Information Notice No. 90-23: Improper Installation of Patel Conduit Seals

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                                April 4, 1990



All holders of operating licenses or construction permits for nuclear power 


This information notice is intended to alert addressees to a possible safety 
problem caused by the improper installation of Patel conduit seals as a 
result of incorrect selection of grommets based on the wire gauge size.  It 
is expected that 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 do not 
constitute NRC requirements; therefore, no specific action or written 
response is required.

Description of Circumstances:

On November 6, 1989, the staff at the H. B. Robinson plant notified the NRC 
that they had discovered that some of the conduit seal grommets used to seal 
insulated wire conductors entering environmentally qualified instrument 
housings were oversized for the application.  The seals are used to prevent 
moisture from entering safety-related electrical components following 
loss-of-coolant accidents.   Use of oversized seals may create potential 
moisture intrusion paths into safety-related instrumentation inside the 
reactor containment. 

The problem was attributed to inadequate installation instructions that were 
used when the seals were installed in 1986 and 1987.  These instructions 
listed the grommets by wire gauge size and gave maximum wire insulation 
diameters for each wire size (see the first table, page 1, of the enclosed 
EGS Bulletin 90-1).  In accordance with these instructions, the seals were 
selected based on wire gauge alone.  However, since the insulation thickness 
for a given wire gauge varies from one wire type to another, a more 
appropriate parameter for choosing the correct grommet size would have been 
the minimum wire insulation diameter for which a particular grommet will 
achieve an effective seal.  The selection of grommet size based only on wire 
gauge size resulted in the installation of some grommets that were too large 
to provide an effective seal.  As a result, some of the seals failed 
pressure tests that were designed to simulate post-LOCA pressures.


                                                            IN 90-23
                                                            April 4, 1990
                                                            Page 2 of 2

During the investigation of the grommet leakage problem, the Robinson staff 
also checked the torque on the conduit seal union nuts that are used to 
compress the seals.  EGS Corporation recommends that the union nuts be 
torqued to 50 ft-lb (EGS Bulletin 90-1).  On approximately half of the 90 
seals inspected, the union nut moved about 1/4 inch when this torque was 
applied.  One union nut was found to be quite loose.  EGS Corporation 
reports that 1/4 inch of movement does not necessarily indicate a degraded 
seal but recommends that the correct torque be verified on a representative 
sample of installed conduit seals.


In order for these conduit seals to be able to function at accident 
pressures and temperatures and for extended periods following an accident, 
it is important that the correct installation procedures be established and 
consistently applied.  In this case, the installation of the seals according 
to inadequate procedures could have resulted in moisture intrusion into more 
than one safety component during an accident, leading to equipment failures 
or the display of erroneous information to the operators.

Although the vendor's use of wire range sizes to designate the grommets 
appears to have been intended as an aid in choosing the proper grommet for 
the application, differences in insulation thicknesses because of 
differences in insulating materials and type of application were not 
accounted for by either the vendor's original instructions or the licensee's 
initial installation procedures for the grommets.  The necessary minimum 
insulation diameter values for each grommet size were subsequently provided 
by EGS Corporation, the supplier of the Patel conduit seals.  These minimum 
diameters are also included in EGS Bulletin 90-1.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  T. E. Conlon, RII
                     (404) 331-5537

                     N. Merriweather, RII
                     (404) 331-5577

1.  EGS Letter to NRC dated 1/2/90, Enclosing
      Product Bulletin 90-1:  Recent Events 
      Concerning Patel Conduit Seals
2.  List of Recently Issued NRC Information Notices 

Page Last Reviewed/Updated Thursday, March 25, 2021