Radiation problems for insulators in ITER and beyond
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
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| Цитувати: | Radiation problems for insulators in ITER and beyond / E.R. Hodgson // Вопросы атомной науки и техники. — 2002. — № 4. — С. 76-79. — Бібліогр.: 45 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859841892372447232 |
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| author | Hodgson, E.R. |
| author_facet | Hodgson, E.R. |
| citation_txt | Radiation problems for insulators in ITER and beyond / E.R. Hodgson // Вопросы атомной науки и техники. — 2002. — № 4. — С. 76-79. — Бібліогр.: 45 назв. — англ. |
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ITER AND FUSION REACTOR ASPECTS
RADIATION PROBLEMS FOR INSULATORS IN ITER AND BEYOND
E.R. Hodgson
Euratom / CIEMAT Fusion Association, 28040 Madrid, Spain
PACS: 62.40.Hf
INTRODUCTION
Present plans envisage that ITER (International
Thermonuclear Experimental Reactor) will come into
operation during the second decade of this new 21st
century, with the purpose of bridging the gap between the
present day large "physics" machines and the pre-
commercial DEMO reactor. Commercial reactors could
then become available by the middle of the century.
Although ITER will help to solve many problems
remaining in the field of plasma physics, it will present
additional operational and experimental problems due to
radiation damage effects as a result of the intense radiation
field from the “burning” plasma. The ignited plasma will
give rise to a high energy neutron and gamma flux,
extending well beyond the first wall, from which one
foresees a serious materials problem which has to be
solved. In the initial physics phase radiation flux will be of
concern, whereas in the later technology phase both flux
and fluence will play important roles as radiation damage
builds up in the materials. For metallic materials the
problem of radiation damage is expected to be severe,
although tolerable, only near to the first wall, however the
problem facing the insulating components is more serious
due to the necessity to maintain not only mechanical, but
also the far more sensitive physical properties intact. The
need to carry out inspection, maintenance, and repair
remotely due to the neutron induced activation of the
machine is also of concern. Remote Handling will require
machines which use standard components ranging from
simple wires, connectors, and motors, to optical
components such as windows, lenses, and fibres, as well
as electronic devices such as cameras and various
sophisticated sensors. All these components use insulating
materials. We face a situation in which insulating
materials will be required to operate under a radiation
field, in a number of key systems from plasma heating and
current drive, to diagnostics, as well as remote handling
maintenance systems. These directly affect not only
operation, but also safety, control, and long term
reliability of the machine. In the long term, beyond ITER,
the solution of the materials problem will determine the
viability of fusion power.
FUSION RELEVANT RADIATION DAMAGE
Radiation will modify to some degree all of the important
material physical and mechanical properties.
Unfortunately in general these changes do not improve the
materials. Some of the changes are flux dependent, while
others are modified by fluence. Flux dependent processes
are of concern from the on-set of operation, while fluence
affects component and material lifetime. The insulator
properties of concern include electrical resistance,
dielectric loss, optical absorption and emission, as well as
thermal and mechanical properties. Papers discussing
general and specific aspects of radiation damage in
insulating materials for fusion applications are included
[1-11].
The study of intense radiation effects in metals has been
closely associated with the development of nuclear fission
reactors, aso by the 1980's when the urgent need to
consider radiation damage aspects of materials to be
employed in future fusion reactors was fully realised, a
considerable amount of data existed for metallic materials
[12]. This was not so for insulators, due to the fact that
insulators in fission type reactors are limited to low
radiation regions. However despite this considerable
progress has been made in assessing the possible problem
areas and finding viable solutions. Several general reviews
give a good introduction to radiation damage in insulators
[13 - 17].
The materials in ITER and beyond will be subjected to
fluxes of neutrons and gammas due to the ignited plasma.
The intensity will depend not only on the distance from
the plasma, but also in a complex way on the position
within the machine due to streaming along numerous
penetrations required for cooling systems, blanket
structures, heating systems, and diagnostic and inspection
channels, as well as radiation from the cooling water due
to the 16O(n,p)16N nuclear reaction. However models are
available which enable the neutron and gamma fluxes to
be calculated with confidence [18 - 20]. At the ITER first
wall the primary displacement dose rate will be about 10-6
dpa/s, and the ionizing dose rate 104 Gy/s.
The polyatomic nature insulators make them far more
sensitive to radiation damage than metals. While stainless
steel can withstand several dpa and GGy with no problem,
some properties of insulators can be modified by as little
as 10-6 dpa or a few kGy. Radiation damage results in a
change in the electrical and thermal conductivity,
dielectric loss and permittivity, optical properties, and to a
lesser extent the mechanical strength and volume. Hence
insulators may suffer Joule heating due to increased
electrical conductivity or lower thermal conductivity,
windows become opaque from the microwave to the
optical region, and in addition they may become more
brittle and swell. Of the numerous insulating materials the
refractory oxides and nitrides show the highest radiation
resistance. MgO, Al2O3, MgAl2O4, BeO, AlN, and
Si3N4 have received specific attention. In addition SiO2,
and diamond and silicon have been examined for window
and optical transmission applications.
Finally one should mention transmutation. Nuclear
reactions in the materials will give rise to transmutation
products [21]. These build up with time and represent
impurities in the materials which may modify their
properties. Physical properties of insulators are
particularly sensitive to impurities. Some of these
transmutation products are radioactive and give rise to the
76 Problems of Atomic Science and Technology. 2002. 4. Series: Plasma Physics (7). P. 76-80№
need for remote handling and hot cell manipulation in the
case of component removal, repair, or replacement. For
the structural materials, in the present concepts mainly
steel alloys, considerable work has been carried out on the
development of so-called reduced activation materials
(RAM) for use in DEMO and future commercial fusion
reactors [22]. For insulating materials no equivalent study
or development has been carried out, due in part to the
small fraction of the total material volume represented by
the insulators, but also because the important physical
properties of these materials will have degraded before the
transmutation products become of concern. Certainly for
ITER transmutation products, with the possible exception
of hydrogen and helium, are not expected to present a
serious problem.
SIMULATION EXPERIMENTS
At present no entirely suitable irradiation testing facility
exists so experiments are being performed in nuclear
fission reactors and particle accelerators, as well as
gamma and X-ray sources, to try to simulate the real
operating conditions of the insulating materials and
components. This can be justified as long as the influence
of the type of radiation on the physical parameter of
interest is known. This in certain cases is true for radiation
induced electrical conductivity and radioluminescence for
example, where for low total fluences it is the ionizing
component of the radiation field which is important. The
experiments must simulate the neutron and gamma
radiation field i.e. the displacement and ionization damage
rates, the operating environment i.e. vacuum and
temperature, and also the operating conditions such as
applied voltage, or mechanical stress. It is essential that
in-situ testing is carried out to determine whether or not
the required physical properties are maintained during
irradiation. Examples of this include electrical
conductivity which can increase many orders of magnitude
due to the ionizing radiation, or optical windows which
may emit intense radioluminescence.
Fission reactors have the advantage of producing both
neutrons and gammas, although the neutron energy
spectrum and the displacement to ionization ratio are not
those which will be experienced in a fusion reactor. The
main difficulties with in-reactor experiments come from
the inaccessibility of the radiation volume and are
concerned with the problem of carrying out in-situ
measurements and achieving the correct irradiation
environment. While considerable success has been
attained in the in-situ measurement requirement, with
parameters such as electrical conductivity, optical
absorption and emission, and even radiofrequency
dielectric loss being determined, the problem of
irradiating in vacuum still remains, with most experiments
being performed in a controlled He environment. Also
nuclear activation generally means that post irradiation
examination (PIE) has either to be carried out in a hot cell,
or postponed until the material can be safely handled.
Particle accelerators are ideal for carrying out in-situ
experiments in vacuum at controlled temperatures due to
easy access and localised radiation field. High levels of
displacement and ionization can be achieved with little or
no nuclear activation. However the non-nuclear aspect of
the radiation field is a problem. A further disadvantage is
due to the limited irradiation volume and particle
penetration depth. This means that only small thin material
samples or components can be tested.
ELECTRICAL INSULATOR DEGRADATION
Electrical resistance, generally discussed in terms of
electrical conductivity (inverse of resistance), is an
important basic parameter for numerous systems and
components including the NBI (Neutral Beam Injection)
heating system, ICRH (Ion Cyclotron Resonant Heating)
windows and supports, magnetic coils, feed-throughs and
stand-offs, MI cables and wire insulation. Reduction in
electrical resistance of the insulators in these components
may give rise to Joule heating, signal loss, or impedance
change. The main candidate for these applications is
Al2O3, and has been extensively studied, in the
polycrystalline alumina form and as single crystal
sapphire. Four types of electrical degradation in a
radiation environment are recognised and being
investigated, these are; Radiation Induced Conductivity
(RIC), Radiation Induced Electrical Degradation (RIED),
Surface Degradation, and Radiation Induced Electro-
Motive Force (RIEMF).
Fig. 1. Schematic RIC as a function of irradiation time
(dose) and dose rate.
RIC, RIED, and surface degradation are fully discussed
elsewhere [8]. RIC is a flux dependent enhancement of the
electrical conductivity due to excitation of electrons from
the valence to the conduction band. Figure 1 shows
schematically the RIC as a function of irradiation time and
ionizing dose rate (flux). The increase to saturation
depends on the dose rate and in a complex way on
temperature and sample impurity content, see figure 2
where RIC for MgO:Fe at 0.1 Gy/s is given [23]. The
complex behaviour is well predicted by theory [24].
77 Problems of Atomic Science and Technology. 2002. 4. Series: Plasma Physics (7). P. 80-83№
Fig. 2. RIC as a function of irradiation temperature and
impurity content for MgO:Fe. 1.8 MeV Bremsstrahlung
irradiation at 0.1 Gy/s [23].
For the dose rates of interest for fusion, approximately 1
Gy/s to 1000 Gy/s, saturation is reached within seconds to
minutes and it is the saturation level which is of concern.
From available data, one can safely say that RIC is
sufficiently "well understood" to allow this type of
electrical degradation to be accommodated by the design,
and that materials exist which give rise to electrical
conductivities ≤ 10-6 S/m for ionizing dose rates of up to
104 Gy/s. One only expects possible problems near the
first wall. Unfortunately this is the region where magnetic
coil diagnostics, which can tolerate only very low leakage
conductivity, will be employed. RIC is a flux dependent
effect and will be present from the on-set of operation of
ITER. Hence devices which are sensitive to impedance
changes, which will occur for example in MI cables, must
take RIC into account. Furthermore RIC is strongly
affected by impurity content, figure 2 and [8], hence the
build-up of transmutation products will modify RIC with
irradiation time (fluence), although this is not expected to
be of serious concern for ITER.
RIED, see figure 3, is more serious, not only from the
point of increasing the electrical conductivity beyond that
of the RIC, but also because this type of degradation is
still not fully understood, nor even is there general
agreement as to whether RIED exists as a real volume
degradation [8, 11]. The most recently completed in-
reactor RIED experiment in HFIR at ORNL [25 - 27]
helps to throw light on the complex RIED problem. Initial
results indicated no significant increase in electrical
conductivity for the 12 different samples. However
moderate to substantial electrical degradation was
observed in some of the sapphire and alumina samples, so
material type may be an important parameter [27]. Despite
the purely academic distinction, for the insulating
components, surface degradation is just as serious as
volume degradation. Two types of surface degradation
have been reported, one related to surface contamination
caused by poor vacuum, sputtering, or evaporation [28 -
30] and real surface degradation of the material related to
surface vacuum reduction and possibly impurity
segregation [31 - 33]. To illustrate all these problems,
figure 4 shows the leakage current measured for a vacuum
gauge 99.7 % alumina insulated feedthrough component
electron irradiated at 300 C, 700 Gy/s [34]. The initial
large increase in conductivity is due to RIC, and the slow
permanent increase is due to either RIED or surface
degradation.
Fig. 3. Schematic RIC and RIED as a function of
irradiation time (dose), showing the underlying
permanent degradation.
Fig. 4. 99.7 % alumina feed-through leakage current as a
function of irradiation time, with 1.8 MeV electrons, 700
Gy/s, 300 C. Leakage is due to RIC, and RIED and/or
surface degradation.
Strictly speaking RIEMF is not a degradation, but an
induced voltage / current which "degrades" the signal
quality carried by the mineral insulated (MI) coaxial
cables in a radiation field. RIEMF can produce several
volts between the inner and outer conductors, or supply
tens of microamps of current, and has been known and
employed in reactor control since the early 70's [35]. The
effect is due to electron producing reactions such as (n, β),
(n, γ, e) etc. causing an unbalanced charge distribution
between the inner conductor and the outer sheath of MI
cables. Judicious choice of the inner and outer materials
together with their diameter and thickness can minimize
the effect for a given neutron and gamma flux and
spectrum, however the rapidly varying radiation field
expected for next step fusion devices means that RIEMF
will have to be tolerated rather than eliminated.
While considerable concern has been expressed about the
possible radiation induced degradation of solid insulating
materials under a fusion radiation environment, and by
implication in those required for the ITER NBI accelerator
system, little or no attention has been paid until very
recently to the problem of the insulating gas which will be
required around the NBI high voltage feed line, ion source
and accelerator. This gas, in the present design SF6, will
be in a radiation field of the order of 1 Gy/s due to the
fusion plasma and the NBI accelerator itself. The radiation
will cause ionization in the gas, and hence an increase in
the gas electrical conductivity. As this is a source of power
loss due to the corresponding leakage current which in
addition will produce heating and possibly breakdown, the
radiation effect must be quantified and taken into account
in the engineering design of the NBI system. Results show
that the gas does not behave like a solid insulator, but that
the leakage current is a function of the gas volume due to
the possibility of collecting all the generated charge
carriers. For the 1 MV ITER NBI system this implies that
up to megawatts of power could be lost due to this
radiation induced leakage current [36]. To limit this the
use of vacuum insulation is being considered.
DEGRADATION OF OPTICAL PROPERTIES
78
Finally quick mention should be made of another area of
concern related to the effects of radiation on the optical
properties of materials to be used as transmission
components (windows, lenses, and optical fibres) for the
UV, visible, and IR wavelengths [11, 37, 38].
Fig. 5. Radioluminescence for KU1 and KS-4V. 1.8 MeV
electrons, 700 Gy/s, 70 C
For remote handling applications the optical components
are expected to maintain their transmission properties
under high levels of ionizing radiation (1 - 10 Gy/s) during
many hundreds of hours. Here radiation induced optical
absorption imposes the main limitation. However in the
case of diagnostic applications, in addition to a higher
level of ionizing radiation (tens to hundreds Gy/s), the
material will be subjected to atomic displacements of the
order of 10-10 dpa/s. For these applications both radiation
induced optical absorption and light emission
(radioluminescence) impose severe limitations on the use
of SiO2 and sapphire, present day ITER candidate
materials, making it extremely difficult to separate out the
plasma emission from the window emission and
absorption [39]. Work on KU1 and KS-4V quartz glasses
provided by the Russian Federation for the ITER
programme has shown that suitable materials do exist in
which the radioluminescence can be reduced to a
minimum, as may be seen in figure 5 where the
radioluminescence from KU1 is almost at the Cherenkov
limit [40]. However one must remember that with
irradiation displacement dose the optical absorption
related to oxygen vacancies in SiO2 quickly renders this
material opaque in the UV and visible range [41 - 44]. Of
course some radiation effects can be put to good use and
this is the case of radioluminescence, which while a
problem for optical windows can be employed as a
detector / converter for X-ray and UV emission from the
plasma. This is illustrated in figure 6 where the intense
radioluminescence from Al2O3 : Cr is shown. Such
emission has been used for many years in ceramic
fluorescent screens for accelerator beam alignment [45],
and is now being developed for improved radiation
resistance and rapid decay times for fusion applications.
CONCLUSIONS
The problems of electrical and optical degradation in
insulating materials for next step fusion devices have been
briefly presented. Although the task ahead is difficult,
important advances are being made not only in the
identification of potential problems, but also in the
understanding of the radiation effects as well as materials
selection and design accommodation to enable the
limitations to be tolerated or even employed.
Fig. 6. Intense Cr radioluminescence for Al2O3:Cr. 1.8
MeV electrons, 700 Gy/s, 30 C.
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80
INTRODUCTION
FUSION RELEVANT RADIATION DAMAGE
SIMULATION EXPERIMENTS
ELECTRICAL INSULATOR DEGRADATION
DEGRADATION OF OPTICAL PROPERTIES
CONCLUSIONS
Fig. 6. Intense Cr radioluminescence for Al2O3:Cr. 1.8 MeV electrons, 700 Gy/s, 30 C.
REFERENCES
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| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T15:37:00Z |
| publishDate | 2002 |
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| spelling | Hodgson, E.R. 2015-04-14T05:21:48Z 2015-04-14T05:21:48Z 2002 Radiation problems for insulators in ITER and beyond / E.R. Hodgson // Вопросы атомной науки и техники. — 2002. — № 4. — С. 76-79. — Бібліогр.: 45 назв. — англ. 1562-6016 PACS: 62.40.Hf https://nasplib.isofts.kiev.ua/handle/123456789/80257 en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники ITER and fusion reactor aspects Radiation problems for insulators in ITER and beyond Article published earlier |
| spellingShingle | Radiation problems for insulators in ITER and beyond Hodgson, E.R. ITER and fusion reactor aspects |
| title | Radiation problems for insulators in ITER and beyond |
| title_full | Radiation problems for insulators in ITER and beyond |
| title_fullStr | Radiation problems for insulators in ITER and beyond |
| title_full_unstemmed | Radiation problems for insulators in ITER and beyond |
| title_short | Radiation problems for insulators in ITER and beyond |
| title_sort | radiation problems for insulators in iter and beyond |
| topic | ITER and fusion reactor aspects |
| topic_facet | ITER and fusion reactor aspects |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/80257 |
| work_keys_str_mv | AT hodgsoner radiationproblemsforinsulatorsiniterandbeyond |