a review of dust in fusion devices

A review of dust in fusion devices: Implications for safety andoperational performance

J.P. Sharpe a,, D.A. Petti a, H.-W. Bartels b

a Fusion Safety Program, Idaho National Engineering and Environmental Laboratory, PO Box 1625, MS 3860, Idaho Falls, ID 83415

3860, USAb ITER International Team, IPP-Garching, Garching, Germany

Abstract

Dust is produced in fusion devices by energetic plasma/surface interactions. As the amount of dust increases,potential safety and operational concerns arise. The dust may contain tritium, may be radioactive from activation

products, and may be chemically reactive and/or toxic. Possible accidents in large fusion reactors could mobilize the

dust and threaten public safety. Dust also poses potential problems to device operation. For example, plasma startup

could be impeded, particulate injected from flaking deposits may disrupt the fusion plasma, and tritium retention in

dust will affect fuel recovery systems. The current understanding of dusts role in fusion devices is reviewed in this paper

by discussing mechanisms of dust production, considering ways dust impacts device safety and operation, and

comparing characteristics of dust collected from existing fusion plasma research devices.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Tokamak dust; Safety analysis; Fusion aerosols; Plasma/surface interaction

1. Introduction

By the very nature of its operation, a fusion

device generates aerosol particulate, broken flakes,

globules, chunks, and other debris, that may

ultimately affect its safety and operational perfor-

mance. Particulate matter of this type, commonly

referred to as dust, does not strongly adhere to

surfaces and is capable of being mobilized. Evi-

dence of the existence and impact of dust is readily

found in current fusion plasma research devices.

Streaking particles are often observed on images of

tokamak plasma discharges, particularly during

startup and periods of intense plasma/surfaceinteraction. Break-up of flakes or particles from

arcing are injected into the plasma and rocket

along field lines. Films of re-deposited material are

found in the proximity of intense plasma/surfaceinteraction. Disruptions and other off-normal

events cause significant erosion, and a sizeable

portion of the eroded material generates dust.

Several different mechanisms produce dust of

various shapes and characteristic sizes, as demon-

strated in Fig. 1. Dust generated from the material

may be copious, radioactive, chemically reactive,

Corresponding author. Tel.: /1-208-526-9830; fax: /1-208-526-2930

E-mail address: [email protected] (J.P. Sharpe).

Fusion Engineering and Design 63/64 (2002) 153/163

www.elsevier.com/locate/fusengdes

0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 1 9 1 - 6

and/or chemically toxic, thereby, posing significant

safety hazards should the dust be mobilized in an

accident or during maintenance. Shape, size, and

number (mass) are key factors in determining

dusts role in safety and its impact on machine

operation. Even for normal operation, a dirty,

dusty device is undesirable. Deposits of dust may,

for example, accumulate in the grooves of first-

wall components, diminishing its effectiveness and

possibly shortening its lifetime. The effectiveness

of wall conditioning may be compromised by the

presence of foreign particulate matter. Off-normal

events, such as vertical displacement events in

magnetic fusion systems of or low-yield shots in

inertial fusion systems, may generate dust concen-

trations locally that are mobilized and adversely

affect the overall system. The impact of dust in the

chamber has recently been recognized as an issue

that must be addressed for future fusion devices.

This paper reviews our current knowledge about

the role of dust in fusion devices. Various mechan-

isms responsible for dust production are ad-

dressed. Safety and operational concerns are

discussed for next step fusion devices. Results of

recent collection activities are compared to show

similarities and differences in dust characteristics.

Dust from a variety of fusion research devices has

been analyzed: the tokamaks of DIII-D, TFTR,

ALCATOR-Cmod, Tore Supra, and ASDEX-

Upgrade, the LHD heliotron stellarator, and the

Nova laser chamber representative of an IFE

system. Finally, directions of future dust research

are considered.

2. The origin of dust in fusion devices

Many mechanisms are responsible for genera-

tion of particulate in fusion devices, and different

mechanisms can dominate in various circum-

stances. The effectiveness of each dust generation

mechanism is characterized by the amount of

energy available for mobilization. Possible me-

chanisms in magnetic fusion systems include

blistering and fracturing of deposited layers, gen-

eration of reactive species in edge plasmas, arcing,

explosive ejection and brittle destruction of surface

imperfections, and nucleation of vaporized mate-

rials. Similar mechanisms are expected in inertial

fusion systems, since large heat loads will be

incident on surrounding surfaces, and additional

mechanisms are being identified for various wall

protection schemes. Many more magnetic fusion

devices are presently available to study dust;

hence, we will focus on the origin of dust in these

devices.

Materials exposed to a flux of energetic plasma

particles erode by physical and chemical sputtering

[1], and erosion rates can approach 1 m per year

for carbon machines [2]. Most of the eroded

material is re-deposited at or close by its origin,

although some material is transported and depos-

ited in layers at more distant, and generally cooler,

locations. These layers are generally stratified from

deposition during cycles of plasma operation,

weakly adhered to the underlying surface, and

subject to break-up by mechanical and thermal

stresses. Properties of re-deposited layers in carbon

Fig. 1. Representative SEM micrographs demonstrating various particulate shapes of dust collected from fusion devices.

J.P. Sharpe et al. / Fusion Engineering and Design 63/64 (2002) 153/163154

machines strongly depend on local plasma and

wall conditions; the coatings may be brittle and

contain mostly carbon atoms, or softer polymeric

coatings of less adhesion. Metals or other materi-

als will deposit along with carbon, so that flakes

are often composed of mixed materials. Thin films

are also applied for the purpose of wall condition-

ing. Spallation and flaking of the layers of

deposited material produce dust particles whose

size depends on the stress mechanisms and local

deposit structure. Dust produced by flake fractur-

ing generally does not display indications of

particle agglomeration or growth.Dust growth may occur in the edge plasma,

where conditions resemble those of reactive plas-

mas [3]. Significant quantities of carbon, oxygen,

and impurities from wall conditioning are located

in the edge plasma, and hydrocarbons are present

from chemical erosion of graphite walls. Low

electron temperature (B/5 eV), low plasma density(/1017 m3), and high neutral concentrationslead to possible growth of macromolecules by

multiple ion-molecule interactions, leading to

nucleation and growth of particles up to 1 mm insize. Negative ions formed by electron attachment

to the particles are confined in the plasma by the

sheath at the wall [3]. This process is balanced by

photo-detachment from ultraviolet photons to

establish a local, geometry-dependent equilibrium.

Helium discharge cleaning and plasmas used for

thin-film wall conditioning produce a similar

environment for dust production. The amounts

and characteristics of dust formed by this mechan-

ism can be investigated with dedicated diagnostics

in current tokamaks.

Unipolar arcs, often generated during the

plasma startup or rapidly varying plasma currents

in tokamaks, locally deposit significant energy

onto a material surface (e.g. first wall). The

interaction results in melting and vaporization of

the material, a process that liberates large particles

and molten drops. Field interactions move the arc

along the surface, creating tracks of damaged

material. Dust produced from arcing is more likely

to be spherical and composed only of material in

the source because the impact of mixed materials

on growth for this mechanism is negligible.

Off-normal plasma events (e.g. edge localizedmodes, vertical displacement events, or full dis-

ruptions) that deposit enormous amounts of

plasma energy on a surface are known to generate

particulate. Heat fluxes of several GW m2 for

100 ms have been observed from edge localizedmodes in Joint European Torus (JET), and the

The International Thermonuclear Experimental

Reactor (ITER) divertor is designed to withstand100 GW m2 for 10 ms during disruptions. Rapid,

intense heating of exposed material results in

vaporization and melting. Dust particles may be

created by in-flight condensation of the vaporized

material [4], pressure-driven ejection of melt layer

material [5], and explosive brittle destruction by

heating of gas pockets near the materials surface

[5]. Significant amounts of small dust particles (ca.B/1 mm) are produced from the violent plasma/surface interactions, and these particles can ag-

glomerate into larger, more complex structures [4].

The design of next step devices must limit the

number off-normal events to maintain dust in-

ventories at reasonable levels, and/or develop in-

situ techniques to remove the dust.

3. Safety and operational performance issues

The existence of dust in current fusion plasma

devices is well known and accepted as a matter of

little concern. Only when a few machines ran D-T

experiments did the role of dust become important

because of its ability to retain tritium. It is now

recognized that in next step devices, dust will playan important role in determining their safety and

operational performance. Much greater plasma

energies and subsequent material erosion rates will

produce quantities of dust much greater than what

is found in machines today. Realization of the

possible public safety consequences from acciden-

tal dust mobilization prompted greater attention

to dust in the safety analyses of high energydensity machines (e.g. ITER, FIRE, NIF). Dusts

impact on the operation of such machines remains

uncertain.

Safety concerns of dust in future machines

include radiological hazard, chemical toxicity,

and chemical reactivity [6]. The public and per-

J.P. Sharpe et al. / Fusion Engineering and Design 63/64 (2002) 153/163 155

sonnel radiological hazards derive largely from

activation of structural material by fusion neu-

trons and retention of the tritium fusion fuel.

Various materials needed in future fusion reactors

contribute to activation, tritium retention, and

chemical reactivity in varying degrees. For exam-

ple, ITER is proposing to use differing amounts of

tungsten, carbon, and beryllium in different re-

gions as plasma facing components. With tungsten

dust, the greatest concern is its radiotoxicity

because of the high activation of tungsten. For

carbon dust, retention of large quantities of

tritium is problematic. For beryllium dust, its

chemical reactivity with steam leading to the

production of hydrogen is of primary importance,

followed by the chemical toxicity of beryllium

oxide. The specific radiological hazard is deter-

mined by the amount of dust mobilized and

transported from the facility in a given accident

scenario. Mobilization and transport of the dust

are determined by properties such as size and

shape. Therefore, accurate safety analyses must

reliably consider dust quantities and morpholo-

gies.

Dust is also chemically reactive if combustible

gas is generated during interaction with coolants

or air from a vacuum leak. Reaction rates are

dependent on dust amount, size, exposed surface

area, and temperature. Dust generation mechan-

isms in tokamaks give rise to particles of large

surface area. High temperatures associated with

some postulated accidents in water-cooled fusion

devices can cause significant hydrogen generation

rates, leading to explosive concentrations. Fig. 2

shows the relative generation rate of hydrogen

versus temperature for beryllium, graphite, and

tungsten dust reacting with steam. Significant rates

are achieved by beryllium at moderate tempera-

tures possible in some accidents. Another chemical

reactivity concern is rapid dust oxidation upon

exposure to air, similar to dust explosions that can

occur in carbonaceous dust in mines and agricul-

tural dust in grain silos. Availability of large

reactive surface areas enhances this reaction.

Determination of dust inventory limits for future

fusion reactors will require careful consideration

of these issues.

ITER design project was the first time that

safety concerns of a larger fusion reactor were

addressed at levels appropriate for regulatory

review. Knowing dust will exist in the ITER

vacuum vessel, and recognizing the associated

radiological and chemical hazards, safety analysts

took an approach to ensure public safety by

developing strategies to confine the dust and limit

chemical reactions by limiting the total dust

inventory [7]. The safety approach developed for

ITER is based on assigning administrative guide-

lines for the maximum tolerable amount of dust

mass at locations inside the vacuum vessel. Two

inventory limits have been defined, the total

amount of dust in the vacuum vessel available

for accidental release is limited due to radiological

and toxicological concerns and the quantity of

dust residing on hot surfaces is limited due to

steam chemical reactivity concerns. Success of this

safety strategy requires development of reliable

methods for dust monitoring and removal. The

administrative guidelines are compared with esti-

mates of the dust production rates for material

type, location, plasma pulse length, and disruption

frequency. Historically, there was a trend estab-

lished during the ITER Conceptual Design Activ-

ity to describe the dust hazard by the cumulative

amount of dust mass in the machine. The initial

Fig. 2. Hydrogen generation rate per gram of 0.5 mm dustparticles reacting with steam at various temperatures likely in

an accident. Be-1 and Be-2 represent two different fits to data.

Adapted from [6].


expectation was that the mass might be limited to

about the kilogram level. This was proven to be

too optimistic, since in a single disruption several

kg of dust may be produced [7]. Even a 10 kg dust

limit seemed an unreasonable constraint on ITER

operation. Therefore, the 100 kg level was intro-

duced early in the ITER Engineering Design

Activity as the dust limit. Distributed over an

area of 200 m2, or roughly the area below the

divertor, this amount of dust would result in a

layer of thickness (assuming 50% dust density

compared with the bulk density of the original

material): tungsten dust, /50 mm; beryllium andgraphite, /500 mm. Layers of this thickness aregenerally observable with optical systems. A

comprehensive safety analysis has shown that the

ITER confinement design keeps environmental

releases well below established release guidelines,

e.g. 50 mSv dose limit, used to address hypothe-

tical accidents in the assessment of the ultimate

safety margins. The ITER confinement releases no

more than 50 mSv dose at the site boundary for a

total dust inventory of 100 kg. In addition, an

assessment for 350 kg of tungsten dust (of

significant radiological hazard in ITER) has been

performed to add margin to the uncertainties

associated with the 100 kg limit. The safety reports

of ITER are now based on this higher assessment

value of 350 kg, and therefore, add conservatism

and margin for operation of ITER. Smaller

inventory limits have also been placed on hot

surfaces where hydrogen generation could become

a concern. For example, only 6 kg of beryllium

dust may be on a hot surface such that the

hydrogen combustion limit is not exceeded. These

values are based on observed dust size distribu-

tions and specific surface areas measured from

existing tokamaks and plasma gun experiments,

and the extensive chemical reactivity databases for

beryllium, carbon, and tungsten (see, for example,

[6]). These dust limits are based on dust with a

count median diameter (CMD) of 0.5 mm, ageometric standard deviation (GSD) of 2, and a

specific surface area of 4 m2 g1. Safety factors are

incorporated into the limits to account for un-

certainties in the current understanding of dust in

fusion machines.

In addition to safety issues, dust may presentoperational difficulties in future fusion reactors.

Retention of tritium is not only a safety concern

but also a concern for machine operation because

fuel captured in the walls and in dust is not

utilized. Tritium recovery/clean-up systems have

historically not considered tritiated particulate;

therefore, on-site tritium inventories are greater

than previously estimated.Another issue arises from dust collecting in gaps

and crevices on an engineered surface, strongly

affecting the performance of the surface. Effi-

ciency of heat transfer from the surface of an

actively cooled component may be greatly reduced

when layers of dust with poor thermal contact are

present. Dust present on windows or view ports

may diminish the effectiveness of optical diagnos-tics, valve binding is problematic as dust deposits

on valve seats [8], and fouling of cyropumps may

occur since dust is not removed during pump

regeneration.

Particles flaking off of upper tiles during a

plasma discharge could induce a disruption. Ex-

periments have been performed [9] to study the

effect of carbon particles dropped from aboveonto a plasma discharge. Although roughly 106

particles with diameters B/2 mm did not affect afully developed discharge, particles present during

plasma startup increased impurity radiation. Most

dust particles fall to the bottom of the device

following the plasma discharge [2]. Smaller parti-

cles may be re-injected into the plasma by electric

and magnetic forces, creating difficulties forplasma breakdown and burn through. As the

dust particles are vaporized, the partially ionized

atoms radiate power and significantly increase

plasma resistivity. Greater resistivity requires a

larger loop voltage for startup; the superconduct-

ing coils likely to be used on next step machines

may be incapable of the increased voltages [2].

Plasma initiation by the decay of tritium re-tained in dust particles is another possibility [10],

an effect that complicates controlled plasma

startup. Investigations of these and other issues

of dusts impact on fusion reactor operation are

currently underway on dedicated experiments;

there is much to be gained in developing a detailed

understanding of dust interactions on fusion


plasmas. Given the potential for large quantities ofdust in next step devices, methods for in-vessel

clearing of dust are also being examined.

4. Comparison of dust from various fusion machines

Dust has been collected and characterized in a

number of fusion research devices, including:DIII-D [11/13], TFTR [13,14], Alcator-Cmod[13,15], JET [16], TEXTOR [2,3], Tore Supra

[17,18], ASDEX-Upgrade [19], LHD [20], and

the NOVA [21] laser facility. Collection opportu-

nities occurred during periods of scheduled main-

tenance, when the vacuum chamber is vented and

personnel may gain access. In addition, a few

dedicated experiments to investigate dust in fusionsystems have been carried out [22], and several

more are being planned.

Systematic collection routines were used to

sample various locations within these devices,

with specific attention to the lower regions of their

chambers. Experience has shown these areas

collect the greatest amount of particulate during

plasma operation. Positions were selected in alldevices based on the expected mass concentration

due to gravitation settling, proximity to important

structures (e.g. antenna armor or NBI beam

dumps), and significant thermal, magnetic, or

power flux gradients in the vicinity of the surface.

Comparison of particle size distribution, surface

mass concentration distribution, composition and

shape, and specific surface from various machinesarea are discussed in this section.

4.1. Particle size distributions

Distribution of the sizes of dust particles cap-

tured by filtered vacuum collection from different

fusion devices is generally obtained by optical

microscopy imaging. Sizing and counting a popu-

lation of particles with the optical microscopeprovides a size distribution of particles projected

area equivalent diameters, and moments (CMD

and GSD) of this distribution are reported. Count-

based distributions based on projected area mea-

surements do not, however, include the effect of

particle shape. A distribution derived from parti-

cles that are generally flat (e.g. small flakes) or

fibrous would produce errors in transport calcula-

tions. Most dust found in present fusion devices,

however, does not strongly deviate from spherical

surface-to-volume ratio (see Fig. 1). Agglomerates

are the exception, although generally their consti-

tuents are also nearly spherical and they do not

constitute a large fraction of total aerosol mass.

A size distribution generated by optical micro-

scopy and count analysis of typical dust from a

fusion machine appears in Fig. 3. This distribution

was obtained with dust collected at the lower

divertor region of ASDEX-Upgrade. The figure

displays measured size frequency data along with

the fitted lognormal distribution; agreement of the

analytical fit is acceptable (linear correlation

coefficient R2/0.99132). Nearly 5700 individualparticles were sized and counted to build this

distribution, providing a statistically accurate

representation of the underlying distribution of

particles collected on the filter substrate. The large

majority of dust samples analyzed with this

technique yielded nearly lognormal distributions,

although some samples showed what could be

interpreted as bi-modal distributions (two resol-

vable peaks in size frequency).

Comparison of average dust sizes from different

regions of the various fusion devices is given in

Fig. 3. Typical count-based size distribution of dust from a

fusion device. This particular distribution was obtained from

dust collected at the very bottom of the ASDEX-Upgrade

vessel, behind the lower divertor structure.


Table 1

Comparison of average dust sizes and surface concentrations from various fusion devices

Machine Lower regions Middle regionsa Upper regions

Collected

mass (mg)

CMD (mm)9/GSD

Surface mass den-

sity (mg m2)

Collected

mass (mg)

CMD (mm)9/GSD

Surface mass

density (mg m2)

Collected

mass (mg)

CMD (mm)9/GSD

Surface mass

density (mg m2)

DIII-D 3.34 0.669/2.82 23.5 18.5b 0.609/2.35 896b 4.20 0.899/2.92 8.40TFTR 32.0 0.889/2.63 / 67.0 1.609/2.33 / / / /Alcator-

Cmod

40.2 1.589/2.80 5470 1.73 1.539/2.80 87.0 0.48 1.229/2.03 66.5

JET / 279/(/) 1300 / / / / / /TEXTORc / 5.209/(/) / / / / / / /Tore Supra 14.3 2.689/2.89 595 1.97 2.989/2.94 31.9 0.55 3.329/2.94 5.33ASDEX-Up-

grade

116 2.219/2.93 1300 2.54 3.699/2.81 55.7 1.25 3.599/3.08 28.3

LHD 0.80 8.599/2.67 58.7 0.76 6.319/2.39 168 1.07 8.739/2.09 247NOVA 1165 1.129/1.90 24 800 3.4 0.769/2.03 36.8 4.45 0.909/1.93 48.0

Data are grouped according to relative poloidal location.a Middle regions include inside surfaces of ports*/a location where debris can easily settle.b This value resulted from shards of glass being collected from a diagnostic port where a plasma probe had broken.c Observation of much smaller particulate occurred, but analysis details are not given in [3].

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Table 1. The overall average particle size for thisdata is 2.89/2.4 mm, however, because of thelognormal nature of the size distribution dust

particles in fusion devices from 0.5 to 10 mm insize (by count) may be expected. If mobilized in an

accident, these particles are easily transportable.

The similarity in dust size among these different

machines indicates similar processes are likely

involved in their production, primarily via con-densation of material eroded during plasma/sur-face interactions.

4.2. Surface mass density

The total dust mass collected from individual

locations when divided by the sampled area gives

the surface mass density at that location, and the

product of surface mass density and the totalcomponent area (e.g. vacuum vessel floor area,

port area, etc.) provides an estimate of the

quantity of dust existing at that component.

Summing these dust inventories from various

components roughly indicates the total amount

of dust in the fusion device.

Table 1 includes average values of collected

mass and the associated surface mass density forseveral fusion machines. Comparatively large

quantities of dust were collected in the TFTR,

ASDEX-Upgrade and Alcator-Cmod chambers,

with nearly all mass collected in the lower region.

The collection of material from a broken probe

resulted in greater than expected mass concentra-

tions in the middle region of the TFTR vessel.

With the exception of JET, TEXTOR and NOVA,total sampled areas of the other devices were

similar; hence, the reported collected mass is a

direct indication of the relative amount of dust

among the different machines. These values do

not, however, reflect differences in operating

history, machine configuration, or cleaning efforts

in the devices.

Consider for example the measured dust surfacemass density from regions of ASDEX-Upgrade

[19]. Most of the dust mass was found at the very

bottom of the chamber. No apparent trends are

found for the middle and upper regions, however,

dust deposits in the pumping duct increase at

greater distances from the main chamber. Values

of the total areas associated with each region inASDEX-Upgrade are presently being obtained,

thus, an estimate of the dust inventory for this

device does not yet exist. Estimates based on

collection in Tore Supra and DIII-D are 27 and

90/120 g, respectively. Variation in the surfacemass density for several devices is shown in Fig. 4.

The plot shows how CMDs vary with increasing

surface mass density. No statistically significanttrend is apparent*/indicating the likelihood thatdust collected at a given location is similar in size

to dust collected at all other locations, regardless

of the collected mass. Large data scatter from

LHD results from the very small amount of dust

collected.

4.3. Dust composition, shape, and tritium content

Dust found in fusion devices nominally shares

the composition of walls that interact with the

plasma. Many of the devices studied to date arecarbon machines, thus, carbon particulate is

pervasive. Metal machines such as Alcator-Cmod

produce metallic dust. Wall surface treatment also

affects dust composition. Experiments performed

following boronization of Alcator-Cmod walls

generated particulate containing significant

amounts of boron. Only a few machines have

Fig. 4. Comparison of dust sizes vs. surface mass density for

various fusion devices. A linear fit is provided from the data to

aid viewing the relative sizes. LHD and ASDEX-Upgrade have

comparatively large scatter in the size data.


operated with metal and carbon components (e.g.

JET and ASDEX-Upgrade), and the composition

of dust from mixed material machines has not yet

been studied. ITER will likely utilize various

materials exposed to plasma; experiments will

likely improve the understanding of dust from

mixed materials and provide validation of schemes

developed for monitoring and removing dust from

ITER.

Large variations in particle shape are not

generally observed. Although not necessarily sphe-

rical, dust shape is often described as granular or

globular, meaning a particles aspect ratio (max-

imum to minimum dimension) is relatively low. An

exception is the presence of large, thin flakes on

surfaces affected by co-deposition (usually cooler

regions away from plasma exposure). Upon mo-

bilization (e.g. during collection), these flakes are

normally broken up into fragments with low

aspect ratios [22]. Fibers are rarely observed in

collected particulate, except possibly at locations

where known failures of electrical insulation

occurred. An important concern for transport of

dust with low aspect ratios is the influence of

particle shape on effective drag forces.

Tritium content of dust and flakes collected

from JET and TFTR was measured to estimate the

tritium inventory held in debris. Dust collected

from the inner wall of the TFTR vacuum vessel

contained tritium at a measured concentration of

4.0/1010 Bq g1, and material (mostly flakes)from a lower view port window held 2.1/1010 Bqg1. At the beginning of DT operation, JET dust

was measured [16] and had a concentration of

1.33/106 Bq g1, whereas, the flakes had tritiumat a concentration of 8.8/106 Bq g1. Differentoperating histories of the two machines accounts

for the large difference in measured tritium

inventories. Similarity is evident in that tritium

concentrations are roughly the same (within a

factor of 10) in dust and flakes, suggesting that

breaking-up of flakes is a dominant source of dust

particles in these machines. Surface adsorption is

the main route for tritium uptake, and the exposed

area available for uptake in dust is nominally

greater than the area of flakes. If dust were not

predominately from flake break-up, the expected

tritium inventory in dust would be (/50 times)greater than that for flakes.

4.4. Specific surface area

Measurement of the specific surface area (m2

g1) of dust from several fusion devices was

performed using the BET gas adsorption techni-

que. Fig. 5 gives a comparison of the measure-ments, displaying specific surface area versus mean

volume-surface diameter (dMVS/CMD exp[2.5 ln2(GSD)]). Curves for fully dense

carbon (graphite density of 1.760 g cm3) and

molybdenum (density of 10.2 g cm3) are shown

to illustrate that measured dust from carbon and

metal machines generally have some degree of

porosity. With the exception of TFTR, there islittle variation of specific surface area among

carbon machines. Also shown for comparison is

the measured specific surface area of ATJ graphite

dust generated in a plasma gun configured to

simulate disruption heat loads [23]. This data point

fits well within measurements of dust from most

operating fusion devices. Causes for the outlying

TFTR data are not presently known. One factorthat will affect variation in specific surface area

measurements is dust composition. For example,

the smaller measurement from Alcator-Cmod was

obtained with dust from non-boronized walls, and

approaches the value of fully dense molybdenum

Fig. 5. Specific surface area of dust and flakes plotted against

the dusts mean volume-surface diameter, dMVS.


spheres. The other measurement, four timesgreater, was taken following runs with boronized

walls. Such an effect was not found in other

devices, possibly because the effect is greater for

metal and non-metal mixtures (as with Alcator-

Cmod) or the other machines routinely condition

their walls so that unconditioned wall dust has not

been characterized. Future work that attempts to

characterize the metallic and non-metallic compo-nents of the dust would be of great benefit.

5. Directions of future dust work

There remain many questions and uncertainties

regarding dusts impact on future fusion devices.

Research must continue to focus on many aspects

of the role dust plays in the safety and operation of

these devices. Detailed studies on the origination

of the dust must be performed with dedicatedexperiments on devices that simulate fusion

plasma conditions or with the existing tokamaks.

Characterizing the effects from mixtures of mate-

rials on dust generation requires significantly more

work. Understanding the dust source will improve

modeling for estimates of dust inventory in large

power reactors, reducing the uncertainties in safety

analysis and safety limits, as well as verifyingmethods for monitoring and removal. In addition,

benchmark experiments must be performed to

study effects of dust re-suspension and transport

in accidents.

Practical issues of dealing with dust are being

addressed for ITER. In-vessel dust monitoring and

removal techniques are presently under evaluation.

Experiments and prototypes to test such systemsmust be built before applying the designs in ITER.

Dust impact on the operation of other next step

devices (FIRE, IGNITOR) have not been consid-

ered, although it must be eventually addressed.

References

[1] G. Federici, et al., Plasma/material interactions in currenttokamaks and their implications for next-step fusion

reactors, External Report PPPL-3512, IPP-9/128, January,

2001.

[2] J. Winter, Dust: a new challenge in nuclear fusion research,

Phys. Plasmas 7 (2000) 3862/3866.[3] J. Winter, G. Gebauer, Dust in magnetic confinement

fusion devices and its impact on plasma operation, J. Nucl.

Mater. 266/269 (1999) 228/233.[4] J.P. Sharpe, et al., Modeling of particulate production in

the SIRENS plasma disruption simulator, J. Nucl. Mater.

290/293 (2001) 1128/1133.[5] A. Hassanein, I. Konkaskbaev, Macroscopic erosion of

plasma facing and nearby components during plasma

instabilites: the droplet shielding phenomena, J. Nucl.

Mater. 290/293 (2001) 1074/1078.[6] K.A. McCarthy, et al., The safety implications of tokamak

dust size and surface area, Fusion Eng. Des. 42 (1998) 45/52.

[7] S.J. Piet, et al., ITER tokamak dust*/limits, production,removal, surveying, Proceedings of the 17th IEEE/NPSS

Symposium on Fusion Engineering, (1998) 167/170.[8] H. Tsujikawa, et al., Preliminary studies on large metal-

sealed gate valves for the International Thermonuclear

Experimental Reactor (ITER), Vacuum 47 (1996) 639/646.

[9] K. Narihara, et al., Nucl. Fusion 37 (1997) 1177/1189.[10] J. Winter, V.E. Fortov, A.P. Nefedov, Radioactive dust

levitation and its consequences for fusion devices, J. Nucl.

Mater. 290/293 (2001) 509/512.[11] W.J. Carmack, et al., DIII-D dust particulate character-

ization, INEEL External Report INEEL/EXT-97-00702,

November 1997.

[12] W.J. Carmack, et al., DIII-D dust particulate character-

ization (June 1998 Vent), INEEL External Report INEEL/

EXT-99-00095, January 1999.

[13] W.J. Carmack, et al., Characterization and analysis of

dusts produced in three experimental tokamaks: TFTR,

DIII-D, and Alcator C-Mod, Fusion Eng. Des. 51/52(2000) 477/484.

[14] W.J. Carmack, et al., Analysis and characterization of

TFTR tokamak dust, ITER Engineering Design File

ITER/US/98/TE/SA-8, April 1998.

[15] S.V. Gorman, et al., C-Mod dust particulate characteriza-

tion, ITER Engineering Design File ITER/US/97/TE/SA-

17, August 1997.

[16] A.T. Peacock, et al., Dust and flakes in the JET MkIIa

divertor, analysis and results, J. Nucl. Mater. 266/269(1999) 423/428.

[17] P. Chappuis, et al., Dust characterization and analysis in

Tore Supra, J. Nucl. Mater. 290/293 (2001) 245/249.[18] J.P. Sharpe, P. Chappuis, D.A. Petti, Characterization of

tokamak dust collected from Tore Supra, Fusion Technol.

39 (2001) 1061/1065.[19] J.P. Sharpe, Collection and characterization of particulate

from the ASDEX-Upgrade tokamak (July 2000 Vent),

INEEL External Report INEEL/EXT-01-00544, April

2001.

[20] J.P. Sharpe, Collection and characterization of particulate

from the large helical device, INEEL External Report

INEEL/EXT-02-00301, February 2002.


[21] J.P. Sharpe, W.J. Carmack, D.A. Petti, Characterization

of dust and debris collected from the NOVA facility,

Fusion Technol. 39 (2001) 1066/1070.[22] M. Rubel, et al., Dust particles in controlled fusion

devices: morphology, observations in the plasma, and

influence on the plasma performance, Nucl. Fusion 41

(2001) 1087/1099.[23] J.P. Sharpe, M.A. Bourham, J.G. Gilligan, Generation

and characterization of carbon particulate in disruption

simulations, Fusion Technol. 34 (1998) 634/639.


A review of dust in fusion devices: Implications for safety and operational performanceIntroductionThe origin of dust in fusion devicesSafety and operational performance issuesComparison of dust from various fusion machinesParticle size distributionsSurface mass densityDust composition, shape, and tritium contentSpecific surface area

Directions of future dust workReferences

a review of dust in fusion devices

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