Environmental Engineering Reference
In-Depth Information
potentially both. Power cable failures have resulted in reactor trips, weak-
ened engineered safety features, loss of redundancy, and reduced power
operation (U.S. NRC, 2001).
For I&C cables, conductor short-circuit to ground failure and conductor-
to-conductor-short-circuit failure interrupt the transmission of control sig-
nals through the cable.
Degraded insulation resistance failure can impair the functioning of I&C
cable and/or increase the rate of error (U.S. NRC, 2010a). The transmit-
ted signal may become erratic, causing errors in measurement, spikes, noise,
and other problems. When cables become bare, shunting and short circuits
can occur, and if the cable insulator is degraded, the insulation material can
become brittle and fl ammable (Hashemian, 2010). I&C cables are the most
susceptible to ageing degradation (U.S. NRC, 2001).
In addition to signal anomalies and problems with plant control and safety
systems, cable ageing has resulted in loss of critical functions and fi re (AMS
Corp., 2011). In light water reactors, the most severe cable failure scenario
is loss of normal function during a LOCA when hot steam under pressure
can cause cables to malfunction if insulation ageing, cracks, or other damage
allow moisture to enter the cable. Hot steam combined with high pressure
is the primary cause for cable malfunction in a LOCA, because steam pen-
etrates smaller cracks more easily than water. Such consequences explain
why the Hungarian Paks Nuclear Power Plant has described cable ageing as
' the most signifi cant I&C ageing issue' in its plant (Hashemian, 2010).
A 2007 U.S. NRC report found that 93% of reported cable failures
occurred in normally energized power cables: 'More than 46% of the fail-
ures were reported to have occurred while the cable was in service and more
than 42% were identifi ed as 'testing failures' in which cables failed to meet
testing or inspection acceptance criteria' (U.S. NRC, 2010a). The majority of
these cable failures occurred between 11 and 30 years of service - less than
the typical 40-year licensing period of a plant (U.S. NRC, 2010a).
While many cases of cable failure are identifi ed through routine cable
testing, some occur before a failure is identifi ed (e.g. on cables that are not
normally tested or powered). This fact underscores the importance of imple-
menting a cable condition monitoring program (U.S. NRC, 2010b).
6.3
Analysis and assessment methods
Cable components such as the conductor wires, insulation, shielding, and
jacket material can all be tested to reveal signs of degradation. By applying
the right testing method or combination of methods effectively faults that
typically occur at cable connections can be confi rmed; these include termi-
nations, penetrations, and/or splices that have been exposed to mechani-
cal stress, oxidation, or corrosion. Other faults include end-device failure
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