Radiation induced degradation of materials is a well-known phenomenon.
Many ceramic components are expected to be used as insulators
in a fusion reactor, i.e., for diagnostics and for the reduction
of the electromagnetic force on various in-vessel components.
The degradation of the electrical resistance is an important consideration
in the design of a fusion reactor. This degradation occurs from
two causes. One is the permanent degradation resulting from crystalline
disorder induced by radiation. The other, radiation-induced conductivity
(RIC), is induced during radiation. The latter is a spontaneous
phenomenon, but is an important cause of such degradation. The
RIC is depicted schematically in Fig. 8-5 on the basis of the
energy level of an insulator. Electrons filling the valence band
are excited to the conduction band by radiation. In this band
they contribute to electrical conductivity, which leads to the
degradation of the resistance of the insulator.
Our first in situ experiments of the change of the electrical
resistance were performed during irradiation with 14-MeV neutrons,
as described below. The degradation of the electrical resistance
of a high purity Al2O3 single crystal, which is representative of insulators, is shown
in Fig. 8-6. This irradiation was done by neutrons from the Fusion
Neutron Source (FNS). The resistance of the specimen decreases
immediately after the start of the irradiation (see the time around
550 seconds on the abscissa) and recovers in a step-wise manner
after termination of the irradiation. The gradual increase of
the resistance after the stop is caused by residual gamma rays
induced by neutron irradiation. When the relationship between
the radiation-induced degradation of resistance and the neutron
dose rate, which was obtained in the present study, can be extended
to the fusion-relevant neutron dose rate, i.e., 103 - 104 gray/sec., the electrical resistance of Al2O3 will decrease significantly, to a level of 106-107 ohmmeter. |
