S6-2
Properties of Tungsten and its Damaging Scenarios
タングステンの性質と損傷シナリオ
Akihiko Kimura1, Hiroaki Kurishita2
木村晃彦1, 栗下裕明2
1

Institute of Advanced Energy, Kyoto University, Gokasho, Uji 611-0011, Japan
1
京都大学エネルギー理工学研究所 〒611-0011 宇治市五ケ庄
2
Oarai branch, Institute for Materials Research, Tohoku University, Narita, Oarai, Ibaragi 311-1313, Japan2
東北大学金属材料研究所大洗研究センター 〒311-1313 茨城県東茨木郡大洗町成田町
Plasma facing material (PFM) plays an important role in the protection of both plasma and divertor devices.
Worldwide fusion program is now focused on Tungsten (W) as PFM of divertor. W may suffer damages
with a variety of scenarios, of which the level depens on plasma conditions and material performance of W.
Physical, chemical and mechanical properties of W are summarized and its damaging scenarios aredeveloped on the bases of the summary and plasma characteristics, such as heat load, helium implantationand displacing damages. Consideration of detailed inspection methodologies and establishment ofstandards of effective inspection techniques are demanded to keep an enough machine time andexperimental schedule for operation of ITER..

1. Introduction
Plasma facing material (PFM) plays an importantrole in the protection of both plasma and devices.
Worldwide fusion program is now focused on
Tungsten (W) as PFM of divertor, since W features
several advantageous properties for fusion
application, high melting points, good thermalconductivity, high creep resistance, good hightemperature strength and low vapor pressure. W has
a high resistance against sputtering and low tritium
retention in fusion environment and the decay time
for activation is acceptable. However, there are
drawbacks: low oxidation resistance, low ductility
at room temperature (RT) and the high ductile tobrittle transition temperature (DBTT). Furthermore,
properties are sensitive to irradiation.
W-PFM may suffer from damages with a varietyof scenarios, which is expected to depend on
plasma conditions and material performance of W.
Physical, chemical and mechanical properties of W
are summarized in this symposium and its
damaging scenarios are developed on the bases ofthe summary and plasma conditions, such as heatload, helium implantation and displacing damages.
2. Motivation
Inspection technology development of a varietyof damages in W-PFM, such as cracking, melting
and self-castellation is indispensable for continuous
operation of divertor of fusion devices. For soundand continuous operation of the divertor, aninspection standard to evaluate damaged level ofW-PFM is necessary to determine operation
schedule of the divertor, which is influenced byrepairing and exchanging process of W-PFM.

3. Properties of W
Specific features and basic properties of W aresummarized in Table 1, indicating that the melting
temperature is highest, and the vapor pressure andthermal expansion of W are the lowest in metals.
Since W is BCC metal, it has ductile to brittle
transition temperature (DBTT) of fracture
toughness or ductility. And, because of very highmelting temperature, the DBTT of W is rather
higher than 673K in impact test. Tensile test data
indicates that tensile elongation becomes large
enough only above 473K.
Table 1: Summary of specific features and basic
properties of W.

4. W-damaging scenarios
There are several types of damages expected for
W-PFM under fusion plasma exposure whichcauses high heat loading, helium implantation anddisplacement damage on W-PFM. The heat loadingof 1000 cycles with 20 MW/m2 resulted in melting
the so-called iter-grade W (Fig.1(a)). Solidification
after melting resulted in the formation of extremely
large grains (Fig.1 (d)) that may cause embrittlelentof the materials. The loading conditions similar toITER-ELM with 1.1 GW/m2 (pure W: ΔT ≈2000℃) at Tbase = 100℃, n = 100 on a exposurearea of 16 mm2 for 1 ms caused grain boundary
cracking (Fig.1(b)). Crack often initiates at the top
part of the monoblock with evidence for a ductile
fracture mode around the initiation site and a brittle
fracture mode with typical intergranular cracks
closer to cooling tube (Fig.1 (e)).
Low energy (50 eV) helium implantation with aflux above 2 x 1025 He+/m2 induced a castellation on
the surface (Fig.1(c)). It was also reported that
self-castellation typically appeared after several 100
loading cycles at 20 MW/m2 but never appeared
during lording 10 MW/m2, 5000 cycles. Fatigue
crack rather than crack due to brittleness might becritical. Thus, higher fatigue resistance and higher
resistance to recrystallization as well as radiation
tolerance are essential for W-PFM.
Since cracks influence on cooling potential of
W-PFM, mechanical property databases aredemanded, which are the temperature dependenceof tensile properties including yield stress, ultimate
tensile stress and total elongation as well as thetemperature dependence of impact fracture behavior
or DBTT in the temperature region from RT to1000K. The investigation of the effect ofrecrystallization on the above and fatigue isnecessary.

5. Inspection methodology
In order to operate fusion devices more
effectively, the damaged W-PFM could be reused
when the damage level is low enough to operate
divertor. Even before the exposure to plasma,
W-mono-blocks may suffer from damages during
production. The production makers provide themono-block under the ordering contract. In general,
the importance is the contents of the specificationsof the products. However, in case of considerationof product management of W-mono-blocks after
practical utilization under fusion plasma, it should
be sophisticate to define the specifications to reuse
the W-mono-block with some damages.
ITER diverter consists of more than 300,000
pieces of W-mono-blocks which would beinspected after disruptions or ELMs, and possibly
some damaged parts could be repaired orexchanged. It is expected that exchanging thedamaged parts requires a rather long time and thecost is not inexpensive. It is also expected that thedamaged parts could be reused even without
repairing depending on the level of the criticaldamage. Thus, it is demanded that a standard
inspection methodology would be established basedon the experiments which provide informationnecessary to classify the damages into some levels.

Fig.2 Inspections of productions and a part(s) ofW-mono-blocks for operation of fusion devices.

Fig.1 Damages of pure W induced by heat loadingor helium implantation.

6. Summary
Because of the occurrence of disruptions andELMs that generate extremely severe plasma,
W-PFM is imposed requirements that overwhelm
the performance of current available W. This means
the W-PFM is often damaged during operation offusion devices like ITER and beyond.
Consideration of detailed inspection methodologies
and establishment of standards of effective
inspection techniques are demanded to keep anenough machine time and experimental schedule
for operation of ITER.