Environmental Engineering Reference
In-Depth Information
dependable power cables enable pumps and valves to be activated or deac-
tivated as the plant recovers (Hashemian, 2010; U.S. NRC, 2001).
In normal but particularly in accident situations, cables and their coatings
can be exposed to a range of stressors, from heat and humidity to ther-
mal and mechanical shock and radiation. When instrumentation cables and
power cables become exposed to or submerged in water, chemicals, high
pressure steam or other environments during an accident, accident recovery
and mitigation can be severely endangered (Hashemian, 2010).
Because cables have been regarded as passive, durable components that
have proven their ability to evade the need for replacement; they have tra-
ditionally received insuffi cient attention (U.S. NRC, 2010b). This neglect
was partly because the wholesale replacement of cables is almost impos-
sible; cables are only readily replaceable in short sections (AMS Corp.,
2011). However, the issue of cable ageing and degradation has intensifi ed
as the trend toward license renewal has enabled existing plants to operate
for up to 60 or even 80 years - twice their original planned life. The accident
at the Fukushima nuclear power plants - the oldest of which is more than
40 years old - has raised questions about the advisability or practicality
of extending the life of old plants. But whether plants are operated for 40,
60, 80 years or more, the Fukushima tragedy has underscored the impor-
tance of safety, in which cables play a ubiquitous and important role (AMS
Corp., 2011 ).
Given that wholesale replacement of cables is neither prudent nor prac-
tical, cost-effective ageing management strategies and techniques that
objectively assess cable condition and remaining life are essential (AMS
Corp., 2010). This chapter addresses the ageing of nuclear power plant
cables - especially I&C, low-voltage, and medium-voltage cables - and
methods for testing these cables to manage their ageing and verify their
reliability.
6.1.1 Cable component types and properties
Several hundred different cable types and sizes are used throughout a typ-
ical light water plant. Plant conditions determine which type of cable is
used; for example, cables for control rod drive mechanisms must withstand
higher temperatures and have additional shielding capacity (IAEA, 2011).
I&C cables are by far the most common cable type (Hashemian, 2010).
Instrumentation cable, which includes thermocouple (T/C) extension wires,
is a low-voltage (< 1 kV), low-ampacity cable used to transmit digital or ana-
log measurement signals from transducers such as resistance temperature
detectors (RTDs) and pressure transmitters. Control cable, also low-voltage
and low-ampacity, is used in the circuits of control (rather than monitoring)
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