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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 .. 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. . 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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