Speaker
Mr
Ezzat Eisawy
(Nuclear and Radiological Regulatory Authority, Egypt)
Description
Due to recent renewed interest in reactor safety and many reactors
approaching end of useful lifetime, emphasis on durability of power
and instrumentation electrical Insulating Materials is growing. While
current materials have shown suitable radiation tolerance in lab
testing, combined effects of radiation, temperature, and water at
normal or abnormal conditions have led to cable failures. Effects of
radiation types and dose rates on selected cable insulating materials
have been studied. Effects of dose-rate temperature during radiation
on service endurance are considered.
Dielectric materials used to fabricate various parts of electrical
equipment systems, nuclear and electronic devices often operate in
ionizing radiation fluxes, problems of radiation resistance and
changing of insulating materials and devices are urgent. It is
necessary to develop ways to improve the radiation resistance.
Radiation damage to dielectric and insulating materials is a function
of temperature and atmospheric conditions as well as the radiation
environment. Many materials are more resistant to radiation in the
absence of oxygen or moisture and at lower temperatures. Because of
this influence of environmental conditions it is impractical to
attempt to compile detailed information that would be directly
applicable to all circuit requirements and environmental conditions.
The fabrication method used by the manufacturer can also be a factor
in the amount of damage that occurs from radiation. Both temporary
and permanent changes occur in the characteristics of organic
insulating and dielectric materials as a result of exposure to a
radiation environment. Enhancement of the electrical conductivity is
the most important of the temporary effects with increases of several
orders of magnitude being observed. The conductivity increases
exponentially in response to ionizing radiation until it reaches
equilibrium at a value that is determined by the rate of exposure and
ambient temperature for a specific material. Following the
termination of the irradiation the induced conductivity gradually
decreases. Other temporary effects, in addition to the enhanced
conductivity, are a reduction in breakdown and flashover voltages,
increases in AC loss characteristics, and variations in dielectric
constants. These changes in electrical characteristics, however, are
often not large enough to prevent the use of the insulators in a
radiation environment, particularly if allowances are made to minimize
their effect on the circuits’ performance.
Permanent effects of radiation on organic insulating materials are
normally associated with physical changes, including decreases in
hardness, tensile strength and melting point, and greater
solubility. This physical degradation in the advanced stages is
disastrous in that the insulating material breaks, crumbles, or
powders thus losing structural integrity and causing failure. Changes
in dissipation factor and insulation resistance have also occurred as
permanent effects, but they are normally quite small and offer few
problems except in the most uncommon applications. A comparison of the
relative radiation resistance of organic insulating materials to
permanent effects is presented. Gas evolution, a secondary reaction
that occurs when organic insulators are irradiated, is a problem
because of pressure build up in confined enclosures.
| Country/Organization invited to participate | Egypt |
|---|
Primary author
Mr
Ezzat Eisawy
(Nuclear and Radiological Regulatory Authority, Egypt)
