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Fire barriers are continuous construction assemblies that are designed and constructed to achieve specific fire resistance ratings, and to limit the spread of heat and fire and restrict the movement of smoke. Fire barriers may be structural (walls, ceilings, floors) or non-structural (radiant energy shields, raceway fire barriers).
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Electrical Raceway Fire Barrier Systems (ERFBSs) are non-structural fire-rated assemblies that protect the electrical cables they enclose.
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In nuclear power plant applications, ERFBS are required to have a fire resistance rating of either 1 or 3 hours, based on the specific application.
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One-hour ERFBSs require detection and automatic suppression to be installed. For some areas, licensees have requested exemptions to these requirements based on the specific area configuration and combustible loading. These exemptions are reviewed by the U.S. Nuclear Regulatory Commission (NRC) under the agency's normal exemption process, and are approved or disapproved by the NRC as appropriate.
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Common ERFBSs in use at nuclear power plants include Thermo-Lag, Darmatt, Hemyc, MT, and 3M Interam.
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Hemyc and MT ERFBSs are credited by plant-specific fire protection programs as 1-hour and 3-hour rated fire protection barriers, respectively.
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In 1999 and 2000, NRC inspections revealed that one licensee's fire testing and acceptance criteria used to determine the fire ratings of the Hemyc and MT fire barrier systems installed to separate safe-shutdown functions within the same fire area were insufficient to qualify Hemyc for cable trays or conduits, or MT for conduits, as rated fire barriers.
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The Hemyc and MT ERFBS fire testing was completed in Omega Point Laboratories in March and April 2005. This testing determined the rating of both the Hemyc and MT ERFBSs.
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The tests indicated that the Hemyc and MT ERFBSs may not provide their designed fire rating. In April 2005, the NRC staff informed licensees of the test results in Information Notice 2005-07, "Results of Hemyc Electrical Raceway Fire Barrier System Full-Scale Fire Testing." This allowed licensees to implement appropriate compensatory measures and develop plans to resolve any noncompliances. The NRC staff also held a public meeting in April 2005 to announce the test results.
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Generic Letter 2006-03, "Potentially Nonconforming Hemyc and MT Fire Barrier Configurations," was issued in April 2006.
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All licensees have responded to the Hemyc Generic Letter (GL 06-03), and all of the licensees' responses have been closed out by the staff.
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Hemyc, although not rated for the full hour as required, has been tested and provides a minimum fire resistance capability of approximately 30 minutes. Because NRC regulations require fire detectors and automatic fire suppression systems when 1-hour fire barriers are used, the agency does not expect degraded conditions to result in high-risk-significant situations. Furthermore, even at its degraded rating, Hemyc protection is expected to provide sufficient time to use redundant fire suppression capabilities (e.g., fire suppression by operators, fire brigade) to extinguish fires. Testing has enhanced the NRC's knowledge of the actual capability of Hemyc ERFBSs. Based on the perceived low-risk-significance, the NRC did not pursue resolution of the Hemyc issue as a high priority.
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The NRC has confidence that the MT barrier provides significant protection based on the following considerations: (1) the fire barrier lasted at least 87 minutes in all tested configurations before reaching the minimum cable failure temperature, (2) furnace-like temperatures would have to be achieved in plant areas and maintained for an extended period of time, (3) even if areas reached and maintained these temperatures, there would be significant time available for manual suppression to occur before cables become damaged.