ML18213A093
| ML18213A093 | |
| Person / Time | |
|---|---|
| Issue date: | 07/31/2018 |
| From: | Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute |
| To: | |
| Shared Package | |
| ML18213A072 | List: |
| References | |
| Download: ML18213A093 (21) | |
Text
Module III - Fire Analysis Task 11 - Special Models Part 2: Main Control Board Fires, Turbine Generator Fires, Hydrogen Fires Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
Module III - Fire Analysis Task 11 - Special Models Part 2a: Fires in the Main Control Board Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
Module III-11, Pt. 2: Special Models Part 2 Scope of this Module Module 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) 3
Module III-11, Pt. 2: Special Models Part 2 Main Control Board Damage Likelihood Model The main control board (MCB) presents many analysis challenges
- Design practices vary widely Configuration of the boards themselves Relay rack room versus main control room Separation 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 4
Module III-11, Pt. 2: Special Models Part 2 Main Control Board Damage Likelihood Model The 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 components This 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 5
Module III-11, Pt. 2: Special Models Part 2 Main Control Board Damage Likelihood Model To use the model you must first identify your target set
- Example: two control switches on the MCB Determine 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 damaged The resulting number includes BOTH the severity factor AND the probability of non-suppression
- It does not include fire frequency!
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Module III-11, Pt. 2: Special Models Part 2 Main Control Board Damage Likelihood Model Example:
- Target set is two switches located Probability of Target Damage: [SF*Pns](d) 0.5 m apart from each other 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1.00E-01
- Inspection shows that the cables Qualified leading to each switch are routed 1.00E-02 Unqualified in opposite directions such that 0.5 m is the minimum separation 1.00E-03 distance between the switches The MCB contains only IEEE-383 certified low-flame-spread cables 1.00E-04
- The conditional probability that a 1.00E-05 fire occurring somewhere in the Damage Distance [m]
MCB will damage the target set is approximately 3.0E-3 7
Main Control Room Fire Analysis Step 8: Fire Growth . . . (contd)
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 EPRIs 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 ) = MCB [ SF Pns ](d )
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Main Control Room Fire Analysis Step 8: Fire Growth . . . (contd)
The likelihood is a combination of severity factors and non-suppression probabilities integrated over all possible fire events inside the panel that may damage the postulated target set.
All possible fire origin locations d
H h
(d ) = MCB [ SF Pns ](d )
w W
HW 1
[ SF Pns ](d ) =
H W SF (d , w, h ) P 0 0 ns (d , w, h)dwdh 9
Module III - Fire Analysis Task 11 - Special Models Part 2b: Turbine Generator Fires Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
Module II-11, Pt. 2: Special Models Part 2 Turbine Generator Set Fires Four 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) 11
Module III-11, Pt. 2: Special Models Part 2 Turbine Generator Set Fires: Exciter Fires Exciter 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 12
Module III-11, Pt. 2: Special Models Part 2 Turbine Generator Set Fires: Hydrogen Fires Fire 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 range For 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) 13
Module III-11, Pt. 2: Special Models Part 2 Turbine Generator Set Fires: Hydrogen Fires For 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 14
Module III-11, Pt. 2: Special Models Part 2 Turbine 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 15
Module III-11, Pt. 2: Special Models Part 2 Turbine Generator Set Fires: Catastrophic Failure (2/4)
Screening approach: assume the conditional probability that, 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 Building Possible failure of exposed structural steel as well Related SRs: FSS-F1, F2, F3
- Estimate screening CDF contribution, refine as appropriate 16
Module III-11, Pt. 2: Special Models Part 2 Turbine 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 credit Manual backup is also assumed to improve suppression success Timely 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 components Generic screening value given the above: Pns = 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 17
Module III-11, Pt. 2: Special Models Part 2 Turbine 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 0.02 For 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.
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Module III - Fire Analysis Task 11 - Special Models Part 2c: Hydrogen Fires Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
Module III-11, Pt. 2: Special Models Part 2 Hydrogen Fires This 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 effects The discussion stops short of recommending modeling approaches.
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Module III-11, Pt. 2: Special Models Part 2 Hydrogen Fires Two general types of fires:
- Jet fires originating at point of a H2 leak Critical question will be flame length About 90% of the events are of this type (Page N-10 of NUREG/CR-6850)
- Explosions If there is a mechanism for the release of large quantities of H2 (e.g.,
a large leak, a prolonged leak that might not be ignited early), then likelihood of a hydrogen explosion is high References provide additional resources for assessing damage potential for an explosion scenario Critical question will be the severity of the overpressure 21