Recent progress in research on tungsten materials for nuclear fusion applications in Europe

TL;DR: In this article, the progress of work within the EFDA long-term fusion materials program in the area of tungsten alloys is reviewed, with a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments.

About: This article is published in Journal of Nuclear Materials.The article was published on 2013-01-01 and is currently open access. It has received 599 citations till now.

Summary

1. Introduction

• The use of tungsten and tungsten alloys for the helium-cooled divertor and possibly for the protection of the helium-cooled first wall in reactor designs going beyond ITER has been discussed and investigated for several years (see, for example, [1-8]).
• To make a continuous operation possible, the plasma has to be cleaned from hydrogen isotopes, helium (the ―exhaust‖ of nuclear fusion) and from impurities (such as particles of the blanket first wall) which are unavoidable.
• Furthermore, the divertors are the highest thermally loaded components of a fusion power plant.
• Since the requirements for both applications (armour or structure) are quite different, in this paper the authors distinguish between armour and structural materials which in both cases could be tungsten.
• Thermal load, heat conductivity, or recrystallization, however, are typical properties which restrict the design significantly on the upper temperature limit while brittleness and irradiation damage narrow the structural materials of question on the low-temperature range.

2. Fabrication

• The helium-cooled finger design [3] has so far been used as a reference for component fabrication issues.
• The most important questions in this field are:.

2.1. Fabrication process development

• Due to its unfavourable grain orientation, rod material cannot be used to machine a thimble by turning or milling.
• The most promising machining route consists of deep drawing of a W plate, which results in grains that follow the contour.
• For more details on the deep drawing assessment see [10].
• Powder injection moulding (PIM) was investigated as a mass fabrication option for the tungsten armour tiles which had to be joined to the thimbles.
• Figure 3 shows the result of first preliminary tests via insert-two-component W-PIM.

2.2. Joining and transitions from steel to tungsten

• One result of the investigations for using electrochemistry for brazing layers fabrication is that the brazing material can be built up from a sequence of electrochemically deposited layers.
• Investigations were performed on the applicability of this technique using low activation vanadium interlayers.
• Hot pressing of tungsten and steel powder mixtures was used as an alternative approach.
• Plasma spraying offers the advantage of easy controlling the compositional profile and the ability to cover much larger areas than the other techniques [14].
• For brazing tungsten, pure titanium, Ti-Fe and ternary Ti-Cr-Fe filler materials were used to fabricate the according joints.

3. Structural Tungsten Materials

• The main requirements of tungsten materials for structural divertor applications comprise properties like high thermal conductivity, high temperature strength and stability, high recrystallization temperature, and enough ductility for an operation period of about two years under enormous neutron load.
• The investigations made during recent years have shown that creep strength and recrystallization can be improved with only little effect on thermal conductivity by the use of dispersed oxides such as lanthana or yttria which stabilize the grains.
• On the contrary, inter-crystalline fracture is enhanced even more.
• The brittleness (measured by Charpy or by fracture mechanics tests) of tungsten materials is still the main problem for their use as structural materials.

3.1 Ductilisation

• In principle, there are three ductilisation strategies that were followed in the present programme, which are (1) alloying/solid solution, (2) nanostructuring, and (3) producing composite materials.
• Examination of the fracture surfaces shows an intergranular fracture mode in both materials, with the crack propagating from the notch straight through the sample.
• At higher temperatures in the W5Ta material the samples failed by a ‗delaminating‘ mechanism (Fig. 8C).
• That is why in the present programme powder metallurgical fabrication routes were followed, which include mechanical alloying and hot isostatic pressing and/or hot/cold forming.
• An alternative route to fabricate nanostructured tungsten-based materials is by chemical powder metallurgical methods.

3.2 Characterization for W data base

• For clear comparability all the produced materials have to be characterized by basic, standardized methods which are (1) DBTT measurement either by Charpy or by fracture mechanics tests, (2) creep/tensile/indentation tests up to 1300 °C, (3) thermal conductivity measurements, (4) determination of re-crystallization temperature, and (5) microstructure and fracture analysis.
• Here is an example for the fracture toughness (FT) measurement of a round blank of pure tungsten where the specimens were oriented longitudinally (L-R), radially (R-L) and circumferentially (C-R) [38].
• For the longitudinal orientation the material exhibited a low room temperature FT of about 7 MPam1/2 and a steep increase of fracture toughness above 200 °C (see Fig. 15).
• For the radial and circumferential orientations, in contrast, the material exhibited relatively high room temperature FT of about 15 MPam1/2.

4. Tungsten Armour Materials

• The range of operating temperature and load conditions (pulse, fatigue, shock, flux, etc.) depend strongly on the power plant design and cannot be exactly specified yet.
• The lowest armour temperatures can be expected to be somewhat higher than the maximum coolant temperature, i.e. about 800-900 °C in the case of the helium-cooled finger design and about 500 °C for the blanket‘s first wall.
• The maximum temperatures will certainly be higher than 1700 °C on the divertor armour surface.
• With that, the most important questions in this line of research are:.

4.1. Heat flux, shock and high temperature testing

• Besides basic material characterization in an extended high temperature range (including thermal fatigue), thermal shock tests in the operation relevant parameter range (JUDITH electron beam facility, FZ Jülich [40]) and thermal fatigue tests under hydrogen and/or helium neutral beam loading (GLADIS, IPP, Garching [41,42]) with divertor relevant power fluxes were carried out.
• Moreover, the JUDITH 2 facility was used for investigations on high cycle thermal shock tests (up to 106 cycles) with the tungsten reference material.
• Effects of erosion, gas retention and void formation depend on both the loading conditions and the operating temperature.
• Particle fluences up to 3·1025 atoms/m2 under stationary temperature conditions were reached by repeated pulses of 30 s each.
• Thermally induced cracks in the bulk material did not occur.

4.2. First wall armour materials

• The use of tungsten as first wall (FW) armour of a fusion power reactor represents an important safety concern in the event of a loss of coolant accident with simultaneous air ingress into the reactor vessel.
• Thin films of such alloys showed a strongly reduced oxidation rate compared to pure tungsten.
• Other than in the WCr10Si10 material, no intergranular ODS phase inhibiting grain growth was detected in this system, and thus the grain size is slightly larger but still below 500 nm.
• The coating provided by PLANSEE is made of a top layer (1.8 mm) of porous W and an interlayer region (also plasma-sprayed) where steel and W droplets were co-deposited on the actively cooled substrate [52].
• A relatively high porosity was observed, in particular at the interface to VPS-W (up to 35 vol. %).

5. Materials Science and Modelling

• As already mentioned, at present only Re (and probably Ir) are known to form ductile tungsten alloys.
• The intrinsic brittleness of tungsten is primarily due to the high activation energy of screw dislocation glide.
• Furthermore, radiation damage data – especially under the relevant divertor operation conditions (i.e. in the temperature range 600-900 °C and up to dose levels of 5-10 dpa) – are very rare.
• Therefore, the main objective for this research effort is to assist and guide the materials development process.
• The most striking question that has still not been addressed is:.

5.1. Plasticity Studies

• It is well known that the impurity content (e.g., of S, P, O, C) is among the parameters influencing the fracture behaviour of tungsten.
• Sintered, hipped, rolled, recrystallized, deformed by high pressure torsion (HPT) and a high purity single crystal, which was HPT-deformed and annealed at different temperatures, resulting in different grain sizes, also known as The manufacturing conditions are.
• Combining all data on the ratio of inter- vs. transcrystalline fracture, grain shape and grain size leads to inconsistencies when postulating a pronounced grain boundary weakening for high impurity contents.
• There, temperature-dependent fracture experiments using technically pure tungsten featuring recrystallized and deformed (elongated grains) microstructures are described.
• In addition, pre-deformation of the material and higher test temperatures increase the fracture resistance, i.e., the starting values of the R-curves are higher.

5.2. Simulation

• It has been realized that the plastic behaviour and mechanical properties of crystalline materials depend strongly on the properties of dislocations.
• The validity of the numerical model has been extensively checked by computing several structural properties of pure W and its alloys.
• After the calculation of the total energy of the unloaded material, the crystal has been elongated along the loading axis (the [001] direction) by a fixed amount ε, which is equivalent to applying some tensile stress σ.
• Fig. 24 presents the 5 keV/D implantation RE result compared with the experimental D concentration.
• He clusters and the influence of impurities have been investigated using Density Functional calculations and the VASP code.

6. Summary and outlook

• The long-term goal of the EFDA programme on divertor materials is to provide structural and functional materials together with the necessary production and fabrication technology for future fusion reactors beyond ITER.
• While fabrication issues are far advanced and well investigated, the most critical part of the programme is still the development of a material for structural divertor parts.
• Joining of tungsten materials is possible, but design routes, cost and low-activation criteria have most probably to be redefined for brazing materials.
• The results clearly show that the calculated sputtering rates might be far too low compared to the real in-service conditions.
• But for further, more specific designs at least, basic irradiation damage data have to be produced.

Figures

Fig. 28

Figure 16

Fig. 26

Figure 7

Figure 6

Figure 4

Figure 2

Table 1: Composition and result of the screening oxidation tests with different tungsten-based alloys at 1073 K for 3 hours.

Figure 9

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Recent progress in research on tungsten materials for nuclear fusion applications in europe" ?

In this paper, the authors present a detailed review of the use and properties of tungsten materials for the first wall of a fusion tokamak. 

Q2. What are the main requirements of tungsten materials for structural divertor applications?

The main requirements of tungsten materials for structural divertor applications compriseproperties like high thermal conductivity, high temperature strength and stability, high recrystallization temperature, and enough ductility for an operation period of about two years under enormous neutron load. 

Q3. How can a low activation vanadium-carbide layer be used for joining?

It was demonstrated that diffusion bonding at only 700 °C can be successfully performed with a minimal alteration of the microstructure of the base materials and strongly reduced formation of vanadium-carbide. 

Q4. How much tungsten oxides would be released in a single hour?

The linear oxidation rate of tungsten at 1000 °C is about 1.410-2 mg cm-2 s-1 [43], which in the approximately 1000 m2 DEMO first wall would correspond to a release of half a ton of tungsten oxides per hour. 

Q5. What was the common method of machining tungsten armour tiles?

Powder injection moulding (PIM) was investigated as a mass fabrication option for the tungstenarmour tiles which had to be joined to the thimbles. 

Q6. How long does the D flux take to the surface?

After the implantation has stopped (region III), the D retention drops and a remnant D flux to the W surface takes place for about 10 min. 

Q7. What is the impact of a loss of coolant in a fusion power plant?

The use of tungsten as first wall (FW) armour of a fusion power reactor represents an importantsafety concern in the event of a loss of coolant accident with simultaneous air ingress into the reactor vessel. 

Q8. What are the main problems related to the use and properties of tungsten materials?

But even neglecting the irradiation effects (due to the large gaps in the knowledge of properties of these materials), there are still unsolved problems related to the use and properties of tungsten materials. 

Q9. Why is the brittleness of the tungsten alloys so severe?

Due to the fabrication route, missing mechanical working and/or an increased impurity level could also be an additional explanation for this severe brittleness. 

Q10. What are the main uses of a tungsten divertor?

In helium cooled divertor designs tungsten materials are also considered for structural use (e.g. as pressurized pipes or thimbles). 

Q11. What is the reason for the formation of small He clusters in tungsten alloys?

The formation of intermetallic compounds in tungsten alloys is just one of the factors responsible for their increased hardening, the other being the conventional solute hardening that gives rise to the embrittlement of the alloys that occurs even in the limit where the concentration of the alloying elements is small. 

Q12. What is the effect of the vacancy formation energy in W-Ta alloys?

It shows that the vacancy formation energy in W-Ta alloys depends sensitively on the lattice site at which a vacancy is formed, whereas in W-V alloys it is almost independent of the location of the vacancy site. 

Q13. What was the first technique used for manufacturing the fillers?

In the first case, pure metallic powders were mechanically mixed, compacted, and molten to allow for brazing filler materials with homogeneous compositions. 

Q14. What is the recent development in tungsten?

In a recent development, precursor powders are fabricated under certain solution conditions where the particle growth could be controlled to produce uniformly yttrium doped nano-sized tungsten oxides. 

Q15. How many atomic parts per million are produced in W?

In W, the production of impurities, such as Re, Ta, and Os, is fairly significant, being of the order of a few thousand to tens of thousands of atomic parts per million (appm) over a typical DEMO-like first-wall 5-year neutron exposure. 

Q16. How strong was the shear strength of the W-W joints?

Preliminary results of mechanical characterization of these W-W joints using Ti-Fe fillers gave rise to an average shear strength of 140 ± 8 MPa. 

Q17. What is the way to improve the ductility of tungsten?

So far, only rhenium is known to improve the ductility of tungsten by solid solution but its usefor fusion energy applications has been ruled out for various reasons (cost, irradiation embrittlement). 

Q18. What is the oxidation rate of the WCr12Ti2.5 alloy?

The oxidation behaviour of the WCr12Ti2.5 alloy is similar to that of the WCr10Si10 material; in this case the oxidation rate is similar to that of thin films of same composition at 600 °C but higher at 800 and 1000 °C [48]. 

Q19. What are the effects of the surface modification processes of the pure hydrogen loaded materials?

Only grain growing and physical sputtering were identified as the surface modification processes of the pure hydrogen loaded materials (Fig. 17). 

Q20. What is the possibility of designing alloys where vacancies form within a desired range of temperature?

The results show the possibility of designing alloys where vacancies form within a desired range of temperature, suggesting the possibility of developing alloys with improved stability under irradiation. 

Q21. What is the way to characterize the fracture resistance of tungsten?

the fracture resistance may increase with crack propagation, which implies that it is not always possible to characterize the material‘s toughness with one single value such as plane strain fracture toughness KIC or critical energy release rate GIC. 

Q22. What are the main subtopics of the W&WALLOYS programme?

In what follows, the results, conclusions, and outlooks are summarized for each of theW&WALLOYS programme‘s main subtopics, which are (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. 

Q23. What is the effect of the grain boundary crystallography on the fracture properties of tungsten?

Further investigations into the effect of grain boundary crystallography and chemistry are currently on-going, but it appears that tantalum has no beneficial effects, and may even have detrimental effects, on the fracture properties of tungsten.