ML19060A243
| ML19060A243 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 03/01/2019 |
| From: | Randy Hall Plant Licensing Branch II |
| To: | Heacock D Virginia Electric & Power Co (VEPCO) |
| Hall R, | |
| References | |
| Download: ML19060A243 (7) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 REQUEST FOR ADDITIONAL INFORMATION RELATED TO LICENSE AMENDMENT REQUEST TO ADDRESS OPEN PHASE CONDITIONS VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 DOCKET NOS. 50-338 AND 50-339 INTRODUCTION By application dated April 30, 2018 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML18127A073), the Virginia Electric and Power Company (Dominion Energy Virginia, the licensee) submitted a license amendment request (LAR) for the North Anna Power Station (NAPS), Units 1 and 2. The proposed amendment would revise the Technical Specifications (TS) Limiting Condition for Operation (LCO) of TS 3.3.5, "Loss of Power (LOP)
Emergency Diesel Generator (EOG) Start Instrumentation." Specifically, the licensee is proposing to add surveillance requirements (SRs) to reflect the addition of new negative sequence voltage relays designed to detect an open phase condition (OPC) that could exist on one or two phases of a primary off-site power source and that would not currently be detected and mitigated by the existing station electrical protection scheme.
In 2012, the U. S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, "Design Vulnerability in Electric Power System," which requested addressees to submit specific information regarding plant design and operating configurations relative to the regulatory requirements of "capacity and capability" described in General Design Criterion 17, "Electric Power Systems." In partial response to NRC Bulletin 2012-01, a proposed voltage unbalance protection system is being installed on the 4160 V emergency buses at NAPS Units 1 and 2.
The NRC staff has identified the need for the additional information identified below to complete its review of the LAR.
REGULATORY BASIS Title 1O of the Code of Federal Regulations (10 CFR) Part 50.36(c)(2), "Limiting conditions for operation," provides the requirement for the establishment of TS limiting conditions for operation (LCO). Paragraph 50.36(c)(2)(ii) requires that a TS LCO of a nuclear reactor be established for each item meeting one or more of the criteria listed. Criterion 3 applies to this LAR, which states: "A structure, system, or component that is part of the primary success path and which functions or actuates to mitigate a design basis accident [OBA] or transient that either assumes the failure of or presents a challenge to the integrity of a fission product barrier."
10 CFR Part 50.36(c)(3), "Surveillance requirements," states, "Surveillance requirements are requirements relating to test, calibration, or inspection to assure that the necessary quality of systems and components is maintained, that facility operation will be within safety limits, and that the limiting conditions for operation will be met."
Request for Additional Information (RAI)
RAI-EEOB-1 In the LAR, Attachment 2, the licensee proposed to revise SR 3.3.5.1 as follows [change is shown in BOLD below]:
SR 3.3.5.1 ---------------------------------NOTE-----------------------------------
Verification of setpoint is not required.
Perform TADOT for LCO 3.3.5.a and LCO 3.3.5.b UV/DV Functions.
Please clarify whether UV/DV abbreviations have been defined in the TS elsewhere. If not, please provide the description of the abbreviation "UV/DV" In TS.
RAI-EEOB-2 In the LAR, Attachment 1, Page 5, the licensee stated:
While many OPCs would be addressed by the existing UV relays, some consequential OPCs are not readily detectable by the existing station electrical protection scheme at North Anna Units 1 and 2.
Please clarify what is meant by "consequential" in the above sentence.
RAI-EEOB-3 In the LAR, Attachment 1, Page 15, the licensee stated:
The negative sequence relay [BE1-47N] includes an inverse timing characteristic feature that is adjustable from 01 to 99 [time dial] in increments of 1. The timing is based on the percent difference from the nominal system voltage. The calculated results show that with a time dial setting of 10.0, the negative sequence voltage relay was secure (i.e., would not issue an alarm) for the simulated unbalanced faults on the medium-voltage and low-voltage systems.
The time dial setting of 10.0 provides sufficient time to allow existing overcurrent relaying to trip on the unbalanced fault condition.
Please demonstrate that the BE1-47N relay time coordination with downstream protective devices is bounding (a sample calculation may be useful in this response). Please explain why the time dial setting of the BE1-47N relay does not need to be specified in TS.
RAI-EEOB-4
In the LAR, Attachment 1, Page 17, the licensee stated:
A feature that blocks actuation of the voltage unbalance (open phase) protection function is also included in the logic scheme. This feature enhances the reliability of the protection system and prevents the protection scheme from automatically starting and loading an EOG in the event of a failed fuse on a PT or failed PT. To achieve this feature, one ASEA Brown Boveri (ABB) 60 relay will be installed per bus in the new voltage unbalance circuit.
The ABB-60 balance relay is a differential voltage monitoring relay. It receives two different voltage source inputs and compares them to each other. When the voltage of one input changes with respect to the other beyond the setpoint, the relay will energize, and only its set of contacts will change state.
A Cutler Hammer ARD relay will be installed downstream of the ABB-60 relay.
One output from the ARD relay will be used to block the negative sequence two-out-of-three circuits from energizing. This relay will also send a signal to an existing spare annunciator window to indicate a 4KV PT fuse is blown.
Please provide the connection/schematic diagrams(s) illustrating the connection of PT fuses, BE1-47N and ABB-60 relays, as described above, for one of the 4160 V safety-related busses.
RAI-EEOB-5 In the LAR, Attachment 1, Page 10, the licensee considered open phase conditions at the following locations:
Single open phase without a ground connection Single open phase with a 350 ohm grounded connection Single open phase with a solid grounded connection Please describe whether the 350 ohm grounded connection represents a high impedance ground fault connection, as mentioned in NRC Bulletin 2012-01, and how this value was calculated.
RAI-EEOB-6 In the LAR, Attachment 1, Page 10-11, the licensee stated that analysis was performed for OPCs at the following locations:
High side terminals of the Switchyard Transformer TX-1 High side terminals of the Switchyard Transformer TX-2 High side terminals of the Switchyard Transformer TX-3 High side terminals of the RSST A Transformer High side terminals of the RSST B Transformer High side terminals of the RSST C Transformer High side terminals of the Unit 1 Generator (Main) Step-up Transformer (GSU-1)
High side terminals of the Unit 2 Generator (Main) Step-up Transformer (GSU-2)
Please provide a justification for why open phase conditions were not considered on the low side of above transformers, or not considered credible.
RAI-EEOB-7 In the LAR, Attachment 1, Page 11, the licensee provided a description of various generating conditions and loading conditions (and large motor starts), considered for performing the OPC analysis.
The relationships between various generating conditions and loading conditions considered are not clear to the NRC staff. For each generating condition, please list the corresponding loading conditions considered. Also, explain how for each of the generating and loading conditions considered, the large motor start scenarios were also considered.
RAI-EEOB-8 In the LAR, Attachment 1, Page 13, the licensee stated:
For cases where the voltage unbalance is greater than 5%, the results indicate that the BE1- 47N relays trip and isolate the motor loads before the integrated (12 x t) values reach 20 pu [per unit] and before the motor load's associated overcurrent relay trips (when applicable).
Please describe the relationship between the percentage voltage unbalance and negative sequence current. Quantify the time duration for which the motor may be operated from the OPC source before the negative sequence current heating capability of the motor is exhausted, and before the motor associated overcurrent trips (A sample calculation may be useful in this response).
RAI-EEOB-9 In the LAR, Attachment 1, Page 13, the licensee stated:
The BE1-47N relays did not actuate for many ungrounded open phase events on the high voltage side of switchyard transformers TX-1 and TX-2. For these conditions, the analysis determined that thermal damage may occur to the Auxiliary Feedwater (AFW) Pump Motors and Inside Recirculation Spray (ISRS)
Pump Motors if they are operated continuously under accident loading conditions. For these cases, the Alstom open phase detection relays will detect the open phase event and initiate a trip of its associated transformer within 5 seconds to provide protection to the downstream loads and to be within the time frames considered in the accident analysis.
Please confirm that the non-Class 1E Alstom Open Phase detection/protection at switchyard transformers TX-1 and TX-2 is not in the scope of this LAR.
RAI-EEOB-10 In the LAR, Attachment 1, Page 13-14, the licensee stated:
BE1-47N relays did not actuate for ungrounded open phase events on the high voltage side of GSU 1 and GSU 2 when Class 1E Bus is aligned and Units are offline. For these conditions, additional analysis was performed to determine if thermal damage may occur to the AFW Pump Motors and ISRS Pump Motors if they are operated continuously under accident loading conditions. Further analysis documents that the AFW and ISRS pump motor insulation will be capable of performing their design function on the most limiting case. Therefore no additional protection is required for this condition.
Please clarify in the first sentence, whether the words "Units are offline" should actually be "after a Unit is tripped." Please provide a more detailed description of the analysis described above.
(A sample calculation may be useful in this response).
RAI-EEOB-11 In the LAR, Attachment 1, Page 15, the licensee provided a Table which summarizes the minimum and maximum Negative Sequence Voltages calculated at each of safety related buses (1 H, 1J, 2H, and 2J) for open phase locations at TX-1, TX-2, TX-3, RSST-A, RSST-B, RSST-C, GSU 1-1H, GSU 1-2J, GSU 2-1J, and GSU 2-2H.
Please explain why two open phase locations have been considered for each GSU (e.g., GSU 1-1 H, GSU 1-2J), whereas according to the LAR, Attachment 1, Page 11, only one open phase location has been considered on high side of each GSU. For the various operating conditions, please provide one-line diagrams highlighting the power feed from the above transformers/switchyard to the safety related buses.
RAI-EEOB-12 In the LAR, Attachment 1, Page 40, the licensee stated:
A UFSAR change request has been initiated to revise the UFSAR to describe the voltage unbalance analysis and detection/protection scheme implemented by this modification.
For any portions of the UFSAR change that are within the scope of this LAR, please provide a proposed markup of the UFSAR change.
RAI-EEOB-13 In the LAR, Attachment 1, Page 20, the licensee stated:
For single ungrounded open phase events on the high side of transformers GSU 1 and GSU 2, there is insufficient negative sequence voltage to actuate the BE1-47N relays. However the negative sequence voltage is greater than 1%
when the open phase event is coincident with a pre-existing 3% switchyard unbalance.
Please provide the basis for the pre-existing 3% switchyard unbalance assumption.
RAI-EEOB-14
Please provide a logic diagram and provide a discussion that clearly demonstrate the negative sequence voltage relay scheme (detection, alarm, and protection), including the feature that blocks actuation of the negative sequence voltage (open phase) protection using the ABB 60 (voltage balance) relay. Also discuss how that scheme interfaces with the existing undervoltage (UV) and degraded voltage (DV) relay protection schemes, which trip the offsite power circuits at the ESF bus level and initiate the EDG starts.
RAI-EEOB-15 In the LAR, Attachment 1, Page 16, the licensee stated:
For open phase conditions on Transformers TX-3, RSST A, RSST B, and RSST C (Delta-Wye Transformers) the minimum negative sequence voltage on the impacted emergency buses is above the minimum possible setting for the BE 1-47N relay. The minimum observed negative sequence voltage was 12.318V (17.78%), located on Bus 1H for an open phase condition on RSST C.
Please confirm whether the above negative sequence voltage of 12.318 V (17.78%) can be considered as Analytic Limit for calculating various margins with respect to the nominal 4%
pickup setting of the BE1-47N relay.
RAI-EEOB-16 In the LAR, Attachment 1, Page 12-13, the licensee stated:
This thermal capability also has to consider that Class 1E motors restart on the EDGs after tripping from the unhealthy source. To account for this motor starting sequence, a total thermal limit (12 x t) of less than or equal to 20 per unit (pu) (40 pu/2 starts) for Class 1E motors during the open phase event concurrent with a LOCA is used as a bounding condition to ensure the motors have enough thermal capability to perform their safety functions.
Please describe the basis for the assumption "a total thermal limit (12 x t) of less than or equal to 20 per unit (pu) (40 pu/2 starts) for Class 1E motors.
RAI-EICB -1 In the LAR, the licensee proposed to add new Surveillance Requirements (SR) 3.3.5.3.c, as follows [changes are shown in BOLD below]:
SR 3. 3. 5. 3 --------------------NOTE-----------------------------
Negative Sequence Voltage is calculated as a percentage of nominal voltage.
Perform CHANNEL CALIBRATION with Allowable Values as follows:
- c. Negative Sequence Voltage ~ 2.894 % and :S 5.106 % for LCO 3.3.5.a and LCO 3.3.5.b Functions In the LAR, Attachment 1, Page 16, the licensee provided the maximum uncertainty (i.e., the Channel Statistical Allowance, or CSA) for the Basler BE1-47N relay at the 4 kV emergency buses, which was calculated to be +/- 1.106% of span. The instruction manual for the Basler BE1-47N Negative Sequence Voltage Relay stated that, "Pickup accuracy is within+/- 1 unit of the percent setting of the negative sequence voltage." The LAR indicates that a 4% relay pickup setting, with a time dial setting of 10.0, would be implemented for the relay settings.
The NRC staff notes that a similar license amendment for Surry to modify the plant Technical Specifications to address surveillance requirements for an identical manufacturer and model number negative sequence voltage relay used a 6% relay pickup setting with a +/- 2.4%
CSA. This LAR specifies a CSA of +/- 1.106% for North Anna with identical relays. It is unclear if the small operating margin provided by this CSA sufficiently accounts for all the uncertainties that could lead to spurious trips of the power supply system as a result of routine fluctuations in power conditions.
Please provide a description regarding each of the uncertainties included within the proposed CSA of+/- 1.106% of span, such as instrument accuracy, calibration accuracy, drift, etc. The description should demonstrate that the setpoint plus-or-minus the calculated total performance uncertainty would not result in frequent spurious trips.
This information is needed to enable the NRC staff to verify the requirements of 10 CFR 50.36(c)(2) and implementing guidance within Regulatory Guide 1.105 are met regarding the selection of the setpoints and allowable values to protect against an unbalanced voltage potentially leading to an open phase condition.