S6-4Radiation Effects on Tungsten
タングステンの照射影響
Akira Haegawa
長谷川晃
Tohoku University, Faculty of Engineering
6-6-01-2, Aramaki-aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan
東北大学工学研究科 〒980-8579 仙台市青葉区荒巻字青葉 6-6-01-2
To understand the irradiation response of tungsten (W) in fusion reactors, irradiation effects on
microstructure development, hardening and electrical resistivity of pure W and W alloys were summarized basedon fission reactors irradiation up to about 1-1.5 dpa in the temperature range from 400 to 800C. Voids were themajor damage structure in pure W, and irradiation data of W-Re alloys showed that Re clearly suppressed void
formation. Based on these results, a damage structure map of W in fusion reactor was suggested.

1. Introduction
Tungsten (W) is considered to be a candidate for theplasma-facing component (PFC) materials ofmagnetic confinement fusion reactors such as the firstwall of a blanket and a diverter plate because of its
high melting temperature and high resistant tosputtering. During fusion reactor operation, as a resultof high-energy neutron exposure, not only
displacement damage but also transmutation elements
are produced in W. For example after 5 years ofoperation in DEMO-like reactor, the average neutron
fluence would be 10 MWa/m2 at the first wall and 5
MWa/m2 at the diverter, the displacement damage andRhenium (Re) content would be 30dpa and 6% in Wof the first wall, and 15 dpa and 3% in the diverter,
respectively[1,2]. Re is one of major solidtransmutation elements of W in fusion reactor
conditions [3], and also it is well known that Re
affects physical properties of W [4].
In ITER, a displacement damage would be 0.7 dpa
and transmutation of Re would be 0.15% in the W
diverter after a neutron fluence of 0.15 MWa/m2 at
200–1000C[1]. The transmuted Re concentration
would be small in ITER, therefore, transmutation may
not be main concern of W in ITER, but neutron
irradiation causes microstructure change andirradiation hardening even after the 0.7dpa irradiation.
The irradiation behavior of W has been studied but thedetails have not been clarified yet. The irradiationinduced defects affect various physical properties ofW including mechanical property, size stability andthermal conductivity and so on. The defects anddefects clusters are also considered as trapping sites ofhydrogen (Tritium), therefore, damage structure
information is required to estimate soundness, life
time and Tritium inventory of W during neutron
irradiation.
2. Neutron irradiation behavior
Previous neutron irradiation works on W using
various fission reactors including JMTR, JOYO(Japan), EBR-II ad HFIR (USA) at temperature
range from 400 to 800C and dpa level was in
the range from 0.15 to 1.54 dpa (Ed=90eV)
were summarized[5-7]. The results oftransmission electron microscopy showed thatthe major defect clusters of pure W under theseirradiation conditions were voids. Dislocation loops
were also observed in lower dpa region but its numberdensity was 1/10 to 1/100 of void. Ordered voidarray structure, which was called void lattice, wereobserved above 1dpa at 538 and 750C in JOYO. Voidswelling of these irradiated specimens estimated bymicrostructure observation were below 0.1%[8]. The
void lattice had also reported in the previous works
carried out in 1970s in USA [5]. The void lattice
formation conditions were clarified using these data.
Void swelling data of heavy irradiated (9.5dpa) W
obtained by density measurement was also reported[9].
The swelling peak temperature of W was 700-800Cand the swelling was around 1.6%. The microstructureof this irradiated W was not reported, but these data
showed that void swelling of W up to 10dpa
irradiation would not be so significantly.
It is well known that Re improves W properties andone of major transmuted element of W, therefore, Re
effects on microstructure development of W after
neutron irradiation have been studied. The irradiation
data of the W-Re alloys showed that the voidformation behavior up to 1.5 dpa were strongly
affected by the Re concentration in W. Results of
JOYO irradiation showed that the void size andnumber density tended to decrease significantly withincreasing Re content [10]. The void number densityof the W-Re alloys compared to pure W was less thanabout 1/10, and void size was also smaller than that ofpure W. The defect types of 5 and 10%Re alloys inthis irradiation conditions were similar, these were
small amount of voids and fine precipitates. In thecase of 26% Re content alloys, severe irradiation
hardening was observed because of high density
precipitates of W-Re intermetallic compounds ( or χVickers hardness. Vertical axis in Figure 2
corresponds to yield stress increase qualitatively byneutron irradiation. With increase displacement
damage, yield stress increase and the hardening
depend on the Re concentration in W. Irradiationinduced defects such as void and loop affect theincrease of hardness. Precipitates of W-Re also
induce large hardening of W-Re alloys. Ductility
loss cannot be estimated by hardness test, therefore
other evaluation methods were used to obtain
ductility change. Irradiation effects on various
property changes were also summarized and someexamples will be presented.
1200.
W-26Re (Prec.)1000

rradiation hardening, Hvphase) [11]. These results suggest that a small amountof Re (3-5%) may enhance microstructural radiation
resistance of W. The mechanism of Re effects on void
formation behavior of W is under investigating using
first-principal study [12].
The microstructural development with Re
concentration change of irradiated pure W and W-Re
alloys by fission reactors irradiation were shown in
Figure 1 [7]. The arrows indicate Re compositionchange during neutron irradiation. Schematic
illustrations of defect structure such as voids andloops are also shown. Based on the fission reactor
irradiation data (gray arrows), qualitative prediction ofdamage structure development of pure W in ITER andDEMO-like fusion reactors with lower transmutation
rate of W to Re are also shown in Figure 1 as blackarrows. The qualitative predictions of damage
structure development in pure W are as follows: finevoids form at early stage of irradiation and void lattice
structure appears at around 1 dpa at 600–800C. This
structure is expected to be stable up to several dpa
because formation and annihilation balance ofvacancy and interstitial atom is considered tocompensate by the void lattice structure. With
increasing neutron fluence up to around 10 dpa, theconcentration of transmuted Re increases to several
mass % even in a fusion reactor, the void lattice
structure may be changed by Re effects on voidformation. Precipitate of W-Re may form in the void
lattice and disordering of the lattice and shrinking ofthe void may occur.

PM SR W (HFIR)
Prec. / No void

800600★
★

400

W-10Re (Prec.)

W-5Re (Prec.)W(Void)W (Arc, R)
W-5Re (Arc, R)
W-10Re (Arc, R)
W-26Re (Arc, R)

200

W (SR)
W-5Re (SR)
W-10Re (SR)
W-26Re (SR)00Figure 2.

123
Irradiation damage, dpa4Irradiation hardening of W and W-Re.

References

Figure1.
Predictionofmicrostructuraldevelopment of W under fusion reactor conditions
based on fission reactor irradiation data [7].
These
microstructuralchanges
causedmechanical property changes. Figure 2 shows
irradiation damage (dpa) dependence on hardness ofW and W-Re alloys [13]. Hardness is the resistanceto permanent indentation and a relationship has
been established between the yield stress and[1] Bolt, H., et al., J. Nucl. Mater. 307–311 (2002)
43–52.
[2] Bolt, H., et al., J. Nucl. Mater. 329–333 (2004)
66–73.
[3] Noda, N., et al., J. Nucl. Mater. 258–263 (1998)
934–939.
[4] Tietz, T.E., & Wilson, J.W., “Behavior of Refractory
Metals”, Chapter 8 Tungsten, Stanford University
Press (1965).
[5] Sikka, V.K. and Moteff, J., J. Appl. Phys. 43 (1972)
4942–4944.
[6] Hasegawa, A. et al., Materials Transactions, 54(2013) 466-471.
[7] Hasegawa, A., et al., Fusion Eng. & Design, 89(2014) 1568-1572.
[8] Tanno, T. et al., Mater. Trans., 52(2011)1447-1451[9] Motolich, J., et al., Scripta Metallurgica, 8 (1974)
837-842[10] Tanno, T. et al., Mater. Trans., 49(2009)2259-2264[11] Nemoto, Y. et al., J. Nucl. Mater. 283–287 (2000)
1144–1147.
[12] Suzudo, T. et al., Modeling and Simulation inMater. Sci. Eng. 22 (2014) 075006-075019[13] Hasegawa, A., et al., J. Nucl. Mater., 417 (2011)491-494.