Ageing of electric cables in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 228

chemical and mechanical tests like the cable indenter, were developed spe-
cifi cally for evaluating the condition of the protective jacket or insulation on
a cable (AMS Corp., 2010).

6.3.1 Visual and mechanical measurements

Visual inspection involves examining the cable throughout its length during
a formal plant walkdown, a useful practice when, as is often the case, degra-
dation is visible to the naked, well-trained eye (IAEA, 2011). Visual inspec-
tion can identify changes in physical/visual appearance, surface texture, and
damage as a result of manufacturing or operation (U.S. NRC, 2010b). More
sophisticated techniques can then be used to determine the degree of age-
ing more accurately.
The advantages of visual inspection are that it is low cost and easy to
perform, requires no specialized equipment, does not require that samples
be removed from the cable, and can be performed on operating equipment
in-situ . Its disadvantages include the requirement that the cable be accessi-
ble and visible, inspectors must be trained to evaluate what they are looking
at (subjectivity), it generally only provides information on the cable jacket,
and it does not provide quantifi able results (no trending possible) (AMS
Corp., 2010 ; IAEA, 2011 ).
Mechanical testing is a subset of life-testing techniques that involves
inspecting cables for cracks or changes in color, texture or hardness, mass loss,
visco-elasticity properties, or size (swelling, shrinkage, deformation). Among
the most conventional and popular means of mechanical cable testing are mea-
suring the elongation-at-break of the cable and its tensile strength when pulled
apart. The elongation-at-break test measures the strain on the cable when it
breaks and is a recognized standard for assessing the health, integrity, and func-
tionality of a cable insulation material (IAEA, 2011). This test is performed by
stretching a 'dog bone'-shaped cable sample until it breaks. The elongation-at-
break test yields information on the tensile strength and modulus of elasticity
of the cable, but the percentage of elongation is the most important criterion in
evaluating cable health. When the percentage elongation-at-break is less than
50%, the cable is considered to be unhealthy - potentially unable to survive
DBA conditions (AMS Corp., 2010; IAEA, 2011).
The tensile test measures the stress needed to break the cable. For poly-
meric materials like thermoplastics, tensile strength only begins to fall after
substantial ageing has already occurred. Both the elongation-at-break and
tensile strength tests can be performed using a tensile testing machine.
A third mechanical test, measuring compressive modulus, involves check-
ing the ductility of the cable insulation or jacket material to determine if the
cable has become dry, brittle, or prone to crack. Developed in the mid-1980s

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