United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-41: Potential Problems with the Use of Freeze Seals

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C.  20555

                                  June 27, 1991


Information Notice No. 91-41:  POTENTIAL PROBLEMS WITH 
                                   THE USE OF FREEZE SEALS


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice is intended to alert addressees to the potential 
consequences associated with failure of freeze seals used to perform mainte-
nance in piping systems.  Of particular concern would be a failure when used 
in a portion of the reactor coolant system pressure boundary where failure 
could lead to core uncovery.  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 April 19, 1989, at the River Bend Station, a freeze seal failed on a 
6-inch service water line.  The freeze seal was used to allow inspection and 
repair of manual isolation valves to a safety-related auxiliary building 
cooler.  The bonnet of one manually operated valve was off the valve and the 
service water system was in operation at the time of the event.  The failure 
of the freeze seal resulted in the flooding of portions of the auxiliary 
building.  Approximately 15,000 gallons of service water were discharged 
through the disassembled isolation valve, covering portions of the floor at 
the 141-foot level of the auxiliary building.  A portion of the water flowed 
through holes in the floor under safety-related 480-Vac motor control 
centers onto nonsafety-related cabinets on the 114-foot level containing 
disconnect links and a 13.8-kV/480-Vac transformer.  As the cabinets were 
not designed to shed water, they allowed the water to enter, shorting 
circuits and causing a fire that damaged one cabinet and its components.  A 
13.8-kV supply breaker opened and deenergized the cabinet and two others, 
causing the loss of the operating residual heat removal (RHR) system, normal 
spent fuel cooling, and normal lighting in the auxiliary building, the 
control building, and the reactor building.  The operators isolated the 
service water system in 15 minutes and restarted the RHR system in 
17 minutes.  Further details may be found in NRC Augmented Inspection Team 
Inspection Report No. 50-458/89-20 and Licensee Event Report 50-458/89-020.


9106210168 

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                                                            IN 91-41
                                                            June 27, 1991
                                                            Page 2 of 3


While the Oconee Nuclear Station, Unit 1, was shut down for a refueling 
outage in 1987, the licensee used a freeze seal to enable plant personnel to 
replace a 3-inch-diameter section of low-pressure injection piping because 
no valves were available to isolate the affected piping.  The freeze seal 
was in a line connected to the borated water storage tank, which supplies 
borated water for the low-pressure injection system.  The freeze seal 
failed, and approximately 30,000 gallons of slightly radioactively 
contaminated water leaked into various areas of the auxiliary building.  A 
portion of the water from the borated water storage tank drained through the 
station yard drainage system and flowed past the site boundary before the 
leak was brought under control 8 hours after the freeze seal failed.  
Further information may be found in NRC Inspec-tion Report No. 50-269/87-51.


Discussion:

Freeze seals are used to isolate components (such as inboard isolation 
valves) for maintenance in locations that cannot otherwise be isolated.  The 
seal is created and maintained by applying a cooling agent such as liquid 
nitrogen to the exterior of the pipe.  The cooling agent freezes the water 
within the pipe section, thus sealing the pipe.  When used in the reactor 
coolant system (RCS) pressure boundary, these freeze seals become a 
temporary part of the pressure boundary.  Therefore, if a freeze seal fails, 
it can result in an immediate loss of primary coolant.  Of particular 
concern would be a failure of a freeze seal in a system connecting to the 
vessel's lower plenum region, such as the reactor water cleanup (RWCU) 
system at boiling water reactor (BWR) facilities.  The staff has estimated 
that the reactor core could be uncovered in less than 1 hour if the freeze 
seal failed completely in the RWCU system. 

Freeze seal failures in secondary systems can also be significant because of 
the potential for consequential failures, such as the loss of decay heat 
removal in the River Bend event.  The licensee for the River Bend Station 
attributed the causes of the event to procedural inadequacies that resulted 
in a failure to install and monitor a temperature detection device, and to a 
lack of personnel training in the use of freeze seals. 

Some licensees have used piping mockups to thoroughly evaluate freeze seal 
applications prior to their use on reactor system piping.  Important 
considerations include examining training, procedures, and contingency plans 
associated with the use of freeze seals, and evaluating the need for and 
availability of additional water makeup systems and their associated support 
systems.  

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                                                            IN 91-41
                                                            June 27, 1991
                                                            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 NRR project manager.




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


Technical Contact:  Amy E. Almond, NRR
                    (301) 492-0875


Attachment:  List of Recently Issued NRC Information Notices
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