High Energy Arcing Faults (HEAF) and Their Impact on PRA for a Nuclear Power Plant Latest International Operating Experience and Research Activities

Transcription

1 High Energy Arcing Faults (HEAF) and Their Impact on PRA for a Nuclear Power Plant Latest International Operating Experience and Research Activities Marina Roewekamp GRS ggmbh, Germany Nicholas Melly, Mark Henry Salley US NRC Office of Nuclear Regulatory Research PSA 2017 International Topical Meeting on Probabilistic Safety Assessment and Analysis Pittsburgh, PA, USA, September 24-28, 2017

2 Presentation Overview Background Current HEAF PRA guidance OECD/NEA HEAF test program Methodology Test results Influence of Aluminum Test 23 Aluminum influence Test 26 Aluminum influence Aluminum Testing Conclusions HEAF operating experience Maanshan HEAF event Onagawa HEAF Event Fort Calhoun HEAF Event Conclusions 2

3 Background (1) International NPP OPEX data clearly show occurrence of a significant number of high energy arcing faults (HEAF) events worldwide 48 out of 415 fire events collected in the international NPP database up to mid-2012 represent HEAF induced fire events (over 10 %), see Clearly identified the need for imporoved methods to analyze the broad spectrum of hazards 3

4 Background (2) HEAF events often create complications impairing the plant s ability to safely shutdown the reactor and maintain it in a safe condition Electrical disturbance initiating HEAF often causes loss of essential electrical power and physical damage Large amounts of smoke provide significant challenges to the operators and fire brigade members handling the emergency In many U.S. plants, internal fire risk is can be a dominant hazard contributor HEAF-initiated scenarios are significant contributors to the overall fire risk based on a preliminary assessment of ten U.S. NFPA 805 NPPs risk assessment Fire risk contributed by HEAF initiated fire scenarios ranged from 1 % to 27 % on a unit basis Average per unit risk contribution was approximately 15 % 4

5 Current HEAF PRA Guidance There are currently 2 methods available for modeling HEAF events Electrical enclosure guidance (NUREG/CR-6850, Vol. 2, Appendix M: very limited model, largely derived from empirical evidence based one single well-documented HEAF event at SONGS, Unit 3, in February 2001 Bus duct HEAF events (NUREG/CR-6850, Supplement 1, Section 7) Both methods employ a one size fits all zone of influence (ZOI) methodology prescribing a damage zone around an initiating component ZOIs prescribe damage to potentially vulnerable electrical or electromechanical components nearby (cables, transformers, etc.) International OECD/NEA experimental HEAF Project was initiated to take an exploratory, scientific approach in an attempt to better understand HEAF phenomena and produce data to better inform fire modeling techniques 5

6 OECD/NEA HEAF Test Program (1) Purpose and objectives Perform experiments to obtain scientific data on HEAF phenomena Test methodology 26 full scale experiments HEAF events exhibit two major damage phases: 1. Short, rapid release of electrical energy o o Catastrophic failure of the electrical enclosure Ejection of hot projectiles and/or fire(s) o Damage to overhead cable trays or nearby panels 2. Ensuing fire o Ignition of combustible material within the HEAF zone of influence (ZOI) 6

7 OECD/NEA HEAF Test Program (2) Experimentally controlled parameters Arc location Nominal arc current Nominal arc voltage Arc duration Measurements Heat release rate (HRR) Temperature Heat flux Pressure Damage zone o Blast damage vs. enduring fire damage Extent of damage o o Thermal (i.e. ensuing fire / smoke damage) Physical ( i.e. thrown cabinet door, shrapnel) 7

8 Arc Initiation/Location Arcs generated at KEMA were initiated by means of a copper shorting wire 2.6 mm in diameter (10 AWG) as described in IEEE C HEAF events can quickly progress to three phase faults. This is evident from a number of LERs: The Kewaunee HEAF event (LER ) involved a phase-toground fault, which progressed to a phase-to-phase fault which accounted for the extensive bus damage The Prairie Island HEAF event (LER ) involved a C-phase ground arcing event, which quickly involved all phases The Zion HEAF event (LER ) states that the failure started as a single phase to ground fault which rapidly evolved into a three phase to ground fault 8

9 Arc Current /Voltage Testing was informed by OPEX and NPP configurations: Actual NPP plant equipment was used in testing Voltage, current, arc duration within the bounds observed in licensee event reports (LERs) and within ratings of the actual NPP equipment tested LERs describe numerous three-phase arc faults with failure of an upstream breaker Damage observed comports with LERs H. B. Robinson Steam Electric Plant, Unit 2; March 28, 2010 OECD HEAF, Test 20 October 13,2015 9

10 Duration OPEX indications Protective devices have not always worked as designed Incorrect breaker settings and fuse sizing due to design errors can increase the likelihood of a HEAF to occur and allow for extended HEAF duration Fault can be initiated in a location not protected by fault clearance devices, which allows for extended fault exposure times Properly implemented protection coordination, would limit arc fault duration to just a few cycles Many of HEAF events contributing to HEAF frequency involve one or more breaker malfunctions that fail to clear the fault Usually little information available on HEAF durations from OPEX KEMA s largest short-circuit generator can deliver instantaneous 2200 MVA Arc durations from 1 s to 8 s, mean value ~ 3 s, medium voltage components only 4 s All tests performed in excess of 4 s conducted at 480 V 10

11 Test Results Key observations Experiments involving Aluminum as a component of the electrical device resulted in more severe physical damage to equipment than those containing only copper and steel o Reactions were so violent that measurement devices were damaged r maximum measuring range was exceeded One of the most severe electrical enclosure damage was observed as result of low voltage (480 V) HEAF in enclosure with aluminum bus bars Increased duration arcing events were more likely to create an ensuing fire HEAF events involving Aluminum were seen to produce conductive product of combustion in the smoke coating test facility causing short circuits and unintended current paths in electrical systems All medium voltage enclosures, > 4.16 kv, maintained arc for > 2 s Large variability in 480 V enclosures for creating sustained arcing event; variables important to sustaining an arc at low voltages were enclosure configuration and design, and bus bar material (e.g., Aluminum vs. Copper) Experiments with arc durations less than 2 s did not result in ensuing fires 11

12 Influence of Aluminum Aluminium material impact Potentially much larger ZOI Potentially greater likelihood of maintaining arcing at low voltage levels Higher risk of fire propagation Testing indications: Introducing Aluminum into arcing fault events significantly larger energy release than expected Sources of Aluminum can include components that form parts o of the normal current carrying pathway or o of the current pathway as a result of a ground fault 12

13 Test 23 Aluminum Influence Low voltage 480 V AC switchgear cabinet utilizing Aluminum bus bar Failure of cabinet walls such that the plasma wave front created by the arcing fault was expelled out of the cabinet a significant distance of several feet Melting and destruction of the Inconel temperature sensors located 1m (3 ft.) from enclosure Conductive Aluminum byproducts coating facility after testing o Shorting out equipment and causing damage to electrical circuits All other low voltage tests (without Aluminum): Temperature sensors not damaged Minimal electrical enclosure breach 13

14 Test 26 Aluminum Influence Bus duct section removed from the 4.16 kv switchgear room of a decommissioned US NPP consisting of segmented (non-segregated) copper bus bars enclosed within aluminum bus duct housing Arc initiated on the copper conductors and progressed to the grounded aluminum bus duct housing as it became involved in the arc path Hot gas mixture and arc jetted out of the bus duct, distributing hot gas/molten metal linearly from the bus duct opening 1.50 in. copper bus bar material and 7 in. aluminum enclosure material were vaporized OPEX shows this same phenomena (e.g., Diablo Canyon NPP bus duct fire in May 2000) 14

15 Aluminum Testing Conclusions Physical damage to test specimens, measurement devices, and testing facility in Test 23 and Test 26 was attributed to presence of Aluminum Presence of Aluminum caused a more energetic plasma development when consumed during the arcing process HEAFs can produce temperatures of 19,426 C (35,000 F) It appears that Aluminum is igniting and contributing to the energy released Test results significantly exceeded expected ZOI postulated in current Fire PRA guidance New, previously unidentified failure mechanism: Conductive Aluminum byproduct coating enclosure environment caused short circuits and unintended current paths in electrical systems Scale of that impact was unexpected and is an area that warrants further investigation 15

16 HEAF Operating Experience - Maanshan HEAF Event HEAF event and subsequent loss of external and internal power supply at Taiwanese 2 unit PWR plant on March 18, 2001 Salt deposit on insulators due to foggy weather caused instability of 345 kv high voltage grid leading to HEAF Breaker opened the connection to the 161 kv external supply During switch to the 345 kv grid, short circuit and HEAF occurred in 4 kv power switch of one redundancy emergency power lines causing a cable fire 2 emergency diesels of Unit 1 were unable to provide power to both essential buses After > 2 h, the so-called swing emergency diesel generator, which can provide power to either one of the units, was successfully connected to Unit 1 16

17 HEAF Operating Experience - Onagawa HEAF Event Tohoku district earthquake caused arcing fault in 2 (No. 7 and No. 8) of 10 enclosures of the non-emergency M/C switchgear cabinets of the Onagawa NPP in Japan on March 11, 2011 Affected cabinet was installed in the underground floor of the turbine building Arcing started at high voltage electric component (6.9 kv M/C) initiated by short to ground Heat due to arcing resulted in ensuing fire inside cabinet affecting all 10 sections of enclosure lineup Control cables for non-emergency components directly above cabinet were affected by heat generated by the fire Fire caused trip of over-current relay start-up transformer causing loss of offsite power in combination with switching over of power supply system and loss of auxiliary transformer 17

18 HEAF Operating Experience Fort Calhoun HEAF Event Switchgear fire occurred at Fort Calhoun NPP on June 7, 2011, while the plant was shut down for planned refueling outage Fire was caused by the catastrophic failure of the feeder breaker for 480 V AC load center with aluminum present Fire resulted in loss of power to 6 of 9 safety related 480 V AC electrical distribution buses and 2 of 4 safety related 4160 V AC buses Event resulted in the loss of the spent fuel pool cooling Combustion products caused fault across open bus-tie breaker on an island bus Feeder breaker tripped unexpectedly resulting in loss of power to opposite train bus resulting in grounds on both trains of safety related direct current power used for breaker operation and electrical protection Large quantity of soot and smoke migrated into non-segregated bus duct connecting island buses causing arcing Maximum conditional CDP: 3.4 x 10-4 at the time of the fire 18

19 Conclusions HEAF-initiated scenarios contribute significantly to fire risk OPEX shows: HEAFs continue to occur, despite the extensive electrical protection against such fire-initiated scenarios built into the plant design Significant complications to shutting down the plant can occur during and after a HEAF HEAF events generate large quantities of dense smoke, cause significant equipment damage, and challenge operators with scenarios unlikely to have been trained on OECD HEAF testing program identified a potential issue where existing regulations, guidance, and analytical models used for PRA applications may not bound the hazard if aluminum is present Severity and risk impact of these events and consequences highlight need for greater understanding of the underlying phenomena and treatment of HEAF events 19

20 Information Notice (IN) 2017-XX High Energy Arc Faults in Electrical Equipment Containing Aluminum Components OECD/NEA international test program insights 6 U.S. operating experience events involving aluminum components Plant Date Fort Calhoun June 7, 2011 Columbia August 5, 2009 Diablo Canyon May 15, 2000 Zion April 3, 1994 Shearon Harris October 9, 1989 Kewaunee July 10, 1987 Issued August 21,

21 PIRT & HEAF Phase 2 Testing HEAF Phase 2 testing to be conducted as an OECD/NEA International Project with more focused objectives International Phenomena Identification and Ranking Table (PIRT) exercise held in February 2017 Early PIRT Insights: Aluminum oxidation and byproducts Pressure effects Target characterization and sensitivity Mitigating factors ( HEAF shields ) PIRT NUREG to be issued in Summer 2017 OECD/NEA HEAF Phase 2 Test Program currently under development 21

22 Thank you for your attention! 22