DCL-07-007, Response to NRC Request for Additional Information Regarding NRC Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power
| ML070380321 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 01/31/2007 |
| From: | Jacobs D Pacific Gas & Electric Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| DCL-07-007, GL-06-002, OL-DPR-80, OL-DPR-82 | |
| Download: ML070380321 (5) | |
Text
Pacific Gas and Electric Company Donna Jacobs Diablo Canyon Power Plant Vice President P. 0. Box 56 Nuclear Services Avila Beach, CA 93424 805.545.4600 January 31, 2007 Fax: 805.545.4234 PG&E Letter DCL-07-007 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Response to NRC Request for Additional Information Regarding NRC Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power"
References:
- 1. NRC Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," dated February 1,2006
- 2. PG&E Letter DCL-06-042, "60-Day Response to NRC Generic Letter 2006-02, 'Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power,"' dated March 31, 2006
- 3. NRC Letter, "Request for Additional Information Regarding Resolution of Generic Letter 2006-02, Grid Reliability and the Impact of Plant Risk and the Operability of Offsite Power (TAC Nos. MD0947 through MD1050)," dated December 5, 2006
Dear Commissioners and Staff:
Generic Letter (GL) 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," (Reference 1) requested licensees to determine if compliance is being maintained with NRC regulatory requirements governing electric power sources and associated personnel training. Pacific Gas and Electric Company (PG&E) responded to GL 2006-02 in Reference 2.
In Reference 3, the NRC staff requested additional information required to resolve GL 2006-02 issues, and indicated Questions 3 and 5 are applicable to Diablo Canyon Power Plant, Units 1 and 2. PG&E's responses to the staff's Questions 3 and 5 are provided in the Enclosure.
PG&E makes no regulatory commitments (as defined by NEI 99-04) in this letter.
This letter includes no revisions to existing regulatory commitments.
A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway e Comanche Peak
- Diablo Canyon
- Palo Verde
- South Texas Project
- Wolf Creek
F!E Document Control Desk January 31, 2007 Page 2 PG&E Letter DCL-07-007 If you have any questions, or require additional information, please contact Stan Ketelsen at (805) 545-4720.
I state under penalty of perjury that the foregoing is true and correct.
Executed on January 31, 2007.
Sincerely, Donn acobs Vice Pr ident - Nuclear Services kjs/4328 Enclosure cc: Edgar Bailey, DHS Bruce S. Mallett Terry W. Jackson Diablo Distribution cc/enc Alan B. Wang A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway
- Comanche Peak e Diablo Canyon
- Palo Verde
- South Texas Project
- Wolf Creek
Enclosure PG&E Letter DCL-07-007 Response to NRC Request for Additional Information Regarding Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power" NRC Question 3:
Verification of RTCA PredictedPost-Trip Voltage Your response to question 2(g) indicates that you have not verified by procedure the voltages predictedby the online grid analysis tool (software program) with actual real plant trip voltage values. It is important that the programs used for predictingpost-trip voltage be verified to be reasonablyaccurate and conservative. What is the range of accuracy for your [Grid Operator's]GO's contingency analysis program? Why are you confident that the post-trip voltages calculated by the GO's contingency analysis program (thatyou are using to determine operabilityof the offsite power system) are reasonablyaccurate and conservative? What is your standardof acceptance?
PG&E Response:
Generic Letter 2006-02 used the term "Transmission System Operator" (TSO) throughout the letter. The term TSO was defined to denote various entities associated with preserving the reliability of the local transmission system; however, the term "Grid Operator" (GO) was not included among them. For the purpose of this response, it is assumed that GO is included within the original TSO definition. As originally stated in the response to Generic Letter 2006-02 question 1(a), Diablo Canyon Power Plant (DCPP) Units 1 and 2 primarily communicates with the Electric System Operations (ESO) Department of the Energy Delivery division of the Pacific Gas and Electric Company (PG&E) regarding offsite power.
As stated in the response to Generic Letter 2006-02 questions 2(a) and 2(g), the ESO analysis tool is based on periodic bounding transmission studies versus an online software program. The direct comparison of actual post trip voltages to analysis results is not practical because DCPP unit trips are very infrequent compared to grid model updates and the grid operating conditions at the time of a trip are not as severe as the worst case conditions analyzed. During an August 2005 Strategic Teaming and Resource Sharing Alliance and Institute of Nuclear Power Operations assessment of switchyards, large transformers, and grid reliability, several recommendations were made. Two of these recommendations focused on the sensitivity associated with the grid related parameters used in the offsite power analysis. A sensitivity study was performed to address reasonable variations in DCPP grid preloading, total grid load, and local grid load distribution. It was concluded that the load values in the offsite power analysis are adequate for the intended purpose because the DCPP switchyard is relatively insensitive to minor variations.
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Enclosure PG&E Letter DCL-07-007 NRC Question 5:
Seasonal Variationin Grid Stress (Reliability and Loss-of-offsite Power (LOOP)
Probability)
Certainregions during certain times of the year (seasonalvariations)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 probabilisticrisk assessment (PRA) and Maintenance Rule evaluations for various seasons? If you do not considerseasonal variations in base LOOP frequency in your [probabilisticrisk assessment] PRA and Maintenance Rule evaluations, explain why it is acceptablenot to do so.
PG&E Response:
The estimates for LOOP initiating event frequencies used in the DCPP PRA model are not adjusted for seasonal variation. These LOOP estimates are based on the latest NRC data from draft NUREG/CR (INEEL/EXT-04-02326), "Evaluation of Loss of Offsite Power Events at Nuclear Plants: 1986-2003 (Draft)", October 2004, and represent an averaged yearly LOOP frequency for grid, severe weather, plant-centered, and switchyard-related LOOP events. This NRC data is further updated by using plant specific data.
PG&E believes that the use of average LOOP frequencies provides better overall insight than using a higher-than-average estimate for certain seasons and a lower-than-average frequency for others. The use of different LOOP frequencies for different seasons could result in either an underestimation of risk (if LOOP frequency were to be decreased for certain seasons to maintain a correct average LOOP frequency) or mask the risk contribution from other plant configurations (if LOOP frequency was increased).
Currently, 10 CFR 50.65(a)(4) Maintenance Rule evaluations use a yearly average for LOOP frequency to provide a quantitative measure of risk. These evaluations account for actual grid conditions by qualitatively considering external trip risks or grid conditions. For example, plant risk status is elevated for California Independent System Operator Stage 3 Alerts, controlled fires near transmission lines, or miscellaneous grid risks. Such emergent external risk conditions are addressed by implementing the appropriate compensatory measures.
Although PG&E believes that using an average LOOP frequency for 10 CFR 50.65(a)(4) evaluations provides better overall risk insights year round, DCPP plans on implementing Safety Monitor. Safety Monitor is an enhanced on-line risk assessment tool that has the ability to adjust initiator frequencies according to emergent external conditions. Safety Monitor users (e.g., operations, scheduling, and PRA engineering personnel) will be able to perform sensitivity analyses, if appropriate, for 2
Enclosure PG&E Letter DCL-07-007 emerging or predicted conditions, including the conditions mentioned above, by increasing LOOP frequency and performing a quantitative risk assessment.
The intent of the 10 CFR 50.65(a)(4) evaluation is to obtain risk insights based on the best available information and, if necessary implement risk-reducing measures. PG&E believes that the current approach of combined quantitative and qualitative risk assessment is adequate to meet the intent of the required 10 CFR 50.65(a)(4) risk assessment. Additionally, the planned move to Safety Monitor, an enhanced on-line risk assessment tool, will further increase the ability to perform realistic assessments of maintenance-related activities.
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