ML11188A072

NRC Meeting with NEI and Licensees June 28, 2011 Workshop Engineering Approach Rev F

ML11188A072
Person / Time
Site:	Palo Verde
Issue date:	07/06/2011
From:	Leake H Nuclear Energy Institute
To:	Office of Nuclear Reactor Regulation
Basturescu S
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ML11187A009	List: ML11188A019 ML11188A072
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Download: ML11188A072 (13)

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A Systematic Engineering Approachto Degraded Voltage Harvey LeakePalo Verde Nuclear Generatin g Station gIEEE Subcommittee 4IEEE 741 Working Group Degraded Grid Voltage Protection Workshop June 28-29, 2011 General Principles"Our regulatory system must protect public health, welfare, safety, and our environment-. It must be based on the bestavailablescience

"(ExecutiveOrder best available science- (Executive Order 13563)"Regulations should be based on the best available knowledge from research and operational experience." (NRC Principles

of Good Regulation) 2 DefinitionsDegraded Voltage:

Voltage < minimum needed to ensure essential load equipment operates without damage or malfunctionDVR Dropout Setting:

Nominal setpoint at which, during a decrease in voltage, the degraded voltage relay begins timing outDVR Reset Setting:

Nominal setpoint at which, during an increase in voltage, the degraded voltage relay ceases timing outInterrupted Flow Event:

Design basis event in which injection is established from offsite power supply, then interrupted 3

LOCA with Degraded Voltage Draft RIS Approachy BusQuestions to consider:1.Is flat voltage profile credible? Bdi?DVR dropout voltage c urs 1 2 Voltage at Safet B oun di ng?2.Is assumed timing of DVR actuation (accident analysis time) credible?3.Injection flow established prior to DVR timeout will be interrupted.

Accident analysis for this event?

Will resequencing be successful?TimeLOCA / degraded voltage oc cDVR timeoutAssume loads resequence onto diesel generator 3

LOCA with Degraded VoltageComponents of Voltage Profile 0.8 1 0 40VoltageTime(seconds)Switchyard Voltage Degradation due to Generator Trip (Millstone effect) 5 0.9 1 0 40VoltageEnergization of Safety LoadsTime (seconds)Time (seconds)0.9 1 0 40VoltageFast Bus TransferTime (seconds)

LOCA with Degraded VoltageConsidering Best Available Sciencey BusInsights: 1VltfilitittfltDVR dropout voltage 1 2DVR reset voltage Voltage at Safet 1.V o ltage pro fil e i s trans i en t , no t fl a t2.DVR timeout unlikely to start at T=0, so actuation time > time delay setting3.How to analyze interrupted flow effect?

-Accident analysis is for simultaneous LOOP/LOCA, not interrupted flow scenario

-Hydraulic effects (GSI-171)?

-Will safety equipment successfully resequence onto EDG?(GSI-171)?TimeLOCA occursDVR timeout 3 6 LOCA with Degraded Voltage ConclusionsDVRs provide unsatisfactory protection against this scenario

-Actuation would result in an unanalyzed interruptedflowevent interrupted flow eventPreventative strategy:

-Interrupted flow events

-Degraded voltage events 7

Prevention StrategyConsidering Best Available Sciencey BusDVR dropout voltageDVR reset voltage

• Criteria Voltage at SafetTimeLOCA occurs

• CriteriaMinimum credible steady-state voltage following automatic load actuation DVR reset voltage settingLongest transient voltage excursions below reset voltage setting < DVR time delay setting

• Effectively prevents interrupted flow scenario 8

DVR Voltage Setting Analysis Draft RIS Guidance"-calculation of voltages at terminals or contacts of all safety related equipment with the voltage at the DVR monitored bus attheDVRdropoutsettingprovidingthe at the DVR dropout setting , providing the necessary design basis for the DVR voltage settings. In this manner, the DVR ensures adequate operational (starting and running) voltage to all safety related

equipment-."

9 DVR Voltage Setting Analysis "Bottom Up" Approachy BusDVR dropout voltage

• Does not demonstrate that "the DVR ensures adequate Voltage at SafetTimeLOCA occursoperational (starting and running) voltage to all safety related equipment"?DVR assumed not to actuate

• Flat voltage profile provides no insight regarding protection during transient conditionsMotor startingLOCA/generator trip 10 DVR Voltage Setting Analysis Consequence if DVR Does Not Resety BusDVR dropout voltageDVR reset voltage Voltage at Safet

• Interrupted flow event will occur if steady-state voltage following automatic load actuation is < DVR reset voltage setting

• Draft RIS suggests LOCA with degraded voltage should be analyzed, but does not discuss transient voltage characteristic nor interrupted flow effectTimeLOCA occursDVR timeout 11 Systematic Engineering Approach 1.Begin with clear descriptions of postulated initiating events and conditions

• Consider operating experienceGenerator trip was the initiating event for the 1976 Millstone event 2Identifydesignrequirementsandobjectives 2.Identify design requirements and objectives 3.Perform necessary research to obtain pertinent information

• Develop credible and bounding voltage profiles for each postulated scenario 4.Establish viability of proposed solutions by analyzing effect of voltage profile 12 Probabilistic Considerations Example Figure100105%)"-studies conducted during resolution of the issue showed that the contribution to CDF from the sequence of events was far less than originally anticipated- Thus, the issue was RESOLVED with no new requirements." (GSI-171)"-studies conducted during resolution of the issue showed that the contribution to CDF from the sequence of events was far less than originally anticipated- Thus, the issue was RESOLVED with no new requirements." (GSI-171)Probability of being at given steady-state voltage following sequencing 85 90 9500.20.40.60.81Voltage (%Probability (given a postulated accident)DVR Tolerance BandProbability of consequential LOOP & interrupted flow not high enough to be a concern per GSI-171Should consequential degraded voltage region be a concern?

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