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 Approach to Degraded Voltage Harvey Leake Palo Verde Nuclear Generating g Station IEEE Subcommittee 4 IEEE 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 best available science science (Executive Order 13563)

Regulations should be based on the best available knowledge from research and operational experience. (NRC Principles of Good Regulation) 2

Definitions Degraded Voltage: Voltage < minimum needed to ensure essential load equipment operates without damage or malfunction DVR Dropout Setting: Nominal setpoint at which, during a decrease in voltage, the degraded voltage relay begins timing out DVR Reset Setting: Nominal setpoint at which, during an increase in voltage, the degraded voltage relay ceases timing out Interrupted Flow Event: Design basis event in which injection is established from offsite power supply, then interrupted 3

LOCA with Degraded Voltage Draft RIS Approach DVR dropout voltage 1

2 Questions to consider:

Voltage at Safety Bus LOCA / degraded voltage occcurs

1. Is flat voltage profile credible?

B Bounding?

di ?

2. Is assumed timing of DVR actuation 3 (accident analysis time) credible?

3. Injection flow established prior to DVR timeout will be interrupted.

Accident analysis for this event?

Will resequencing be successful?

Assume loads resequence onto diesel generator DVR timeout Time

LOCA with Degraded Voltage Components of Voltage Profile Switchyard Voltage Degradation due to Generator Trip (Millstone effect) 1 Voltage 0.8 0 40 Time (seconds)

Energization of Safety Loads 1

Voltage 0.9 0 40 Time (seconds)

Fast Bus Transfer 1

Voltage 0.9 0 40 Time (seconds) 5

LOCA with Degraded Voltage Considering Best Available Science 1 DVR reset voltage 2

DVR dropout voltage Voltage at Safety Bus Insights:

1. V 1 Voltage lt profile fil iis ttransient, i t nott fl flatt LOCA occurs

2. DVR timeout unlikely to start at T=0, so actuation time > time delay setting 3

3. 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)?

Time DVR timeout 6

LOCA with Degraded Voltage Conclusions DVRs provide unsatisfactory protection against this scenario

- Actuation would result in an unanalyzed interrupted flow event Preventative strategy:

- Interrupted flow events

- Degraded voltage events 7

Prevention Strategy Considering Best Available Science DVR reset voltage DVR dropout voltage Voltage at Safety Bus

• Criteria LOCA occurs Minimum credible steady-state voltage following automatic load actuation DVR reset voltage setting Longest transient voltage excursions below reset voltage setting < DVR time delay setting

• Effectively prevents interrupted flow scenario Time 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 at the DVR dropout setting, 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 Approach DVR dropout voltage Voltage at Safety Bus

• Does not demonstrate that the DVR ensures adequate operational (starting and running) voltage to all safety related equipment?

LOCA occurs DVR assumed not to actuate

• Flat voltage profile provides no insight regarding protection during transient conditions Motor starting LOCA/generator trip Time 10

DVR Voltage Setting Analysis Consequence if DVR Does Not Reset DVR reset voltage DVR dropout voltage Voltage at Safety Bus

• Interrupted flow event will occur if steady-state LOCA occurs 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 effect Time DVR timeout 11

Systematic Engineering Approach

1. Begin with clear descriptions of postulated initiating events and conditions

• Consider operating experience Generator trip was the initiating event for the 1976 Millstone event

2. Identify design requirements and objectives 2

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 Figure 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) 105 Probability of being at given steady-100 state voltage following sequencing Voltage (%

%)

Probability of consequential LOOP & interrupted flow not high enough to be a concern per GSI-171 95 90 DVR Tolerance Band Should consequential degraded voltage region be a concern?

85 0 0.2 0.4 0.6 0.8 1 Probability (given a postulated accident) 13