Environmental Engineering Reference
In-Depth Information
may appear as a problem with the conductor in test data and may be
the result of corrosion, loose terminations, and other faults (AMS Corp.,
2010 ).
Cable manufacturers qualify cables for a specifi c service life (e.g. 40 years
for nuclear power plant cables) and specifi c voltage class at a given max-
imum ambient temperature (U.S. NRC, 2010a). Service life is affected by
everything from voltage and temperature rating of the cable and the mate-
rial and thickness of its insulation and conductor jacket, to the conductor
size and construction (e.g. solid or stranded), the type of metal and coat-
ings used in the conductor, the cable confi guration (e.g. single or multiple),
and the presence of ground conductors, shields, braids, or binding and fi ller
material (U.S. NRC, 2001).
Because I&C cables are used at low current, their typical operating tem-
peratures are between about 40°C and 65°C (IAEA, 2011). In contrast,
power cables can operate at 80-90°C because of continuous current fl ow,
which generates ohmic self-heating, and the higher voltages and currents
used to power medium- and high-voltage equipment such as pump motors
(IAEA, 2011 ).
Because of their typical length, cables can experience multiple operating
environments as they travel through different areas of the plant, including
harsh temperature, radiation, humidity, and moisture conditions, which may
include submersion in water (IAEA, 2011; U.S. NRC, 2001).
6.2
Cable degradation issues
As long as cables are installed properly and not exposed to environmental
conditions beyond their design basis, they are generally durable and rel-
atively long lived, typically lasting 40-50 years (Hashemian, 2010; IAEA,
2011). In fact, compared to other I&C components, cables have historically
experienced few problems. A Japanese study, for example, found that most
nuclear power plant I&C cables will maintain their electrical function capa-
bilities over 60 years of operation (Hashemian, 2010).
The IAEA defi nes a 'mild' operating environment as one that 'would at
no time be signifi cantly more severe than the environment that would occur
during normal plant operation, including anticipated operational events.'
In contrast, a 'harsh' environment is one that results from a design basis
accident (DBA) involving, for example, a loss-of-coolant accident (LOCA)
or the failure of a high-energy line or main steam line (IAEA, 2011). Mild
and harsh operating environments can be distinguished from unanticipated
operating conditions such as those caused by poor installation, operation or
movement of the cable. All of these can accelerate cable ageing and degra-
dation (IAEA, 2011).
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