ML11277A270

Response to Request for Additional Information Regarding Earthquake on August 23, 2011 and Restart Readiness Determination Plan

ML11277A270
Person / Time
Site:	North Anna
Issue date:	10/03/2011
From:	Grecheck E Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
11-544A
Download: ML11277A270 (12)

Text

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 October 3, 2011 10 CFR 100, Appendix A U.S. Nuclear Regulatory Commission Serial No.:

11-544A Attention: Document Control Desk NL&OS/ETS RO Washington, DC 20555 Docket Nos.:

50-338/339 License Nos.:

NPF-4/7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNITS 1 AND 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING THE EARTHQUAKE ON AUGUST 23. 2011 AND RESTART READINESS DETERMINATION PLAN On August 23, 2011, at 1351 hours0.0156 days <br />0.375 hours <br />0.00223 weeks <br />5.140555e-4 months <br />, with North Anna Power Station Units 1 and 2 operating at 100% power, a Magnitude 5.8 earthquake occurred approximately 5 miles from Mineral, Virginia. The epicenter was approximately 11 miles WSW of North Anna Power Station. Ground motion was felt and recognized as an earthquake by the Main Control Room operators at the station.

Dominion has confirmed that the August 23, 2011 earthquake exceeded the spectral accelerations for the Operating Basis Earthquake (OBE) and Design Basis Earthquake (DBE) for North Anna Power Station Units 1 and 2.

In accordance with 10 CFR 100, Appendix A, in a September 17, 2011 letter (Serial No.11-520), Dominion submitted a Summary Report of the August 23, 2011 Earthquake Response and Restart Readiness Determination Plan to permit NRC evaluation of, and concurrence with, Dominion's readiness for restart of North Anna Units 1 and 2 following completion of the identified near-term action items.

In a September 14, 2011 letter, sent prior to the submittal of Dominion's Restart Readiness Determination Plan, the NRC requested additional information regarding the earthquake as discussed in the public meeting held on September 8, 2011.

Some of the requested information was provided in the September 17, 2011 letter; however, Dominion is providing a specific docketed response to address the requested information.

To facilitate NRC review, Dominion provided responses to several of the NRC questions in a September 27, 2011 letter (Serial No.11-544). Specifically, a response to Fuels questions 2, 4, 5, 6, 7, and 9 and Reactor Systems questions 1 through 5 were provided. Dominion's response to the Fuels questions 3 and 8 and updated information

/ PoL

Serial Number 11-544A Docket Nos. 50-338/339 Page 2 of 3 for Fuels question 2 are provided in the attachment to this letter. Fuels question 1 will be addressed following completion of an ongoing evaluation of the fuel assemblies.

If you have any questions or require additional information, please contact Mr. Thomas Shaub at (804) 273-2763.

Sincerely, E. S. Grecheck Vice President - Nuclear Development Attachment Response to Request for Additional Information Regarding the Earthquake on August 23, 2011 and Restart Readiness Determination Plan There are no Commitments made in this letter.

COMMONWEALTH OF VIRGINIA COUNTY OF HENRICO The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by E. S. Grecheck who is Vice President -

Nuclear Development, of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this day of

!jg

, 2011.

My Commission Expires:

!13012D*

1 6 Gingor Lynn Rutherford NOTARY PUBLIC Commonwealth of Virginia Reg. # 310847 My Commission Expires 4/30/901F N ary Public

Serial Number 11-544A Docket Nos. 50-338/339 Page 3 of 3 cc:

Regional Administrator U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257 NRC Senior Resident Inspector North Anna Power Station M. Khanna NRC Branch Chief-Mechanical and Civil Engineering U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9E3 11555 Rockville Pike Rockville, MD 20852-2738 R. E. Martin NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 P. G. Boyle NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 J. E. Reasor, Jr.

Old Dominion Electric Cooperative Innsbrook Corporate Center 4201 Dominion Blvd.

Suite 300 Glen Allen, Virginia 23060

Serial Number 11-544A Docket Nos. 50-338/339 Attachment Response to Request for Additional Information Regarding the Earthquake on August 23, 2011 and Restart Readiness Determination Plan Fuels Questions Virginia Electric and Power Company (Dominion)

North Anna Units I and 2

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 1 of 8 BACKGROUND To facilitate NRC review, Dominion provided responses to several of the NRC questions contained in a letter dated September 14, 2011.

Specifically, Dominion responded to Fuels questions 2, 4, 5, 6, 7, and 9 and Reactor Systems questions 1 through 5 in a letter dated September 27, 2011 (Serial No.11-544).

Dominion's response to Fuels questions 3 and 8 and updated information for Fuels question 2 are provided below.

Question 1 will be addressed following completion of an ongoing evaluation of the fuel assemblies.

NRC REQUEST FOR INFORMATION 4.0 Fuels

2. Describe the extent of fuel assembly inspections which will be performed to confirm the structural integrity of the fuel. Provide specific information on how the inspections will determine that there is no distortion of the fuel lattice array or rod cluster control assembly (RCCA) guide tubes that occurred as a result of the seismic event. Also provide information on guide tube drag and rod drop testing.

Dominion Response When the units were tripped during the recent earthquake, the control rods [rod cluster control assemblies (RCCAs)] fully inserted. When control rods are removed from fuel assemblies, the design of the RCCA handling tool prevents inspection of the RCCA rodlets.

To confirm that there was no distortion of the RCCA rodlets or the fuel assembly guide tubes, and that the RCCAs could still freely travel within the fuel assembly guide tubes, RCCA drag loads were measured in the spent fuel pool. From the perspective of ensuring that the control rods can be inserted, the drag force in the upper portion of the guide tubes is the measurement of concern, which is also reinforced by the fact that the Technical Specifications limit on control rod drop time is a limit on the time to the dashpot region of the guide tubes.

Therefore, the remainder of this discussion will focus on the drag forces measured above the dashpot region of the guide tube. However, the results of drag load measurements in the dashpot region are also tabulated for completeness.

Measurements were first performed for the forty-eight (48) Unit 2 fuel assemblies in which the RCCAs resided during Cycle 21. Following movement of the RCCAs into their host assemblies for Cycle 22, drag loads were also measured for the forty-eight (48) fuel assemblies in which the RCCAs will reside during Cycle 22. AREVA recommended using a screening criterion of 40 lbs. for these drag tests, and most of the measured drag forces met this criterion. In the Cycle 21 host assemblies, with an average burnup of about 42 GWD/MTU, there were five (5) fuel assemblies that had drag forces in excess of 40 lbs., and in the Cycle 22 host assemblies, with an average burnup of about

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 2 of 8 22 GWD/MTU, there was only one (1) fuel assembly where the drag force exceeded the screening criterion. There were no instances of any single RCCA where its respective drag force exceeded the screening criterion in both its Cycle 21 and Cycle 22 host assemblies, indicating there were no mechanical issues with the RCCAs.

Dashpot Note I Upper Portion of Guide Tube Maximum Minimum Average Maximum Minimum Average Measured Measured Measured Measured Measured Measured Drag (Ib)

Drag (Ib)

Drag (Ib)

Drag (Ib)

Drag (Ib)

Drag (Ib)

Cycle 21 Host 65 4

23.8 57 1

18.3 Assemblies Cycle 22 Host Ass 74 6

23.6 56 0

14.6 Assemblies Note 2 Overall 74 4

23.7 57 0

16.4 Notes:

1. Drag forces in the dashpot region are typically higher than in the upper part of the guide due to tighter clearances.

2 Based on the drag loads measured in the fuel in the North Anna spent fuel pool, the BOC in-core RCCA drag loads are expected to meet the AREVA criteria for BOC in-core drag loads, and control rod drop times are projected to meet the Technical Specifications limit.

Experiencing a small number of fuel assemblies with RCCA drag force measurements higher than the screening criteria is consistent with recent North Anna operating experience.

Further, RCCA drag forces are not normally measured in the spent fuel pool. However, it is normal practice during refueling outages to check the control rod drag forces in core after the rods have been latched, to verify that the rods move freely.

During these beginning of cycle (BOC) drag tests, most RCCA drag forces above the dashpot region are observed to be less than 40 lb.

However, in recent North Anna cycles with Advanced Mark-BW fuel at both Units 1 and 2, there have been a small number of cases (1-2 per cycle) where the RCCA drag forces have exceeded 40 lb.

Similar behavior has been observed in other units using the Mark-BW type fuel design.

At North Anna there has been no pattern to these BOC elevated drag force observations in terms of either core location or RCCA. For the Cycle 21 fuel, there is also no direct correlation between the observations of elevated drag forces during the BOC test and the observations of elevated drag during the measurements in the spent fuel pool following the earthquake.

Further, AREVA BOC drag force criteria allows for a small number of locations to have RCCA with drag forces above the 40 lb. criterion, while still ensuring acceptable RCCA drop time results at BOC and for subsequent plant operation. In the cases where elevated drag forces have been measured after latching at BOC, the BOC RCCA drop times in the high drag locations have met the Technical

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 3 of 8 Specifications drop time limit, and the drop times have been comparable to those observed in core locations where normal drag forces were measured.

The elevated drag forces observed during recent North Anna refueling outages have typically been about 47 lb., although in one case the measured drag was 63 lb. Even higher drag forces have been measured at another utility with Mark-BW type fuel. For North Anna, the BOC drag loads are generally consistent with, although slightly lower than, those measured in the North Anna Unit 2 fuel in the spent fuel pool. However, drag forces measured in the spent fuel pool are generally expected to be higher than those measured in core during the BOC testing, as fuel assemblies have more room to bow in the storage cells in the spent fuel pool than when positioned in core.

The alignment of the tools when measuring RCCA drag forces in the spent fuel pool may not be as accurate as the alignment over the RCCAs in core, also potentially contributing to slightly higher measured drag forces. RCCA drag forces have been measured in the spent fuel pool of another utility that uses Mark-BW-type fuel, and the drag forces measured in their spent fuel pool have also generally been higher than those measured in core during their BOC testing.

The results of the RCCA drag force measurements in the spent fuel pool at North Anna -

the average magnitude of the control rod drag forces, the number of fuel assemblies with higher than average drag forces above the dashpot region, and the magnitude of the higher drag forces in those assemblies - are consistent with behavior observed in recent cycles at North Anna. Similar results have been observed in Mark-BW-type fuel used at another utility. The drag test results to date indicate that there has been no seismic induced impact on the RCCAs, and no distortion of the fuel assembly guide tubes beyond the normal bow that is expected with Advanced Mark-BW fuel based on observations from recent North Anna cycles. Final confirmation of the Unit 2 RCCAs and fuel assembly guide tube operability will be obtained through BOC RCCAs drop time measurements.

The fuel and insert components in North Anna Unit 1 are of the same design as the Unit 2 fuel and insert components that were inspected, and are constrained in the core in the same manner as the Unit 2 fuel and inserts. The effect of the seismic loads on the Unit 1 fuel and inserts would therefore be similar to the effect on the Unit 2 fuel and inserts.

No damage to the Unit 2 fuel or inserts was identified by the visual inspections that were performed, and the control rod drag measurements are consistent with North Anna experience using the Advanced Mark-BW fuel. It is therefore concluded that the North Anna Unit 1 fuel and inserts were similarly not subjected to any loads or vibrations that would adversely impact their ability to continue to safely perform their design functions.

Hot rod drop time measurement of the Unit 1 RCCAs will be performed in accordance with station procedures as a final confirmation of the full functionality of the RCCAs and fuel assembly guide tubes.

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 4 of 8

3. Describe the extent of fuel assembly inspections and supporting analyses which will be performed to confirm the thermal-hydraulic performance of the fuel. Provide specific information on how the inspections will determine that there is no deflection of any fuel grid mixing vanes or any other component that will alter the thermal-hydraulic performance of the fuel bundle as a result of the seismic event. If any deficiencies are detected, provide information on the impact on the fuel departure from nucleate boiling ratio (DNBR).

Dominion Response Fuel assembly grids are designed to maintain their configuration during normal shipping and handling operations (up to 4g lateral and 6g axial loads), as well as during normal operation in core, which includes the fuel experiencing vibration throughout its operating life (typically two or three 18-month cycles) from interaction with the high coolant flow rates. Because the mixing vanes are located interior to the grid, once the grids are incorporated into fuel assemblies, damage to the vanes from contact with other fuel assemblies or core baffle plates is precluded. Any forces exerted on the mixing vanes during the earthquake would have been caused by the coolant acting on the mixing vanes. These forces on the vanes during the earthquake would not significantly differ from the forces withstood by the vanes during normal operation, and would not result in deformation of the mixing vanes.

The grid strap features (springs and hardstops) that prevent rod contact with the mixing vanes during normal handling and operation in the core similarly precluded contact between the rods and mixing vanes during the August 23, 2011 earthquake. Based on grid crush testing, plastic deformation of the Advanced Mark-BW grids would be expected to take the form of deformation across one or more entire rows of cells.

Because there is no deformation of inner grid cells without deformation of the outer cells in the same row, such deformation would be evident in examinations of the inner and outer grid straps from the exterior faces of the fuel assembly.

Detailed visual inspections were performed on thirty-five (35) fuel assemblies from the North Anna Unit 2 core, including twenty (20) assemblies that were located on or near the core periphery, in locations judged to be most susceptible to deformation during a seismic event. The grids were examined from the four faces of the assembly. In addition to looking directly at the outer strap for any indications of deformation, grids were also examined from above and below the mid-plane to allow better examination of the inner straps and mixing vanes. These inspections did not reveal any indications of damage or plastic deformation of the grid straps or mixing vanes on either structural grids or mid-span mixing grids.

Preliminary calculations performed by the fuel vendor are consistent with the results of these visual inspections, showing that the spacer grids on the North Anna Unit 1 and Unit 2 fuel remained within the elastic region during the August 23, 2011 earthquake.

The greatest elastic deformation for Advanced Mark-BW grids occurs on the intermediate grids at beginning of life. The predicted maximum grid and cell deformation

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 5 of 8 on the intermediate grid is less than the amount of deflection required to induce rod to vane contact on either the structural grids with mixing vanes or the mid-span mixing grids. Therefore no interaction between the mixing vanes and fuel rods is predicted to have occurred.

Since the spacer grid remained in the elastic region, the grids would return to their original condition following the earthquake.

Therefore, the critical heat flux (CHF) correlation remains applicable for this fuel, and no adverse impact on the fuel thermal-hydraulic behavior is postulated as a result of the August 23, 2011 earthquake.

8. Describe the extent of inspections on the core shroud to investigate possible changes in local flow conditions (e.g., baffle jetting, change in core bypass flow).

Dominion Response As stated in Enclosure 2 of the submittal dated September 17, 2011 (Serial No.11-520),

evidence of inspections performed to date is consistent with Damage Intensity 0 on the EPRI seismic damage scale. No specific inspections of reactor internals or associated components are specified in EPRI NP-6695 for Intensity 0 earthquakes.

Since the earthquake produced only minimal damage to non-seismically designed equipment, and since there was no observable significant physical or functional damage to seismically designed structures, systems, and components (SSCs) that were examined following the event, there is a reasonable assurance that there was no significant physical or functional damage to the reactor vessel internals (RVIs), and that the RVIs remain functional and capable of performing their design functions. Additional evaluations of the RVI design margins, as discussed in Enclosure 3 of Dominion's letter dated September 17, 2011 (Serial No.11-520), and the attachment to the October 3, 2011 letter (Serial No.11-566), have been performed based on existing design analyses of the structural integrity of the RVIs.

These evaluations, in addition to providing the reasonable assurance of continued functionality described above, support the conclusion that the earthquake resulted in no significant physical or functional damage to the RVIs, and that the RVIs remain capable of performing their design bases functions.

Although not required by EPRI NP-6695, Dominion, in collaboration with Westinghouse (the NSSS vendor), identified several inspections of the RVIs to supplement the above conclusion that the RVIs remain capable of performing their design bases functions.

The inspections are listed below, with the results or current status.

1. Record the height of the upper internals prior to removal using the typical refueling plumb bob process and compare the height to previous outages.

This measurement, when compared to the previous measurement (2010), confirms that

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 6 of 8 fuel assembly nozzle springs lift the upper internals to the nominal position (i.e.,

there is no significant binding of components).

Result: Measurements taken on Unit 2 upper internals agreed with data from the 2010 refueling outage.

2. Note any difficulty engaging the internals lift rig with the threaded holes of the upper support plate.

Result: No anomalies were identified on Unit 2.

3. Verify that no RCCA elements or drive shafts are raised during reactor disassembly. Also note the maximum load during the upper internals removal.

Result: No RCCA element or drive shaft anomalies were identified on Unit 2. The maximum load was within normal limits.

4. Perform a visual inspection of selected parts of the upper internals during handling of the upper internals from the reactor vessel to the storage stand.

Result: Submarine camera observations confirmed no fuel assembly interferences in Unit 2. The camera was also used to observe the upper internals during the placement in the storage stand. No anomalies were identified.

5. Note any difficulty during retraction or insertion of the flux mapping system detectors or thimbles.

Result: The flux thimbles were retracted for the Unit 2 refueling outage.

No anomalies were identified.

In addition, the flux mapping system will be used during the Unit 1 and Unit 2 power ascensions to perform a flux map at 30% power.

6. Maintain a high sensitivity to the presence of foreign material during fuel movement. Particularly perform visual inspections for lock bars either on the fuel assembly bottom nozzles or the fuel movement path.

Result: No foreign material issues were discovered that could be attributed to the earthquake. Two fuel assemblies were observed to have debris, but the debris was not earthquake-related.

7. Inspect peripheral fuel assembly grids for indications of any grid crush.

Result: Detailed video inspections were performed on thirty-five (35) North Anna Unit 2 fuel assemblies, twenty (20) of which had been in core locations on

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 7 of 8 or near the periphery where the potential for seismic damage was judged to be greatest. No grid damage was observed.

8. Perform video inspections of baffle plate visible gaps.

Result: These inspections will be performed prior to the Unit 2 core onload.

9. Perform video inspections of accessible baffle bolts. Look for missing lock bars, heads and loose parts on the lower core plate.

Result: This inspection will be performed prior to the Unit 2 core onload.

10. Perform video inspections of core barrel pin surfaces that restrain the upper core plate from moving laterally. Note any indications of indentation on the pin contact surfaces from the upper core plate keyway clevises.

Result: This inspection will be performed prior to the Unit 2 core onload.

11. Perform a visual inspection at the bottom of the reactor vessel by lowering a camera through a few core plate flow holes down to the bottom of the reactor vessel to view the bottom area for any foreign loose parts.

Result: This inspection will be performed prior to the Unit 2 core onload.

12. Step the RCCAs with the RV Head installed to aid in confirmation that the drive line components and guide tubes have not deformed.

Result: This test will be performed during the Unit 1 and Unit 2 restart.

13. Perform a drag test of the RCCAs with the upper internals installed.

Result: RCCA drag testing on North Anna 2 fuel was performed in the spent fuel pool for confirmation of fuel assembly and RCCA dimensions. Drag testing was performed in the fuel assemblies in which the RCCAs resided in Cycle 21 and also in the fuel assemblies in which the RCCAs will reside in Cycle

22. The results of the spent fuel pool RCCA drag testing confirmed that the fuel assembly guide tubes and RCCA dimensions were not impacted by the August 23, 2011 earthquake.

(Refer to the response to RAI Question 2 of this letter for additional information on RCCA drag testing.)

RCCA drag testing will also be performed with the upper internals installed as part of the normal Unit 2 refueling outage process.

Differences in a few design features exist between the RVIs of North Anna Unit 1 and Unit 2 in regard to local flow conditions. Most notably, Unit 1 was converted to "upflow" in the baffle-former region in 1996, while Unit 2 remains in the original "downflow"

Serial No. 11-544A Docket Nos. 50-338/339 Attachment Page 8 of 8 configuration. Because of this difference, Unit 2 is more susceptible to baffle bolt failure and baffle jetting, and it is appropriate to select Unit 2 for baffle region inspections. If no degradation is identified in the North Anna Unit 2 RVIs and fuel inspections, Westinghouse concurs that the inspection requirements for EPRI damage intensity 0 can be applied to Unit 1 to restart the current fuel cycle. Baffle bolt video inspections are ongoing in the Unit 2 reactor vessel as discussed in inspection Item 9 above.

The inspections detailed above, in combination with the assessment discussed in of the submittal dated September 17, 2011 (Serial No.11-520), will identify any anomalous condition of the North Anna Unit 2 RVIs. Assuming these inspections find no anomalous conditions, local flow conditions are not expected to change in either Unit 1 or Unit 2. If any anomalous conditions are found, an assessment of the impact on local flow conditions will be performed.