IE Circular No. 79-05, Moisture Leakage in Stranded Wire Conductors
CR79005
March 14, 1979
MEMORANDUM FOR: B. H. Grier, Director, Region I
J. P. O'Reilly, Director, Region II
J. G. Keppler, Director, Region III
K. V. Seyfrit, Director, Region IV
R. H. Engelken, Director, Region V
FROM: Norman C. Moseley, Director, Division of Reactor
Operations Inspection, Office of Inspection and
Enforcement
SUBJECT: IE CIRCULAR NO. 79-05, MOISTURE LEAKAGE IN STRANDED WIRE
CONDUCTORS
The Draft Information Notice covering the above subject matter has been
upgraded to IE Circular status following consideration of several comments
from NRR and IE Staff. The Circular is transmitted for issuance on March 20,
1979. This Circular should be issued to al holders of Reactor Operating
Licenses or Construction Permits. Also enclosed is a draft copy of the
transmittal letter.
Norman C. Moseley, Director
Division of Reactor Operations
Inspection
Office of Inspection and Enforcement
Enclosures:
1. Draft Transmittal Letter
2. IE Circular No. 79-05
CONTACT:
V. D. Thomas, IE
49-28180
.
(Transmittal letter for Circular 79-05 to each holder of an NRC Operating
License or Construction Permit.)
IE Circular No. 79-05
Addressee:
The enclosed Circular 79-05 is forwarded to you for infomation. If there are
any questions related to your understanding of the suggested actions, please
contact this office.
Signature
(Regional Director)
Enclosure:
IE Circular No. 79-05
.
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 20, 1979
IE Circular No. 79-05
MOISTURE LEAKAGE IN STRANDED WIRE CONDUCTORS
Description of Circumstances:
During a staff review of the results of environmental qualification test of
certain electrical equipment, the phenomenon of water penetration between an
electrical conductor and its loosely fitting insulation sleeve was
identified. The staff determined it to be prudent to investigate this
phenomenon and initiated an equipment checkout test at Sandia Laboratories
using a few typical conductors used in nuclear plants.
The equipment checkout test for cable leaks performed at Sandia Laboratories
in August 1978, has shown that most stranded wire conductors, when subjected
to a differential pressure across the conductor ends, will leak steam or
moisture through the interstices of the strands of wire. The test has also
shown that solid conductors, under similar conditions, do not leak.
Steam/moisture leakage through stranded conductors can occur during a Loss
of Coolant Accident/Main Steam Line Break (LOCA/MSLB), if a differential
pressure were to develop across the conductor ends; e.g., one end of the
conductor is exposed to the accident environment (typically, in an unsealed
junction box) and the other end is isolated from the environment (typically,
inside a sealed component such as a sensor/transmitter).
In order to determine whether as-installed configurations exist in nuclear
power plants which may result in degradation of safety-related functions due
to moisture leakage in stranded wire conductors, inspection visits by NRC
personnel were made in October to two nuclear plants, (one under
construction, and the other operating). During the inspections the detailed
configuration of stranded conductor cables, and their terminations at
splices, penetrations, junction boxes, transmitters, motors, motor
1 of 4
.
IE Circular No. 79-05 March 20, 1979
operated valves, and valve limit switches were reviewed. Special attention
was given to termination details of stranded conductors at the terminal
blocks installed inside junction boxes and at the connections to the various
equipment. Additional infomation was provided by both licensees of the power
plants visited and by the Architect-Engineer regarding design and
construction details of such equipment. Based on observations made during
these site visits and information obtained, the following items have been
identified as areas where the possibility of the steam/ moisture incursion
phenomenon exists:
1. Terminal Blocks/Junction Boxes - Junction boxes in containment that are
used to house terminal blocks where cables were terminated, usually
have dust tight and drip proof housings with vent and drain holes.
Cables terminated in this fashion are generally stranded conductor
type. The terminal blocks and cable terminations inside these boxes
will be exposed to the containment environment during normal operation
and a LOCA. No special provisions are generally made in the termination
of stranded conductors in terminal blocks/junction boxes to prevent
moisture or steam incursion.
2. Sensor Transmitters - Sensor transmitters such as Foxboro, Fisher and
Porter, Barton, etc. are typically contained in sealed housings that
are designed and constructed to maintain their internals isolated from
the environment during a LOCA. The electrical connection to the
transmitter is generally made with solid conductors which pass through
qualified pressure seal connectors. However, if stranded conductors are
used to make such connections, moisture incursion is a possibility.
With this configuration, a differential pressure can develop between
the exposed end of a cable in a terminal block or junction box and the
end terminated in a sensor transmitter.
3. Motors - The cable terminations at motors are normally housed in
terminal boxes which are not sealed and cables are brought into them by
conduits which are open at, the other end. Hence, no differential
pressure will exist across the cable. However,
2 of 4
.
IE Circular No. 79-05 March 20, 1979
if the cable terminations at the motor are sealed and stranded
conductors are used to make such connections, then moisture incursion
is a possibility.
4. Electric Motor Operated Valves (MOV's) and Limit switches The cables
are brought into the housing of these components through open conduits
which were not designed to keep out the environment. Since these
components were qualified with the housings vented, steam/moisture
incursion does not appear to be of concern.
These items were observed and no potential for steam/moisture incursion was
found:
1. Splices - Qualified heat-shrink butt splices are used on terminations
in place of terminal blocks or connectors in several nuclear plants.
Where there are no interposing junction boxes or other possible
moisture entry points in a cable run between the penetration and the
equipment (and only splices are used), then moisture incursion into the
splice is not possible; i.e., if no differential pressure can exist
then moisture incursion is not possible.
2. Penetrations - The containment end and the end outside the containment
will experience a differential pressure during a LOCA event. However,
no leakage will occur across the conductor associated with the
penetration since the penetrations are designed, constructed, and
installed to prevent leakage. (The containment intergrated leak test
with all penetrations installed verifies this satisfactorily.)
Based on the above observations, it is our view that qualification of Class
IE equipment is generally done on a component basis. The "as-installed"
configuration of the equipment and its interface connections have not been
fully considered. In the case of sensor transmitters and connectors, the
possible incursions of steam/moisture through the interstices of stranded
conductors of a cable has not been specifically considered during LOCA
qualification. However, NRC is only concerned if the steam/moisture
incursion could affect the operation of the equipment.
3 of 4
.
IE Circular No. 79-05 March 20, 1979
All holders of operating licenses or construction permits should be aware of
the potential problems of the types discussed above. Because cable
termination details are plant specific, it is recommended that both
licensees of operating facilities and holders of construction permits
conduct a detailed review and analyses of their "as-installed"
safety-related transmitters and connectors to ascertain the possibility of
steam/moisture incursion through stranded conductors which could adversely
affect the operability of safety-related equipment during a LOCA. If the
possibility exists, then the facility should verify the qualification of the
affected equipment for the service intended.
No written response to this Circular is required. If you require additional
information regarding these matters, contact the Director of the appropriate
NRC Regional Office.
Enclosure:
List of IE Circulars
Issued in Last
Twelve Months
4 of 4
Page Last Reviewed/Updated Thursday, March 25, 2021