NUREG-2262, Comment Resolution

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Comment Resolution
ML22353A079
Person / Time
Issue date: 07/29/2022
From: Kenneth Hamburger
NRC/RES/DRA/FXHAB
To:
References
NUREG-2262, NRC-2022-0130
Download: ML22353A079 (1)
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Text

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph 1 General The document does not specifically indicate if it is This new method provides significant advances to modeling approaches over superseding prior guidance, or is provided as a guidance for the previous methodologies. Consistent with other methodological refinement of prior modeling. Superseding prior guidance advancements and data improvements, licensees should evaluate this new would require fire model revisions during the next scheduled information as part of their periodic update process as committed to per the update. PRA standard. In general, NUREG documents are not compulsory methodologies, and licensees can choose to implement these methods per their normal procedures.

Further clarification will be provided as needed during a planned workshop activity in the Spring of 2023.

2 General All references to Thermoplastic cable or Thermoset cable Revised as suggested.

should specify if referencing the jacket or insulation material.

Recommend consistently using thermoplastic jacketed cable, as an example.

3 General Although the guidance is comprehensive, it provides a The new methodology was developed to be implemented as a replacement to detailed analysis methodology which will require significant the current tools rather than to supplement pieces of a current analysis on an man-hours to research and implement. Besides the simple as needed basis. The aspects of the new methodology should be applied summary in the Abstract (page vii lines 27-35), is there any together and not partially implemented, as HEAF likelihoods, durations, guidance for a utility to partially implement the guidance as definitions and ZOIs are tied together. The methodology relies upon a level of necessary for high risk HEAF scenarios, allowing current detail not developed in the NUREG/CR-6850 approach.

modeling per NUREG/CR-6850 & Supp 1 to remain bounding for other scenarios?

Could the guidance provide very specific instances where The new HEAF PRA methodology has aspects which may produce a larger NUREG/CR-6850 & Supp 1 are non-bounding? increase in risk than 6850, as well as aspects which can reduce risk. For Must the guidance on application of revised ZOIs, IGFs, and example, larger ZOIs, particularly for longer FCTs for bus ducts, can result in NSPs be implemented together? more extensive cable damage and increased risk; however, credit for ERFBS can lead to reductions in risk. Thus, the overall impact of risk from the new HEAF PRA methodology will depend upon the plant configuration and be plant specific.

The guidance provides the tools for the analysist to determine if their current model is non-bounding, but the determination will be plant specific and Page 1 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph application specific. For high-risk scenarios, the FCT for the HEAF scenarios should be analyzed to determine if the energetic ZOI is larger/smaller than currently analyzed. Section 10.3 provides a starting point where the original NSBD/MV switchgear ZOIs may be challenged.

4 General Throughout document, equation values should not have an Revised to tighten up the text for the numbers displayed in scientific notation apparent space before and after the - in the exponent. for Appendix G.

E.g., Page G-12, Lines 5-14. This may be a result of the selected italic font.

Regarding: The 15 & 30 MJ/m2 criteria apply beyond the thermoplastic and thermoset "15 MJ/m2 target fragility" & "30 MJ/m2 target fragility" cabling. See Section 6.2 which discusses additional targets such as junction boxes, electrical equipment, bus ducts, and instrument air.

5 General Throughout the document, "15 MJ/m2 target fragility" & "30 MJ/m2 target fragility" should be changed to use the more No changes are made globally since the fragility thresholds apply to more than common terms "Thermoplastic" and "Thermoset". just TS and TP-jacketed cabling.

6 General Rotate wide pages in PDF for ease of reading electronically This is a publication issue.

Section 6 provides general guidance for switchgear/load centers and NSBDs. For example, Section 6.1.2 is the damage footprint for NSBD Regarding: (describes the waterfall). Section 6.5 (Post-HEAF ensuing fire) is only X Section 6 provides general guidance on the energetic applicable to switchgear and load centers, so added the following Lines 13 portion of the HEAF ZOI, how to determine fault clearing bolded text to the executive summary.

7 & 14 times, and characteristics of the post-HEAF ensuing fire.

  • Section 6 provides general guidance on the energetic portion of Exec the HEAF ZOI, how to determine fault clearing times (FCTs), and Summary State that the energetic portion of the HEAF ZOI discussed characteristics of the post-HEAF ensuing fire (for switchgear and in Chapter 6 is specific to switchgear and load centers. load centers).

Page 2 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Regarding: Agree. Comment incorporated The overall discussion on Section 8 regarding fragility.

X Use of "and" for defining the 15 and 30 MJ/m2 target types 8 Exec may be misleading in lines 38-47. For example, as written it Summary reads that 15 MJ/m2 should be used when you have thermoplastic targets AND aluminum enclosed bus ducts.

One solution here would be to use commas separated values in the parenthesis with an "etc." at the end since there are more than 2 criteria to apply each fragility level.

The scope of the overall report seems to ignore a design Added discussion of configurations where ESF transformers exists between configuration wherein an additional transformer exists Zone 1 and Zone 2. Added in definition (Section 3), Zone 1 and Zone 2 between Zone 1 and Zone 2. In such configurations the size switchgear (Section 3) and a new section 6.4.3.

of the associated transformer (e.g., 13kv - 4kv) can be Ix relatively small and consequently the maximum available 9

General fault current can be well below the designed capability of the switchgear. It is unclear whether such a configuration was considered or addressed within the framework of this NUREG. It would be beneficial if some discussion for such a configuration could be provided.

Section 1.2 Approach indicates the guidance is a Removed the word update from the sentence. Not in the scope to determine Section methodology update. Is this to be interpreted as the if the report is considered an update or an upgrade.

10 1.2 methodology is not an upgrade which would require peer-review?

In Section 2, terminology - the definition of target fragility is added as:

Target Fragility should be a defined term in Section 2 11 Section 2 Target Fragility: The condition when targets external to the HEAF are likely to Terminology.

be damaged.

Page 3 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Added definition (in Section 2) and discussion of bus differential protection in Section 3.6.3.

The discussion of switchgear main bus bar faults does not address the design configuration wherein a bus differential is applied. The discussion in the report should acknowledge If bus differential works, there is no HEAF-level consequences. The treatment Section of bus differential is similar to the treatment of the SAT differential protection 12 that if such a design exists and the breaker(s) actuated by 3.6.3 (assumed failed). Footnote added in Section 8.2.2 to address.

such a differential relay successfully opens (trips), then a HEAF event would not occur. This conceptual approach should apply to all instances where an 87 device is applied.

Page 3-32 The 9th branch down (Supply side of supply breaker, Updated Figure 3-14 Section fails/stuck, SAT, with zone of differential (87), one 13 3.6.5 switchyard breaker fails/sticks) should be light grey text Figure 3- since the duration is less than or equal to 0.2s, and 14 therefore wouldnt be expected to cause a HEAF.

Page 3-34 Figure 3-16 updated with 0.4 to 5.2 s Section The last branch should have a duration of 0.4 to 5.2s 14 3.7.1 instead of 0.4 to 5s.

Figure 3-16 Page 4 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Regarding statement: Regarding the second comment, the analyst should not multiply by the upstream breaker failure probability. Revised the text near Figure 6-6 (old It is possible to use both the known transition point and the figure 6-5) to remove the word end state to eliminate confusion. Also added unknown transition point method in the same analysis if the in the footnote to clarify the working groups intentions:

frequency is conserved within the respective NSBD bin. The three independent failures are not explicitly modeled in the PRA method.

However, through the use of the Zone 1/Zone 2 weighting factor (Section Section 5.2.3 discusses the development of the ignition 5.2.2.3) and the apportionment of split fractions in the Zone 2 event trees Section 15 frequency weighting factors applied to Zone 1 and Zone 2 (Figure 8-7 [5%] and Figure 8-10 [1%]), scenarios depicted in Figure 6-6 are 5.2.3 MV switchgear. These are based on OE, and thus appear to expected to be of low likelihood.

implicitly include the breaker protection failure likelihood for the Zone 2 events. It is not exactly clear that modeling a Zone 2 event should not (or should) multiply the Zone 2 HEAF ignition frequency by the probability of upstream supply breaker failures (as illustrated in Figure 6-5). A statement could be added to clarify the intent.

Page 5 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The OPEX has shown a propensity for vents, hatches, and wall penetrations to be the initiation point for arcing faults, however, this is typically associated with outdoor events. After reviewing the operating experience, the working Regarding: group decided to remove the requirement to consider vents, hatches and wall In addition to the transition points, fire PRA targets in penetrations inside (but still consider outside if these features exist).

locations with a propensity to allow for degradation of the bus bar insulation - vents, hatches, or wall penetrations -

should be captured and included with scenarios structured The revised text is as follows:

around the nearest transition points. Reviewing operating experience also highlighted the potential for a HEAF to occur in outdoor locations where environmental access to the bus bar Page 5-10 Lines 3-6 and Page 5-15 Lines 11-13; Inclusion insulationsuch as ventilation openings, mechanical hatches, or external wall of bus duct vent/drain/pens/hatch targets with the closest penetrations (e.g., yard-to-turbine-building penetration)occurs and could Page 5-10 transition point could result in capturing targets from both allow accelerated degradation of the bus bar insulation.

Lines 3-6 Engineered Safety Feature trains in one scenario, even 16 and Page though the train targets are well separated and farther apart For known transition points, the analysis should look for fire PRA targets (i.e.,

5-15 Lines than the bus duct ZOI. fire PRA equipment and cables) within the ZOI at the transition points and 11-13 Did the OE indicate that events related to postulate scenarios consistent with Supplement 1 to NUREG/CR-6850 [2]. For vents/drains/pens/hatch occur at that location or at the outdoor locations with features that may allow degradation of the bus bar nearest transition point? (Page 5-9 Lines 36-39) If at the insulation (e.g., vents, hatches, and wall penetrations), fire PRA targets near vent/... location, then recommend including the these features should be captured and included with scenarios structured vents/drains/pens/hatches in the count, and mapping targets around the nearest transition points or alternatively considered as transition exclusive to those locations. points.. Openings, such as vents, drains, or hatches located inside buildings Inclusion of the additional targets at the nearest transition is (protected from weather elements) are not expected to increase the likelihood more conservative than the method provided for unknown that the bus bar will degrade and do not need to be included in a scenario.

transition points, which does not address Locations outdoors with a propensity to allow degradation of the bus bar vents/drains/pens/hatches (Page 5-16, Example 2). insulation do not need to be counted as transition points for the purposes of counting segmented bus ducts, but the fire PRA targets located in the ZOI of one of these locations should be included in a scenario involving the closest transition point.

Page 6 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments The method of mixing counting with in a bin, appears to allow an analyst to selectively assign risk as desired based on targets. This does not appear to meet the original treatment of bus duct counting, based on the following sentences in supplement 1 of NUREG/CR-6850:

As noted above, arc faults generally occur at the transition points. When the actual location of the transition points is not known, the approach assumes that a fault might occur at any point along the duct length. point. By the same token, the approach partitions fire frequency equally along the length of the bus duct, whereas in reality faults would be more frequent at the actual (but unknown) transition points However, the following sentences allow for the removal of targets from consideration based on a more detailed look at the bus duct:

It is recommended that in assessing analysis results, these observations be As written, the guidance does not allow a mixed treatment treated as a part of the uncertainty and sensitivity analyses, and that the within a bus duct type bin. For example, assume a plant analysis be refined for cases where risk-significant fire scenarios develop (i.e.,

has 500 feet of non-segregated bus duct and the location of by examining the bus duct to determine if any transition points are actually transition points are not readily apparent. Therefore, the present in the segment of bus duct associated with a significant approach described in 5.2.3.2 is used. However, during analysis development and/or refinement it is discovered that scenario). This is discussed further below. Note that in either approach, the a key critical target is within the ZOI for a short length of analysis can always be refined by examining the bus duct to Section 17 bus duct. If it can be determined there are no transition determine if one or more transition points actually lie within the applicable bus 5.2.3.3 points or features described in Section 5.2.3.1 within that duct segment. If no transition points are identified within that particular duct short length, the guidance would not allow the HEAF event section, then a fault scenario need not be postulated and the scenario goes to be precluded using the guidance in Section 5.2.3.1. away.

Section 5.2.3.3 should be modified to allow a mixed Revised text in in 5.2.3.3 to state:

treatment within a bin - i.e., the HEAF event can be excluded provided the frequency for the short length is re- In summary, the analyst may choose different apportioning strategies for Bins apportioned to the remaining lengths of bus duct. 16.1-1 and 16.1-2. Supplement 1 to NUREG/CR-6850 [2] identifies the following refinement which may still be utilized if the unknown transition point method is used for one of the bins:

Note that in either approach, the analysis can always be refined by examining the bus duct to determine if one or more transition points actually lie within the applicable bus duct segment. If no transition points are identified within that particular duct section, then a fault scenario need not be postulated and the scenario goes away. If one or more transition points are identified within a particular duct section, then the analysis can be refined based on the known locations (i.e., both the fire frequency and the impacted target set may be refined once transition points are identified).

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Listing the old numbers creates a chance for the wrong number. Change in values can be compared by referencing NUREG-2169. Added discussion below Table 5-8 that discusses the frequency changes. The text includes:

The mean frequencies for bin 16.a (HEAF for low-voltage electrical cabinets) and bin 16.2 (HEAF for iso-phase bus ducts) increased from the mean values It would be beneficial to include the frequency previously in NUREG-2169 [38]. The mean frequencies for bins 16.a and 16.2 have 18 Table 5-8 assigned for a quick assessment of increases or decreases increased by 250% and 71%, respectively. The significant increase in bin 16.a in the generic frequency. is driven by the limited number of fire events in the industry experience in NUREG-2169 and an event that occurred post NUREG-2169 (the impact of adding or removing a single event is more apparent). However, the frequency for bin 16.a is the low (second lowest following Bin 1 - batteries). The frequency for bin 16.b has decreased by 7%. Splitting the segmented bus duct frequency results in an increase in frequency by 137% for bin 16.1-1 and a decrease of 18% for bin 16.1-2 from the previous combined bin 16.1 mean.

19 Table 5-8 Bin 16.a 95th percent is missing -, should read 1.69E-03. Corrected.

it would be helpful to provide the uncertainty values for the Updated Table 5-8. This now includes the Mu, Sigma, and EFs for each HEAF distribution (Mu, Sigma, EF) similar to NUREG-2169 Table bin.

20 Table 5-8 4-4, and/or indicate the distribution function [assume log-normal per NUREG-2169].

Page 8 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Agree that the draft report was not clear on what to do when the GCB operates. Added the following text after Table 5-9:

Regarding:

The value of 3.5E-05 associated with the air-blast type GCB bounds the failure results for the three different GCB If the GCB operates as designed (1 - 3.5E-05), the GCB prevents the technologies. Credit for the GCB interrupting the faulted conditions can be applied when the fault is within the GCB main generator coast-down energy from feeding a fault within the GCB zone of differential protection. This credit can be applied to zone of protection. The working group determined that plants with Page 5-21 the following fault zones: installed GCBs are expected to have a better than average 21 Section performance as compared to plants without GCBs. Therefore, for an 5.3.1 Application of 1-GCB factor is not discussed here. The other end state where the GCB is credited, the scenario frequency is not frequency modifiers in this document function like split conserved, since the 1 - 3.5E-05 when applied to the branch end state fractions, where applicable, and add up to 1.0. The does not result in HEAF-type consequences.

difference here should be clarified to avoid issues where it will appear frequency is not conserved.

Editorial issue corrected Additional number events should read number of events

  • Also added text in Section 8.5 (Zone 1) for the 1-GCB reliability and in Section 9.2.1 (IPBD), and 9.2.2 (BDUAT)

Regarding: Table A Fire Event Data provided details on the suppression time.

The primary difference is the number events counted for the Reviewing the events in this table, three events are excluded due to determination of the suppression rate results from the suppression discharge (old 434, 50935, and 51765). Old 678 is self-inability to count events with no suppression time (self- extinguished and the remainder of the events with no suppression time are extinguish), automatic suppression, or unknown suppression bus duct events (which do not always necessitate manual suppression efforts).

Page 5-23 times.

22 Section There are no load center or switchgear HEAFs that are self-extinguished (and 5.4 by definition HEAFs in load centers and switchgear have an ensuing fire). No If there are known fire events where self-extinguishment changes to the HEAF suppression approach are made.

occurred, this could be developed into a split fraction where no ensuing fire occurs. This would benefit the overall results given that the HEAF suppression curve has historically been one of the most challenging.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The ZOIs as depicted in Figures 6-1 through 6-4 are as intended. The ZOIs for electrical enclosures are squared and are extended from the corresponding Page 6-1; Figure 6-1 (as well as others) illustrates HEAF enclosure faces. These are the regions around the enclosure where an arc ZOIs and implies that no damage occurs in the areas plasma jet could be located due to ventilation openings, access doors, or extending from the corners between the front/back zones breaches. The space outside these regions would not be located within an arc and the side/side zones or the top zone, as opposed to bus plasma jet, and would the radiant view factor to the arc would be small given duct ZOI shape of rounded corner square. Is this the arc is within the enclosure. The ZOI for bus ducts is rounded since Page 6-1 intentional? breaches in the housing tend to occur on all sides resulting in minimal radiant 23 obstruction between faces and potential exposure to the arc plasma jet. As the Figure 6-1 bus duct ZOIs are drawn and developed in this report they capture the 360° As an example, in Figure 11-19 (page G-8), imagine that around the bus duct. For switchgear, the ZOI is intended to be squared based cable A penetrates the floor at u in the title Figure 11-19, on the FDS results and the analyst should consider this difference in defining and Cable C penetrates the floor at y in the title supply, the energetic and ensuing fire ZOIs.

these cables would be outside the HEAF front or side ZOIs.

Added additional text for the ZOIs in Section 7, 8, and 10 for loads centers and switchgear.

Regarding: Text on Page 6-2 already eludes that the ensuing fire may be larger than the energetic phase (for short FCTs). Additional it could get larger if additional Note that the ensuing fire ZOI may expand beyond the ZOI combustibles are involved, HGL, or additional sections are ignited.

associated with the HEAF if secondary combustibles are involved, a damaging hot gas layer forms, or adjacent Revised text to state:

electrical enclosure sections are ignited.

Page 6-2 Note that the ensuing fire ZOI may also expand beyond the initial HEAF ZOI if Lines 11- Per Section 6.5, the ensuing fire is required to utilize a secondary combustibles are involved, if a damaging hot gas layer forms, or if 24 13 170kW fire. The ZOI from the 170kW fire is larger than the adjacent electrical enclosure sections are ignited.

Section majority of the HEAF specific ZOIs when utilizing NUREG-6.1 1805 FDTs.

The statement in question implies that the ensuing fire ZOI will only be larger than the HEAF ZOI if the ensuing fire involves secondary combustibles, creates a HGL, or ignites adjacent sections. If this is true, Section 6.5 should be revised to be consistent with this statement in Section 6.1.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Page 6-5 Corrected.

25 change bents to vents Line 34 Section 6.3.1, page 6-6 line 44, introduces the term VL-L Added a footnote to explain this, footnote reads:

Section which I interpret as a line-line (or phase-phase) short arc 6.3.1 VL-L represents phase-to-phase arc voltage. Phase-to-phase arc fault testing is the voltage. predominant industry standard (i.e., IEEE Std C37.20.7 [60]). In addition, NRC open box 26 Was a phase to ground short also considered for developing Page 6-6 testing (RIL 2021-18 [53]) shows that phase-to-phase and phase-to-ground faults rapidly Line 44 the simulation of MV or LV HEAF scenarios? develop into three-phase faults. Due to the wide variability of reduced fault current in resistance grounded systems, phase-to-ground fault testing is not used.

Equation 6-2, page 6-7, implies that power(W)

  • time(s) = Corrected Equation 6-2 to Joules (rather than Joules/m2). Also corrected 68 Joules/m2, rather than Joules. MJ/m2 to 68 MJ.

Page 6-7 If intending to calculate radiant exposure or radiant fluence (MJ/m2), then terms are missing from the equations, and the 27 Equation example on line 17 is inaccurate.

6-2 By the context, it appears that the units on the Arc energy on line 11 should be Joules versus J/m2, and on line 17 should be MJ versus MJ/m2.

Page 6-12 lines 21-25, is there a reason that the Zone 1 See the comment response to comment #29. The analyst can credit load breaker feeding the faulted Zone 2 bus supply breaker the load breaker if it has overcurrent protection and use smaller is not being credited (refer to Figure 6-8)? Is there some energy end state if the time is less than the default. Added discussion Page 6-12 basis for the settings of the load breaker being equivalent or in the supply breaker limited section and linked to Section 8.6.1 Zone less conservative than the Zone 1 supply breaker? Or, is the 2: Refinement Level 1 and Section 8.6.2 Zone 2: Refinement Level 28 Lines 21- Zone 1 load breaker considered equivalent to the Zone 2 2 if the Zone 1 to Zone 2 load breaker has active protective relaying 25 supply breaker, in which either could be credited to interrupt for credit. See also Section 3.2.3 and 3.8.

the fault as depicted in Figure 6-9? Could crediting the Zone 1 load breaker limit the fault duration to 2 seconds (versus 4 seconds) as discussed in lines 27-30.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The analyst can credit a load breaker interrupting the faulted Zone 2 bus supply breaker if the load breaker has overcurrent protection. See Section 8.6.1 Zone 2: Refinement Level 1 and Section 8.6.2 Zone 2:

Refinement Level 2 if the Zone 1 to Zone 2 load breaker has active protective relaying for credit. See also Section 3.2.3 and 3.8.

To be comprehensive, starting in the SBL discussion on Page 6-14, There is no stated justification for the non-credit for the Zone Page 6-12 1 load breaker feeding the faulted Zone 2 bus supply added a note about crediting the load breaker in the PRA method in 29 Section 8 with the following text for SBL 3s and SBL2 end states: This Figure 6-8 breaker (refer to Figure 6-8). This justification should be included, or the credit explicitly described. refinement can also be used for Zone 2 if the Zone 1 switchgear has a load circuit breaker with overcurrent protection (see Section 8.6.1 and 8.6.2).

Change: Added text to Figure 6-9 (old Figure 6-8) to show the Zone 1 load breaker feeding Zone 2 as a No Relay Protection -

Maintenance Switch.

Page 6-12 30 change interpolated to interpolation is.

Line 24 Page 6-18 Line 6. Should read: Fault Current: 1.043 Units

  • Revised to:

Page 6-18 40.276kA/Unit = 42kA (brown vertical line) 31 Fault current: 1.043 units x 40.276 kA/unit = 42 kA (brown vertical line Line 6 on Figure 6-11)

Page 6-19 I spotted one error on page 6-19 line 30 where it refers to Agree. Corrected Figure 6-1 to Figure 6-11 (figures got moved by one place 32 due to addition of Figure 6-4).

Line 30 figure 6-1. It really should be figure 6-10.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The order of magnitude where a HEAF becomes a concern is 2 seconds or greater. Accounting for circuit breaker interrupting time (3 - 8 cycles) and relay sensing time (e.g., 0.5 cycles) for HEAF events is not necessary since it is very small (Ref: IEEE Std 242-2001, Section 9.4.3). For fast fault clearing For fault clearing time (I had accounted for relay operating times (FCT) less than a half a second, these can be factored in; however, the Section time and breaker operating time). The breaker operating margin of error in calculating the ZOIs is greater and the ZOIs at these much 33 6.4 time is small. Guidance should clarify if we need to account shorter times are minimal.

for both.

Text added at the end of Section 6.4 to address this (e.g., the analyst does NOT need to account for relay sensing time and circuit breaking opening time in the FCTs.

Page 6-21 Revised as follows:

34 Lines 29- correct grammar, or replace with with where. Figure 6-13 shows the total HRR for the vertical section where the HEAF 30 originates and the propagation to the adjacent sections Page 13 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The following two notes are added to the end of Section 6.4 to clarify the use of the three-phase bolted fault.

1. In the early stages of the project, proof of concept used three phase bolted (zero impedance) faults for medium voltage. IEEE 1584 shows that medium voltage arc faults are consistently around 85% of the three phase bolted (zero In section 6.4 step 4 where they instruct to obtain the impedance) fault (this is also supported by HEAF OE). Since the objective is to available short circuit current (ASC), I am assuming they ensure all the energy is accounted for ZOI determination, use of three-phase mean maximum 3 phase bolted fault. Note that you may bolted fault at the available short-circuit current (ASC) is considered have minimum/maximum fault currents calculated. Guidance acceptable. Due to the design of inverse time overcurrent relays (51), the total Section should clarify which fault current needs to be used to integrated energy is slightly less (or the same) given a 15% reduction in fault 35 6.4 determine FCT. Is it arcing current (see next comment) or 3 current, even though the fault clearing time (FCT) is slightly increased for an phase bolted fault current and if it needs to be done for both arc fault by approximately 0.1 to 0.2 seconds. This was verified against four Step 4 minimum and maximum fault conditions? It appears section NPP medium voltage protection and coordination calculations. The only 3.2.3 used arcing current to determine FCT (see next exception would be for existing station design vulnerabilities where the time comment). overcurrent (51) relays are not optimally set (that is, FCTs are in excess of 5 seconds at the ASC). In these cases, the arc fault current using IEEE 1584 may need to be used for FCT determination.
2. With respect to the question regarding minimum or maximum currents, since the fire PRA focuses on operating plants (e.g., Mode 1), Per Section 6.4.1, step 4.b, the maximum ASC associated with Mode 1 operation should be used.

I noted that section 6.4.2 for determining supply breaker Step 3 converting ASC to arcing current) is in error and has been removed in limited FCT refers to section 3.2.3. However in 3.2.3 it the report. Table 3-2 was updated to reflect the 3-phase bolted fault clearing seems to have additional step in that the 3 phase bolted times.

fault current was used to determine arcing current before determining FCT. This would be another step then just obtain available short circuit current (ASC). Section 3.2.3 Section 36 step 3 has you use IEEE 1584-2018 [24] to estimate the 6.4.2 corresponding arcing fault current from the ASC current in step 2. Then the arcing current was used to determine FCT.

Is there a disparity between 6.4.1 and 6.4.2? For 6.4.2 are we supposed to use the arcing current determined per IEEE 1584-2018 [24] to determine FCT? This is different from 3 phase bolted fault.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Potentially a rephrase of above question: See response to comment #36, copied below:

Section 3.2.3 summarizes a review performed for a sample of United States NPPs and an upper bound of 4 seconds was determined for the time it takes for the Zone 1 MV Step 3 converting ASC to arcing current) is in error and has been removed in switchgear bus supply circuit breaker to operate. the report. Table 3-2 was updated to reflect the 3-phase bolted fault clearing times.

Section Section 3.2.3 has an apparent additional step, in that the 3 37 6.4.2 phase bolted fault current was used to determine arcing current before determining FCT. Section 3.2.3 step 3 references use of IEEE 1584-2018 to estimate the corresponding arcing fault current from the ASC current in step 2. Then the arcing current was used to determine FCT.

This is different from 3 phase bolted fault. Please resolve the disparity between 6.4.1 and 6.4.2.

Added the following text in Section 6.5 Section The fire elevation to be used in the ensuing fire should be addressed. It is not clear where the ensuing fire should start The elevation (location) of the ensuing fire should be modeled following 38 6.5 existing practices as described in Supplement 1 to NUREG/CR-6850 [2]

for a cabinet in a lower section, or if the cubicle height be Page 6-19 utilized in some way. considering the expected condition of the load center or switchgear post-HEAF.

The application of this guidance calls for the ensuing fire Given the limited data available for characterizing the ensuing fire, the original having a maximum HRR at time 0 for switchgear and load- guidance provided in NUREG/CR-6850 was retained, which assumes the fire centers creates a zone of influence that exceeds the has a heat release rate corresponding to the 98th percentile and the fire has no majority of the energetic ZOIs (Sections 7 & 8) immediately. growth stage. Note that although the ZOI for the ensuing fire could be larger Section Much of this guidance identifies HEAF events that will be than the energetic phase, target damage is not immediate. Heat soak 39 6.5 significantly smaller than previously expected. Assuming, for methods, THIEF, and other tools may be applied to develop a damage Page 6-19 all cases, that the ensuing fire will immediately reach the progression timeline and credit for suppression may be integrated. Targets 98th% HRR significantly limits the use case for this new within the energetic phase ZOI are damaged at time zero and there is no guidance. Less conservative guidance around the potential for suppression credit.

application of the ensuing fires should be pursued. More realistic cabinet to cabinet propagation addressed in comment 42.

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Paragraph This is because the HEAF is considered a more severe Agree that NUREG-2178 Volume 2 is not the correct reference for the MV ignition factor and the side breach pattern for medium symmetric breaching patterns. The correct reference is the HEAF FDS report.

Section voltage switchgear is generally symmetric [46]. This reference has been updated.

40 6.5.1 Page 6-21 Reference 46 (NUREG-2178, Volume 2) doesn't appear to be the correct reference for this statement. NUREG-2178, Volume 2 doesn't discuss HEAF breaching patterns.

Reviewed the operating experience as part of this comment and the totality of the cabinet to cabinet propagation comments. For event 434; this event is in a load center (and the methodology does not recommend propagation for load The referenced HEAF events in Appendix A of this draft centers). Events 106 and 112 were reviewed. Both events describe report mention damage to adjacent sections, however, they challenging firefighting - centered around the initiating vertical section. The Section do not make any inference of sustained ignition to the working group debated the level of fire / soot spread in these events, but 41 6.5.1 adjacent cabinet sections. These events are Incident agreed that from a hot gas layer contribution, the effect was negligible for Page 6-21 Numbers 434 (Page A-2), 106, and 112 (Page A-7). These these two events.

incident reports do not support considering propagation to adjacent cabinet sections for the ensuing fire.

As discussed in comment 42, Section 6.5.1 was re-written to credit double wall construction (no spread through 200 MJ) and a probabilistic approach for cases were spread is not ruled out.

At 101 MJ, the sides of the enclosure breach/open and expose adjacent The assumption that a HEAF above 101 MJ allows for section internals to the arc energy. Previous experience at Onagawa suggest Section sustained ignition, and requires ignition of both adjacent it is possible to involve adjacent vertical sections. However, the working group 42 6.5.1 vertical sections that must be modeled as a 98th% HRR is acknowledged that guaranteed (1.0) spread and two additional 98th percentile Page 6-21 overly conservative based on the guidance and provided fires was too conservative. Section 6.5.1 was re-written to credit double wall testing data. construction (no spread through 200 MJ) and a probabilistic approach for cases were spread is not ruled out.

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Paragraph Regarding: Given the limited state of knowledge, an assessment of available information Testing by Sandia [53] indicate that sustained ignition after a and insights from the calculated ZOIs, enclosure breach areas, and the target HEAF requires energy fluxes greater than 30 MJ/m². fragilities was used to define a minimum arcing energy for propagation.

Updated to include the correct references to the Sandia fragility testing and the The basis for the methodology to ignite the adjacent vertical subsequent working group analysis. Sentence revised as:

section does not seem to be in alignment with the referenced documents. The referenced Sandia testing was performed on open box enclosures, and with no secondary

  • Testing by Sandia [14], subsequently analyzed by the working combustibles adjacent to them. The conclusions of this draft report are focused on the characteristics of a HEAF (i.e.,

group [15] indicates that sustained ignition after a HEAF requires thermal energy, mass loss, air conductivity, surface energy fluxes greater than 30 MJ/m². The 30 MJ/m² fragility ZOI Section 6.5.1 conductivity, and electromagnetic interference), not on for HEAFs with an arc energy of 101 MJ or less is 0.5 ft (15 cm) 43 propagation. While there is some severe damage to the (see Table 8-4 and Table 8-6), which indicates that the exposure Page 6-21 enclosures for HEAFs over ~22MJ (e.g., Test OBMV06, is marginally greater than the sustained ignition threshold at the Section 4.2.4), it doesnt consider that the vertical sections boundary of the adjacent vertical section. Given this, sustained could have more than one layer of steel between them, and ignition in this section is not expected.

none of the tests looked at secondary combustibles or the possibility of ignition in adjacent cabinet vertical sections.

Therefore, how this testing was used to develop the basis See response to comment 42 for improvements in HEAF cabinet to cabinet for requiring 30 MJ/m² is unclear. Recommend revising the propagation.

guidance to allow screening the possibility of adjacent cabinet spread.

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Paragraph Regarding: Revised statement to:

Fire propagation is likely in the adjacent vertical sections.

Fire propagation is possible in the adjacent vertical section.

None of the testing performed for this issue supports the conclusion that adjacent cabinet propagation is realistic, and The full-scale testing was primarily performed on single cabinet vertical this statement should be removed. Per NUREG-2178, sections and only of energies up to ~148 MJ so the state of knowledge is low Volume 2, Section 4.2.2.7, switchgears and load centers originally, and there is a lack of both operational experience and testing at the have limited fuel loads (low/very low) and are normally higher energies (e.g., 233 MJ & 300 MJ).

ventilated. Propagation to adjacent cabinet sections is not considered for HEAF scenarios since the mechanical force associated with the HEAF event will cause the cabinet doors The propagation of concern is to the relay/metering cabinet where the bulk of to blow open. This will result in venting in the cabinet the combustibles are located. The working group agrees that breach can occur preventing an internal hot gas layer from forming which is into adjacent cubicles, but those cubicles may be sparsely loaded (e.g., the what NUREG/CR-6850 states is the basis for adjacent primary cable compartment bus bar propagating into the adjacent primary cabinet spread. Section 4.2.2.2 of NUREG-2178, Volume 2 cable compartment bus bar does not pose a secondary combustible/hot gas Section states that cabinets with open top configurations allow the layer concern). In the new Section 6.5.1 the operating experience and 6.5.1 heat generated by the fire to be transferred outside of the potential locations of spread are considered in the probabilistic model.

44 enclosure reducing the likelihood of propagation. This would Page 6-21 also be true for a cabinet whose doors have been blown Section 6.5.1 was re-written to credit double wall construction (no spread open by the HEAF. Also, many Switchgear cabinets have through 200 MJ) and a probabilistic approach for cases were spread is not open vents at the top, by design, which would also support ruled out.

venting and meet RF2 criteria for screening adjacent cabinet propagation.

The double wall with air gap is now explicitly part of the factors to consider if The cables within switchgear cubicles are typically a larger propagation is necessary. The 101 MJ is derived from FDS simulations which diameter which would be difficult to ignite from radiant use a single wall construction. With the second layer, the amount of energy energy. There is usually a double wall with an air gap required to melt/breach two panels is expected to be linear (2x as much between the switchgear sections as well. Even if the HEAF energy). For double wall construction, propagation only remains for 233 and provided a greater than 5% opening between adjacent 300 MJ cases.

vertical sections, there is no combustible continuity between the vertical sections for the ensuing fire to spread. All of these factors would exclude propagation to adjacent vertical sections under normal circumstances.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph The counting guidance was changed for MV switchgear as the operating experience determined that there was not equal likelihood of a HEAF in each vertical section. The supply sections are much more likely to experience a HEAF - as noted in the operating experience, potential single circuit breaker points of failure, and in the event trees in Section 8. Nevertheless, a HEAF can still occur in the main bus bar portion of the switchgear or the load circuit breakers (15% in Zone 1, 14% in Zone 2) that consider the remaining portion of the MV switchgear.

Section 5 provides the Task 6 counting guidance/scenario frequency for the MV switchgear (by vertical section). (note: this section does not provide The guidance provided earlier in Section 5 of the NUREG guidance for the EDG supply breakers since ignition source counting is at the modifies the counting for switchgears such that each bank level).

switchgear is counted as a unit. The guidance provided in Section 6 seems to suggest that the HEAF must be In Section 8 the scenario frequency can be used with the screening values in assumed to originate anywhere along the footprint of the Table 8-2.

Section 6 entire switchgear. While the guidance in Section 5 provides 45 General some guidance in varying the treatment of incoming versus outgoing breakers, it is unclear how the assigned frequency When more detail is necessary the analyst can enter the corresponding event is to be partitioned for each of the HEAF scenarios for the tree (Zone 1, Zone 2: refinement 1, Zone 2: refinement 2) with the scenario switchgear. It is also unclear if the EDG supply breaker is frequency. The partitioning of frequency is on a vertical section level for the considered a load or supply breaker in the context of this supply section (s). When analyzing the load portion the analyst can take the NUREG. remainder of the load bank cabinets and partition the frequency into one or more sub-scenarios as determined by the analyst. Added this to Section 8.2.2, page 8-11 (Zone 1 conf specific), and 8.6.2 (Zone 2 refinement level 2)

In regards to EDG supply breaker treatment; the document in Section 8.2.2 states:

Some switchgear may contain a vertical section associated with the EDG, as shown in Figure 3-1. The energy associated with the EDG is not sufficient to produce damage on a similar scale as a generator- or switchyard-supply-fed fault. Therefore, EDG supply vertical sections are analyzed with the load vertical sections.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Defined the following terms in Section 2 and Section 7.

Load Center Supply Circuit Breakers: These are the low voltage circuit breakers that supply power from the load center transformer to the low voltage switchgear (i.e., 480Vac or 600Vac).

Provide definitions the types of circuit breakers that are Load Circuit Breakers: These are any medium or low voltage circuit breakers discussed in Section 7 if they impact the application of the that serve a load, such as:

Page 7-1 analysis. For instance, "load center supply circuit breakers" 46 and "load breakers" are not defined in Section 2, but could

  • Motor load Section 7.1 change the location where a HEAF is postulated. These
  • Motor control center (MCC) terms should be explicitly defined to ensure the user
  • Step down transformer understands the terminology adequately.
  • Another switchgear
  • Bus Tie Reviewed and edited Section 7 to use terms consistently.

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REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Revised the introduction for Section 7.3 to clarify that the fire ignition frequency is assigned per load center supply circuit breaker. Load center HEAFs are no longer counted by vertical section. The location-specific ZOIs are provided for the most realism, however, an analyst could select the most conservative ZOI (location A) if analyzing the entire load center as one Include guidance about how to apportion the frequency of a scenario. Revised text as follows:

fire occurring at the different circuit breaker locations.

Page 7-3 There is no specific instruction about how to apportion the Load center HEAFs are modeled with the fault initiated at load center supply Section frequency of the fire if incorporating the Table 7-1 guidance. circuit breakers. Scenario frequency for load centers is apportioned to each 47 load center supply circuit breaker as described in Section 5.2.1.2 and Section 7.2 & For example, if a vertical section has 3 cubicles, two would Figure 7-1 be "lower elevation" and one "upper elevation." It is not clear 5.2.5.1. For example, in Figure 7-2, consider the red colored boxes (B and D) if the ZOI from Table 7-1 would apply to 2/3 of the vertical both contain supply breakers. The assigned frequency for the load center is section's frequency and 1/3 to the upper elevation's two over the total number of load center supply circuit breakers at the plant.

frequency. The supply circuit breaker at location B has a scenario frequency of one out of the total number of load center supply circuit breakers at the plant with a ZOI corresponding to location B in Table 7-1. Similarly, the supply circuit breaker at location D also has a scenario frequency of one out of the total number of load center supply circuit breakers at the plant with a ZOI corresponding to location D in Table 7-1.

Agree - added this to the section on the post-HEAF ensuing fire. New text It would be helpful to state in this section that spread added:

Page 7-3 48 between adjacent vertical sections should not be applied to Consistent with the conclusions in Section 6.5.1, the low-voltage HEAF energy Section load centers. While this is stated in Section 6.5.1, consider of 90 MJ is below the threshold to require fire propagation to adjacent 7.3 also including it in Section 7 for clarity. cabinets. Fire propagation to adjacent load center cubicles should not be postulated.

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Paragraph Agree that calling out the ensuing fire in only the last bullet could call into Regarding: question the ensuing fire for the first three bullets. Since the objective of the "A supply circuit breaker in the mid or lower elevation, bulleted list was to provide insights on the ZOI, the mentioning of the post-location E or F in Figure 7-2, does not have a ZOI external HEAF ensuing fire (for all supply circuit breaker locations) should be moved to the switchgear. For this case, an ensuing fire should still (and clarified that it is intended to capture all low-voltage HEAFs). Revised text be postulated at the supply circuit breaker. See Section 6.5 as follows:

Page 7-3 for modeling the ensuing fire."

49 Section 7.3 This bullet specifies that an ensuing fire should still be In addition to the energetic ZOIs an ensuing fire is postulated at the supply modeled for this situation, however the ensuing fire is not circuit breaker. See Section 6.5 for modeling the ensuing fire. Consistent with addressed for the previous three bullets. Should the ensuing the conclusions in Section 6.5.1, the low-voltage HEAF energy of 90 MJ is fire therefore not be modeled for the situations in the below the threshold required to model fire propagation to adjacent vertical previous three bullets? sections. Fire propagation to adjacent load center cubicles should not be postulated.

Table 8-3 Added GF to acronym list and also added note at the bottom of Tables 8-3 Acronym GF is used in Tables 8-3, 8-4, 8-5, and 8-6, but is through 8-6.

Table 8-4 not defined until a footnote in Table 9-2.

50 Table 8-5 Table 8-6 Page 8-8 Regarding: Corrected.

Lines 9 & " (shown in Figure 8-2 and Figure 8-3) The screening" 51 10 Section A period should be placed after "Figure 8-3)." to end the 8.4 sentence.

Page 9-5 Keep Figure 9-1 Title with the figure on page 9-5 Together in the public comment version and also confirmed together in the 52 final version.

Figure 9-1 Page 9-4 Corrected Page 9-7 Remove the extra page breaks on Pages 9-4, 9-7, 9-8, and 53 Page 9-8 9-12 Page 9-12 Page 22 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Change: Corrected Do not postulated fire propagation for arc energies of 101 Section MJ and below.

54 10.2.2 To:

Change to "Do not postulate fire propagation for arc" Table A-1 Corrected as part of tech edit Pages A- There are multiple instances where there is a typo for 55 2, A-4, A- "ensuing" fires where it's spelled "ensuring".

11, A-12 Added links in the main body of the report to the metric tables in Appendix E.

This is summarized as follows:

Table E-1 to Table 7-1 Table E-2 to Table 8-2 Appendix Table E-3 to Table 8-3 56 Request note to correlate to tables in body of report.

E Tables Table E-4 to Table 8-4 Table E-5 to Table 8-5 Table E-6 to Table 8-6 Table E-7 to Table 9-2 Row 20: Corrected "For the post-HEAF ensuing fire, the first cable tray ignites Page G-6 dur to the ensuing fire and eventually propagates to the third 57 Section tray in the cable tray stack."

G.1.4 Typo: "dur" should be "due" Page 23 of 24

REVIEW / COMMENT DOCUMENTATION Activity: HEAF - PRA Report - Resolution of Public Comments Comment Document Review Comments (Print)/Basis for Comment Disposition No. Number Section /

Paragraph Regarding: Corrected The ignition frequency for each scenario is split between the Page G- supply sections and supply breaker limited fault in Error!

58 11 Reference source not found. (shown below as Figure 11-Line 18 23).

Correct reference error Page 24 of 24

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