ML12163A052
| ML12163A052 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 06/07/2012 |
| From: | - No Known Affiliation |
| To: | Division of Operating Reactor Licensing |
| References | |
| Download: ML12163A052 (51) | |
Text
WBN2Public Resource From: Boyd, Desiree L [dlboyd@tva.gov]
Sent: Thursday, June 07, 2012 1:33 PM To: Epperson, Dan; Poole, Justin; Raghavan, Rags; Milano, Patrick; Campbell, Stephen Cc: Arent, Gordon; Hamill, Carol L; Boyd, Desiree L
Subject:
TVA letter to NRC_06-07-12_Response to SSER Open Item 30 Attachments: 06-07-12_Response to SSER Open Item 30_Final.pdf Please see attached TVA letter that was sent to the NRC today.
Thank You,
~*~*~*~*~*~*~*~*~*~*~
Désireé L. Boyd WBN Unit 2 Licensing dlboyd@tva.gov 423-365-8764 1
Hearing Identifier: Watts_Bar_2_Operating_LA_Public Email Number: 700 Mail Envelope Properties (7AB41F650F76BD44B5BCAB7C0CCABFAF2DB5BC81)
Subject:
TVA letter to NRC_06-07-12_Response to SSER Open Item 30 Sent Date: 6/7/2012 1:32:41 PM Received Date: 6/7/2012 1:33:27 PM From: Boyd, Desiree L Created By: dlboyd@tva.gov Recipients:
"Arent, Gordon" <garent@tva.gov>
Tracking Status: None "Hamill, Carol L" <clhamill@tva.gov>
Tracking Status: None "Boyd, Desiree L" <dlboyd@tva.gov>
Tracking Status: None "Epperson, Dan" <Dan.Epperson@nrc.gov>
Tracking Status: None "Poole, Justin" <Justin.Poole@nrc.gov>
Tracking Status: None "Raghavan, Rags" <Rags.Raghavan@nrc.gov>
Tracking Status: None "Milano, Patrick" <Patrick.Milano@nrc.gov>
Tracking Status: None "Campbell, Stephen" <Stephen.Campbell@nrc.gov>
Tracking Status: None Post Office: TVANUCXVS2.main.tva.gov Files Size Date & Time MESSAGE 274 6/7/2012 1:33:27 PM 06-07-12_Response to SSER Open Item 30_Final.pdf 3142038 Options Priority: Standard Return Notification: No Reply Requested: Yes Sensitivity: Normal Expiration Date:
Recipients Received:
Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381-2000 June 7, 2012 10 CFR 50.34(b)
U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Watts Bar Nuclear Plant Unit 2 Docket No. 50-391
Subject:
Watts Bar Nuclear Plant (WBN) Unit 2 - NUREG-0847 Supplemental Safety Evaluation Report (SSER) Related to the Operation of Watts Bar Nuclear Plant, Unit 2, Appendix HH Open Item 30 - Power System Degraded Voltage
References:
- 1. TVA letter to NRC dated August 12, 2011, Watts Bar Nuclear Plant (WBN) Unit 2 - Safety Evaluation Report Supplement 22 (SSER22) -
Response to NRC Required Action Item
- 2. NRC Regulatory Issue Summary (RIS) 2011-12, Revision 1, "Adequacy of Station Electrical Distribution System Voltages" The purpose of this letter is to update the information provided in Reference 1 related to the offsite and onsite power system degraded voltage studies for safety-related electrical equipment. The enclosed information provides a study that shows the impact of using a methodology that meets the intent of the guidance provided in Reference 2. The information provided in the enclosure and associated attachments demonstrates that the plant design conforms to the applicable regulatory requirements.
There are no new regulatory commitments contained in this letter. If you have any questions, please contact Gordon Arent at (423) 365-2004.
U.S. Nuclear Regulatory Commission Page 3 June 7, 2012 bcc (Enclosure):
Stephen Campbell U.S. Nuclear Regulatory Commission MS 08H4A One White Flint North 11555 Rockville Pike Rockville, Maryland 20852-2738 Fred Brown, Deputy Regional Administrator for Construction U. S. Nuclear Regulatory Commission Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257
ENCLOSURE WBN Unit 2 Degraded Voltage Study SER Open Item 30 TVA should confirm that all safety-related equipment (in addition to the Class 1E motors) will have adequate starting and running voltage at the most limiting safety-related components (such as motor-operated valves (MOVs), contactors, solenoid valves or relays) at the DVR setpoint dropout setting. TVA should also confirm that (1) the motor-starting transient studies are based on the dropout voltage value of DVR and time delay, (2) the steady-state voltage drop studies are carried out by maximizing running loads on the Class 1E distribution system (bounding combination of safety systems loads), with the voltage at 6.9-kV Class 1E buses (monitored by the DVRs) at or just above the DVR dropout setting, and (3) the DVR settings do not credit any equipment operation (such as LTC transformers) upstream of the 6.9-kV Class 1E buses. TVA should also confirm that the final technical specifications (TSs) are properly derived from these analytical values for the degraded voltage settings. This is Open Item 30.
TVA response:
TVA has performed an auxiliary power system study to update our response to this open item.
A copy of the study and two appendices and one attachment to the study are provided in below. This study was performed as a sensitivity analysis to determine if there is any impact on the Degraded Voltage Relay (DVR) setpoint and/or plant response if the current methodology is changed. This study did not credit any equipment operation (such as Load Tap Change [LTC] transformers) upstream of the 6.9 kV Class 1E buses. The methodology used in this study was developed to meet the intent of Regulatory Issue Summary (RIS) 2011-12, Adequacy of Station Electrical Distribution System Voltages. The RIS states that the protective function of the DVR setpoint is to ensure adequate starting voltage to all Class 1E equipment for all operating and accident conditions. Additionally, the RIS states that the DVR setpoint must be based on the starting voltage requirement at the terminal of the most limiting component and must account for the effect of all components which could affect voltage; this includes offsite power circuits, the plant distribution system, as well as all Class 1E and non-Class 1E loads. However, the RIS does not prescribe a specific methodology to be used for the motor starting analysis, and there is no industry consensus as to how to best address DVR protection for motor starting scenarios. In actuality, the ability to start motors cannot be determined by monitoring voltage alone. The ability to start motors can only be determined by power system capacity, which is the ability to maintain voltage while providing the required starting current. Therefore, the premise of using a voltage relay to protect motor starting ability requires an assumption of the power source capacity, which seems inconsistent with the stated purpose of the DVR protective function.
Considering these constraints, TVA used an analytical approach to determine if the DVR setpoint provides the minimum required voltages at the terminals of the Class 1E loads during motor starting (automatic starting during a design basis event or individually) while still connected to the preferred offsite power source. The following Class 1E motor starting scenarios were evaluated:
- 1. Dynamic Motor Starting - Safety Injection Signal Phase A (SIA)
- 2. Dynamic Motor Starting - Safety Injection Signal Phase B (SIB)
- 3. Static Motor Starting (starting individual motors). This analysis is performed with SIA since Safety Injection Signal Phase A provides the worst case voltages on the 480 V system. For this analysis, a motor starting study case is generated to individually start each Class 1E motor.
E-1
ENCLOSURE WBN Unit 2 Degraded Voltage Study To perform the above analysis, the 6.9 kV shutdown boards were disconnected from all offsite power source(s) and a dedicated fixed voltage source was added to each 6.9 kV shutdown board (6.9 kV shutdown board was used as a swing bus).
The source voltage was set to the DVR analytical dropout limit of 6555 V.
This study concludes:
- 1. That adequate starting voltage is available to each Class 1E load when the 6.9 kV Shutdown Boards are at the DVR dropout setpoint (analytical limit) for the following cases:
x During a design basis event (DBE): all Class 1E loads that are automatically actuated for a safety injection signal (SI-Phase A or SI-Phase B).
x When individually starting a motor (single motor start): all Class 1E loads, even if not SI-actuated.
- 2. The study shows that for a sustained degraded voltage condition, overcurrent protective devices associated with Class 1E loads will not trip during SI-actuated motor starting and subsequent start on the emergency diesel generator.
- 3. The bounding voltage recovery analysis demonstrates that any possible voltage transient caused by DBE motor starting, including drop to the loss of voltage (LOV) setpoint, will also result in successful recovery above DVR reset within 4 seconds.
With respect to the contactors, relays and solenoid valves, adequacy of pickup voltage for these control components was performed as part of the Control Circuit Voltage Drop (CCVD) analysis.
This analysis was performed considering a steady state minimum voltage of 432 V at the motor control center (MCC) bus. All the components were determined to have adequate pickup voltage upon implementation of issued design changes as identified in the CCVD calculation.
Implementation of these design changes is scheduled to be completed prior to Unit 2 fuel load.
The MCC transient bus voltage under degraded voltage conditions (at DVR dropout voltage of 6555 V) drops below 432 V due to starting of large motors on the 480 V switchgear. This voltage recovers to a value of >432 V within 4 seconds. Since the startup of the safety-related equipment may not have adequate starting voltage due to transient voltage conditions, Westinghouse evaluated the impact of an additional 5 second delay for the startup of the safety injection pumps and feedwater isolation valves with offsite power available, and concluded the following: This safety evaluation concludes that an additional five second delay for the startup of the safety injection pumps and an additional five second delay in the closure of the feedwater isolation valve does not impact the conclusions of the safety analysis that form the Watts Bar licensing basis (SECL-92-029; RIMS No. T33930330990).
Based on the above, TVA considers that the analysis performed to verify adequacy of available voltage for the contactors, solenoid valves, and relays is adequate and no further analysis under the transient voltage conditions needs to be performed.
In conclusion, TVA confirms that safety-related equipment will have adequate starting and running voltage and that the Technical Specifications for degraded voltage settings are properly derived and appropriate.
E-2
ENCLOSURE WBN Unit 2 Degraded Voltage Study Attachment Excerpts from Sensitivity Study of Degraded Voltage Relay(DVR) Protection During Motor Starting STUDY-EEB-WBN-12-001, Rev. 1
- 1) Write up for Sensitivity Study
- 2) Appendix A for Evaluation of Class 1E Motor Starting Voltages
- 3) Appendix B for Degraded Voltage Relay LOV Voltage Recovery Analysis
- 4) Attachment 2 for Degraded Voltage Relay Settings E-3
STUDY-EEB-WBN-12-001 APPENDIX A Page i Appendix A Introduction This appendix is a summary tabulation of the results for the various motor starting scenarios described in Section 6.0. The analyses in Appendix A were developed using the methodology and applicable inputs from References 2.2, and 2.3. Voltage values were obtained from the applicable ETAP files and output reports in this study. The key information and results are determined as follows:
A. Minimum Motor Starting Voltage Minimum motor starting voltages are based on Attachments 1, 3 and 6 of this study. The minimum motor starting voltages used are:
6600 Volt Motors - 5280 Volts (80%)***
460 Volt Motors - 391 Volts (85%)**
460 Volt Air Compressors - 368 Volts (80%)**
89-10 MOVs - Valve thrust and torque design margin calculations/evaluations based on motor terminal voltage as documented in Attachments 4 and 5 of this study.
Non 89-10 MOVs - 368 Volts (80%)**
- Percentages are based on 460 Volt rated motor voltage. For motors rated other than 480V, the same percentages are applicable. The minimum starting voltage may be different from the above voltage if it is based on manufacturers data or test report as documented in Attachments 3 and 6 of this study.
- 80% for all motors except for the ERCW Pump motor which requires 90% and the Auxiliary Feedwater Pump motor which requires 85%
B. Motor Starting Evaluation Results Individual motor starting evaluation results are tabulated for all class 1E safety-related motors and motor operated valves. As summarized in Section 6.1 of this study the following steps were performed:
- 1. Motor terminal voltage with Phase A loading is compared against the minimum motor starting requirements above. If acceptable voltage is obtained, no further action is required.
- 2. Motor terminal voltage with Phase B loading is compared against the minimum motor starting requirements above. If acceptable voltage is obtained, no further action is required.
- 3. Single motor start terminal voltage with Phase A loading or with Phase B loading provided the Phase B criteria in section 3.4 of this study applies is compared against the minimum motor starting requirements above. If acceptable voltage is obtained, no further action is required.
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STUDY-EEB-WBN-12-001 APPENDIX A Page ii
- 4. Individual circuit operating evaluations are performed based on when the motor is in service such as the ERCW Screen Wash Pump.
- 5. For 89-10 MOVs, See Section C below.
C. 89-10 Motor Operated Valve Evaluation Results For Unit 1 and Unit 2 89-10 MOVs that did not meet the required valve motor starting voltage criteria, as a first step the calculated starting voltages for these MOVs were provided to Mechanical discipline (valve group) to evaluate if the revised voltages were acceptable.
Based on the new thrust and torque calculations and calculated new design margins by mechanical valve group, the new available starting voltage for some MOVs was determined to be acceptable. However, some of the MOVs still did not meet either the required voltage or the minimum design margin criteria. Available voltage for these MOVs were re-evaluated further using the hammer-blow (HB) feature (see Section 3.5) which resulted in improved starting voltage available at the motor terminals. The available voltage with HB feature was again evaluated by the mechanical valve group and determined to be acceptable.
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