S-P1-6
Status and prospect of international collaboration on PWI research:
Behavior of hydrogen isotopes in neutron-irradiated plasma-facing materials
国際共同研究によるＰＷＩ研究の進展と今後の展開：
中性子照射プラズマ対向材料中の水素同位体挙動
Yuji Hatano
波多野雄治
Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555, Japan
富山大学水素同位体科学研究センター 〒930-8555 富山市五福3190
Tritium retention and release by plasma-facing materials is a critical issue in safety assessment of fusion
reactors. The influence of neutron irradiation on hydrogen isotope retention and release by tungsten has
been examined in Japan-US Joint Project TITAN Task 2-1. The obtained results indicated significant
change in both retention and release by trapping effects of radiation damages. The overview of this projectis given in this presentation together with lessons learned for future international collaborations.

1. BackgroundPlasma-facing materials (PFMs) of fusion
reactors are subjected to extensive irradiation of
14 MeV neutrons. Various types of defects, suchas vacancy, interstitial loops and voids, arecreated by the irradiation, and physical andchemical properties PFMs are modified with theaccumulation of defects. From the viewpoints ofsafety assessment, increase in tritium retention bytrapping effects of defects is a critical issue. It
should be noted that PFMs emit significant decay
heat after extensive neutron irradiation, and PFMs
retaining a large amount of tritium and heated tohigh temperature could be a major source oftritium release under accidental conditions. The
accumulation of data on hydrogen isotope
behavior in neutron-irradiated PFMs is therefore
indispensable for the development of fusion
reactors.
International collaboration is really necessary
to understand neutron irradiation effects on PFMs
because (1) availability of neutron source islimited in most countries/areas, and (2) facilities
for post-irradiation examination (PIE) of nuclear
materials are not necessarily equipped with
instruments suitable for tests of PFMs. For
example, in the author’s knowledge, a high flux
plasma machine being able to accept radioactive
specimens is available only in Idaho National
Laboratory (INL), US at the present.
In the Japan-US Joint Project TITAN Task 2-1(Irradiation-Tritium Synergism), the retention andreleaseofhydrogenisotopesbyneutron-irradiated tungsten has been examinedbecause tungsten is currently recognized as a
primary candidate of PFM in DEMO reactor [1-3].
The objectives of this presentation are to provide
the overview of this activity and to extract theissues important for future international
collaboration in this direction.
2. Japan-US Joint Project TITAN Task 2-1
In this subtask of TITAN project, disk-type
specimens of pure tungsten prepared in Japan areirradiated in High Flux Isotope Reactor (HFIR),
Oak Ridge National Laboratory (ORNL). Then, thespecimens are shipped to INL to examine retention
and release of hydrogen isotopes including tritiumwith the liner plasma machine called Tritium
Plasma Experiment (TPE). A part of specimens arealso transported to University of Wisconsin,Madison, for depth profile measurements withnuclear reaction analysis (NRA) technique, andInternational Research Center for Nuclear Materials
Science, Institute for Materials Research, Tohoku
University for microstructure examination.
The specimens were prepared in University ofToyama by slicing the rods of pure tungsten(99.99 %) supplied by A. L. M. T. Co., Japan understress-relieved (SR) and recrystallized conditions.
The grains in the specimens were elongated to thedirection perpendicular to the surface like
ITER-grade tungsten. The size of specimens forhydrogen isotope retention/release measurements
were limited to be φ6 × t 0.2 mm by the volumeof irradiation capsule and radioactivity induced byneutron irradiation, and those for microstructure
examination wereφ3 × t 0.2 mm. The conditions

for neutron irradiation in HFIR are summarized inTable I. Irradiation at higher temperature iscurrently under discussion.
The neutron-irradiated specimens were shipped
to INL and tested with TPE. Figure 1 shows thecomparisonofdeuterium
retentioninneutron-irradiated specimens (SR) [3] and that in
ion-damaged specimens [4-7]. It should be notedthat the retention in neutron-irradiated specimens
was relatively high especially at high temperature inspite of lower dpa value. Significant increase indesorption temperature by neutron irradiation wasalso observed. These results suggested theformation of strong trapping sites by neutronirradiation. The binding energy between those
trapping sites and deuterium was about 2 eV, andthis binding energy corresponds to trapping byvoids [7,8]. The specimens irradiated to higher dpa
values will be examined in the near future.
Table I. Conditions of neutron irradiation in HFIR

Temperature (oC)50300

Dose (dpa)
0.025, 0.3
2.5of PIEs.
(2) As described above, most of PIE facilities are
not well equipped with instruments necessaryfor PFM tests such as plasma machines and
NRA device. Hence, the radioactive specimens
have to be shipped from one facility to anotherfrequently. Good coordination of specimen
shipping is indispensable for timely PIEs.
(3) Control of surface states is important for PFM
study but it may be difficult during neutron
irradiation. Hence, methods for surface
treatments of radioactive specimens should beconsidered in advance.
4. Conclusions
The observations in TITAN project clearly
showed significant change in hydrogen isotope
retention and release by neutron irradiation. This
project will be over at 2012 Japanese fiscal year.
On the other hand, importance of neutron
irradiation effects on PFMs is recognized not onlyin Japan and US but also in Europe [9] and othercountries. Establishment of new international
collaboration framework is really necessary toobtain sufficient understanding of neutron
irradiation effects before starting the design ofplasma-facing components of DEMO reactor.

D concentration in W (at.%)

Fukumoto et al. [4] (1 dpa)
Wright et al. [6] (2 dpa)

010Neutron(0.025 dpa)
-1

Ogorodnikova
et al. [7] (0.28 dpa)

10Shimada et al. [3](Undamaged)

Wampler& Doerner [5](0.6 dpa)

-210300 400 500 600 700 800 900 1000
Temperature (K)Fig.1. Comparison of deuterium retention between
neutron-irradiated tungsten specimens (SR) [3] andion-damaged specimens [4-7].

3. Lessons Learned
Post-irradiation experiments of PFMs aresignificantly different from those of common
nuclear materials. The special requirements for
PFM study we learned from the experiences in
TITAN Task 2-1 are listed below.
(1) Because of large mass number and highdisplacement energy, damage rate (dpa/s) intungsten under fission neutron environment isrelatively small. The irradiation to 2.4 dpa takes
roughly 1 year in HFIR. Neutron irradiation
should be started well in advance of target dateAcknowledgments
The author would like to express sincere thanks
to all collaborators of TITAN Task 2-1 for theirindispensable works to obtain the world first dataon retention and release of hydrogen isotopes byneutron-irradiated tungsten.
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