Ageing of electric cables in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 222

Cable ageing is a subset of cable degradation and primarily consists of
cracking, embrittlement, or other changes to the cable jacket or insulation
material. In most cases, these changes are produced by a combination of
physical age and environmental stressors such as temperature or radiation
exposure (AMS Corp., 2010). Cable circuits can be subjected to any or a
combination of the following stressors: oxidation, water intrusion, contami-
nation, vibration, thermal variations, electrical transient, voltage variations,
temperature, installation damage, and handling and physical contact (AMS
Corp., 2010). However, the three principle ageing factors for cables are (1)
elevated ambient temperature or humidity; (2) cyclic mechanical stress; and
(3) exposure to radiation (Hashemian, 2010). Cable degradation is mainly
dependent on environmental factors such as temperature, radiation, humid-
ity, or contaminants (IAEA, 2011).

6.2.1 Individual cable stressors: temperature, humidity,
mechanical stress, and radiation

Elevated temperatures cause the polymers in the cable insulation to degrade
through loss of elongation, embrittlement, and cracking (U.S. NRC, 2001).
Cable polymers are primarily degraded by thermal oxidation in the pres-
ence of oxygen, accelerating with increases in temperature as defi ned by the
modifi ed Arrhenius equation (IAEA, 2011):

k = A exp( − EA/ RT ) [6.1]

where EA is the activation energy, A is the frequency factor, and R is a con-
stant. Temperature is the most important ageing stressor for most cables in
a light water reactor (IAEA, 2011).
As a result of internal ohmic self-heating, power cables age uniquely,
depending on how long the cable carries electric current, which current it
carries, and the specifi c confi guration of the cable installation itself. Treeing
(the appearance of small tree-shaped cracks in the insulation caused by
electrochemical reactions) and the loss of the dielectric properties of cable
insulation are characteristic results of power cable ageing (IAEA, 2011; U.S.
NRC, 2001 ).
Exposure to moisture can also degrade cables that have been installed
directly in the ground or in ducts or conduits where water has access.
'Wetting' describes conditions in which a cable is exposed to moisture or
high humidity for extended periods of time, including limited periods of
complete submergence. Submersion describes conditions when the cable is
completely submerged in water for extended periods. So long as the insula-
tion and outer jacket are not damaged, intermittent wetting will not damage

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