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 pres­ence of oxygen, accelerating with increases in temperature as defined by the modified Arrhenius equation (IAEA, 2011):

к = A exp(-EA/RT) [6.1]

where EA is the activation energy, A is the frequency factor, and R is a con­stant. 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 specific configuration 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 insula­tion and outer jacket are not damaged, intermittent wetting will not damage most cables, but extended submersion is beyond the qualified operating conditions for most cables (U. S. NRC, 2010a).

Moisture can cause water treeing where voids or contamination are pre­sent in the cable. This combination of water and electrical stress degrades the insulation’s dielectric properties (U. S. NRC, 2001). In fact, the U. S. NRC (NUREG 6704) identified wetting as the primary ageing-related cause of failure (specifically, short circuit) for medium-voltage cables, in particu­lar the insulation (U. S. NRC, 2001). Such failure could allow currents and voltages to spread into the adjacent power distribution system, potentially causing other degraded power cables to fail too (U. S. NRC, 2010b). For this reason, cables in hard-to-access underground ducts and conduits, covered trenches, bunkers, and manhole vaults are the subject of special concern (U. S. NRC, 2010b).

Both power cables and I&C cables are directly affected by mechanical stress including bending, abrasion, cutting, contact, deformation, and per­foration, as a result of installation and maintenance, for example. Cables connected to vibrating machines are also subjected to stress, leading to chafing, cutting, or cracking of the cable insulation material (AMS Corp., 2011). Cable jacket and insulation material as well as cable conductors can be damaged by electromechanical forces caused by high levels of short cir­cuit current passing through a power cable (U. S. NRC, 2010a).

Radiation is another significant cause of cable degradation. During nor­mal operation, gamma and neutron radiation cause oxidative degradation in increasing (nonlinear) relation to the radiation dosage absorbed by the cable. During accidents, beta radiation may also affect cables unprotected by a conduit (IAEA, 2011).