success criteria and time windows
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IAEA.PNRA.05.05 Stetkar & Associates 1
THERMAL / HYDRAULIC ANALYSES, SUCCESS CRITERIA, TIME WINDOWS
byJohn W. Stetkar
Presented toNational Training Course on Probabilistic Safety Assessment
Islamabad, PakistanMay 2 - 6, 2005
IAEA Project C7-PAK/9/028-001
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TIME WINDOWS
• OFFSITE POWER RECOVERY
• LOSS OF HVAC / ROOM HEATUP
• POST-INITIATOR OPERATOR ACTIONS
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OFFSITE POWER RECOVERY
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LOSS OF OFFSITE POWER
• INTEGRAL MODEL FOR POWER FAILURE AND RECOVERY
• OFFSITE POWER RECOVERY CURVE
• ELECTRIC POWER RECOVERY TIME WINDOWS
• INITIATING EVENT FREQUENCIES
• PSA MODEL IMPACTS
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ELECTRIC POWER RECOVERY
• "BASELINE" PSA MODEL DETERMINES UNAVAILABILITY OF ONSITE POWER IF DIESEL GENERATORS MUST RUN FOR ENTIRE 24-HOUR MISSION TIME
• DIESEL GENERATORS MUST ACTUALLY RUN ONLY UNTIL OFFSITE POWER IS RESTORED
• PLANT CAN SURVIVE BLACKOUT FOR SOME TIME
– WHEN DO DIESEL GENERATORS FAIL?
– WHEN IS OFFSITE POWER RESTORED?
– HOW MUCH TIME IS AVAILABLE TO RESTORE POWER DURING EACH SCENARIO?
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INTEGRAL MODEL FOR ELECTRIC POWER FAILURE AND RECOVERY
Q(T) = ∫0
24
ΦF(t) * [ 1 - ΦR(t + T) ] dt
where
Q(T) = Unavailability of power for longer than T hours during the 24-hour period after a loss of offsite power event at time t = 0.
ΦF(t) = Frequency of onsite power failure between times t and t + dt.
ΦR(t + T) = Fraction of power failures that are recovered within T hours after time t.
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INTEGRAL MODEL FOR ELECTRIC POWER FAILURE AND RECOVERY
The term [1 - ΦR(t + T)] quantifies the fraction of power failures that are NOT recovered within T hours after failure of onsite power at time t.
The complete integral quantifies the total likelihood that onsite power fails and is not recovered within a time window of T hours during the 24-hour period after loss of offsite power at time t = 0.
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ELECTRIC POWER RECOVERY FACTOR
RE = ∫0
24
ΦF(t) * [ 1 - ΦR(t + T) ] dt
∫0
24
ΦF(t) dt
where
RE = Unavailability of power after accounting for possible recovery within a time window of T hours, divided by the baseline unavailability of power from the diesel generators during the nominal 24-hour mission time before recovery is included in the model.
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ELECTRIC POWER RECOVERY MODELS
• ONSITE POWER FAILURE FUNCTION ΦF(t) DETERMINED BY DIESEL GENERATOR MODELS
• PSAs TYPICALLY DO NOT INCLUDE CREDIT FOR REPAIRS OF FAILED DIESEL GENERATORS
• POWER RECOVERY FUNCTION ΦR(t + T) DETERMINED BY OFFSITE POWER RECOVERY CURVES
• RECOVERY TIME WINDOW (T) DETERMINED BY EVENT SEQUENCE MODELS AND THERMAL / HYDRAULIC ANALYSES
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DIESEL GENERATORSPOWER UNAVAILABILITY OVER TIME
• FAILURES AT TIME t = 0
– FAILURE TO START
– UNAVAILABILITY DUE TO MAINTENANCE
– UNAVAILABILITY DUE TO TESTING
– UNAVAILABILITY DUE TO PRE-INITIATOR HUMAN ERROR
• FAILURES AFTER TIME t = 0
– FAILURE TO RUN
– LOSS OF FUEL AFTER DAY TANK IS DRAINED
– ROOM HEATUP
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OFFSITE POWER RECOVERY CURVES
• AVOID DETAILED MODELS FOR TRANSMISSION LINES, GRID CONNECTIONS, SWITCHYARDS
• DETAILED MODELS ARE TYPICALLY OPTIMISTIC, COMPARED WITH ACTUAL EXPERIENCE
• PLANT-SPECIFIC LOSS OF OFFSITE POWER EXPERIENCE TYPICALLY VERY LIMITED
• DERIVE RECOVERY CURVES FROM REGIONAL TRANSMISSION LINE OUTAGE DATA
• USE GENERIC EXPERIENCE FOR "SANITY CHECK"
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OFFSITE POWER RECOVERY CURVES
• SIMPLIFIED MODEL FROM REGIONAL TRANSMISSION LINE OUTAGE DATA
• ONLY FORCED OUTAGES
• OMIT DURATIONS LESS THAN ~ 5 MINUTES– SINGLE LINE AUTOMATIC SWITCHING– TIME TO RECONNECT POWER TO ONSITE BUSES
• TRANSMISSION LINE CORRIDORS– VOLTAGES AND ROUTING NEAR PLANT– COMMON RIGHT-OF-WAY– SIMILAR DIRECTIONAL ROUTING
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OFFSITE POWER RECOVERY CURVES
• MODEL EACH CORRIDOR AS A SINGLE LINE
• DERIVE UPPER BOUND CURVE FROM "N" INDEPENDENT CORRIDORS
– 95TH PERCENTILE RECOVERY
– CONSERVATIVE IF TRUE INDEPENDENCE APPLIES
• DERIVE LOWER BOUND CURVE FROM FULL CORRELATION OF ALL CORRIDORS
– 5TH PERCENTILE RECOVERY
– ACCOUNTS FOR REGIONAL / GRID IMPACTS
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OFFSITE POWER RECOVERY CURVES
RecoveryFraction 95th Percentile
Median
5th Percentile
Time
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ELECTRIC POWER RECOVERYTIME WINDOWS
• DC BATTERY LIFE
• PLANT THERMAL / HYDRAULIC RESPONSE
– STEAM GENERATOR DRYOUT
– CORE UNCOVERY
• PSA SCENARIOS / SUCCESS CRITERIA
– REACTOR COOLANT PUMP SEAL FAILURE
– BLEED-AND-FEED COOLING
– HIGH PRESSURE / LOW PRESSURE INJECTION
– CONTAINMENT COOLING
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LOSS OF OFFSITE POWERDURATION-BASED INITIATING EVENTS
• SIMPLIFIED APPROACH
• DISCRETE TIME INTERVALS FOR DURATION OF OFFSITE POWER FAILURE
• ALLOCATE INITIATING EVENT FREQUENCY ACCORDING TO CRITICAL EVENT DURATION TIME WINDOWS (e.g., FOUR INITIATING EVENTS)
– LESS THAN 30 MINUTES
– 30 MINUTES - 1 HOUR
– 1 HOUR - 2 HOURS
– MORE THAN 2 HOURS
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LOSS OF OFFSITE POWERDURATION-BASED INITIATING EVENTS
• PSA MODEL IMPACTS
– DIESEL GENERATOR OPERATING MISSION TIMES
– AVAILABLE SYSTEMS
– OPERATOR ACTIONS
• FAULT TREE HOUSE EVENTS FOR RECOVERY CONDITIONS
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LOSS OF HVAC / ROOM HEATUP
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LOSS OF VENTILATION / ROOM COOLING
• USUALLY MOST IMPORTANT FOR ELECTRICAL / ELECTRONICS EQUIPMENT
– SWITCHGEAR ROOMS
– PROTECTION / CONTROL / LOGIC CABINET ROOMS
– MAIN CONTROL ROOM
• MECHANICAL EQUIPMENT IN SMALL ENCLOSED ROOMS
• VERY IMPORTANT FOR SOLID-STATE EQUIPMENT
– ENCLOSED CABINETS
– ENCLOSED ROOMS FOR FIRE / FLOODING HAZARDS
– UPGRADES / BACKFIT DESIGNS
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LOSS OF VENTILATION / ROOM COOLING
• ROOM HEATUP ANALYSES
• EQUIPMENT THERMAL FRAGILITY ANALYSES
• RECOVERY TIME WINDOWS
• RECOVERY SUCCESS CRITERIA
• PSA MODEL IMPACTS
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ROOM HEATUP ANALYSES
• REALISTIC ANALYSES OFTEN NOT AVAILABLE
• DESIGN-BASIS CALCULATIONS
– CONSERVATIVE HEAT LOADS
– ASSUMPTIONS ABOUT RECOVERY
• PSA MODELS REQUIRE TEMPERATURE VS. TIME AFTER LOSS OF VENTILATION
• SIMPLIFIED CALCULATIONS PROVIDE REASONABLE ESTIMATES
• ACTUAL TESTS
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EQUIPMENT THERMAL FRAGILITY ANALYSES
• FAILURE LIKELIHOOD AS A FUNCTION OF TEMPERATURE
• TYPICALLY NOT AVAILABLE FROM MANUFACTURER
• POSSIBLE INFORMATION
– QUALIFICATION TEMPERATURE
– RATED OPERATING TEMPERATURE
– MAXIMUM PERMISSIBLE OPERATING TEMPERATURE
• CONSTRUCT FRAGILITY CURVES FROM AVAILABLE INFORMATION
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VENTILATION RECOVERY SUCCESS CRITERIA
• EXTENSION OF ROOM HEATUP ANALYSES
• RECOVERY OPTIONS
– OPEN DOORS
– PORTABLE FANS
– ALTERNATE CHILLED WATER / FORCED COOLING
• SIMPLE LOCAL ACTIONS MAY NOT BE ADEQUATE IF ALL COOLING IS FAILED FOR THE WHOLE BUILDING
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VENTILATION RECOVERY TIME WINDOWS
• PROBABILITY DISTRIBUTION FOR TIME UNTIL EQUIPMENT FAILURE
– ROOM HEATUP CURVES
– EQUIPMENT THERMAL FRAGILITY CURVES
• TIME WINDOWS FOR OPERATOR ACTIONS
• SEQUENTIAL IMPACTS AS ROOMS HEAT UP
– PSA EQUIPMENT
– OPERATOR ACTION DEPENDENCIES
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POST-INITIATOR OPERATOR ACTIONS
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HUMAN ERROR RATETIME-RELIABILITY CORRELATION
• ANALYSIS OF POST-INITIATOR DYNAMIC ACTIONS
• TWO CONTRIBUTIONS TO HUMAN ERROR
– COGNITIVE: ACCOUNTS FOR IDENTIFICATION, DIAGNOSIS, EVALUATION, AND DECISION ERRORS DURING COGNITIVE PHASE OF RESPONSE
– IMPLEMENTATION: ACCOUNTS FOR MISOPERATION OF EQUIPMENT DURING EXECUTION PHASE OF RESPONSE ("SLIP" ERRORS)
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MODEL FOR COGNITIVE RESPONSE ANDIMPLEMENTATION ACTIONS
Td Ta
Tm
T0
T0 = ANNUNCIATION (OR OTHER COMPELLING SIGNAL) OF ABNORMAL EVENT
Tm = ESTIMATED MAXIMUM ALLOWABLE TIME TO COMPLETE COGNITIVE AND REQUIRED IMPLEMENTATION ACTIONS TO SATISFY PSA SUCCESS CRITERIA ("AVAILABLE TIME WINDOW")
Td = ESTIMATED ALLOWABLE TIME FOR CORRECT DIAGNOSIS THAT PERMITS SUFFICIENT TIME TO COMPLETE IMPLEMENTATION ACTIONS BEFORE Tm
Ta = ESTIMATED TIME TO PERFORM REQUIRED ACTIONS AFTER CORRECT DIAGNOSIS
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TIME WINDOWS
• INITIATION TIME (T0) AND TOTAL AVAILABLE TIME WINDOW (Tm)
– SCENARIO-SPECIFIC THERMAL-HYDRAULIC ANALYSES
– SCENARIO-SPECIFIC SIMULATOR RUNS
– FSAR OR DESIGN-BASIS ANALYSES
– SIMPLIFIED ENGINEERING CALCULATIONS
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IMPLEMENTATION TIME
• IMPLEMENTATION TIME (Ta)
– EQUIPMENT MANIPULATIONS
– RESPONSE TIME FOR LOCAL ACTIONS
– ACTIVE CONTROLS (TIME TO COOL DOWN, REDUCE PRESSURE, ETC.)
– TASK ANALYSES, WALKDOWNS
– DISCUSSIONS WITH PLANT OPERATORS
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