of Cl- in a two-phase flow were also studied. Results of this study reveal that impurities concentration is influenced by the effective diffusion coefficient, which is further influenced by the boiling intensity and flow velocity of the coolant. The combined effect of the boiling intensity and flow velocity of the coolant has a synergistic effect on the migration and diffusion of Cl-. In conclusion, this study provides valuable insights into the flow heat transfer impurity migration under a multi-tube coupled heat transfer condition of the steam generator, which can be used as a scientific basis for the design optimization and operation of steam generators.

Emission and time-resolved migration rates of aromatic diamines from two flexible polyurethane foams.

Performing risk assessments (RA) on household use of flexible polyurethane (PU) foams requires access to reliable data about emission and migration of potential diamine impurities. A toluene diisocyanate (TDI) and a methylene diphenyl diisocyanate (MDI) based foam were thermally treated to enable measurements on samples with defined concentrations of the corresponding diamines, toluene diamine (TDA), and methylene dianiline (MDA). The thermally treated foams used for emission testing contained up to 15mg.kg-1 of TDA and 27mg.kg-1 of MDA. Those used for migration testing contained 5.1mg.kg-1 of TDA and 14.1mg.kg-1 of MDA. Stability of the thermally generated diamines was sufficient for testing over a 37-day period. Analytical techniques that did not decompose the polymer matrix were applied. Emission rates for TDA and MDA isomers were less than the limit of quantitation (LOQ) of 0.008-0.07μg.m-2.h-1. Migration was studied using samples of the same thermally treated foams over a 35-day period. Quantifiable migration of MDA from the MDI-based foam was only observed on Days 1 and 2. From Day 3 onward, migration rates were less than the LOQ. Quantifiable migration of TDA from the TDI-based foam rapidly decreased with time and was only observed on Days 1 thru 3. From Day 4 onward, migration rates were less than the LOQ. Theoretically, the migration rate should be inversely proportional to the square root of time (t) as t-0.5. This relationship was confirmed by the experimental data and enables extrapolating migration values to more extended time periods to conduct RAs.

The Synergistic Promotion Effect of In‐situ Formed Metal Cationic Vacancies and Interstitial Metals on Photocatalytic Performance of WO3 in CO2 Reduction

AbstractMicrostructure modulation of photocatalysts is an effective way to improve photocatalytic performances. The synergistic effect of metal cationic vacancies and interstitial metals on photocatalysts is rarely studied. In‐situ formations of metal cationic vacancies and interstitial metals are achieved in WO3 via acid treatments, resulting from the migration of metal impurities in WO3. Characterizations of structures, components, photo‐responsive and photoelectric properties identify the differences of WO3 photocatalysts via different treatments. The presence of both metal cationic vacancies and interstitial metal impurities exhibits the positive synergistic effect in photocatalytic reaction. Photocatalytic activities in CO2 reduction are enhanced with the high production rates of CO and CH4. Experimental characterizations of WO3 photocatalysts indicate the increased photoinduced electrons and photocurrent intensities, and the decreased electrochemical impedances. Theoretical simulations confirm the changes of electronic structures of WO3. The calculated band gaps of WO3 after acid treatment decrease and the defect energy levels form, which favor the separation and the transfer of photoinduced carriers for the promotion of photocatalytic activities. To utilize the dissolution difference of metals is feasible to fabricate metal cationic vacancies and interstitial metals simultaneously, an option for other metal‐containing photocatalysts with the in‐situ introduced impurities.