S-P1-2
Reminiscence of PWI studies performed under international co-operations andsuggestions for future works.
国際協力によるPWI研究
Tetsuo Tanabe
田辺哲朗
Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Science,
Kyushu University、6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
九州大学総合理工研究院 〒812-8581 福岡市東区箱崎6-10-1
Plasma materials interactions (PMI) with DT fuels in a reactor are so hard to realize and hence no one has everexperienced. PMI studies require interdisciplinal R&D, i.e., physics, chemistry and engineering to handle extremely
low temperature materials (Ice pellet of hydrogen) to high temperature (burning plasma) including radioactive materials(tritium and neutron activated materials) and the PMI phenomena are synergistic. Based on the author’s experience oninternational cooperational research, necessary future international cooperation is suggested.

1. General
In recent days, “globalization” is often
reiterated, and even in scientific community
international cooperation is in some sense forced
to apply fund and/or budget.
TEXTOR cooperation under IEA agreement
among EU, Japan, Canada and US was initiatedby prof. A. Miyhara at 1987. Since then lots ofbrilliant achievements have been produced
particularly in a field of plasma materials
interactions (PMI), because TEXTOR has been
dedicated to PMI studies.
The author has joined the program and initiateda new research program of “high Z limiter test inTEXTOR”, since then more than 10 Japanese
researchers have joined and more than 100 papers
have been published. The program has been
extended to other subjects like production,transport and deposition of impurities, tritium
retention and others. Furthermore, cooperation
with JET and ASDEX is now ongoing. Happily
the program can be categorized as one of the mostsuccessful international cooperation program.
The PMI study considering tritium usage is now
one of the key issues to establish burning plasma.
And even JET is focusing this issue modifying
PFM from full carbon to ITER like wall, i.e. Be andW. As described later, test beds to use W as PFM isquite limited, and we need strong cooperation orinvolvement in JET experiments.
In this presentation, based on the author’s
experiences of international cooperation, necessary
future international cooperation is suggested.
2. Problems remaining or requested urgent
solution in PMI filed.
All problems are originated from divertor andhandling tritium. How to handle or what material
can tolerate steady state heat load of 10-20 MW/m2
and transient heat load of a few MJ given by giant
ELM and/or disruption?
Because of safety concerns of tritium retention,
EU seems exclude carbon and select W. However,utilization of brittle materials below their DBTT
(ductile brittle transition temperature), (ITER is justthe case) can easily results materials fault due totheir brittlement or easy crack propagation. Fig.1 isa photograph of cracked W. The crack occurred
unintentional use of the W limiter below its DBTT,(Usually the limiter was preheated) in the TEXTOR.
The result itself was not a good example of thecooperation. Nevertheless, such kind of new finding
encourages and brought bitter smile. The results
irrespective of whether it is successful or not, areimportant.
Before starting the W limiter experiments inTEXTOR, most of plasma physicists were against
to use W as PFM. In Japan, no tokamaks allowed us
to use high Z materials in their PFM. After the longand hard discussion with scientists in TEXTOR,

Fig. 1 Brocken W test limiter owing tounintentional usage below DBTT

they allowed us to start the Mo limiter test.
From the results, we noticed that plasma cantolerate high Z PFM but without additional heating
like NBI, high Z impurities accumulate in plasma
center to result in minor or major disruption [1].
The plasma behavior was quite similar for Mo andW, even for Kr and Xe gas puffed in.
Once people noticed new results, irrespective of
successful or not, they are willing to precede
experiments further and hot discussion on newevents encourages to realize more severe
experiments to understand the new results.
Followed by the high Z limiter experiments inTEXTOR, ASDEX changed its carbon PFM to fullW. Now even “full-W divertor from day first in
ITER” is proposed. But this seems dangerous.
Referring the cracking damage in fig.1, one shouldnote how dangerous of W usage below its DBTT.
TEXTOR cooperation was really useful for bothof Japanese side in that we could use only one
tokamak in the world which has been dedicated toPMI studies.
Not only the ideas, but also a new technique canbe brought into an international cooperation. One
good example is tritium measurements. A tritiumprofile on TFTR bumper limiter, which was done
just by a voluntary base without any official
procedure, but scientists from our side and PPPL
agreed to make experiments and got excellent
results were obtained.
3. Necessary works in future
3-1.W relating works
Problems arise from heat load and He pumping
in divertor and tritium usage.
In particular,understanding of W behavior in ITER divertor likecondition is urgent. Unfortunately, JT-60U is closed
and JET will provide opportunity to study W,
though its heat load is still far low. ASDEX could
be also the test bed, but in order to avoid W
accumulation in the plasma center, their operational
plasma conditions are rather limited and studies on
intentional disruptions or ELM have not been doneyet. To study W accumulation, high temperatureplasma is necessary. Referring O accumulation, lowtemperature plasma has been hard to tolerate theradiation of oxygen, so as W radiation.
Nevertheless high Z accumulation must be avoided.
We have to wait experiments with “ITER like wall”
in JET, now just started.
Hopefully, newinternational cooperation with JET will be done.
3-2. Carbon related works
Because of sever radiation damage and large
tritium retention, utilization of carbon as PFM in

ITER and a reactor seems to be minimized oravoided. As mentioned above, however, concerns
on materials properties of W do not allow relying
on W as PFM. We should not forget low
activation and tolerance of high heat load of C.
Furthermore, possible in-situ repairing by carbon
deposition and low hydrogen uptake at higher
temperature attracts C. Hence R& D for C must
be continued. The author personally very muchconcerns manufacturers of carbon, in particular,
CFC, leaving to other large markets. The R&D ofPFM must be done internationally. Domestic
market is too small to encourage them.
3-3. Tritium in burning plasma
Burning efficiency in a fusion fuel cycle is,unfortunately, very poor; i.e. only a few % or lessof input tritium burns and the majority must be
recovered to recycle. In addition, large in-vessel
fuel retention rate would result in the huge in-vessel
tritium inventory, which is hard to remove andrecover. Since exhausted fuel from the vessel
includes all hydrogen isotopes (H, D, and T), He
and other impurities like water and hydrocarbons,the hydrogen isotopes must be refined andisotopicaly separated with each other to be recycled
as fuels.
One should note that the number of tritium
handling facilities is decreasing. And only one or
two laboratories can be handle massive amount oftritium. Our experience of handling the massive
amount of tritium in large volume like tokamak isvery poor, and multi-step contaminations reallyconcerns us.
4. Conclusion
PMI with DT fuels in a reactor are so hard to
realize and hence no one has ever experienced.
PMI studies require interdisciplinal R&D, i.e.,
physics, chemistry and engineering to handle
extremely low temperature materials (Ice pellet ofhydrogen) to high temperature (burning plasma)including radioactive materials (tritium and neutron
activated materials) and the PMI phenomena aresynergistic.
There are numbers of apparatus including
tokamaks, high heat load test machines, tritium
plasma source etc. Although each apparatus cansimulate certain phenomena expected in ITER
divertor, no one can realize conditions appearing in
ITER divertor simultaneously. In addition, no one
laboratory or institute has all apparatus, and these
are widely distributed.
The conclusion is quite simple. PMI studies dorequire international cooperation considering real
PMI phenomena in a reactor.