Materials Transactions, Vol. 54, No. 4 (2013) pp. 437 to 441
Special Issue on Materials-System Integration for Fusion DEMO Blanket
© 2012 The Japan Institute of Metals
OVERVIEW
Retention of Hydrogen Isotopes in Neutron Irradiated Tungsten
Yuji Hatano1, Masashi Shimada2, Yasuhisa Oya3, Guoping Cao4, Makoto Kobayashi3,
Masanori Hara1, Brad J. Merrill2, Kenji Okuno3, Mikhail A. Sokolov5 and Yutai Katoh5
1
Hydrogen Isotope Research Center, University of Toyama, Toyama 930-8555, Japan
Fusion Safety Program, Idaho National Laboratory, Idaho Falls, ID 83415, USA
3
Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan
4
Department of Engineering Physics, The University of Wisconsin, Madison, WI 53706, USA
5
Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2
To investigate the effects of neutron irradiation on hydrogen isotope retention in tungsten, disk-type specimens of pure tungsten were
irradiated in the High Flux Isotope Reactor in Oak Ridge National Laboratory followed by exposure to high flux deuterium (D) plasma in Idaho
National Laboratory. The results obtained for low dose n-irradiated specimens (0.025 dpa for tungsten) are reviewed in this paper. Irradiation at
coolant temperature of the reactor (around 50°C) resulted in the formation of strong trapping sites for D atoms. The concentrations of D in nirradiated specimens were ranging from 0.1 to 0.4 mol% after exposure to D plasma at 200 and 500°C and significantly higher than those in nonirradiated specimens because of D-trapping by radiation defects. Deep penetration of D up to a depth of 50­100 µm was observed at 500°C.
Release of D in subsequent thermal desorption measurements continued up to 900°C. These results were compared with the behaviour of D in
ion-irradiated tungsten, and distinctive features of n-irradiation were discussed. [doi:10.2320/matertrans.MG201204]
(Received August 31, 2012; Accepted November 5, 2012; Published December 14, 2012)
Keywords: plasma-facing material, tungsten, hydrogen isotope, irradiation, trapping effect
1.
Introduction
Because of favorable physical properties such as high
melting point and low sputtering rate, tungsten (W) and its
alloys are recognized as primary candidates for plasma-facing
materials (PFMs) of fusion reactors. PFMs will be exposed to
the high fluxes of deuterium (D) and tritium (T) particles
under irradiation of 14 MeV neutrons (n). From the point of
view of radiation control, inventory of T in reactor core
should be limited to a certain low value. Solubility of
hydrogen in W is low in comparison with other metals,1) and
hence retention of hydrogen isotopes in W is considered to be
dominated by trapping effects by defects such as vacancies
and voids. Therefore, understanding n-irradiation effects on
hydrogen isotope retention in W is an important issue
assessing the safety of fusion reactors.
The effects of displacement damage on the retention of
hydrogen isotopes in W have been simulated using ionirradiation techniques by various researchers.2­10) In those
studies, W specimens were irradiated with various types of
high-energy ions (³ MeV) to moderate damage levels (³ a
few dpa) and exposed to relatively low energy, high-flux
(eV­keV, 1020­1024 m¹2 s¹1) D plasma or ions. Typical
thickness of damaged zone corresponding to the range of
implanted ions is 0.5­3.5 µm. A significant increase in D
retention caused by ion-irradiation was observed; the
concentration of D, CD, in the damaged zones was around
1 mol% at least at exposure temperature of 200°C or below.
However, retention of hydrogen isotopes in n-irradiated W
was scarcely examined.
The authors have irradiated the disk-type specimens of
pure W with neutrons in the High Flux Isotope Reactor
(HFIR) in Oak Ridge National Laboratory (ORNL) to 0.025,
0.3 and 2.4 dpa at coolant temperature of the reactor (around
50°C) and 300°C in Japan-US collaboration research.11­15)
The irradiated specimens have been shipped to Idaho
National Laboratory (INL) to examine the retention of
hydrogen isotopes by using a high flux linear plasma
machine, Tritium Plasma Experiment (TPE). The objective
of this paper is to review the results obtained for the low
dose (0.025 dpa) n-irradiated W specimens and discuss
distinctive features of n-irradiation effects in comparison
with ion-irradiation.11­15)
2.
Experimental Procedures11,12)
Disk-type specimens (º 6 © 0.2 mm) were prepared by
slicing a rod of pure W (99.99 mass%) supplied by A. L. M.
T. Co., Japan, under stress-relieved conditions (900°C, 1 h).
The surfaces of specimens were polished with diamond
powders (9 and 3 µm) and mirror-finished with colloidal
silica suspension (40 nm). Then, the specimens were
annealed in vacuum (10¹6 Pa) at 900°C for 0.5 h to relief
the stress induced during the polishing and finishing.
The specimens were irradiated with neutrons in HFIR for
33 h at coolant temperature of the reactor (around 50°C).
The fluence of fast neutrons was 1.1 © 1024 m¹2, which
corresponded to 0.025 dpa for W. The irradiated specimens
were shipped to INL after cooling for 300 d to examine
the retention of D. First, the surface of specimens were
gently polished with a lapping film (3M, aluminum oxide,
0.3 µm) to remove oxide layers formed during neutron
irradiation. Then, the specimens were exposed to high-flux
deuterium plasma at 100, 200 and 500°C up to (5­7) ©
1025 D m¹2 in TPE. The flux and incident energy was
(5­7) © 1021 D m¹2 s¹1 and 100 eV, respectively. Depth
profiles of D were measured by a technique of nuclear
reaction analysis (NRA) in the University of Wisconsin,
Madison (UW). Retention of D and detrapping energy
were examined by a technique of thermal desorption
spectroscopy (TDS). The rate of the temperature ramp was
10°C min¹1.
438
Y. Hatano et al.
9)
100
D concentration, C D (mol%)
Alimov et al. (0.89 dpa)
D concentration, C D (mol%)
10-1
10-2
100
10-1
10-2
100
5)
0
Wright et al. (2 dpa)
-1
Neutron
(0.025 dpa)
10
12)
10
6)
12)
Shimada et al.
(non-irradiated)
10
-2
Tyburska et al.
(0.25-0.3 dpa)
0
Wampler
4)
& Doerner
(0.6 dpa)
7)
100 200 300 400 500 600 700
Exposure temperature, Tex / °C
10-1
Fig. 2 Comparison of temperature dependence of deuterium concentration
CD in n-irradiated and ion-irradiated W. The conditions for irradiation and
plasma exposure are summarized in Table 1.
10-2
Depth,d / μm
Fig. 1 Depth profiles of deuterium in n-irradiated and non-irradiated
tungsten after exposure to high flux deuterium plasma at 100°C (a), 200°C
(b) and 500°C (c). [Reproduced from Ref. 12): “The deuterium depth
profiles in n-irradiated tungsten exposed to plasma”, M. Shimada, G. Cao,
Y. Hatano, T. Oda, Y. Oya, M. Hara and P. Calderoni: Phys. Scr. T 145
(2011) 014051, 16 December 2011, IOP Publishing.]
3.
Ogorodnikova et al.
(0.28 dpa)
Results and Discussion
Depth profiles of deuterium at different exposure temperatures, Tex, are shown in Fig. 1.12) The D atoms detected in
the non-irradiated specimens corresponded to those trapped
in intrinsic defects formed during fabrication process of the
specimens (such as dislocations and vacancies) and plasmainduced defects (such as blisters and bubbles). In the case of
the n-irradiated specimens, D atoms were present also in
radiation defects in addition to the intrinsic and plasmainduced defects. The concentration of D in a solid solution
state should be negligibly small because of desorption
during the specimen shipping from INL to UW at room
temperature. The diffusivity of D in a solid solution state
DSS at room temperature is evaluated to be 5 © 10¹14 m2 s¹1
from the data reported by Frauenfelder.16) The mean
diffusion distance, (2DSSt)1/2 where t is time, reaches the
specimen thickness (0.2 mm) within just 2 d. In Fig. 1,
no significant difference in CD was observed between
non-irradiated and n-irradiated specimens at Tex = 100°C.
Nevertheless, at Tex = 200°C, clear difference was observed
up to a depth, d, of 3 µm; CD in n-irradiated specimen
(0.1­0.4 mol%) was higher than that in non-irradiated one
(0.1 mol%). At Tex = 500°C, CD in non-irradiated specimen
was very low (¯ 0.01 mol%) because of enhancement of
associative emission of implanted D. On the contrary, CD
in n-irradiated specimen was 0.2 mol% and comparable
with that at Tex = 200°C except a near-surface region at d
below 1 µm. These observations clearly indicated that the
Table 1 Experimental conditions for data shown in Fig. 2.
Ref.
Irradiation conditions
Particles
Temperature
Plasma exposure
Flux (m¹2 s¹1)
12
®
®
(5­7) © 1021
12
Neutrons
around 50°C
(5­7) © 1021
4
12 MeV Si ions
room temp.
about 2 © 1022
5
12.3 MeV W ions
300 K
1.1 « 0.2­2.7 « 0.3 © 1024
6
5.5 MeV W ions
around 290 K
7 5.5 and 20 MeV W ions
room temp.
9
room temp.
20 MeV W ions
1022
22
10
on opposite side
1022
neutron irradiation induced strong trapping sites. It should
be noted that the penetration depth of D in n-irradiated
specimen increased with increasing Tex, which is reflecting
temperature dependence of diffusion process into the bulk
of specimen with reducing emission flux from the surface;
D diffused beyond the detection depth of NRA (³ 5 µm)
at Tex = 500°C.
In Fig. 2, CD in the n-irradiated specimens obtained with
NRA was compared with that in ion-irradiated regions of W
reported by various researchers.4­7,9) The values for ionirradiated W plotted in this figure were taken from literatures
in which the temperature dependence of CD was measured
in relatively wide temperature range. The conditions for
irradiation and plasma exposure are summarized in Table 1.
Low D concentrations in the n-irradiated specimen in
comparison with ion-irradiated W at Tex = 100 and 200°C
can be ascribed to lower damage level for the former
(0.025 dpa). As described above, CD in the n-irradiated
specimen at Tex = 500°C was comparable with that at
Tex = 200°C. Such weak temperature dependence of CD
was also reported by Wright et al.5) as shown in this figure.
However, many other researchers reported significant drop
at Tex = 500°C.4,6,7,9) The mechanisms underlying such
discrepancy will be discussed later.
n-irradiated, Tex = 500°C
1
439
Bulk
Surface
10
18
D desorption rate, φ D / 10 m s
-2 -1
Retention of Hydrogen Isotopes in Neutron Irradiated Tungsten
n-irradiated,
Tex = 200°C
non-irradiated,
Tex = 200°C
Ed: Activation energy
for diffusion
H
1
H2
2
Ebin: Binding energy
Edet: Detrapping energy
0.1
ES: Heat of solution
0 100 200 300 400 500 600 700 800 900
Fig. 4
Schematic potential diagram for hydrogen-tungsten system.
Temperature, T / °C
Fig. 3 Thermal desorption spectra of D from n-irradiated (0.025 dpa for W)
and non-irradiated W after exposure to D plasma in TPE at 200 and
500°C. (Reproduced from Ref. 15): “Hydrogen isotope trapping at
defects created with neutron and ion-irradiation in tungsten”, Y. Hatano
et al.: Proc. 24th IAEA Fusion Energy Conf., Oct. 8­13, 2012, San
Diego, USA.)
Thermal desorption spectra of D from n-irradiated specimens after plasma exposure at 200 and 500°C are shown
in Fig. 3 together with that from non-irradiated specimen
(Tex = 200°C).11­15) The desorption from non-irradiated
specimen was completed at around 400°C. On the other
hand, the release of D from n-irradiated specimens continued
to high temperatures above 900°C. These observations also
suggested the presence of strong trapping sites in the nirradiated specimens. In addition, the desorption peak after
the plasma exposure at 200°C was very broad, indicating
the presence of several different types of trapping sites
with different binding energies with D atoms as discussed
below. The D retention in the n-irradiated specimens were
1.2 and 6.4 © 1021 m¹2 at Tex = 200 and 500°C, respectively. By assuming that the D concentration in the
penetration zone was uniform and 0.1­0.2 mol% (6.3­
12.6 © 1025 m¹3), the thickness of penetration depth at
Tex = 500°C was evaluated to be 50­100 µm. Such deep
penetration of trapped D is one of the distinctive features
of n-irradiated specimens. In the previous paper, Shimada
et al.14) irradiated the disk-type W specimens with 0.7 MeV
H ions (0.48 dpa), 2.8 MeV Fe ions (0.025, 0.3 and 3.0 dpa)
and 20 MeV W ions (0.3 and 3 dpa). Then, they exposed
these ion-irradiated specimens to D plasma under the same
conditions as the n-irradiated specimen (Tex = 200°C) and
compared TDS spectra. The ranges of those ions were
evaluated to be 1.25­3.9 µm [see Fig. 1 in Ref. 14)]. In spite
of the lowest damage level, the desorption from n-irradiated
specimen continued up to the highest temperature among
those specimens. As shown in Fig. 1 the penetration depth
of D at Tex = 200°C (3­4 µm) is comparable with the
maximum thickness of damaged zone created by the ionirradiation (1.25­3.9 µm). In addition, empty trapping sites
are present beyond the penetration depth of D in the case of
n-irradiated specimens; a part of detrapped D atoms diffused
into to the deeper region of bulk to be trapped again.
Therefore, the continuation of D release from n-irradiated
specimens to higher temperatures than ion-irradiated ones
can be ascribed to the deeper penetration depth of D and
the presence of strong trapping sites beyond the penetration
depth.
Schematic description of potential diagram of tungstenhydrogen system is shown in Fig. 4. In this figure, ES is the
heat of solution of hydrogen in normal interstitial sites, Ebin
is the binding energy between hydrogen atom and trapping
site, and Edet is the activation energy for detrapping process.
The difference between Ebin and Edet is considered to be
comparable with the activation energy for diffusion of
hydrogen atoms, Ed. The values of Ed and ES reported by
Frauenfelder16) are widely accepted. Irradiation of neutrons
induces various types of defects such as vacancies, vacancy
clusters and dislocation loops with different Ebin. In addition,
one defect can trap several hydrogen atoms with different
Ebin. For example, Ohsawa et al.17) reported that 12 hydrogen atoms can be accommodated in monovacancy in W
though Ebin for a single hydrogen atom decreases with
increase in number of trapped hydrogen atoms. The
probability of occupation of trapping site, ªt, is expressed
as follows:
ª t,i =ð1 ª t,i Þ ¼ ª L =ð1 ª L Þ expðEbin,i =kTex Þ;
ð1Þ
where i is a type of trapping state with the binding energy of
Ebin,i, ªL is the occupation probability of normal interstitial
site, and k is the Boltzmann constant. If the concentration of
D in a solution state is sufficiently low and the concentration
of trap i is Ct,i, CD is written as
CD ¼ i Ct,i ª t,i :
ð2Þ
Shimada et al.12) tried to reproduce the broad desorption
spectrum of D from n-irradiated specimen after plasma
exposure at 200°C by assuming the presence of 6 different
types of trapping sites with different Edet,i (0.9, 1.1, 1.3, 1.5,
1.7 and 2.0 eV) with TMAP7 program.18) This program could
handle only 3 types of trapping sites, and hence they needed
to separate trapping sites into two groups [low Edet,i group
(0.9, 1.1 and 1.3 eV) and high Edet,i group (1.5, 1.7 and
2.0 eV)] and evaluate the effects of these two groups
separately. Later, Merrill et al.19) have evaluated the effects
of 6 types of trapping sites simultaneously by improving
the TMAP program. They could reproduce the desorption
spectrum relatively well by assuming 6 types of trapping
Y. Hatano et al.
6
n-irradiated W,
Tex = 500 °C
18
Desorption flux, φ D / 10 m s
-2 -1
440
4
TMAP,
Total
2 TMAP, Front side
TMAP, Back side
0
100
300 500 700 900 1100
Temperature, T / °C
Fig. 5 Example of simulation of thermal desorption spectrum with TMAP
program. Conditions for simulation are described in text. (Reproduced
from Ref. 15): “Hydrogen isotope trapping at defects created with neutron
and ion-irradiation in tungsten”, Y. Hatano et al.: Proc. 24th IAEA Fusion
Energy Conf., Oct. 8­13, 2012, San Diego, USA.)
sites with smaller values of Edet,i than Shimada et al.12)
(0.9, 1.0, 1.1, 1.2, 1.4 and 1.6 eV). They also proposed to
assume bimodal lognormal distribution of Edet,i (and Ebin,i)
to reproduce the desorption spectra. No complete model,
however, has been established to explain the desorption
spectrum observed after the exposure at 200°C. In these
analyses, the recovery of defects during TDS measurements
was not taken into account. The maximum temperature in
the TDS measurements (900°C) corresponds to stage V of
recovery processes in which disappearance of some defect
clusters or formation of voids are possible.20) It is therefore
necessary to improve computer program such as TMAP to
simulate desorption under variations of trap density and
Edet,i with time and temperature. Figure 5 shows a result of
simulation with TMAP4 program together with desorption
spectrum obtained after plasma exposure at 500°C.15,21) In
the simulation, it was assumed that a single type of trapping
site is uniformly distributed at concentration of 0.2 mol% in
a 200 µm-thick specimen, trapping sites up to a depth of
50 µm from a plasma-exposed surface are filled with D while
those in a deeper region are empty, and Edet = 1.83 eV.
Although the peak position of simulated spectrum agreed
with the experimental data, the former had a shoulder in
high temperature side. This shoulder appeared by the release
of D from the opposite side (unexposed side) surface. Such
disagreement in a high temperature region can also be
attributed to, at least in part, recovery of defects during TDS
measurement. To understand the annealing effects, the nirradiated specimens subjected to TDS measurements were
exposed to D plasma again. The D retention after the postTDS plasma exposure was indeed significantly smaller than
the value before the 1st TDS measurement. Details will be
reported in a separate paper. Note that the above-mentioned
value of Edet (1.83 eV) has uncertainty due to such annealing
effects of defects. In addition, this value should be considered
as an average value of Edet,i for several different types of
defects.
As described above, there is significant disagreement in
temperature dependence of CD in irradiated W. The nirradiated specimens showed weak temperature dependence,
and similar tendency was observed in the data reported
by Wright et al.5) On the other hand, much stronger
temperature dependence was reported by Wampler and
Doerner,4) and Tyburska et al.7) The difference between
Wright et al.5) and Tyburska et al.7) cannot be attributed
to that in microstructure because irradiation conditions in
those studies are similar to each other as shown in Table 1;
the former irradiated 12.3 MeV W ions, and the latter used
5.5 and 20 MeV W ions. Therefore, the attention is focused
here on factors other than microstructure. One of the
important factors is the concentration of D in a solid solution
state.
As indicated by eq. (1), the probability of trapping site
occupancy is sensitively dependent on the concentration of D
in a solid solution state. Trapping sites can be saturated with
D even at high temperatures if the concentration of D in a
solution state is sufficiently high. Under plasma exposure,
the concentration of D in a solution state is determined
by balance between incident flux and rate of associative
emission from the surface. Hence, if the rate of associative
emission is comparable, the probability of trapping site
occupancy increases with increase in incident deuterium flux.
Wright et al.5) exposed ion-irradiated W to D plasma in a
plasma machine called Pilot-PSI in which the incident flux
of D is high and in order of 1024 m¹2 s¹1, as shown in
Table 1. On the other hand, Tyburska et al.7) irradiated one
side of 25 µm-thick W foil and exposed the non-irradiated
side to plasma, in which the incident flux was 1022 m¹2 s¹1.
Although the incident flux was relatively high, Tyburska
et al.7) examined the trapping of permeating D at the opposite
side. Hence, the concentration of D in a solution state in
damaged zone is expected to be much higher in the study of
Wright et al.5) than that of Tyburska et al.7) Such difference
in the concentration of solute D could be one of the reasons
for that in temperature dependence of D retention. The flux of
D in TPE was also relatively high [(5­7) © 1021 D m¹2 s¹1]
and the data shown in Figs. 1 and 2 were taken from the
plasma-exposed side. It is therefore plausible that the
concentration of D in a solution state in the analyzed region
in this work was sufficiently high to maintain high ªt,i even
at 500°C.
Release of D during specimen cooling can also be a
problem in the case of exposure to high flux plasma at high
temperatures. In high flux plasma machines, specimens are
generally heated by plasma, and hence specimen temperature
starts to decrease only after termination of discharge. If
cooling rate is not sufficiently high, a part of trapped D is
released during cooling process. The release of D under
typical conditions for ion-irradiated specimen was calculated
with TMAP4 program. In this calculation, only a single type
of trap was considered, and the thickness of damaged zone,
trap density and cooling rate were adjusted to 2 µm, 0.5 mol%
and 1°C s¹1, respectively. The surface association rate
coefficient was assumed to be sufficiently high. It was shown
that more than 90% of trapped D is released during cooling
if Edet = 1.5 eV or smaller. This result indicates that the
underestimation of D retention is possible under certain
Retention of Hydrogen Isotopes in Neutron Irradiated Tungsten
conditions. In principle, the effects of such D release during
cooling could be less significant for n-irradiated specimens
because a part of released D can be compensated by D
diffusing from deeper region.
Both the concentration of D in a solution state and the rate
of D release during cooling can be dependent on surface
states. It is, however, difficult to examine surface states of
specimens during exposure to high flux plasma, and no
direct information is available in this study and literatures.
Therefore, further discussion on the mechanisms underlying
the difference in temperature dependence in D retention is
difficult at the present. Examination of trapping effects under
a controlled solute D concentration is possible by a technique
of D2 gas exposure; the concentration of D in a solution
state is determined by the solubility and gas pressure.
Experimental campaign with a gas absorption technique
has already been started by the authors for ion-irradiated
specimens.15)
The results obtained for W specimens irradiated to
0.025 dpa with neutrons at coolant temperature of the reactor
showed that deep penetration of tritium is possible under
long-term steady state plasma operation, and the concentration of trapped tritium could be high (³ 0.2 mol%) even
at relatively high temperature under certain conditions. The
measurement of D retention in the specimens irradiated to
higher dose is in progress, and the results will be reported in a
separate paper. Effects of irradiation temperature on hydrogen
isotope retention have to be studied in future research
programs for quantitative evaluation of tritium inventory
in fusion reactors; more moderate irradiation effects are
anticipated at elevated temperatures.8) Development of
tritium removal technique and mitigation of tritium retention
by alloying or microstructure control are also important
issues.
4.
Summary
Retention of D in W irradiated with neutrons in a fission
reactor at the coolant temperature of the reactor was
examined. Clear effects of neutron irradiation were observed
even at 0.025 dpa for W. The concentration of D reached
0.1­0.4 mol% under exposure to high flux D plasma at 200
and 500°C. Deep penetration of D up to a depth of 50­
100 µm was observed after exposure at 500°C. Release of D
in subsequent TDS measurements continued up to significantly higher temperatures than ion-irradiated specimens.
Mechanisms underlying the temperature dependence of D
retention and the significant difference in D release temperature between n-irradiated and ion-irradiated specimens were
discussed.
441
Acknowledgements
The authors express their sincere thanks to Prof. Y. Ueda
(Osaka University) and Dr. V. Kh. Alimov (University of
Toyama) for fruitful discussion. This work was supported by
the Japan-US joint research project TITAN, Kakenhi on
Priority Areas, 476, Tritium for Fusion, from MEXT, Japan
and the Collaboration Research Programs of National
Institute of Fusion Science, Japan (NIFS10KUMR004 and
NIFS11KEMF018).
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