Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 3, 1992
NRC INFORMATION NOTICE 92-01: CABLE DAMAGE CAUSED BY INADEQUATE CABLE
INSTALLATION PROCEDURES AND CONTROLS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission is issuing this information notice to
alert addressees to the possibility that inadequate cable installation
procedures and quality control could cause safety-related cables to fail.
It is expected that the recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances
In June 1989, the Tennessee Valley Authority (TVA) removed the cables from a
conduit in the reactor protection system of the Watts Bar Nuclear Plant,
Unit 2, to inspect for damage. TVA selected this conduit in response to an
employee's concern that a welding arc that struck the conduit during
construction may have damaged cables in the conduit. When the cables were
removed, TVA found significant damage in the insulation of some cables.
This damage was not attributed to heat generated by the alleged welding arc.
The damage was principally attributed to the pulling stresses exerted during
the initial installation of the cables. Some of the cables had insulation
removed down to the conductors. To assess the extent of cable damage and
determine the scope of its investigation, TVA removed more cables from
conduits that constituted the most difficult pulls (worst case) and found
varying degrees of damage that it attributed to pulling stresses.
To fill a conduit at Watts Bar, personnel used pull cords to pull more
cables through the conduits over the top of existing cables. This practice
is called "pull-by." This practice can cause damage to the existing cables
from the
9112270125
.
IN 92-01
January 3, 1992
Page 2 of 3
sawing action of the pull cords and the friction of cables as they are
pulled over existing cables. The University of Connecticut, under contract
for TVA, evaluated damaged cables and determined the cause of damage to have
been cable pull-bys. Usually such damage would be minimized by using the
proper and adequate amounts of lubricants, controlling pulling tension,
choosing appropriate pull cords, limiting the distance between pull points,
and minimizing the number and angle of bends allowed in the conduit.
Industry standards provide no specific guidance for performing multiple
pulls of cables in conduits.
TVA instituted programs to assess the adequacy of cable installation at all
its nuclear facilities and take appropriate corrective actions. At Watts
Bar, TVA replaced cables in conduits which exceeded a calculated threshold
value of side wall bearing pressure (SWBP) and performed high-potential
(hi-pot) tests on a number of other cables in conduits with SWBP below the
calculated threshold value. The hi-pot test voltage was 240 Vdc per mil of
insulation thickness (based on the minimum qualified thickness) not to
exceed 80 percent of the factory's test voltage. Before conducting the
testing, TVA flooded the segments of conduits with the highest SWBP values
with water to establish a continuous electrical ground at the external
surface of the cable. TVA did not flood conduits with shielded cables
because the shield provided adequate ground.
At the Sequoyah and Browns Ferry Nuclear Plants, TVA performed hi-pot tests
on cables in a smaller sample of conduits with high SWBP values. TVA did
not replace, but included in the test sample, cables at these plants that
exceeded the threshold SWBP values of Watts Bar because it had not used the
very abrasive nylon cord (parachute cord) at these plants that was
extensively used at Watts Bar. This cord was used as pull cord and seemed
to have caused the more severe damage to cables. Though TVA observed some
failures during the hi-pot testing, it attributed none of the failures to
the effects of cable pull-bys.
Discussion of Safety Significance
The damage identified in the reactor protection system cables at Watts Bar
demonstrates that the safety function of safety systems could be lost if
damaged cables are located in harsh environments. Such cable damage can be
caused by the pulling stresses exerted during cable installation. If
moisture enters the affected conduits, it can cause cables to short, which
could cause the common mode loss of safety function of systems whose
performance is required to mitigate the consequences of an accident.
.
IN 92-01
January 3, 1992
Page 3 of 3
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contact: H. Garg, NRR
(301) 504-2929
Attachment: List of Recently Issued NRC Information Notices
.