ML18213A093

Joint EPRI/NRC-RES Fire PRA Workshop - Task 11 - Special Models Part 2: Main Control Board Fires, Turbine Generator Fires, Hydrogen Fires

ML18213A093
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Issue date:	07/31/2018
From:	Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute
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Module III

-Fire Analysis Task 11 -Special ModelsPart 2: Main Control Board Fires, Turbine Generator Fires, Hydrogen FiresJoint EPRI/NRC

-RES Fire PRA WorkshopAugust 6-10, 2018A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)

Joint EPRI/NRC

-RES Fire PRA WorkshopAugust 6-10, 2018Module III

-Fire AnalysisTask 11 -Special ModelsPart 2a: Fires in the Main Control BoardA Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES) 3Module III

-11, Pt. 2: Special Models Part 2Scope of this ModuleModule III

-11, Pt. 2 covers the three remaining "Special Models"-Main Control Board Fires (Appendix L)

-Turbine Generator (TG) Fires (Appendix O)

-Hydrogen Fires (Appendix N) 4Module III

-11, Pt. 2: Special Models Part 2Main Control Board Damage Likelihood ModelThe main control board (MCB) presents many analysis challenges

-Design practices vary widelyConfiguration of the boards themselvesRelay rack room versus main control roomSeparation and partitioning within MCB

-MCB may be important to risk, but IPEEE vintage approaches were identified as a weakness of those studies

-Fire models cannot currently predict in

-panel fire behavior, so an alternative approach is needed

-New FAQ! FAQ 14-0008. Provides clarification of what constitutes the Main Control Board. A method is provided to assess the likelihood that a fire in the MCB will grow large enough to damage a specific target set as defined by a specific physical region of the board 5Module III

-11, Pt. 2: Special Models Part 2Main Control Board Damage Likelihood ModelThe MCB model is built on several assumptions that are specific to the MCB and the MCR

-MCB fire frequency partitioning approach

-Suppression times for MCR fires

-Fire characteristics of a MCR type control panel (peak HRR and growth profile)-Damage limits for control componentsThis model applies ONLY to the MCB itself

-Not intended for other electrical cabinets/panels

-Not intended for MCR "back

-panels"-Not intended for the relay room or other similar areas 6Module III

-11, Pt. 2: Special Models Part 2Main Control Board Damage Likelihood ModelTo use the model you must first identify your target set

-Example: two control switches on the MCBDetermine the separation distance between the most remote members of the damage set (those furthest apart)

-Consider cable routing within the panel!Using this distance, go to the probability curve and estimate the conditional probability that given a fire somewhere in the MCB, the specific zone encompassing the target set will be damagedThe resulting number includes BOTH the severity factor AND the probability of non

-suppression

-It does not include fire frequency!

7Module III

-11, Pt. 2: Special Models Part 2Main Control Board Damage Likelihood ModelExample:-Target set is two switches located 0.5 m apart from each other

-Inspection shows that the cables leading to each switch are routed in opposite directions such that 0.5 m is the minimum separation distance between the switches The MCB contains only IEEE -383 certified low

-flame-spread cables

-The conditional probability that a fire occurring somewhere in the MCB will damage the target set is approximately 3.0E

-3Probability of Target Damage: [SF

• P ns](d)1.00E-051.00E-041.00E-031.00E-021.00E-010.00.51.01.52.02.53.0Damage Distance [m]QualifiedUnqualified 8Main Control Room Fire Analysis Step 8: Fire Growth . . . (cont'd)A probabilistic model of fire spread in the main control board estimates the likelihood that a set of targets separated by a predetermined distance would be affected by a fire. Difficult to model fire spread within a cabinet using current state

-of-the-art analytical tools. Probabilistic model based on EPRI's Fire Events Database and cabinet fire experiments reported in NUREG/CR

-4527. The likelihood is a combination of severity factors and non

-suppression probabilities

)]([)(d P SF d ns MCB 9Main Control Room Fire Analysis Step 8: Fire Growth . . . (cont'd)The likelihood is a combination of severity factors and non

-suppressionprobabilities integrated over all possible fire events inside the panel thatmay damage the postulated target set. All possible fire origin locations

)]([)(d P SF d ns MCB W H d h wH W ns ns dwdh h w d P h w d SF W H d P SF 0 0), , (, , 1)]([

Joint EPRI/NRC

-RES Fire PRA WorkshopAugust 6-10, 2018Module III

-Fire AnalysisTask 11 -Special ModelsPart 2b: Turbine Generator FiresA Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES) 11Module II-11, Pt. 2: Special Models Part 2Turbine Generator Set FiresFour types of fires can occur involving the turbine generator set, and each is treated differently:

-Electrical fires in the exciter

-Hydrogen fires

-General oil fires

-Catastrophic failure (e.g., blade ejection) 12Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Exciter FiresExciter fires do occur, but all evidence indicates fires remain small and non-threatening

-No evidence of any exciter fire that led to damage to anything other than the exciter itself

-No attempt was made to estimate likelihood of a severe exciter fire (one that challenges external targets)Recommended Practice:

-Assume exciter fires remain confined to the exciter

-Verify for your application, but should not represent a significant risk contributor 13Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Hydrogen FiresFire event database shows 13 T/G set hydrogen fires, two categorized as severe, with the rest being fires due to small leaks (generally associated with seals) with limited damage rangeFor small fires:

-Assume damage will be limited to within a few feet of the point of release

-Secondary ignitions should be considered and treated if there are nearby combustibles

-See more in Hydrogen Fires discussion (Appendix N) 14Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Hydrogen FiresFor severe fires, widespread damage may occur due to an explosion or detonation of the hydrogen gas.

-Assume fire may damage all Fire PRA cables and equipment within the line of site of the generator and its bearings (including above and below)-Hydrogen explosion could cause some structural damage as well

-For further discussion

-see Hydrogen Fires 15Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Catastrophic Failure (1/4)International experience includes a few fires initiated by catastrophic turbine failure that resulted in widespread damage including structural damage-Examples: Vandellos (1989), Narora (1993), Chernobyl Unit 2 (1991)

-Events involved a combination of turbine blade ejection, hydrogen release, and large oil fires.Domestically, only one event came close to involving all of these elements (Salem, 1991)

-Event involved minor damage due to existence of an automatic suppression system and prompt fire brigade response

-Indicates that both automatic fire suppression systems and fire brigade should be credited to prevent catastrophic consequences 16Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Catastrophic Failure (2/4)Screening approach: assume the conditional probabilitythat, given a T/G set fire, the event will involve catastrophic failure (e.g., blade ejection), hydrogen, and oil fires is:1 over 38 events or 0.025

-With successful fire control, damage would be localized and limited to the T/G system, as was the case at Salem

-In case of failure of all suppression, automatic and manual, assume loss of all Fire PRA cables and equipment in the Turbine BuildingPossible failure of exposed structural steel as wellRelated SRs: FSS

-F1, F2, F3

-Estimate screening CDF contribution, refine as appropriate 17Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Catastrophic Failure (3/4)Comments on the suppression credit for screening case:Approach assumes fixed suppression is available

-Deluge and/or sprinklers are typical for TB

-If no fixed suppression, do not apply the suppression creditManual backup is also assumed to improve suppression successTimely fire control(i.e., before widespread damage) is success here

-Implies a somewhat extended time frame for action, but no specific time available is assumed

-Failure of suppression still means damage to T/G system and nearby componentsGeneric screening value given the above:

P ns= 0.02-This is taken from sprinkler reliability, but is also intended to reflect other fixed suppression with manual backup

-Example: Deluge system with fire brigade backup 18Module III

-11, Pt. 2: Special Models Part 2Turbine Generator Set Fires: Catastrophic Failure (4/4)Table O-2 (page O-5) covers the cases, but is very confusing-To use this table:

-You start with total TG fire frequency* (sum bins 33,34,35):

2.0E-2 events/yr-Assume 1 in 38 is a catastrophic event (split fraction):x 0.025-For a raw frequency of catastrophic events:

5.0E-4 events/yr-Given fixed suppression apply generic suppression credit:x 0.02For a total net frequency of unsuppressed catastrophic events:

1.0E-5 events/yr*We are using the original 6850/1011989 frequencies because that is what table used and we want the numbers here to align with the table.

Joint EPRI/NRC

-RES Fire PRA WorkshopAugust 6-10, 2018Module III

-Fire AnalysisTask 11 -Special ModelsPart 2c: Hydrogen FiresA Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES) 20Module III

-11, Pt. 2: Special Models Part 2Hydrogen FiresThis discussion (Appendix N) applies to general hydrogen fires

-Including T/G set fires

-Also fires from other sources of hydrogen leaks and releases (e.g., recombiners, storage tanks, piping, etc.)The intent was to provide general discussion of hydrogen fires and their potential effectsThe discussion stops short of recommending modeling approaches.

21Module III

-11, Pt. 2: Special Models Part 2Hydrogen FiresTwo general types of fires:

-Jet fires originating at point of a H 2leakCritical question will be flame lengthAbout 90% of the events are of this type (Page N

-10 of NUREG/CR

-6850)-ExplosionsIf there is a mechanism for the release of large quantities of H 2(e.g., a large leak, a prolonged leak that might not be ignited early), then likelihood of a hydrogen explosion is highReferences provide additional resources for assessing damage potential for an explosion scenarioCritical question will be the severity of the overpressure