An Investigation of the Effects of Thermal Aging on the Fire Damageability of Electric Cables (NUREG/CR-5546, SAND90-0696)
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Manuscript Completed: May 1991
Date Published: May 1991
Steven P. Nowlen
Sandia National Laboratories
P.O. Box 5800
Albuquerque, New Mexico 87185-5800
Division of Engineering
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report documents the findings of an experimental investigation of the effects of thermal aging on the fire damageability of electric cables. Two popular types of nuclear qualified cables were evaluated. The two cables tested were (1) a neoprene jacketed, cross-linked polyethylene (XFE) insulated, three conductor, 12AWG, 600V light power or control cable produced by the Rockbestos Corporation and marketed under the trade name Firewall III, and (2) an ethylene-propylene rubber (EPR) insulated, chlorosulfonated polyethylene (CSPE or Hypalon) jacketed, two conductor, 16AWG, twisted shield pair, 600V instrumentation and signal cable produced by BIV Cable Systems, Incorporated, and marketed under the trade name Bostrad 7E. For each cable type, both unaged (i.e., new off the reel) and thermally aged samples were exposed to steady-state elevated temperature environments until conductor-to-conductor electrical shorting was observed. Plots of the time to electrical failure versus the exposure temperature were developed and thermal damage thresholds were determined.
Thermal aging had a different impact on the thermal damageability of each of the two cable types tested. For the Rockbestos cable, the thermally aged cables were less vulnerable to thermal damage than were the unaged samples. This conclusion is supported by (1) an increase in the thermal damage threshold for the aged samples, and (2) an increase in the time to thermal damage for the aged cables at exposure temperatures above the damage threshold. For the BIW cable, a mixed result was obtained. The threshold of thermal damage was lowered somewhat by the aging process, an indication of an increased vulnerability to thermal damage due to aging. However, for the higher temperature exposures, no statistical difference between the damage times for aged and unaged cable samples was noted. For both cable types, the changes in the thermal damage threshold observed were not considered large enough to result in a significant impact on fire risk estimates because the changes in damage threshold observed were not significant in comparison to other analysis uncertainties including uncertainty in the current models used to assess thermal damage times and uncertainties associated with other fire risk assessment input values.