0CAN010702, Response to Request for Additional Information for Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power
| ML070440164 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 01/31/2007 |
| From: | Marlow T Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 0CAN010702, GL-06-002 | |
| Download: ML070440164 (6) | |
Text
Entergy Operations, Inc.
1448 S.R. 333 Entery Russellville, AR 72802 Tel 479-858-4601 Thomas A. Marlow Director, Nuclear Safety Assurance OCAN010702 January 31, 2007 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
SUBJECT:
Response to Request for Additional Information for Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power Arkansas Nuclear One, Units 1 and 2 Docket Nos. 50-313 and 50-368 License No. DPR-51 and NPF-6 REFERENCES 1 NRC letter dated February 1, 2006, Grid Reliability and the Impact on Plant Risk and the Operabilityof Offsite Power (OCNA020601) 2 Entergy letter dated March 29, 2006, Response to Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operabilityof Offsite Power (OCAN030601) 3 NRC letter dated December 5, 2006, Request for Additional Information Regarding Resolution of Generic Letter 2006-02, Grid Reliability and the Impact On Plant Risk and the Operabilityof Offsite Power (OCNA120605) 4 NRC letter dated December 13, 2006, Revised Response Date for Request for Additional Information Regarding Resolution of Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operabilityof Offsite Power (OCNA1 20606)
Dear Sir or Madam:
Per Reference 1, the NRC issued Generic Letter (GL) 2006-02 to request information for determining compliance with regulatory requirements governing electric power sources. The Arkansas Nuclear One, Unit 1 and Unit 2 (ANO) response to the requested information in the generic letter was provided in Reference 2.
The NRC staff reviewed licensee's responses to GL 2006-02 and determined that additional information was needed to resolve the concerns discussed in the generic letter. The NRC request for additional information (RAI) on GL 2006-02 was provided to all licensees in Reference 3. The generic NRC RAIs and the plant specific applicability for each RAI were A12-73
OCAN010702 Page 2 of 2 contained in Enclosures 2 and 3, respectively. Only question 5 was determined by the NRC to require a response for Entergy's ANO units. The response to this question for ANO is contained in the attachment to this letter. The requested information is being made under the requirements of 10CFR50.54(f).
The NRC requested that the additional information be provided within 30 days of receipt of the subject NRC RAIs. However, based on feedback from the nuclear industry and NEI, the NRC response date was extended to January 31, 2007 (Reference 4).
Entergy is not making any commitments as a result of our response to this letter. If you have any questions or require additional information, please contact Steve Bennett at 479-858-4626.
I declare under penalty of perjury that the foregoing is true and correct.
Executed on January 31, 2007.
Sincerely, TAM/sab
Attachment:
Response to NRC Request for Additional information on Generic Letter 2006-02 for ANO-1 and ANO-2 cc: Dr. Bruce S. Mallett Regional Administrator U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 NRC Senior Resident Inspector Arkansas Nuclear One P. 0. Box 310 London, AR 72847 U. S. Nuclear Regulatory Commission Attn: Ms. Farideh Saba MS 0-7 D1 Washington, DC 20555-0001 Mr. Bernard R. Bevill Director Division of Radiation Control and Emergency Management Arkansas Department of Health 4815 West Markham Street Little Rock, AR 72205
Attachment to OCAN010702 Response to NRC Request for Additional information for Generic Letter 2006-02 for ANO-1 and ANO-2
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Attachment to OCAN010702 Page 1 of 3 Response to NRC Request for Additional information (RAI) for Generic Letter 2006-02 for ANO-1 and ANO-2 NRC RAI 5 (Maintenance Rule)
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.
ANO Response NRC Information Notice (IN) 2006-06, Loss of Offsite Power and Station Blackout Are More Probableduring Summer Period,was issued to alert the industry of the recent findings provided in NUREG/CR-6890 during seasonal summer variations. Entergy reviewed this IN as well as NUREG/CR-6890 to determine whether any changes need to be made to the Maintenance Rule (MR) (a)(4) risk assessment process to reflect concerns about increased loss of offsite power (LOOP) risk during the months of May through September (i.e., summer months). Based on our review of these documents for the Entergy Nuclear South (ENS) grid, Entergy's approach to this concern is more broadly addressed in our MR (a)(4) risk assessment than just the seasonal risk increase.
The factors that impact the frequency of LOOP are grid instability, severe weather and maintenance activities in the plant switchyard.
Grid Stability Grid stability is one of the factors cited in NUREG/CR-6890 as increasing the LOOP risk during the summer months. Grid stability is impacted by several factors, such as plant outages, available capacity, and peak electricity usage.
The Transmission Operations Center (TOC) and the System Operations Center (SOC) are centers within Entergy Transmission Operations. These centers are responsible for the operation and monitoring of the grid system. In accordance with Entergy Corporate Procedure ENS-DC-201, ENS Transmission Grid Monitoring, the SOC has established communication channels with ENS nuclear sites to ensure that a process exists to notify the station when the local transmission system parameters indicate a potential degraded condition or abnormal situation such that appropriate actions can be taken to maintain defense in depth. Specific alert notifications are provided to ensure grid reliability is maintained and degraded grid off-site power supply conditions are communicated. The SOC will notify the ENS site control room if critical parameter levels are outside of prescribed operating range. The ENS control room will then evaluate the grid degradation reported by SOC and take appropriate actions.
Attachment to OCAN010702 Page 2 of 3 Site specific procedures provide for on-line risk assessments to satisfy the maintenance rule.
At ANO, MR (a)(4) risk assessments are performed under ANO Operations Directive COPD-24, Risk Assessment Guidelines. Within these guidelines are instructions to calculate the elevated risk from an increased likelihood of loss of offsite power due to grid instability.
ANO Operations staff can calculate the elevated risk by adjusting the slider bar in station's Equipment Out of Service (EOOS) Probabilistic Safety Assessment (PSA) model. Adjusting the slider bar will increase the loss of offsite power frequency and thus address concerns related to seasonal variations.
Switchyard Maintenance Maintenance work in the plant switchyard could also impact the frequency of a loss of offsite power due to inadvertent action that would interrupt transmission of power to the nuclear plant. Scheduled switchyard maintenance is normally performed during periods exclusive of the peak summer months.
When switchyard work is required, transmission work inside the ENS switchyards requires communication between the TOC and the ENS Operations' personnel. Work by ENS maintenance crews is controlled by the ENS work control group and is also communicated to the TOC.
As discussed in our initial response to GL 2006-02, Procedure 1015.033, ANO Switchyard and Transformer Yard Controls, provides the interface between ANO's control room personnel and the TOC for switchyard activities. This procedure provides the actions that the onsite control room personnel will take when notified by the TOC that there is needed maintenance work in the switchyard. Within Directive COPD-24 there are instructions to calculate the elevated risk due to an increased likelihood of loss of offsite power including switchyard work. Operations personnel can determine the elevated risk by adjusting the slider bar in EOOS and quantifying the PSA risk. Adjusting the EOOS slider bar during periods when there is a higher risk for switchyard maintenance will bound the affects from seasonal variations in LOOP risk.
Severe Weather Response Severe weather could also impact the frequency of a loss of offsite power. Some of the severe weather impact could be seasonal, although not always associated with the summer months. Severe weather impacts of particular interest are tornados, high winds and possibly severe thunderstorms.
Each plant has an Abnormal Operating Procedure for Severe Weather or Natural Emergencies. At ANO these procedures are OP-1 203.025, NaturalEmergencies, and OP-2203.008, NaturalEmergencies,for Units 1 and 2, respectively. The entry conditions for these procedures are either a warning issued by the National Weather Service or result from local indications (high wind instruments or tornado spotted locally). These procedures instruct the operators to increase the LOOP risk per the on-line maintenance procedure. Within that procedure there are instructions to calculate the elevated risk due to an increased likelihood of loss of offsite power due to severe weather.
Attachment to OCAN010702 Page 3 of 3 Conclusions Entergy Nuclear South does not specifically increase the LOOP frequency for seasonal variations. However, the ENS sites more broadly address the conditions which would bound the seasonal risk increase. Specific grid conditions that would be projected by Entergy's SOC/TOC will sufficiently address the potential increase in LOOP risk for seasonal variations discussed in Information Notice 2006-06 and NUREG/CR-6890. The ENS actions more accurately estimate the impact to plant risk for incremental and instantaneous risk assessments. Therefore, no additional actions are considered necessary by Entergy to address seasonal variations for the ENS plants including ANO.