ML070330134
| ML070330134 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 01/31/2007 |
| From: | Archie J South Carolina Electric & Gas Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| GL-06-002 | |
| Download: ML070330134 (4) | |
Text
A SCANA COMPANY Jeffrey B. Archie Vicu President, Nuclear Operations 803.345.4214 January 31, 2007 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Dear Sir / Madam:
Subject:
VIRGIL C. SUMMER NUCLEAR STATION (VCSNS)
DOCKET NO. 50/395 OPERATING LICENSE NO. NPF-12 RESPONSE TO THE NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING GENERIC LETTER 2006-02, GRID RELIABILITY AND THE IMPACT ON PLANT RISK AND THE OPERABILITY OF OFFSITE POWER This letter provides South Carolina Electric & Gas Company's (SCE&G) response to the request for additional information (RAI) on the subject generic letter for V. C. Summer Nuclear Station (VCSNS). This NRC letter was distributed industry wide and contained six RAIs, three of which pertained to VCSNS. The responses to these three RAls are contained in the attachment to this letter. The NEI template for the RAls was considered in these responses.
Some of the questions in these RAls seek information about analyses, procedures, and activities concerning grid reliability of which VCSNS does not have first-hand knowledge and which are beyond the control of VCSNS. In providing information responsive to such questions, VCSNS makes no representation as to its accuracy or completeness.
Should you have questions, please call Mr. Bruce L. Thompson at (803) 931-5042.
I certify under penalty of perjury that the foregoing is true and correct.
I
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ExeSuted on effrey B. Archie/
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flcý Vre den Nuclear Operations MWD/JBA/dr Attachment c:
K. B. Marsh S. A. Byrne N. S. Carns J. H. Hamilton R. J. White W. D. Travers R. E. Martin K. M. Sutton NRC Resident Inspector NSRC CER (C-06-0465)
File (815.14)
DMS (RC-07-0016)
A12z SCE&G I Virgil C. Summer Nuclear Station - P 0. Box 88
- Jenkinsville, South Carolina 29065
- T (803) 345.5209
- www.scana.com
Document Control Desk Attachment C-06-0465 RC-07-0016 Page 1 of 3 Requested Information RAI No. 1: Switchyard Minimum Voltage:
In response to question 1 (g) you did not identify specific minimum switchyard voltage limits (KV) that you supplied to the local transmission entity. Please provide the following information:
What is the specific minimum acceptable switchyard voltage included in your protocol agreement with your grid operator (GO) and what was the basis for this value? How is this value related to your technical specifications degraded voltage relay setpoints?
Response
The minimum acceptable switchyard voltage limits (KV) provided to the GO are dependent on the number of buses connected to each offsite source, the arrangement of transformers, and for the 115kV source, on whether the voltage regulator (XTFOO06) is in service. These voltages for the various configurations are contained in a procedure that is available to the GO. The basis for the limits is to ensure the 7.2 kV bus voltages are at least 90.2% of rated voltage to ensure the voltage at motor terminals exceeds the rated minimum motor voltage for steady state operation and Motor Control Center (MCC) contactor pickup. The technical specification limits for the degraded voltage relay settings are at 91.34% for 3 seconds as measured on the 7.2 kV vital buses. The lowest of the minimum voltage limits are for a normal plant configuration and are contained in the table below:
115 KV BUS LOWER LIMIT 230 KV BUS LOWER LIMIT VCSNS ON-LINE 104.2 KV 225.4 KV VCSNS OFF-LINE 101.2 KV 216.2 KV RAI No. 2: Loss of Real-Time Contingency Analysis (RTCA) Capability:
Your response to question 2(f) did not identify the actions that would be taken if the online grid analysis tool (software program) that you rely upon as an input for offsite power operability became unavailable. If the online grid analysis tool that you use to determine if the offsite power is inoperable becomes unavailable, what actions would you take to determine if post-trip voltages will be acceptable, including any other compensatory actions, until the post-trip voltage is confirmed to be adequate? As an example, the actions may include reliance on a backup (third party's) real-time contingency analysis or similar program, or relying on a grid planning study to confirm that the original assumptions bound the existing grid conditions.
Document Control Desk Attachment C-06-0465 RC-07-0016 Page 2 of 3
Response
The GO provides continuous RTCA with an online grid analysis tool (software program) to determine if VCSNS post-trip voltages will be acceptable. In addition to the GO's RTCA tool, an additional RTCA tool (software program) is continuously monitored via a third party, namely the Vacar South Reliability Coordinator Agent. Monitored in conjunction, the GO and the third party provide for redundant RTCA tools.
If the GO encounters a RTCA failure, the third party is notified, and vice versa. At times where both RTCA systems fail, VCSNS is notified that the GO is unable to provide VCSNS post-trip voltage predictions via the RTCA tool(s); however, the GO would continue to operate the transmission system to maintain voltages as specified in the GO's procedure, SOP-602, Voltage Regulation for VC Summer Nuclear Plant. These voltage limits are determined by the VCSNS Design Engineering Department utilizing off-line transient stability studies. Also during the time of redundant RTCA failure, the GO will conduct offline evaluations of system conditions that would prevent VCSNS offsite power operability and notify VCSNS of such conditions. All communications to VCSNS for RTCA capabilities, both in-house and third party, are channeled through the GO.
As noted in our original response, once the GO notifies VCSNS that the RTCA program is not available, VCSNS would monitor the offsite grid voltages and take actions within the plant to minimize risk.
RAI No. 5: Seasonal Variation in Grid Stress (Reliability and Loss-of-offsite Power (LOOP)
Probability):
Certain regions during certain times of the year (seasonal variations) experience higher grid stress as is indicated in Electric Power Research Institute (EPRI) Report 1011759, Table 4-7, Grid LOOP Adjustment Factor, and NRC NUREG/CR-6890. Do you adjust the base LOOP frequency in your probabilistic risk assessment (PRA) and Maintenance Rule evaluations for various seasons? If you do not consider seasonal variations in base LOOP frequency in your PRA and Maintenance Rule evaluations, explain why it is acceptable not to do so.
Response
VCSNS does not adjust the base LOOP frequency in the PRA or the Maintenance Rule evaluations for the various seasons.
VCSNS schedules major refueling outages for the spring and fall when electric demand and replacement power costs are lower. According to EPRI 1009890, 50% of LOOP events at nuclear power plants happen during outages, so scheduling outages during the spring and fall helps limit challenges to the grid during the summer months.
Document Control Desk Attachment C-06-0465 RC-07-0016 Page 3 of 3 The Maintenance Rule (a) (4) risk assessment at VCSNS accounts for emergent conditions in accordance with NUMARC 93-01 (Industry Guidance for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants) guidance. The following conditions that affect the LOOP multiplier are explicitly modeled in the risk-monitor software program: hot line work, hurricane warning, hurricane watch, tornado watch, tornado warning, loss of the RTCA, switching evolutions, switchyard work involving a boom truck, light vehicles, relays, batteries, breakers, transformer oil samples, or opening doors in the switchyard. Selecting any of these "activities" in the plant risk monitor program or manually increasing the LOOP initiating event multiplier will cause a "Grid Risk" status button in the risk-monitor software interface to turn red and the LOOP initiating event frequency to be multiplied by a factor (such as two or four).
The LOOP initiating event is increased by an even higher multiplier and appropriate components are modeled as out of service during scheduled maintenance on either the 230kV or 115kV power lines to the plant. This also turns the Grid Risk button red.
VCSNS procedure OAP 100.5, Rev. 2, Guidelines for Configuration Control and Operation of Plant Equipment, directs operators to ensure System Control has been notified if the Grid Risk status button turns red in order to ensure that the grid is healthy. System control is also notified when the Grid Risk red condition clears.
If System Control notifies the VCSNS control room of a RTCA contingency violation or loss, a LOOP multiplier of 2 or more is applied in the plant risk monitor turning the Grid Risk status button red.
The guidance for Maintenance Rule risk assessments (NUMARC 93-01) does not require applying a seasonal multiplier for the LOOP frequency in the Maintenance Rule (a) (4) assessment tool. Rather, activities or emergent conditions which increase the probability of a LOOP such as switchyard work and grid/weather conditions are addressed in the Maintenance Rule (a) (4) assessment tool.
A review of the LOOP initiator frequency from the EOOS (Equipment Out Of Service) history file at VCSNS for 2006 shows that increases are primarily due to severe weather and switching evolutions. The average LOOP frequency over the cycle was about 4% higher than the base LOOP frequency. This shows that the LOOP adjustment in the online risk assessment process does reflect the overall impact on LOOP frequency.