Comprehensive nuclear materials 4 17 tungsten as a plasma facing material

Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material Comprehensive nuclear materials 4 17 tungsten as a plasma facing material

G Pintsuk

Forschungszentrum Ju¨lich, Ju¨lich, Germany

ß 2012 Elsevier Ltd All rights reserved.

4.17.3.2.4 Component fabrication: CTE mismatch with heat sink 557

4.17.4.1.1 Microstructure, composition, and mechanical properties 561

4.17.4.1.5 Thermal shock during off-normal events: disruptions 563

CFC Carbon fiber composite

CTE Coefficient of thermal expansion

CVD Chemical vapor deposition

DBTT Ductile to brittle transition temperature

DEMO Demonstration fusion reactor

ECAE Equal-channel angular extrusion ECAP Equal-channel angular pressure ELMs Edge localized modes

fpy Full power years FTU Frascati tokamak upgrade

(Frascati, Italy) ICRH Ion cyclotron resonance heating IFE Inertial Fusion Experiment

551

IFMIF International Fusion Materials Irradiation

Facility

ITER Tokamak, Latin for ‘the way’

JET Joint European Torus (Culham, UK)

LPPS Low-pressure plasma spraying

MIM Metal injection molding

NIF National Ignition Facility (Livermore,

SPS Spark plasma sintering

TEXTOR Tokamak EXperiment for Technology

Oriented Research (Ju¨lich, Germany)

Until the mid-1990s, only few fusion devices used

high-Z elements in plasma-facing materials (PFMs).1

These devices either operated at high plasma

cur-rents and high plasma densities such as Alcator

C-Mod2and Frascati tokamak upgrade (FTU)3,4or

used high-Z materials only as test limiters such as

Tokamak EXperiment for Technology Oriented

Research (TEXTOR).5–9

Since then, high Z refractory metals have been

attracting growing interest as candidates for PFMs

because of their resistance against erosion and the

need for low erosion and stability against neutron

irradiation.10 Considerable effort has been made to

study the behavior of high Z impurities in the core

and edge plasmas, erosion/redeposition processes

at the limiter/divertor surfaces, hydrogen isotope

retention, and on material development and testing

In particular, the modification of ASDEX-upgrade

(AUG) into a fully tungsten machine,11–17which was

achieved in 2007, provided positive answers to critical

questions on the reliability of tokamak operation withhigh-Z plasma-facing components (PFCs) and thecompatibility with standard and advanced H-modescenarios and with the available heating methods.10Among the challenges, for tokamak devices, that stillremain are the strong increase of the W source and

W concentration resulting from ion cyclotron nance heating (ICRH) and the need for rigorous mod-eling to support the extrapolation of current results toITER conditions Clearly, not all questions posed

reso-by ITER can be answered reso-by AUG only For example,the effects of material mixing with Be, the melt behav-ior under transients, or the change of the hydrogenretention due to damage by high-energy neutron irra-diation18 cannot be addressed in AUG Answers tosome of these issues may be provided by the ITER-like wall project in Joint European Torus (JET), which

is installing a bulk tungsten component for the strikepoint and physical vapor deposition (PVD)-W-coatedcarbon fiber composite (CFC) tiles for the remainingparts of the divertor.19–21 The remaining questionshave to be answered by dedicated experiments inother plasma devices or can only be assessed by mod-eling However, the results obtained so far do notexclude the use of W in ITER as a standard PFM.10Further investigations related to future fusion powerplants such as demonstration fusion reactor (DEMO)have to focus on the minimization of plasma heat loads

to the PFCs to increase their lifetime In particular,transient heat loads caused by instabilities significantlydecrease the operation domain of PFCs, due to thermalstresses and consequent enhanced erosion.22 There-fore, it is also important to mitigate all instabilities,such as edge localized modes (ELMs), that cause sig-nificant plasma transient heat losses.23Plasma scenar-ios need to be developed, such that the conditions forachieving the required fusion yield are maintained insteady state, while at the same time sustaining tolerableheat loads on the PFCs The above-mentionedupgrades to the JET24 and AUG15 will allow furtheroptimization of the plasma scenarios under these con-ditions, in particular with DEMO relevant tungstenPFCs.25These investigations will show how the iden-tified deficiencies of W can be overcome or how theyhave to be dealt with

In addition to the application of tungsten in ITERand in potential future tokamak devices such asDEMO,26–29tungsten also became an interesting alter-native for the divertor of stellarators, for example,Advanced Reactor Innovation Evaluation Studies –Compact Stellerator (ARIES-CS),30 and as a first

wall material for inertial fusion devices.31 Due to

similar demands on the PFMs during the operation

of all these devices, similar problems have to be solved

for each application

4.17.2 Functional Requirements

In the current design of the ITER divertor32–34for

the start-up phase, tungsten has been selected as

armor for the divertor dome and the upper part of

the divertor vertical targets In addition, due to

exces-sive co-deposition of tritium in CFC raising

regu-latory concerns related to tritium inventory limits,

a full tungsten divertor will be installed before the

D–T phase of operation.32

The PFC design for ITER consists of bulk

W bonded to an actively pressurized water-cooled

Cu alloy heat sink Here W has no primary structural

function However, due to the operating conditions

listed in Table 1, the PFMs face large mechanical

loads particularly at the interface to the heat sink

material during cyclic steady state heat loads (see

Section 4.17.4.2) and at the plasma-loaded surface

during transient thermal events (seeSection 4.17.4.1)

Furthermore, the material response to these loads is

influenced by the material damage or degradation

due to neutron irradiation (see Table 1, Sections

4.17.4.3.3 and 4.17.4.3.4)

Along with thermally induced loads, the tion of the PFM with the plasma, that is, the hydro-gen isotopes D and T as well as the fusion product

interac-He, is of importance (see Section 4.17.4.4) becausethey have an influence on material erosion and near-surface material degradation

The further development of the ITER designled to four conceptual designs for the DEMOdivertor.25,35 These designs include either water(inlet 140
C/outlet 170
C) or, due to the higherachievable efficiency, more probably He-cooling(inlet 540
C/outlet 700
C) In all cases bulk W isforeseen as the armor material that will have to facepeak steady state heat loads of 15 MW m2in case

of the water-cooled design and 10 MW m2for theHe-cooled designs In contrast to ITER, off-normalevents such as disruptions have to be avoidedcompletely and transient thermal events during nor-mal operation, for example, ELMs, have to be miti-gated below the damage threshold of the material(see Section 4.17.4.1) This may be particularlyimportant considering the expected neutron damagethat will amount up to 40–60 dpa during the plannedoperation of the fusion reactor35leading to a signifi-cant amount of transmutation products.36 However,the main limiting factor is expected to be the materi-al’s erosion leading to a maximum lifetime of 2 yearsfor the divertor armor.35

Table 1 Operating conditions assumed for the design of the ITER PFCs during D–T operation

Divertor target Divertor baffle/dome

Normal operation

3

Operation temperature design window during normal operation (
C) 200–1000 200–600

Off normal operation: disruptions

Source: Federici, G.; Wuerz, H.; Janeschitz, G.; Tivey, R Fusion Eng Des 2002, 61–62, 81–94; Loarte, A.; Saibene, G.; Sartori, R.; et al.

In Proceedings of the 22nd IAEA Fusion Energy Conference, Geneva, Switzerland, Oct 13–18, 2008; IT/P6-13; Raffray, A R.; Nygren, R.; Whyte, D G.; et al Fusion Eng Des 2010, 85, 93–108.

a Slow transients lasting 10 s up to 20 MW m2(10%).

b Without replacement.

In comparison to tokamaks, calculations for a

device such as ARIES-CS predict steady state heat

loads between 5 and 18 MW m2.30,37 Similar to

DEMO, a He-cooled W divertor is anticipated with

a maximum heat removal capability of 10 MW m2

The design limits for neutron irradiation at the shield

of ARIES-CS are up to 200 dpa at 40 fpy (full power

years).38The component lifetime limits are similarly

dictated by the material’s expected erosion

Finally, tungsten or more specifically tungsten

coatings find their application also in the dry wall

concept for inertial fusion devices, for example, the

National Ignition Facility (NIF) In future, inertial

confinement devices, thermal loads will occur only in

the form of transient thermal loads (P¼ 0.1 MJ m2,

t¼ 1–3 ms, f ¼ 5–15 Hz, Tbase 500
C).31

These aresimilar to those expected during ELMs and almost

identical to those occurring in an X-ray anode39and,

therefore, affect a thin surface layer only

4.17.3 Material Selection

4.17.3.1 Fabrication and Microstructure

Tungsten and tungsten alloys are commercially

available in many forms, for example, as bulk rods,

plates and discs, or thin coatings on various kinds of

substrates For each of these tungsten products,

opti-mized production routes exist involving mainly

pow-der metallurgical techniques for bulk materials and

PVD and chemical vapor deposition (CVD) as well as

plasma spraying (PS) for coatings Each of these

processes has its own advantages and disadvantages

as well as an individual influence on the material’s

microstructure and subsequently the material

prop-erties In addition to the fabrication method, the raw

materials, the alloying elements and

dopants/impu-rities, pre- and postthermomechanical treatments,

and the final shape/geometry have a strong impact

on the achieved microstructure

Focusing on the powder metallurgy fabrication

route, tungsten powder is obtained from ammonium

paratungstate ((NH4)2WO4), tungsten oxide (WO3),

and tungsten blue oxide (WO3x) by hydrogen

reduc-tion at temperatures in the range of 700–1100
C

Vari-ous grain sizes can be produced depending on the

reduction temperature and the hydrogen dew-point

The purity of the metal powder obtained is>99.97%

In the manufacture of doped or alloyed tungsten

pro-ducts, the dopants or alloying elements are either

introduced into the raw materials before reduction or

they can be added to the metal powder after reduction

Following the reduction stage, the powder issieved and homogenized The initial densification

of the powder in various plate and rod geometriestakes place predominantly through die pressing andcold isostatic pressing The pressed compacts aresubsequently sintered at temperatures between 2000and 2500
C (2273–2773 K), mostly using furnaceswith hydrogen flow This increases the density andthe strength of the pressed blanks.40

After sintering, the products have a rather lowdensity of about 80% of the theoretical value andpoor mechanical properties To increase density andimprove mechanical properties, the sintered productsare subject to a mechanical treatment such as rolling,forging, or swaging at temperatures up to 1600
C.Intermediate annealing, leading to recovery andrecrystallization, is necessary to maintain sufficientworkability The working temperature can bereduced as the degree of deformation increases Inthis way, forged parts such as rods and discs as well assheets and foils are produced.40

The final step, that is, the mechanical treatment,changes the microstructure from isotropic with grainsizes determined by the initially used powder sizeinto anisotropic Depending on the deformationmethod, the grains may show either:

 an elongated, needle-like structure along thedeformation direction for radially forged rodsand uniaxially rolled plates (seeFigure 1(a)), or(a)

(b)

200 mm

200 mm Figure 1 Light microscopy images of etched

cross-sections of (a) a deformed rod and (b) a rolled plate.

 a flat disc-shaped structure for axially forged discs

or blanks and cross-rolled plates (seeFigure 1(b))

In addition to bulk materials, research and

develop-ment is also directed on tungsten coatings One

possi-bility would be the plasma-spraying process, in which

powders are injected into a plasma flame, melted, and

accelerated toward the (heated) substrate The

depos-ited layers are splat-cooled, leading to a flat

disc-shaped microstructure Depending on the atmospheric

conditions, the result may be layers with high porosity

and oxygen content (water stabilized and atmospheric

plasma spraying, APS, see Figure 2(a))41,42 or low

porosity and good thermal contact (low-pressure or

vacuum plasma spraying, LPPS/VPS).26,43–47

In contrast, PVD and CVD coatings show a

columnar structure perpendicular to the coated

sub-strate with grain sizes in the range of the coating

thickness (see Figure 2(b)) PVD coatings, which

are also used as thin intermediate layers below a

plasma-sprayed tungsten top layer,48are deposits of

tungsten vapor on the substrate surface, which is in the

source’s line of sight.43,49CVD coatings are reactions

of a W-containing gaseous phase and have the ability

to coat complex geometries.6,50–52In both cases, a high

density (100%) of the coatings is achieved

The coated substrate can be graphite as used for

AUG (PS),12,13CFC as used for the ITER-like wall

project in JET (PVD),21,53–55or copper and steel as itmight be used for first wall applications in futurefusion devices (PS, PVD, CVD).44,49,50,56–60

4.17.3.2 Advantages and Limitations forFusion Application

For fusion plasma-facing applications, the mostessential properties are thermal conductivity,strength and ductility, thermal shock and thermalfatigue resistance, structural stability at elevated tem-perature, and stability of the properties under neu-tron irradiation The advantages and disadvantages oftungsten for these conditions are manifold andopposed to each other as shown in Table 2 Whilethe advantages of the material are mainly related

to its high temperature-handling capability, thelimitations are associated with manufacturing andhandling at low temperatures (below ductile to brittletransition temperature, DBTT61–63), plasma com-patibility including neutron irradiation, and radio-logical issues

However, with regard to other potential PFMs, forexample, Be (see Chapter 4.19, Beryllium as aPlasma-Facing Material for Near-Term FusionDevices), CFC (see Chapter 4.18, Carbon as aFusion Plasma-Facing Material), and Mo, tungsten

is still the most promising, offering an advantageouscombination of physical properties and, therefore,has become the material of choice for ITER andDEMO Since this decision was made, R&D effortsfor investigating newly developed tungsten grades(a)

(b)

50 mm

300 mm Figure 2 Light microscopy images of etched cross-

sections of (a) atmospheric plasma spraying W and (b)

chemical vapor deposition W on a graphite substrate.

Table 2 Features of W armor materials

 High melting point

 Low erosion (high energy threshold for sputtering)

 High thermal stress resistance

 High thermal conductivity

 Low swelling

 Low tritium retention

 High Z (low allowed concentration in plasma)

 Potential loss of melt layer during transient events

 High radioactivity (short-term waste, decay afterheat)

 Explosion dust potential

 Limited resistance to grain growth

and alloys that are able to overcome or at least

miti-gate some of the above-mentioned disadvantages

have significantly increased

4.17.3.2.1 High atomic number: material

erosion/melting

As the high atomic number is an intrinsic material

property that cannot be changed, the only possibility

to avoid plasma contamination by tungsten is to adapt

to the loading realities, that is, thermal loads and

plasma wall interaction conditions, and the energy of

the incident plasma particles In particular, surface crack

formation, loosening of particles, and particle ejection or

melting are addressed (seeSection 4.17.4) Concerning

the latter, the addition of suitable alloying elements

or dispersoids (see Section 4.17.3.3) reduces the

material’s thermal conductivity causing a reduction

of allowed applied heat fluxes From this point of view

only low-alloyed grades should be considered and the

best grade is tungsten of high purity

4.17.3.2.2 Recrystallization

Recrystallization is a thermally activated process

Therefore, it is expected that the activation energy

of nucleation is dominated by small angle grain

boundaries The activation energy of grain growth is

dominated by large angle grain boundaries.64 The

temperature of recrystallization depends mainly on

the deformation history, that is, the higher the degree

of deformation, the lower the recrystallization

tem-perature,65,66 and the chemical purity When heated

above the recrystallization temperature, the structure

of tungsten is altered due to grain growth causing an

increase in DBTT and reducing other mechanical

properties, that is, strength and hardness.67

There are several possibilities for increasing the

recrystallization temperature Particle reinforcement

and controlled formation of porosity are the best and

most investigated options.68For example, the higher

recrystallization temperature of dispersion

strength-ened alloys results from the interaction between

dispersoids and dislocations during hot-working;

the higher the amount of hot-work, the finer are the

dispersoid particles and the higher is the

recrystalli-zation temperature During recrystallirecrystalli-zation, these

particles prevent secondary grain growth and

conse-quently, the recrystallization temperature of

disper-sion strengthened alloys may increase compared to

pure W.67Another example is highly creep-resistant

doped/nonsag materials with aligned porosity acting

as obstacles for dislocation movement as they are

used in the lighting industry.69

Experience shows that incomplete tion often helps to achieve the desired balance inmaterial properties If the operating temperature

recrystalliza-is well known, controlled recrystallization duringapplication might be feasible as well.67 However,for operational conditions in nuclear fusion devices,

it is expected that the very high thermal strain ratesexperienced in the thin layer heated by plasma dis-ruption or any other transient thermal event willsignificantly affect the material’s microstructure andproperties

4.17.3.2.3 Machinability, mechanicalproperties, and DBTT

Mechanical properties of W strongly depend on ables such as production history, alloying elements,impurity level, thermomechanical treatment, andform of material Depending on the production his-tory and heat treatment, W and W-alloys could haveanisotropic mechanical properties This is expressed

vari-by showing significantly better properties in thedirection of elongated grains (by rolling, forging, ordue to deposition processes for coatings) but poorerproperties in other directions.70While reported data

on single crystals (SCs) (e.g., Gumbsch62) and forisotropic materials (e.g., Kurishita et al.71) give aclear indication of the material’s performance, typi-cally the reported data refer to the best orientation ofthe material as shown for fusion relevant tungstengrades in numerous publications.44,51,57,72–81The prop-erties in other directions, particularly the DBTT, couldsignificantly differ.76 This will affect the operationalperformance, which is reflected by the orientation-dependent thermal shock response.82

Tungsten is a body-centered cubic (bcc) tory metal, with a comparatively low fracturetoughness,61,83high DBTT, and poor machinability,which is directly correlated to the material’s lowductility and low grain boundary strength.67How-ever, DBTT is an ill-defined property and dependsstrongly on purity, alloying elements, thermo-mechanical treatment, and, most essentially, thetesting/loading conditions due to its deformationrate dependence.62,63 The obtained values varyover a broad temperature range from room temper-ature (RT) to several hundreds of degrees Celsius.The exact value depends on the stress state, forexample, a three-dimensional state of stress in thesample leads to a lower DBTT

refrac-Although many other parameters influence thefracture of bcc metals, the DBTT is usually asso-ciated with the thermal activation of dislocation

kink pairs Below this characteristic temperature

the separation of a screw dislocation into three partial

dislocations (which cannot easily recombine and

are therefore more or less immobile) is responsible

for the brittle behavior Increasing temperature leads

to thermal activation of the kink mechanism and

increased ductility due to shielding of the crack

tip.84There is an empirical correlation between

tem-perature and activation energy for brittle-to-ductile

transitions in single-phase materials suggesting that

the ratio between the activation energy and the

DBTT gives approximately a value of 25.63

Another factor is the occurrence of interstitial

solute elements, such as oxygen, carbon, and

nitro-gen, which even in very small amounts tend to

segregate at grain boundaries, promoting

intergran-ular brittleness and increasing the DBTT Two ways

can be used to get rid of or mitigate the negative

effects of interstitial impurities: either a reduction of

the grain size,84 to dilute their effect on a larger

grain boundary surface, or the complete elimination

of grain boundaries, as in SCs The development

of W-alloys essentially follows the first route, as

the SC technique, although effective, is too costly

The conventional method to decrease the grain

size of tungsten or tungsten alloys is to deform the

material at an intermediate temperature, above

the DBTT and below the recrystallization

tempera-ture.81,84–86 The formation of oxides and carbides

of the alloy constituents helps to stabilize the grain

boundaries and to dispersion strengthen the matrix

at high temperature Recently, mechanical alloying

followed by powder densification has been applied

to refractory alloys Materials with a stabilized

fine-grained structure and with the grain boundary

strengthened by even finer dispersoids of TiC

improve the low-temperature impact toughness

of refractory alloys, leading to increased ductility

even down to RT and create superplasticity at high

temperatures.71,87–89

Another reliable method to increase the ductility

at low temperatures and therefore reduce the DBTT

is to alloy tungsten with the rather expensive element

rhenium, which is a substitutional solute in the

W lattice.67,83

As mentioned before, both material deformation

and heat treatment influence the DBTT A heat

treatment slightly below the recrystallization

tem-perature is able to significantly reduce the DBTT

In contrast, annealing above the recrystallization

temperature reduces strength and hardness and

increases the DBTT.67

4.17.3.2.4 Component fabrication:

CTE mismatch with heat sink

A mismatch between the coefficients of thermalexpansion (CTEs) can lead to thermal stresses atthe interface, which are detrimental to the compo-nent lifetime This can occur with either Cu-basedalloys or steels (steel is more likely to be used in case

of coatings) such as that used for water-cooleddesigns, or to W and W-alloys in the He-cooleddesign In particular W and W alloys, in the cold-worked and stress-relieved condition, tend to delam-inate in the direction parallel to the direction ofdeformation Such delamination can occur duringmachining or during operation To avoid failure due

to delamination, the orientation of the texture has to

be perpendicular to the surface of the joints,90raisingthe question of the suitability of plasma-sprayed

W coatings Two possible options are recommended

to mitigate the thermal stresses, that is, reducing thejoint interface by introducing castellations or usingsmaller tiles,91–93or introducing soft and chemicallystable interlayers94,95or graded layers.96–101

Despite the fact that surface finish has no directeffect on the performance of ITER-related compo-nents,94 it is recommended to avoid possible crackinitiators in the armor design, such as castellationsending in the tile and to ensure accurate surfacefinishing.102–104 Designs that have been proven toreduce the tile and interface thermal stresses and toextend the component lifetime beyond the designlimits are the macrobrush or the monoblock Thelatter is the reference design for ITER105because itprovides the most reliable attachment and therefore areduced risk of catastrophic cascade failure.106Finally, the thermal treatment of W during joiningmanufacturing cycles might have an influence onthe material’s properties While the process tempera-tures during joining of W and Cu do not lead to anysignificant change of the W properties, in the case ofhigh-temperature brazing of W to W alloys for theHe-cooled divertor design,102 the recrystallizationtemperature of W has to be taken into account.4.17.3.2.5 Neutron embrittlementThere are little data available for irradiated tungsten(seeChapter1.04, Effect of Radiation on Strengthand Ductility of Metals and Alloys) Based onresults for other refractory alloys and limited data

on tungsten, one would expect neutron irradiation

to increase the strength and decrease the ductility ofthe tungsten armor largely through increases in theDBTT To minimize the neutron-induced material

degradation, it is reasonable to limit the operational

conditions for components in a neutron environment

to temperatures above 900
C where recovery of

tungsten takes place,107as the ductility loss is more

pronounced below about 0.3 Tm This is possible in the

region close to the plasma-facing surface, but it is

impossible in the heat sink region as tungsten will be

in contact with materials that cannot operate at this

temperature and stress concentrations in these ‘cold’

areas have to be avoided.108In the case of ITER, Cu

will be employed in the heat sink while steel is more

likely to be used in DEMO, which has a higher

operating temperature.29 Hence, a greater

under-standing of the irradiation response of tungsten at

temperatures between 700 and 1000
C is

needed.109,110 The effect of embrittlement is

alle-viated when operating above 250
C, although in the

presence of He (produced by transmutation reactions)

somewhat higher temperatures may be required.83

Although at intermediate temperatures (0.3–0.6 Tm),

void swelling and irradiation creep are the dominant

effects of irradiation, the amount of volumetric

swelling associated with void formation in refractory

alloys is generally within engineering design limits

(<5%) even for high neutron fluences (10 dpa)

Very little experimental data exist on irradiation

creep of refractory alloys, but data for other bcc alloys

suggest that the irradiation creep will produce

negligi-ble deformation for near-term reactor applications.110

4.17.3.2.6 Neutron activation and

radiological hazards

The activation and transmutation of tungsten as

a PFM is a critical issue, particularly concerning

long-term storage and recycling times Different

studies on activation issues have been performed

These comprise the analysis of cross-sections for

high-energy neutrons,111,112 studies on the

helium-cooled lithium lead divertor for DEMO,113the inertial

fusion devices,109other benchmark experiments,114,115

and modeling issues, for example, on the self-shielding

ability of tungsten.116,117

Furthermore, it was shown that the long-term

activation behavior is dominated by activation products

of the assumed material impurities while the

short-term behavior is due to the activation of the stable

W isotopes.113 For a short period of a few weeks, the

latter causes a huge amount of decay-induced afterheat

that has to be removed by continued active cooling.67

On the other hand, the accumulation of the highly

radioactive transmutation product 186mRe was

deter-mined to be most critical, limiting the component

lifetime to a maximum of 5 fpy when using pure W or

to 2 fpy when using Re-doped W before the limits forstorage by shallow land burial could be exceeded.109The dose rate limits for recycling after differentapplications are expected to be reached within

5 years115to 50 years113of storage or up to 75 yearsafter end of plant life.118Fischer et al.113take a limit of

100 mSv h1for remote handling into account, whichmight be a problem at the times when maintenanceoperations would be in progress Taylor andPampin118give a value of 20 mSv h1as the limit forallowing tungsten to be categorized as a recyclablematerial The hands-on limit for tungsten should beachieved after about 200 years.115

Besides waste management, tungsten has also beeninvestigated and evaluated according to characteristicradiological hazards that might occur when using it asPFM in tokamak fusion reactors It was found that thetritium permeation into tungsten does not, in contrast

to CFC, appear to be a major problem However, due

to neutron activation, the mobilization of activationproducts, for example, by forming volatile oxide spe-cies in the presence of steam and air, has to be limited

by establishing shutdown requirements to avoidmelting of tungsten in case of an accident The poten-tial exposure from mobilized activation productsfrom the tungsten divertor may be modified by vary-ing the operating conditions of fusion power andchange-out time as well as the thickness of thedivertor armor The dose can be reduced by selectingshorter change-out times However, the total life-cycle waste volume will be increased accordingly

A thinner divertor will produce less mobilized vation products while suffering a more restrictiveshutdown requirement.119

acti-4.17.3.2.7 Material availabilityThe quantity of W needed for the PFCs in a fusiondevice such as ITER or DEMO represents only asmall fraction of the yearly production and theworld’s reserves120 and its production can be easilysatisfied by existing industrial capabilities The samepoint is valid for stellarators and even more for iner-tial fusion devices, which only work with thin coat-ings However, the issue of component lifetime has to

be taken into account Depending on the componentlifetime, the recycling rate, and the storage time until

a hands on level is achieved (seeSection 4.17.3.2.6),the operation of numerous power plants may require

an amount of tungsten that exceeds what is currentlyavailable from the market

4.17.3.3 Tungsten Grades

Within current R&D programs for the selection

and characterization of candidate grades of W and

W alloys for fusion applications, many materials

pro-duced according to the schemes outlined above were

investigated These are discussed in the following

sec-tion, which introduces some of their characteristics

The manifold production processes described below

for pure W are also applicable to W alloys

Pure tungsten (undoped)

 Sintered W is the most readily available and

cheapest grade with a grain size that depends

on the initially used W powder However,

it is characterized by high porosity, low

recrys-tallization temperature (1000–1200
C), and

low strength at elevated temperature.96 The

option of improving the sinterability by

add-ing small amounts of activators (Ni, Fe)121

increases the radiological hazard due to

addi-tional activation products that have to be taken

into account.119

 Forged or swaged W offers an increased density

and a refined microstructure compared to

sin-tered material, resulting in higher ductility and

mechanical strength Forging and swaging are

therefore the industrial production processes

that are typically applied not only for pure

tungsten but also for most kinds of tungsten

alloys (see below) This grade of W is

manu-factured in block shape or more commonly

in the form of rods with different diameters

(90 mm)40

showing an anisotropic structure122 with elongated grains along the

micro-axial direction and an increasing grain size

and porosity with increasing rod diameter

Thus, increasing rod diameter leads to a

decrease in mechanical strength and ductility

For the production of monoblock tiles, such as

those planned for ITER, rods with a minimum

diameter of 30–35 mm are necessary

 Rolled W is applied in the form of plates or foils

with thicknesses from 0.02 to 20 mm.40,123,124

It offers a densified but layered microstructure

that is strongly anisotropic, with flat disc-shaped

grains parallel to the rolled surface affecting the

mechanical properties (see Section 4.17.3.2.3)

and resulting in the risk of delamination

 Double-forged W is in the form of blanks with a

diameter of 140 mm and a height of 45 mm The

double-forging process, first in the radial and

then in the axial direction, provides a more

isotropic microstructure than it is generated bysingle forging This material should act as areference grade for establishing a reliable mate-rials database for finite element calculations.82

 SC W provides higher ductility than talline W, higher thermal conductivity, lowerneutron embrittlement, higher thermal fatigueresistance, and a more stable structure at ele-vated temperatures The disadvantages are highcost and low industrial availability.96,125,126

polycrys- Metal injection molded (MIM)-W127–129

provides

a dense and isotropic microstructure withgrain sizes on the order of the powder particlesizes used A final densification by hot isostaticpressing (HIP) at temperatures>2000
C leads

to an improvement of the mechanical ties; recrystallization and grain growth do notplay a role Furthermore, the production pro-cess offers the possibility of net shaping

proper- Spark plasma sintered (SPS)-W and resistance tering under ultra-high pressure.130–132The mate-rial is characterized by a short fabrication time

sin-of only a few minutes keeping the initial finemicrostructure determined by the powdersused The finer the grain size, the higher themicrohardness and the bending strength but alsothe lower the achievable density The applica-tion of alternatively uni-, two-, or three-directional orthogonally applied forces for thematerial’s densification during the process leads

to internal stresses, which have an influence onthe recrystallization behavior Recrystallizationand grain growth occur at1500
C Depend-ing on the amount of porosity, the finer theinitial grain size of tungsten, the smaller isthe grain growth

 Severe plastically deformed W (and W alloys, seebelow) with ultra-fine grains in the nm rangeare produced by either high-pressure torsion

at 400
C84,133or by the equal-channel angularextrusion or pressure (ECAE or ECAP) pro-cess at high temperatures (1000–1200
C).134The material shows stable, that is, deformation-independent, recrystallization temperatures andexhibits considerably enhanced ductility andfracture toughness.61,85,86,135,136

 Plasma-sprayed W involves, in general, application

of VPS, more precisely also called low-pressureplasma spraying (LPPS), which provides asignificantly reduced oxygen content andimproved thermophysical properties com-pared to atmospheric (APS) or water-stabilized

plasma spraying.42However, LPPS-W is

typi-cally characterized by a lower thermal

con-ductivity (up to 60% of bulk tungsten is

reported67) and a lower strength than bulk

W particularly when deposited on large

sur-faces The recrystallization temperature is

similar to pure W.48,137 Although the

thick-ness of the plasma-sprayed coatings required

for fusion applications are flexible,

coat-ings with 200mm or thicker are commonly

produced.26,43,67,138,139 Furthermore, PS is the

only production method that offers the

possibil-ity to produce and repair W components.57,60,96

 CVD W provides a microstructure with a

columnar grain structure parallel to the

sur-face, high thermal conductivity similar to bulk

W, and a very high density and purity.6,140,141

Thicknesses up to 10 mm were produced,67but

its high cost is a significant drawback for

prac-tical applications.52,96

 PVD W provides a featureless structure that is

extremely dense and pore free In contrast to

plasma sprayed and similar to CVD-coatings,

the deposition rates are low Economic and

process-related restrictions generally limit the

deposited W thickness to 10–50mm.13,54,55,67,142

 W foam for Inertial Fusion Experiment (IFE)

applications provides structural flexibility

dur-ing quasivolumetric loaddur-ing The material is

microengineered with a relative density of

21% and can be simultaneously optimized

for stiffness, strength, thermal conductivity,

and active surface area.143

Tungsten alloys

 Oxide dispersion strengthened W alloys such as

W–La2O3, W–Y2O3, and W–CeO2with oxide

additions2% are processed by powder

met-allurgy methods similar to pure W.40The

insol-uble dispersoids, which are influenced in shape

and distribution by the thermomechanical

treatments during the production process,73,144

improve the grain boundary strength and

machinability and play an important role in

controlling recrystallization and the

morphol-ogy of the recrystallized grains.68 This results

in a higher recrystallization temperature by

100–350 K by suppression of secondary grain

growth (i.e., grain boundary migration), lower

grain size, higher strength after

recrystalliza-tion, and better machinability than sintered

W even at RT This permits fabrication at

lower costs.67 The size of the dispersoids incommercially available alloys is10 mm; how-ever, research on mechanically alloyed materi-als using submicron dispersoids is currentlybeing performed.145However, the presence ofoxide particles with a melting temperaturebelow those of tungsten has a negative effect

on the erosion resistance.146,147

 W–3–5% Re is, compared to sintered pure W,characterized by a higher recrystallization tem-perature and strength even after recrystalliza-tion,148 better machinability, and improvedductility at low temperatures.67 The addition

of Re, which has a high solubility in W, ever, reduces thermal conductivity, increasesembrittlement after neutron irradiation, andsignificantly increases the cost and safety con-cerns because of the high Re activation underneutron irradiation.96

how- W–1–2% Mo (þY and Ti) cast alloy The tion of Mo and the reactive elements Y and Ti,which reduce the amount of free oxygen andcarbon and form obstacles to grain growth,improves the mechanical properties compared

addi-to large grained pure cast W.67,73

 W–TiC produced by mechanical alloying andslow deformation techniques provides, similar

to all other W alloys, higher strength and tallization temperature, better machinability,and improved ductility compared to pure Wwith superplastic behavior at temperatures of1400–1700
C.89The addition of Ti-carbide par-ticles stabilizes the grains during the material’sproduction process This generates an isotropicgrain structure and has the additional effect

recrys-of keeping a fine grain structure even in therecrystallized condition, but the alloy is moreexpensive After recrystallization, the finerdispersoids of TiC particles improve the low-temperature impact toughness of refractoryalloys following low-dose neutron irradia-tion.71,87–89,149–154Other carbides, for example,ZrC155,156or HfC (in combination with Re andMo),96can be used instead of TiC

 K-doped W is a nonsag material that contains amaximum of 40 ppm of potassium.40Originallyknown from the lighting industry, it provides highcreep strength due to its aligned pore structure,high recrystallization temperature >1600
C,and good machinability.68,77,78,157

 W–Si–Cr as a ternary or even by the addition

of another element as a quarternary alloy is a

newly developed and not yet optimized

mate-rial that is being investigated as a wall

protec-tion material due to its favorable oxidaprotec-tion

resistance, preventing excessive material

ero-sion in case of accidental air ingress.158,159

 Severe plastically deformed W alloys offer, similar

to pure tungsten (see above), significantly

improved fracture toughness and ductility.61,84

The addition of alloying elements to the starting

material (any developmental or commercial

produced W alloy), such as Re or dispersoids,

leads to an increasing stability of the grains and

therefore a higher recrystallization

tempera-ture and less grain growth.133

Any of the bulk materials mentioned above could be

used and are being investigated in its cold-worked,

stress-relieved, or recrystallized state The latter is

of particular interest due to in situ recrystallization of

surface near regions during operation.108

In spite of the fact that a large variety of tungsten

grades and alloys already exist, the attempts to

fur-ther optimize these materials are ongoing The

fabri-cation and successful testing of He-cooled divertor

mock-ups for DEMO and ARIES-CS102,160 under

a heat flux of 10 MW m2are important driving forces

for the present development of W alloys with improved

performance in the fusion environment.25 However,

R&D has to address many different issues related

to the performance of the material when exposed to

thermal loads, neutron irradiation, and the plasma;

these will be discussed in the following section

4.17.4 Influence of In-Service

Conditions

4.17.4.1 Thermal Shock Resistance

Tungsten-armored PFCs will be subjected to

differ-ent types of heat fluxes dependdiffer-ent on their field

of application (see Section 4.17.2) Among others,

this includes thermal transient loads (e.g., ELMs

and disruptions) The behavior of the material

under these conditions, that is, the combination of

cyclic steady state and transient heat loads, is a key

factor that has to be considered for the selection of a

suitable grade of W

The machines simulating these operational

conditions are electron and ion beam facilities,

quasi-stationary plasma accelerators, plasma guns, and

high-energy lasers A most critical issue is the

compa-rability of such simulations Therefore, a round robin

test involving some representative facilities was madefor investigating the influence of the different timeregimes and different power density levels The resultsshowed that when compared on the basis of a heat fluxparameter P (MW m2s1/2), which is directly propor-tional to the temperature increase, the cracking andmelting thresholds are almost identical This permits

a direct transfer of the qualitative results obtained

in any of these facilities.161 In contrast, quantitativeresults representative of the operational conditions inlarge fusion devices can only be obtained when theloads are applied in the desired time range The reasonfor this is the heat penetration depth and the relatedstress field that is produced, which influences crackand melting depth

There are several parameters influencing thethermal shock behavior of tungsten that will be dis-cussed in the following sections for the differentmaterials under disruption and ELM-like loads.4.17.4.1.1 Microstructure, composition, andmechanical properties

During thermal shock loads, steep temperaturegradients of hundreds to several thousand degreesCelsius on a length scale of millimeter or evenmicrometer (depending on the pulse length) areformed, influencing only a limited volume near theloaded surface While the heat load is applied, due tothermal expansion and the decreasing strength of thematerial at the surface compared to the bulk material,compressive stresses are formed in the surface plane.These stresses can lead to permanent plastic defor-mation that might, during cool down, generate tensilestresses sufficiently high to initiate crack formationperpendicular to the surface and thereby cause stressrelaxation at the surface

Depending on the mechanical properties in thesurface plane, the amount and starting point ofcrack formation can be influenced Based on thisand on the fact that the mechanical properties arestrongly dependent on the material’s microstructure(see Section 4.17.3.2.3), a grain orientation parallel

to the surface and therefore high strength in thesurface plane might be preferred.162However, grainsoriented parallel to the surface, such as in rolledmaterials or plasma-sprayed coatings, might result indelamination (see Figure 3(a)), which causes over-heating and subsequently surface melting if they have

a lower strength in the depth direction and exhibitpreferential cracking along the weak grain boundaries.Therefore, a grain orientation perpendicular tothe surface and parallel to the direction of the heat

flow is recommended.90 This will cause cracks to

form along the grain boundaries toward the cooling

structure (seeFigure 3(b)) causing no degradation or

only a negligible degradation of the material’s

ther-mal transfer capabilities Due to the lower

mechani-cal properties in the surface plane, more or larger

cracks will form during thermal shocks, running

per-pendicular to the surface and following the grain

orientation

In contrast to deformed materials, crack formation

and crack orientation in materials with isotropic or

almost isotropic grain structures, for example, MIM-W

or recrystallized W, is rather unstable and is strongly

enhanced for the weakened recrystallized material

Depending on the applied power densities, the formed

temperature gradient, and the resultant stress fields

within the material, cracks initially running

perpen-dicular to the surface might deflect at zones with

compressive stresses and keep running parallel to

the surface (seeFigure 4)

4.17.4.1.2 Power density and pulse duration

The material’s response is strongly related to the

applied temperature fields and by this to the absorbed

power density and the pulse duration This results in a

material-related surface temperature increase and

heat penetration depth.163 A classification of the

impact of the temperature field is made by establishing

three parameters: the damage, the cracking, and the

melting threshold While the latter depends on the

ther-mal conductivity and the melting temperature (for

alloys or mixed materials formed during tokamak

operation) of the material, the damage and crackingthreshold are determined mainly by the material’smechanical properties Damage here means that thematerial’s surface has undergone a visible and measur-able modification, for example, by surface roughening,recrystallization, or pore/void formation

4.17.4.1.3 Base temperatureThe base temperature influences the thermal shockbehavior in two ways First, a higher base tempera-ture influences the damage, cracking, and meltingthreshold All of them are essential because theylimit the operational conditions and when exceededcause enhanced material degradation Therefore, life-time estimates based on RT data will yield unrealisticconclusions

Second, crack formation strongly depends onthe plastic deformation at high temperatures andeven more on the stress developed during cooldown To understand the influence of a higher basetemperature, one has to be aware of the typical shape

of the yield and tensile strength curve for W or a

W alloy.105,157While the decrease in strength is ratherhigh at low temperatures, the curve flattens at hightemperatures despite a drop in strength when exceed-ing the recrystallization temperature As a result, thehigh temperature plastic deformation induced by thecombination of a heated surface and ‘cool’ base materialcan be significantly reduced by a small increase inbase temperature Combining this effect with theincreased ductility of W at the given base temperature,brittle crack formation can be avoided when heating

(a)

(b)

200 mm

500 mm

Figure 3 Light microscopy images of the etched

cross-sections of thermal shock–loaded specimens with

grains oriented (a) parallel and (b) perpendicular to the

loaded surface; cracks follow the grain orientation/

deformation direction.

100mm Figure 4 Light microscopy images of the etched cross-sections of thermal shock–loaded metal injection molding tungsten with isotropic grain structure.

the material above a certain threshold.82,157,164,165

This temperature threshold is related to the DBTT

but is not necessarily identical to it

4.17.4.1.4 Repetition rate

In addition to the parameters mentioned above, the

damage, cracking, and melting thresholds are

deter-mined by the number of load repetitions, because of

continuing material degradation such as hardening

and recrystallization This is of particular interest

for short transient events with a high repetition

rate in magnetic (ELMs) and inertial fusion devices

Up to now the simulation of submillisecond events

(ELMs, IFE) has been performed only up to a

rela-tively low number of cycles; large numbers of pulses

(e.g., >106

ELM pulses during the life-time of the

ITER divertor) are not feasible in the majority of

the above-mentioned test facilities

4.17.4.1.5 Thermal shock during off-normal

events: disruptions

Disruptions still occur frequently in operating

tokamaks, and therefore they are also expected for

ITER with an anticipated occurrence in 10% of

the ITER pulses (3000 pulses per expected

compo-nent lifetime) During a disruption in which the

plasma undergoes a partial or full thermal quench,

most of the plasma thermal energy will be dumped

on the divertor plates.166 Taking into account the

resultant loading conditions (see Section 4.17.2),

significant material loss from the tungsten

plasma-facing surface should occur by melting and

evapora-tion particularly in the dome area.167,168In simulating

these events, the amount of melting, the melt motion

and subsequent roughening of the surface, the

mate-rial erosion by droplet emission, the resolidification

behavior, and finally, the crack formation occurring

in the loaded area or at the boundary between melted

and unmelted zone are the most important

para-meters to be determined

The underlying mechanisms for the

above-mentioned material degradation are well described

(see Figure 5).169Thermal loading of tungsten and

metals, in general, at ‘moderate’ energy densities (up to

a few MJ m2) will result in a homogeneous, localized

melting of the sample surface When higher energy

densities are applied, surface evaporation occurs; the

momentum transfer due to evaporating atoms from the

surface generates an effective pressure on the melt

layer, which finally results in the formation of a melting

ridge Increasing the incident energy density even

fur-ther, the material’s response is characterized by intense

boiling and convection of the melt layer resulting indroplet formation and ejection.170–172 Open pores inthe recrystallized material have a strong impact on thethermophysical properties

The melting threshold and subsequently theamount of melt formation depend on the material’sthermal conductivity, which is lower for porousmaterials such as plasma-sprayed tungsten, and fortungsten alloys In particular, it has to be taken intoaccount that dispersoids such as La2O3(Tm¼ 2578 K)have a lower melting temperature than tungsten Thismay result in early melting and increased evaporationcausing the formation of a porous and depleted sur-face layer, which becomes even more important whenapplying loads below the melting threshold (seebelow and Section 4.17.4.1.2) On the other hand,the melting threshold is correlated with the basetemperature of the PFM When the base temperatureincreases, the melting threshold energy decreasesand the amount of melt formation, the obtained cra-ter depth, and the evaporation losses for the sameapplied loading conditions increase significantly.169

As it cools, the material resolidifies in a recrystallizedstate providing a columnar grain structure typical ofPVD or CVD coatings With further cooling, depending

on the base temperature of the material/component(see ‘Base Temperature’ inSection 4.17.4.1.3), brittlecrack formation will not take place above a certainthreshold temperature However, with fast coolingafter loading below this temperature, the materialwill undergo severe cracking with crack lengths thatcan reach the order of millimeters.169

When the qualification of different W grades andalloys108,141,147 is done in combination with thermalfatigue loading,90 materials with high thermal con-ductivity in combination with superior mechanicalproperties, that is, with high ductility, performedbest with regard to melt material loss and crackformation This comprises low-alloyed W materialswith increased ductility such as W–Re or W–Ta, orfine-grained pure W or W alloys

Cracking roughening

Homogeneous melting

Melt ejection Boiling and

droplet formation

Increasing energy density Figure 5 Performance of tungsten and metals in general under transient thermal loads.

Disruption simulation experiments on bulk

tungsten and tungsten coatings have also been

described in the literature These were performed

not only to investigate the melting behavior but

also for the purpose of characterizing the cracking

behavior.26,42,60,101,122,130,131,162,165,173–176 Although

these experiments are more related to those on the

characterization of ELM conditions (see Section

4.17.4.1.2) and were often performed only at RT,

the results indicate that the use of highly ductile

SC materials is preferred.90,177 Alternately, in case

of cheaper polycrystalline materials, it is necessary

for the material to have the proper microstructure

orientation as described above, that is, the grain

ori-entation perpendicular to the loaded surface The

reason for this is that crack formation occurs mainly

along the grain boundaries and follows the

orienta-tion of the deformed microstructure The crack depth

is, in general, related to the applied loading conditions

and therefore the pulse length, which determines the

heat penetration depth and the temperature and

stress gradient induced during loading The

temper-ature gradient also determines the recrystallization

zone, which is generated below the loaded area as a

function of temperature (<Tm) and time

However, the quantification of the applied

condi-tions and by this a comparison of the materials

response is often not straight forward as each testing

facility has its own characteristics Most of the time the

cited incident power density, for example, in Hirooka

et al.,174Linke et al.,178and Makhankov et al.,90does not

correspond to the absorbed power density For

exam-ple, with an electron beam at 10 keV, Pabs 0.62

Pinc179 – with the ratio slightly decreasing at higher

acceleration voltages In a plasma accelerator, Pabs

depends on incident angle and for a perpendicular

impact might only reach 0.1 Pinc.180For a rough

esti-mate of the temperature impact, the given conditions

can be compared to the heat flux parameter introduced

above For a base temperature of RT, this amounts to a

melting threshold of60 MW m2s1/2for pure and

fully dense tungsten Due to the fact that this

parame-ter163is also directly proportional to the thermal

con-ductivityl, the specific heat cp, and the densityr:

P ffiffi

t

p

¼DT2

ffiffiffiffiffiffiffiffiffiffiffiplcprq

a decrease in thermophysical properties consequently

reduces the heat flux parameter and the melting

threshold

As all performed investigations indicated that

melting will cause increased material degradation

and the continuous erosion of the PFM, it will icantly limit the lifetime of the PFCs Therefore, thesafe and economic operation of a future fusion reac-tor requires that scenarios causing melt formationhave to be limited to a minimum

signif-4.17.4.1.6 Thermal shock during normaloperation: ELMs

In contrast to disruptions, ELMs occur during mal operation in the H-mode and are characterized

nor-as instabilities caused by the steep temperature anddensity gradients at the plasma edge, which deposit asignificant amount of energy at a high repetitionrate.181,182In particular, it is the expected high repe-tition rate for ELMs during the lifetime of the PFC(>1 million of events at a frequency of 1–25 Hz183

)that, although yet unexplored, will impose highdemands on the PFMs

While it is the desire of plasma physicists to ate in H-mode regimes with high-energy ELM depo-sition (1 MJ m2), the response of bulk tungsten,tungsten coatings, and tungsten alloys to such loadingconditions, that is, surface melting, melt motion,material erosion, and vaporization,170,171,184–189 isdetrimental To obtain further insight into materialbehavior under these conditions, modeling of experi-mental conditions was carried out.9,167,168,190–195 Ithas been shown that with regard to melt motion/erosion, the results of the different facilities cannot

oper-be directly compared196and none of the testing ities used provides identical conditions to those thatwill occur in a tokamak However, mitigation techni-ques have been explored for reducing the appliedELM energy, which, in general, can only be done atthe expense of a higher repetition rate.183The extent

facil-to which the ELMs have facil-to be mitigated depends onthe melt formation at tile edges due to the shallowplasma impact, which was experimentally found to bebetween 0.4 and 0.6 MJ m2 for pure forged tung-sten.189,197 On the other hand, the effect of crackformation during ELMs on the lifetime behavior ofthe PFCs has to be taken into account As mentionedbefore, this behavior is yet unexplored at high repe-tition rates

Typical investigations on various grades of

W,82,187,189,198 coatings21,54,186,199 and alloys146,157,187were in the range of 10–100 repetitions In a few cases

up to 1000 repetitions and in single experiments even

on the order of tens of thousands of repetitions havebeen obtained, depending on the testing facility used

As the repetition rate is still rather low compared tothe expected millions of events, the main interest of

these investigations was the qualification of different

W grades and alloys (see Section 4.17.3.3) with

regard to their damage and cracking thresholds The

characterization was done as a function of the main

parameters described in Section 4.17.4.1, that is,

microstructure, power density, and base temperature

The results obtained so far showed that crack

formation200 vanishes above a certain base

tem-perature (see Figure 6).82,157,198 This temperature

decreases with increasing material ductility,

indicat-ing that the use of W alloys or fine-grained W is

preferred In the case of an anisotropic microstructure,

this effect strongly depends on the material’s

orienta-tion Better results are obtained for grain orientations

parallel to the loaded surface (seeSection 4.17.4.1),

yielding differences in the threshold temperature

compared to the orthogonal direction of up to several

hundred K (cf Figure 6(a) and 6(b)) Recrystallization

leads to a slight homogenization of the material’s

microstructure and therefore the mechanical ties; however, there is no full convergency of theorientation-dependent thresholds.82

proper-Despite the fact that for the currently limitednumber of applied pulses no crack formation wasobserved above a material and orientation-dependenttemperature, the material is still damaged by plasticdeformation and surface roughening The evolution

of this plastic deformation and of the related materialhardening as a function of the applied number

of loads is still unclear and has to be investigated.However, there are also heat load levels (at least

up to Tbase 800
C), at which no visual materialdegradation could be determined and the futuregoal will be to investigate if these damage thresholdsare still valid for high repetition rates, at higher basetemperatures, and particularly in combination withneutron irradiation (seeSection 4.17.4.3) and plasmawall interaction (seeSection 4.17.4.4)

All the information given above on the effect ofELMs is also directly transferable to the short tran-sient events expected for inertial fusion applicationsand has been verified by IFE-related tests on dif-ferent W-based materials.201–204 There are coatingparameters of high interest besides those mentionedabove; these include the manufacturing-inducedresidual stresses at the surface, which are dependent

on the used substrate, and the coating thickness Asmentioned inSection 4.17.4.1.1, the applied loadingconditions and therefore the pulse length determinethe heat penetration depth.163As a result, the tem-perature and stress gradient induced under IFEapplications should be similar to those in X-rayanodes (seeSection 4.17.2) In case of thin coatings,residual and induced stresses might affect the coating

to substrate interface and could lead to interfacialcrack formation and delamination This leads tominimum requirements for coating thicknesses thatdepend on the applied loading conditions.54 Forexample, in industrially produced X-ray anodes,W–Re coatings are typically used with a thickness

of 200–700mm205,206

to provide better mechanicaland thermal-shock properties compared to pure W.204However, the first experience on the influence ofELMs on coatings under real plasma operational con-ditions will be gained in the ITER-like wall project

in JET, which involves testing relatively thin tungsten coatings (14–20mm) on a CFC substrate thatprovides a strong and anisotropic CTE difference.19,142The behavior of this material under the aboveoutlined transient heat loads is of course a key factorfor the lifetime assessment of PFCs However, the

Figure 6 Thermal shock testing results of double forged

W as a function of temperature and the heat flux parameter;

grain orientation (a) perpendicular and (b) ‘parallel’

(one direction still perpendicular, indicated by the

orientation of the large cracks) to the heat flux.