0CAN101201, Units 1 and 2, 90-Day Response to Bulletin 2012-01, Design Vulnerability in Electric Power System
| ML12299A391 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 10/25/2012 |
| From: | Schwarz C Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 0CAN101201, BL-12-001 | |
| Download: ML12299A391 (13) | |
Text
0CAN101201 October 25, 2012 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
SUBJECT:
90-Day Response to Bulletin 2012-01, Design Vulnerability in Electric Power System Arkansas Nuclear One - Units 1 and 2 Docket Nos. 50-313 and 50-368 License Nos. DPR-51 and NPF-6
REFERENCE:
- 1. NRC letter to Entergy,Bulletin 2012-01 Design Vulnerability in Electric Power System, dated July 27, 2012 (0CNA071207)
Dear Sir or Madam:
On July 27, 2012, the NRC issued Bulletin 2012-01 (Reference 1), requesting that each licensee submit a written response in accordance with 10 CFR 50.54(f) within 90 days of the bulletin to provide requested information. This letter provides Entergy Operations, Inc. 90-day response to Reference 1 for Arkansas Nuclear One (ANO), Units 1 and 2 in the attachment.
There are no new commitments contained in this submittal. Should you have any questions concerning the content of this letter, please contact Stephenie Pyle at 479.858.4704.
Entergy Operations, Inc.
1448 S.R. 333 Russellville, AR 72802 Tel 479-858-3110 Christopher J. Schwarz Vice President, Operations Arkansas Nuclear One
0CAN101201 Page 2 of 2 I declare under penalty of perjury that the foregoing is true and correct. Executed on October 25, 2012.
Sincerely, Original signed by Christopher J. Schwarz CJS/mkh
Attachment:
90-Day Response to NRC Bulletin 2012-01 cc:
Mr. Elmo E. Collins Regional Administrator U. S. Nuclear Regulatory Commission, Region IV 1600 East Lamar Boulevard Arlington, TX 76011-4511 NRC Senior Resident Inspector Arkansas Nuclear One P.O. Box 310 London, AR 72847 U. S. Nuclear Regulatory Commission Attn: Mr. Kaly Kalyanam MS O-8 B1 One White Flint North 11555 Rockville Pike Rockville, MD 20852
Attachment to 0CAN101201 90-Day Response to NRC Bulletin 2012-01
Attachment to 0CAN101201 Page 1 of 10 90-Day Response to NRC Bulletin 2012-01 Overview System Description - Items 2, 1.d, 2.a, and 2.c System Protection - Items 1, 1.a, 2.b, and 2.d Consequences - Items 1.b, 1.c, and 2.e Simplified One-Line Diagram Tables o Table 1 - Engineered Safety Features (ESF) Buses Continuously Powered from Offsite Power Source(s) o Table 2 - ESF Buses not Continuously Powered from Offsite Power Source(s) o Table 3 - ESF Buses Major Loads o Table 4 - Offsite Power Transformers o Table 5 - Protective Devices
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System Description===
Items 2, 1.d, 2.a, and 2.c request system information and are addressed in this section:
- 2. Briefly describe the operating configuration of the ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) at power (normal operating condition).
See simplified one-line diagram.
The main turbine-generator for each unit supplies power to the unit auxiliary transformer (UAT) for Arkansas Nuclear One (ANO), Unit 1 (ANO-1) and Unit 2 (ANO-2). Both ESF buses (480V and 4160V) and non-ESF buses (480V, 4160V, and 6900V) are normally supplied by the UAT for each ANO unit.
The 500kV system provides offsite power via the autotransformer to the 22kV bus that supplies startup transformer #1 (SUT-1) for ANO-1 and startup transformer #3 (SUT-3) for ANO-2. At power operations, these transformers are normally energized but not connected to any ESF bus load. SUT-1 and SUT-3 are normally energized and ready for fast transfer of plant 6900V and 4160V buses from the UAT.
The 161kV system provides offsite power to startup transformer #2 (SUT-2), which is shared by both of the ANO units. At power operations, SUT-2 is normally energized but not connected to any load. SUT-2 is available as a backup offsite source for both ANO units. ESF 4160V buses A-3 and A-4, for ANO-1, are fed from non-ESF buses A-1 and A-2, respectively. During normal power operations, only bus A-1 is aligned for fast transfer to SUT-2 should SUT-1 not be available. The same is true for ANO-2 in that the ESF 4160V buses 2A-3 and 2A-4 are fed from non-ESF buses 2A-1 and 2A-2, respectively. During normal power operations, only bus 2A-1 is aligned for fast transfer to SUT-2 should SUT-3 not be available.
Attachment to 0CAN101201 Page 2 of 10 1.d Describe the offsite power transformer (e.g., startup, reserve, station auxiliary) winding and grounding configurations.
See Table 4 for offsite power transformer winding and grounding configurations.
2.a Are the ESF buses powered by offsite power sources? If so, explain what major loads are connected to the buses including their ratings.
For at power configurations (normal operating condition), ESF buses are not powered by offsite sources. See Tables 1, 2, and 3.
2.c Confirm that the operating configuration of the ESF buses is consistent with the current licensing basis. Describe any changes in offsite power source alignment to the ESF buses from the original plant licensing.
The following at power configurations (normal operating condition) have been confirmed to be consistent with the current licensing basis:
For ANO-1, the ESF buses, A-3 and A-4, are fed from non-ESF buses A-1 and A-2, respectively. Buses A-1 and A-2 receive power from the UAT which is fed from the main turbine-generator. The offsite sources, SUT-1 and SUT-2, are available for supplying power should all onsite alternating current (AC) power be lost. Normally SUT-1 is the preferred offsite source and has fast transfer capabilities to buses H-1, H-2, A-1, and A-
- 2. With the exception of bus A-1, ANO-1 is prevented from automatic transfer to SUT-2 during normal power operations.
For ANO-2, the ESF buses, 2A-3 and 2A-4, are fed from non-ESF buses 2A-1 and 2A-2, respectively. Buses 2A-1 and 2A-2 receive power from the UAT which is fed from the main turbine-generator. The offsite sources, SUT-3 and SUT-2, are available for supplying power should all onsite AC power be lost. Normally, SUT-3 is the preferred offsite source and has fast transfer capabilities to buses 2H-1, 2H-2, 2A-1, and 2A-2.
With the exception of bus 2A-1, ANO-2 is prevented from automatic transfer to SUT-2 during normal power operations.
Both the current licensing basis and the original licensing basis for ANO-1 and ANO-2 indicate that during normal power operations, the ESF buses receive power from the UAT, and that two offsite sources (SUT-1 and SUT-2 for ANO-1)(SUT-3 and SUT-2 for ANO-2) are available for automatic or manual transfer of plant buses if the UAT becomes unavailable.
As noted in Tables 1 and 2 there are no changes in the offsite power source alignment to the ESF buses from the original plant licensing.
Attachment to 0CAN101201 Page 3 of 10 System Protection Items 1, 1.a, 2.b, and 2.d request information regarding electrical system protection and are addressed in this section:
- 1. Given the requirements above, describe how the protection scheme for ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) is designed to detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited offsite power circuit or another power source. Also, include the following information:
Consistent with the current licensing basis and General Design Criteria (GDC) 17, existing protective circuitry would separate the ESF buses from a connected failed offsite source due to a loss of voltage or a sustained, balanced degraded grid voltage concurrent with certain design basis accidents. The relay systems were not specifically designed to detect an open single phase of a three-phase system. Detection of an open-phase condition is beyond the approved design and licensing basis of the plant.
Independence is maintained because the two offsite sources (SUT-1 and SUT-2 for ANO-1 and SUT-3 and SUT-2 for ANO-2) are fed from separate switchyard buses (500kV and 161kV) so that an open-phase condition does not affect both offsite sources.
1.a Provide the sensitivity of protective devices to detect abnormal operating conditions and the basis for the protective device setpoint(s).
Consistent with the current licensing basis and GDC 17, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss-of-voltage or a degraded voltage but were not designed to detect an open-phase condition. See Table 5 for the undervoltage protective devices and the basis for the device setpoint(s).
Analysis has not been performed to determine if the existing electrical protective devices are sufficiently sensitive to detect a high impedance ground fault. Table 5 lists ground protection on the ESF buses and the basis for the device setpoint(s).
ANO-1 and ANO-2 non-ESF 4160V buses have negative sequence overvoltage relays that can detect an open-phase condition and provide an alarm. These relays alarm upon detecting negative sequence voltages greater than 5%. Although it is expected that an open-phase condition would result in this relay alarm, it should be recognized that this benefit is beyond original design intent, and thus, no formal calculation or analysis exits. The ESF 4160V buses are normally fed from the non-ESF 4160V buses.
Thus, when the buses are fed from the startup transformers, an open-phase condition could cause an alarm in the control room. Annunciator corrective action procedures for both units provide operator actions in the event of these alarms.
Attachment to 0CAN101201 Page 4 of 10 2.b If the ESF buses are not powered by offsite power sources, explain how the surveillance tests are performed to verify that a single-phase open circuit condition or high impedance ground fault condition on an offsite power circuit is detected.
Additional log readings are being taken via the weekly operations switchyard log. The readings record an inspection of components on the 161kV bus from the ring bus feeding SUT-2 voltage regulator and lines to SUT-2. Inspections are performed on the 22kV lines and bus work from the autotransformer to the SUT-1 and SUT-3 voltage regulators. The inspections are looking for any open circuits in the associated equipment including disconnected bus work, disconnected lines, and open disconnect stabs. If any discrepancies are found, the Control Room Supervisor is immediately informed.
Daily inspections of SUT-1, SUT-2 and SUT-3 supply lines in the transformer yard are being performed via the operations logsheets. These inspections look for lines that are not connected, disconnects not seated, and damage to lines and insulators.
2.d Do the plant operating procedures, including off-normal operating procedures, specifically call for verification of the voltages on all three phases of the ESF buses?
The startup transformers are not normally connected to the ESF buses and thus, monitoring the voltage on all three phases of the ESF buses would not detect an open-phase condition on the high side of the startup transformer. While at normal operating conditions, an open-phase condition on the high side of a startup transformer has no effect on the ESF buses since they are supplied by the UAT. For off-normal periods when the startup transformers supply power to the ESF buses, existing negative sequence overvoltage relays are available to detect an open-phase condition and provide an alarm in the control room. Annunciator corrective action procedures for both units provide operator actions in the event of these alarms.
Consequences Items 1.b, 1.c, and 2.e request information regarding the electrical consequences of an event and are addressed in this section:
1.b The differences (if any) of the consequences of a loaded (i.e., ESF bus normally aligned to offsite power transformer) or unloaded (e.g., ESF buses normally aligned to UAT) power source.
Installed relays were not designed to detect open-phase conditions. Existing loss-of-voltage and degraded voltage relays may respond depending on load and possible grounds. In general, there would be no plant response for an unloaded (e.g.,
ESF buses normally aligned to UAT) power source in the event of an open-phase condition on a credited offsite power circuit because there is insufficient current to detect an open-phase condition for this configuration.
Attachment to 0CAN101201 Page 5 of 10 The plant response for a loaded power source cannot be determined without specifying the amount of loading and the specific loads involved.
1.c If the design does not detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited offsite power circuit or another power sources, describe the consequences of such an event and the plant response.
A high impedance ground fault on the credited offsite power circuit would have no immediate effect on plant operation since the ESF buses are fed from the UAT during normal operating conditions. Analysis has not been performed to confirm that a ground, sufficiently large enough to affect plant operation, would be automatically detected and isolated by protective relaying.
Regarding detection and response to an open-phase condition:
- 1. ANO-1 and ANO-2 do not credit in the current licensing basis that the Class 1E protection scheme (for the ESF buses) was designed to detect and automatically respond to an open-phase condition on the credited offsite power source as described in the final safety analysis report and technical specifications.
The offsite power circuits for ANO-1 consists of two independent circuits. The preferred offsite source is from SUT-1 to the 4160V buses A-3 and A-4 via A-1 and A-2, respectively. The second offsite power source is from SUT-2 to buses A-3 and A-4 via A-1 and A-2, respectively.
The offsite power circuits for ANO-2 consists of two independent circuits. The preferred offsite source is from SUT-3 to the 4160V buses 2A-3 and 2A-4 via 2A-1 and 2A-2, respectively. The second offsite power source is from SUT-2 to buses 2A-3 and 2A-4 via 2A-1 and 2A-2, respectively.
Both ANO-1 and ANO-2 are prevented from an automatic transfer to SUT-2 during normal power operations for all buses except buses A-1/A-3 (ANO-1) and 2A-1/2A-3 (ANO-2). Procedures administratively control ANO-1 and ANO-2 access to SUT-2.
- 2. Since ANO did not credit the ESF bus protection scheme as being capable of detecting and automatically responding to an open-phase condition, an open-phase fault was not included in the design criteria for either the loss-of-voltage or the degraded voltage relay scheme. Since open-phase detection was not credited in the ANO design or licensing basis, no design basis calculations or design documents exist that considered this condition.
- 3. Without formalized engineering calculations or engineering evaluations, the electrical consequences of such an open-phase event (including plant response), can only be evaluated to the extent of what has already been published by Electric Power Research Institute and Basler, which is a generic overview. The difficulty in applying these evaluations to the ANO-specific response is that these are generic assessments and cannot be formally credited as a basis for an accurate response.
The primary reason is that detailed plant specific models would need to be developed (e.g., transformer magnetic circuit models, electric distribution models,
Attachment to 0CAN101201 Page 6 of 10 and motor models including positive, negative, and zero-sequence impedances (voltage and currents)). In addition, the models would need to be compiled and analyzed for the ANO-specific Class 1E electric distribution system.
- 4. Preliminary analysis has determined that the existing ANO plant protective relays may not be able to provide automatic protection for an open-phase condition on the primary side of SUT-1, SUT-2, and SUT-3. However, during normal power operations, power for the ESF buses at ANO is normally supplied by the UAT.
Therefore, an open phase on the primary side of SUT-1, SUT-2, or SUT-3 has no effect on the ESF buses while the buses are being supplied by the UAT.
- 5. Existing negative sequence overvoltage relays on the non-ESF 4160V buses can detect an open-phase condition and provide an alarm. Thus, when the buses are fed from the startup transformers, an open-phase condition that causes a negative sequence voltage greater than the relay setpoint, would be detected and provide an alarm in the control room. Annunciator corrective action procedures for both ANO-1 and ANO-2 provide operator actions in the event of these alarms.
- 6. Since the startup transformers are not normally connected to the ESF buses, the ANO plant protective relays cannot detect an open-phase condition on the primary side of the transformers. Interim measures taken to aid in the detection of an open-phase condition on the high side of the startup transformers include walkdowns of the transformer yard and switchyard.
2.e If a common or single offsite circuit is used to supply redundant ESF buses, explain why a failure, such as a single-phase open circuit or high impedance ground fault condition, would not adversely affect redundant ESF buses.
Consistent with the current licensing basis and GDC 17, protective circuitry separates the ESF buses from a failed offsite source due to a loss-of-voltage or a sustained balanced degraded grid voltage. The relay systems were not specifically designed to detect an open single phase of a three-phase system. Detection of an open-phase condition is beyond the approved design and licensing basis of the plant. Calculations have not been performed for this scenario.
Attachment to 0CAN101201 Page 7 of 10 Simplified One-Line Diagram 500 KV SYSTEM 7451 TO PLEASANT HILL 8351 TO FORT SMITH 5102 5102 5101 5105 5106 5107 5110 5111 5109 5951 TO MABELVALE 5122 5121 5123 5147 5148 5149 5126 5125 5127 5100 5130 5131 5129 5134 5135 5133 5124 5118 5117 5119 5114 5113 5115 5112 TO RUSSELLVILLE NORTH TO PLEASANT HILL B2512 B1204 B1205 B1206 B1219 B1218 B1217 B1220 161 KV SYSTEM SU#2 VOLTAGE REGULATOR B1250 B1239 B1240 B1291 TO LONDON B1214 B1212 B1211 B1213 B0126 B0125 B1200 AUTO TRANSFORMER 500/161-22 KV SU#1 VOLTAGE REGULATOR SU#3 VOLTAGE REGULATOR STATION SWITCHYARD UNIT 2 MAIN GENERATOR MAIN TRANSFORMER 500-22 KV UNIT AUXILIARY TRANSFORMER 21.5-6.9-4.16 KV REMOVABLE BUS LINKS OVERHEAD SWING LEADS SU#1 TRANSFORMER 21.5-6.9-4.16 KV SU#2 TRANSFORMER 157-6.9-4.16 KV 6.9 KV BUS H2 TO UNIT 2 BUS 2A9 DG1 DG2 A4 A3 CRD SYSTEM X8 2A9 A1 A2 X7 X1 X2 X3 X4 X5 X6 B7 B1 B2 B3 B4 B5 B6 A-B-C SEQUENCE UNIT 1 MAIN GENERATOR MAIN TRANSFORMER 500-22 KV REMOVABLE BUS LINKS UNIT AUXILIARY TRANSFORMER 21.5-6.9-4.16 KV OVERHEAD SWING LEADS 6.9 KV BUS H1 4.16 KV BUSES 480 V BUSES 6.9 KV BUS 2H2 6.9 KV BUS 2H1 SU#3 TRANSFORMER 21.5-6.9-4.16 KV DISCONNECT LINKS X-04 X-03 2X-03 X-02 2X-02 2X-01 X-01 2A9 2DG1 2A4 2A3 2A9 2A1 2A2 2X1 2X2 2X3 2X5 2X6 2B7 2B1 2B2 2B3 2B4 2B5 2B6 2X7 4.16 KV BUSES 480 V BUSES 2DG2 2X4 2X9 2B8 2X8 2X1 2B9 2B10 CLS (bypa ssed) 2A1/2A2 (Non-ESF)
Condensate Pumps Heater Drain Pumps Main Chiller Aux Feedwater Pump (2A1) 2A3/4 (ESF)
EFW Pump (2A3)
HPSI Pump LPSI Pump SW Pump CSS Pump B
88 12 B
12 86 31.5 MVAR B1216 B1215
Attachment to 0CAN101201 Page 8 of 10 Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s)
Description of ESF Bus Power Source ESF Bus Name (normal operating condition)
Original licensing basis configuration (Y/N)
N/A N/A N/A Table 2 - ESF Buses Not Continuously Powered From Offsite Power Source(s)
Description of ESF Bus Power Source ESF Bus Name (normal operating condition)
Original licensing basis configuration (Y/N)
UAT (ANO-1) 4160V bus A-3 Y
UAT (ANO-1) 4160V bus A-4 Y
UAT (ANO-1) 480V bus B-5 Y
UAT (ANO-1) 480V bus B-6 Y
Table 3 - ESF Buses Major Loads Normally Energized by the Startup Transformers ESF Bus Load Voltage Level (V)
Rating (HP)
A3 None 4160 N/A A4 None 4160 N/A B5 None 480 N/A B6 None 480 N/A 2A3 None 4160 N/A 2A4 None 4160 N/A 2B5 None 480 N/A 2B6 None 480 N/A
Attachment to 0CAN101201 Page 9 of 10 Table 4 - Offsite Power Transformers Transformer Winding Configuration MVA Size (OA/FA/FOA)
Voltage Rating (kV)
(Primary/Secondary)
Grounding Configuration SUT-1 (ANO-1 only)
Delta-Wye-Wye (3 Leg)
H - 30/40/50 X - 16/21.3/26.7 Y - 14.4/19.2/24 21.5 - 6.9 - 4.16 X and Y winding neutral resistance grounded SUT-2 (Shared between ANO-1 and ANO-2)
Wye-Wye-Wye (3 Leg)
With embedded delta stabilizing winding H - 27/36/45 X - 15/20/25 Y - 12.6/16.8/21 157 - 6.9 - 4.16 H winding neutral solidly grounded; X and Y winding neutral resistance grounded SUT-3 (ANO-2 only)
Delta-Wye-Wye (3 Leg)
H - 35.2/46.8/58.5 X - 19.7/26.2/32.8 Y - 15.5/20.6/25.7 21.5 - 6.9 - 4.16 X and Y winding neutral resistance grounded OA-Oil-immersed Self-cooled FA-Forced Air FOA-Forced Oil and Air
Attachment to 0CAN101201 Page 10 of 10 Table 5 - Protective Devices Protection Zone Protective Device Undervoltage Logic Setpoint (Nominal)
Basis for Setpoint 4160V ESF Buses Loss of Voltage Relay 1 of 2 2450V (61.25% of 4000V)
To actuate upon complete loss of safety bus voltage 480V ESF Buses Degraded Voltage Relay 2 of 2 429.6V (93.4% of 460V) with a nominal 8 second time delay To actuate upon a degraded voltage condition of all three phases 4160V non-ESF Bus feed to ESF Buses Negative Sequence Overvoltage Relay N/A 5% Negative Sequence Voltage To provide an alarm for negative sequence voltage (factory set at 5%)
SUT-1 Ground Fault Protection (151G-ST1Y)
N/A 320A To coordinate with upstream and downstream protective devices SUT-2 Ground Fault Protection (151G-ST2Y)
N/A 320A To coordinate with upstream and downstream protective devices SUT-3 Ground Fault Protection (151G-ST3Y)
N/A 320A To coordinate with upstream and downstream protective devices