ML070030383
| ML070030383 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 12/19/2006 |
| From: | Progress Energy Co |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUREG/CR-6850 FAQ 06-0017, Rev 0 | |
| Download: ML070030383 (3) | |
Text
FAQ Number 06-0017, Revision 0 Plant: Oconee FAQ # 06-0017 Rev. 0 Submittal Date: 11-6-06 Licensee
Contact:
Kiang Zee Tele/email NRC
Contact:
Tele/email Distribution: Check all that apply (NEI Internal Use)
FPWG RIRWG NSSS OG NFPA 805 TF
Subject:
Clarification/enhancement of Ignition Source counting guidance for High Energy Arcing Faults (HEAF) in NUREG/CR-6850, supporting NFPA-805 Fire PRA application.
Interpretation of guidance? Yes Proposed new guidance not in NEI 04-02? Yes Details:
NEI 04-02 Guidance needing interpretation (include section, paragraph number, and line number):
New attachment on interpretation issues Circumstances requiring guidance interpretation or new guidance:
The guidance provided in NUREG/CR-6850 for Task 6, Fire Ignition Frequency (Section 6.5.6, Bin 16), states:
Bin 16 - High-Energy Arcing Faults (Plant-Wide Components): High-energy arcing faults are associated with switchgear and load centers. Switchyard transformers and isolation phase buses are not part of this bin. For this bin, similar to electrical cabinets, the vertical segments of the switchgear and load centers should be counted.
Additionally, to cover potential explosive failure of oil filled transformers (those transformers that are associated with 4.16 or 6.9kV switchgear and lower voltage load centers) may be included in vertical segment counts of the switchgear.
Pilot discussions and benchmarking of NUREG/CR-6850 for Task 6, Fire Ignition Frequency, has shown inconsistency in the treatment of High Energy Arcing Faults (Bin 16). Strict interpretation of the guidance is that the HEAF count should mimic the electrical cabinet counts for switchgear and load centers. The application of such a counting method is expected to result in reported High Energy Arcing Fault (HEAF) frequency values for an individual plant being inconsistent with industry experience. The industry experience and consequently the HEAF frequency is based on 3 events occurring on medium voltage switchgears and 1/2 event occurring on a 480 VAC Load Center. Because of the relative numbers of switchgears and load centers at an individual plant, it is expected that the resultant frequency may be inappropriately skewed.
There is a concern that the occurrence of a HEAF frequency distribution that departs significantly from the 3 to 1/2 ratio would cause results to be challenged.
There is also a question of counting Bus Ducts. The specific guidance for NUREG/CR-6850, Task 6 does not require any counting of bus ducts. However, the discussion in Appendix M of NUREG/CR 6850 notes that bus ducts are susceptible to HEAF events.
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There is a need to resolve these issues to prevent future rework and to reduce burden associated with uncertainty treatment.
Detail contentious points if licensee and NRC have not reached agreement This topic has impact on the NFPA-805 pilots, non-pilots and other users of NUREG/CR-6850.
Potentially relevant existing FAQ numbers:
This guidance is specific to the characterization of electrical cabinets for Bin 16 HEAF determination. The characterization and counting of electrical cabinets for Bin 15 determination is addressed by FAQ 06-0016.
Response Section It is proposed that the existing guidance in NUREG/CR-6850 that recommends counting based on segments be modified. Since industry experience shows that the medium voltage switchgears are most likely to experience this event, it is proposed that each low voltage switchgear (usually referred to as load centers or unit substations) operating at 440 Vac or higher be counted as a single unit regardless of the number of vertical sections or segments. This treatment would ensure that the majority of the HEAF frequency is allocated to the medium voltage switchgears.
When performing detailed fire modeling, the HEAF should be distributed by vertical section of the applicable load center.
Medium Voltage Switchgear 9 Breakers and Sections Count = 9 for Bin 16 Low Voltage Switchgear 440 Vac (Load Centers or Unit Substations) 16 Breakers in 4 Sections Count = 1 for Bin 16 In the case of bus ducts a possible approach is to count them as being equivalent to an individual section, regardless of length. However, there have been no reported occurrences of a HEAF event originating at (on) a bus duct and the most likely location for such an event if it were to occur is at the termination point of the bus duct at the switchgear and/or transformer. The treatment of a HEAF event at these terminal ends would already bound the anticipated consequences. Because the specific guidance for NUREG 6850 Task 6 does not require the counting of bus ducts, a HEAF event originating at (on) a bus duct has not occurred, and the potential consequences if it were to occur at its terminal end is bounded by the HEAF treatment of that terminal, it is proposed that no specific treatment or counting of bus ducts be required for fire frequency determination.
FAQ Number 06-0017, Revision 0 Basis:
The existing guidance in NUREG/CR-6850 is based on industry data which has only been provided with fidelity adequate to support plant level ignition frequencies for HEAFs. Although the guidance does address the data, it leaves room for variability that can create issues with PRA quality. It is important that the ignition frequency results be of sufficient quality to support not only NFPA-805 transition but also the more broad scope of regulatory inspection and enforcement issues.
The guidance proposed will provide more consistency when determining plant specific electrical cabinet ignition frequencies while working within the bounds of the exiting data provided by the NUREG. This should facilitate the review and acceptability of the results.
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