Environmental Engineering Reference
In-Depth Information
reactive efforts have successfully resolved connector problems, corrected
signal-to-noise ratios, and improved grounding and shielding. However,
they have done little to identify the condition, age, or remaining useful
life of cables, especially the insulation material (AMS Corp., 2010). Not
enough research has been completed to identify a useful, practical method,
procedure or technique for accurately evaluating the ageing condition of
plant wiring or correlating the condition of cables to measurable electrical,
mechanical, or chemical properties (AMS Corp., 2010). In 2010, the U.S.
NRC (NRC DG1240) stated that 'research and experience have shown that
no single, nonintrusive, currently available condition monitoring method
can be used alone to predict the survivability of electric cables under acci-
dent conditions' (U.S. NRC, 2010b).
Because of the safety-related importance of I&C cables functioning effec-
tively on an ongoing basis, efforts to use prognostic techniques to predict
residual life in cables continue.
Such techniques attempt to establish relationships between condition
indicators and ageing stressors (IAEA, 2011). To predict future perfor-
mance, a trendable indicator and a well-defi ned end point are essential.
From them, a trend curve can be used to estimate the time remaining before
the end point is reached (U.S. NRC, 2001). Used with appropriate mate-
rial ageing models and knowledge of environmental conditions, such trend
data can be used to estimate residual cable lifetimes, but only when suffi -
cient data has been generated to validate predictive ageing models (IAEA,
2011). Currently, both the NRC and DOE are sponsoring research at AMS,
national laboratories such as Sandia National Laboratories (SNL), Oak
Ridge National Laboratory (ORNL), Idaho National Laboratory (INL),
and elsewhere to address cable aging and cable qualifi cation issues.
In recent years researchers have developed analytical ageing models
based on experimental data from cable samples that have been subjected
to accelerated ageing. For example, the power law extrapolation model
extrapolates radiation ageing data obtained under isothermal conditions
at several dose rates. Similarly, the superposition of time-dependent data
model combines data from both thermal and radiation ageing to account
for both dose rate effects and the synergistic relationship between radiation
and thermal ageing. The superposition of end-point dose data model also
uses a superposition approach to radiation and thermal ageing data, but
can be used in materials where a single dominant degradation mechanism is
lacking (AMS Corp., 2011).
There have been recent efforts toward integrated cable residual life
analysis systems that combine existing methods to provide cable testing,
ageing assessment and cable management as part of a plant-wide cable age-
ing assessment program (AMS Corp., 2010) (see Table 6.2). For example,
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