This review summarizes surface morphology changes of tungsten caused by heat and particle loadings from edge plasmas, and their effects on enhanced erosion and material lifetime in ITER and beyond. ...Pulsed heat loadings by transients (disruption and ELM) are the largest concerns due to surface melting, cracking, and dust formation. Hydrogen induced blistering is unlikely to be an issue of ITER. Helium bombardment would cause surface morphology changes such as W fuzz, He holes, and nanometric bubble layers, which could lead to enhanced erosion (e.g. unipolar arcing of W fuzz). Particle loadings could enhance pulsed heat effects (cracking and erosion) due to surface layer embrittlement by nanometric bubbles and solute atoms. But pulsed heat loadings alleviate surfaces morphology changes in some cases (He holes by ELM-like heat pulses). Effects of extremely high fluence (∼1030m−2), mixed materials, and neutron irradiation are important issues to be pursued for ITER and beyond. In addition, surface refurbishment to prolong material lifetime is also an important issue.
Functionally graded steel/tungsten layers may be used as interlayers in the first wall of future fusion reactors to balance thermally-induced stress peaks in the steel‑tungsten joint. In this work, a ...modified water-stabilized atmospheric plasma spraying set-up is used to deposit uniform and functionally graded steel/tungsten coatings at elevated substrate temperatures. Uniform coatings were used to characterise individual sublayers of graded coatings in detail. The thermal expansion, thermal conductivity, Young's modulus and yield strength of the layers are promising for the application in steel‑tungsten joints and can be explained by microstructural observations. Only at a substrate preheating temperature of 900 °C the formation of intermetallic precipitates during deposition was observed.
Decades of workplace health promotion research Robroek, Suzan; Coenen, Pieter; Hengel, Karen Oude
Scandinavian journal of work, environment & health,
11/2021, Letnik:
47, Številka:
8
Journal Article
Recenzirano
Odprti dostop
The workplace is a promising setting for health promotion as workers spend a lot of time at work, and existing social networks for social support could be used to change behavior and enhance health. ...Workplace health promotion programs could be a way to improve workers' health and can for example include elements of support, policies, or environmental changes to encourage healthy behavior. Traditionally, programs have focused on providing workers with advice on how to change their behavior. Such programs have been criticized because they do not take a broader perspective such as the environment. However, still many of these traditional programs are offered to employees and evaluated. The effects of such programs remain disappointing thus far. Here, Robroek et al reflect on the body of research regarding workplace health promotion. They also describe gaps in the literature, most notably regarding the need for more targeted workplace health promotion, a systems approach for workplace health promotion, and the delivery of workplace health promotion. Furthermore, they propose some research agenda concerning workplace health promotion.
•Stable crack propagation of tungsten reinforced with tungsten fibers.•Fracture surface observations.•Using fiber reinforcement improves the fracture toughness of tungsten.•The fracture toughness of ...tungsten fibre-reinforced tungsten composites was investigated with different methods.•Discussion of the applicability of the used methods for tungsten fibre-reinforced tungsten composites.
Tungsten fibre-reinforced tungsten composites (Wf/W) have been developed to overcome the inherent brittleness of tungsten, which is a promising candidate for the plasma-facing material in a future fusion power plant. As the development of Wf/W evolves, the fracture toughness of the composite is in the focus of interest for further component design. In this contribution fracture mechanical tests on two different types of chemical vapour deposited (CVD) Wf/W are presented. Three-point bending tests according to ASTM E399 as a standard method for brittle materials were used to get a first estimation of the toughness. A provisional fracture toughness value of up to 241 MPa m1/2 was calculated for the as-fabricated and of up to 20.5 MPa m1/2 for a heat-treated and thus embrittled state. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied for this state. A maximum value of the J-integral of 7.5 kJ/m2 (57.6 MPa m1/2) was determined. A detailed post mortem investigations was used to obtain the active mechanisms.
Tungsten is the most promising first wall material for nuclear fusion reactors. One disadvantage, however, is its intrinsic brittleness. Therefore, tungsten fiber reinforced tungsten (Wf/W) is ...developed for extrinsic toughening. Wf/W can be produced by chemical vapor deposition (CVD), e.g. by reducing WF6 with H2 using heated W-fibers as substrate. However, it still needs to be optimized regarding relative density and fiber volume fraction. The decisive factor is the tungsten deposition rate, which depends on the temperature and the partial pressures. For this dependence, however, there are controversial results in the literature. In this article, a new rate equation is presented, in which different literature equations are partially adapted and combined. It adjusts the WF6 reaction order between one and zero, depending on the temperature and the H2 and WF6 partial pressure. For validation, a simplified experimental setup with a single fiber was designed, which provides very well defined boundary conditions while varying the CVD process parameters heating temperature, pressure, gas flow rate and gas inlet composition. The experimental runs were simulated with COMSOL Multiphysics. The model was successfully validated by measurements of the WF6 consumption rates (< 2 to 100 %), deposited tungsten masses and spatially high-resolved tungsten deposition rates.
•A new rate equation is presented for chemical vapor deposition of W (from WF6 & H2).•Temperature, inlet WF6 & H2 partial pressures, and total gas flow rate were varied.•Temperature and deposition rate were measured in a high spatial resolution.•Validation was achieved by modeling the experimental setup in COMSOL Multiphysics.•Literature controversies regarding the WF6 reaction order have been solved.
Plastic deformation of tungsten wire is an effective source of toughening tungsten fibre-reinforced tungsten composites (Wf/W) and other tungsten fibre-reinforced composites. To provide a reference ...for optimization of those composites, unconstrained pure tungsten wire is studied after various heat treatments in terms of microstructure, mechanical behaviour and fracture mode. Recrystallization is already observed at a relatively low temperature of 1273K due to the large driving force caused by a high dislocation density. Annealing for 30min at 1900K also leads to recrystallization, but causes a rather different microstructure. As-fabricated wire and wire recrystallized at 1273K for 3h show fine grains with a high aspect ratio and a substantial plastic deformability: a clearly defined tensile strength, high plastic work, similar necking shape, and the characteristic knife-edge-necking of individual grains on the fracture surface. While the wire recrystallized at 1900K displays large, almost equiaxed grains with low aspect ratios as well as distinct brittle properties. Therefore, it is suggested that a high aspect ratio of the grains is important for the ductile behaviour of tungsten wire and that embrittlement is caused by the loss of the preferable elongated grain structure rather than by recrystallization. In addition, a detailed evaluation of the plastic deformation behaviour during tensile test gives guidance to the design and optimization of tungsten fibre-reinforced composites.
•Microstructural and mechanical characterization of pure W wire at room temperature•Investigations in as-fabricated and annealed at 1273K and 1900K state•Recrystallization does not necessarily result in embrittlement.•Fibrous grain shape, i.e. a high aspect ratio, key parameter for ductility
For the future fusion demonstration power plant, DEMO, several blanket designs are currently under consideration. Despite geometric and operational differences, all designs suggest a first wall (FW), ...in which tungsten (W) armour is joined to a structure made of Reduced Activation Ferritic Martensitic (RAFM) steel. In thermo-mechanical analyses of breeding blankets, this joint has received limited attention. In order to provide a basis for better understanding of thermally induced stresses and strains in the FW, the thermo-mechanical behaviour of a water-cooled test component is explored in the current contribution. The model aims at providing a simple geometry that allows straightforward comparison of numerical and experimental results, while trying to keep boundary conditions as realistic as possible. A test component with direct RAFM steel-W joint, and a test component with a stress-redistributing, functionally graded RAFM steel/W interlayer in the joint is considered in the current contribution. The analyses take production- and operation-related loads into account. Following a detailed analysis of the evolution of stress components and strain in the model, a parameter study with respect to geometric specifications and loads is presented.
The analyses show that, even in a small test component, a direct RAFM steel-W joint causes enormous plastic deformation. The implementation of a functionally graded interlayer reduces stresses and strains significantly, but vertical normal stresses at the joint's circumference remain considerable. With the component geometry considered here, the graded interlayer should be at least 1 mm thick and contain 4 sublayers to appropriately redistribute stresses. Beyond a component width of 14 mm, stresses increase strongly, which may pose a risk to the applicability of large-scale FW components, too.
As a candidate material for plasma facing material in future fusion reactor, tungsten (W) fiber reinforced tungsten (Wf/W) composite has been recently developed. The crack resistance of Wf/W is ...proven to be significantly higher compared to normal tungsten. However, the W-fibers used always become embrittlement during the powder metallurgy (PM) processes. In order to understand this significant issue, in this work, a series of Wf/W composites have been prepared. Microstructural and mechanical studies revealed that microstructural and mechanical studies revealed that the nanosized carbides in the grains and the carbide-layer on the grain boundaries are formed during PM processes. Especially, the carbide-layer on the grain boundaries can cause the brittle fracture of those W-fibers affected. Meanwhile, W-foil protection of the green body during the sintering process can reduce the carbon contamination effect and allows to preserve the ductility of the tungsten fibers used.
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