ML052500306

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Response to Request No. 2 in NRC Generic Letter 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors
ML052500306
Person / Time
Site: Palo Verde  Arizona Public Service icon.png
Issue date: 09/01/2005
From: Overbeck G
Arizona Public Service Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
102-05336-GRO/TNW/GAM, GL-04-002, NEI 04-07
Download: ML052500306 (22)
v • d • e


Text

Generic Letter 2004-02 LAMS Gregg R. Overbeck Tel (623) 393-5148 Mail Station 7602 Palo Verde Nuclear Senior Vice President Fax (623) 393-6077 PO Box 52034 Generating Station Nuclear e-mail: GOVERBEC@apsc.com Phoenix, Arizona 85072-2034 102-05336-GRO/TNW/GAM September 1, 2005 U.S. Nuclear Regulatory Commission Attn: Document Control Desk 11555 Rockville Pike Rockville, Maryland 20852

Dear Sirs:

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Units 1, 2 and 3 Docket Nos. STN 50-528, 50-529, and 50-530 Response to Request No. 2 in NRC Generic Letter 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors" In Generic Letter (GL) 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors," the NRC requested that pressurized water reactor licensees perform containment sump evaluations and provide information identified in request no. 2 no later than September 1, 2005.

APS has completed the evaluation requested by GL 2004-02 for Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2, and 3. The evaluation was performed in accordance with the guidance in NEI 04-07, "Pressurized Water Reactor Sump Performance Evaluation Methodology," and the associated NRC safety evaluation. As a result of the evaluation, APS plans to increase the size of each of the two containment sump screens in each unit from the current area of approximately 210 ft2 to 2,000 ft2.

This increased sump screen size will provide sufficient area to assure emergency core cooling system (ECCS) and containment spray system (CSS) performance by accommodating any blockage that is postulated to occur following a loss of coolant accident (LOCA), and includes sufficient margin for chemical effects that have been estimated but not yet confirmed. Other corrective actions include removal of installed Fiberfrax insulation and changes to PVNGS programs and procedures to ensure A member of the STJARS (Strategic Teaming and Resource Sharing) Alliance

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U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Response to Request No. 2 in GL 2004-02 Page 2 and/or enhance the control of transportable debris in containment. All corrective actions required to assure conformance with the ECCS and CSS regulatory requirements specified in GL 2004-02 will be implemented by December 31, 2007, as directed in the GL. contains a list of regulatory commitments being made in this letter. is the notarized affidavit required by 10 CFR 50.54(f).

If you have any questions, please contact Thomas N. Weber at (623) 393-5764.

Sincerely, GRO/TNW/GAM

Enclosures:

1. Notarized Affidavit
2. APS Response to Request No. 2 in NRC Generic Letter 2004-02
3. List of Regulatory Commitments : Deviations to NEI 04-07 and WCAP-16406-P cc:

B. S. Mallett NRC Region IV Regional Administrator M. B. Fields NRC NRR Project Manager G. G. Warnick NRC Senior Resident Inspector for PVNGS

Enclosure I Notarized Affidavit STATE OF ARIZONA

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COUNTY OF MARICOPA )

1, Gregg R. Overbeck, represent that I am Senior Vice President - Nuclear, that the foregoing document has been signed by me on behalf of Arizona Public Service Company with full authority to do so, and that to the best of my knowledge and belief, the statements made therein are true and correct.

49 R. Overbeck Sworn To Before Me ThisŽ2 Lj...Day Of (

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, 2005.

N6a r ublic SUSIE LYNN ERGISH Notary Public - Arizona M.rLBopa County My'COMM. Expires Jul 14, 2007 Notary Commission Stamp Arizona Public Service Company Response to Request No. 2 in NRC Generic Letter 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors" for Palo Verde Nuclear Generating Station Units 1, 2, and 3 Executive Summary In Generic Letter 2004-02, the NRC identified that research and analysis suggested a potential susceptibility of pressurized water reactor (PWR) recirculation sump screens to debris blockage during design basis accidents requiring recirculation operation of the emergency core cooling system (ECCS) and containment spray system (CSS). The NRC requested that PWR licensees, using an NRC-approved methodology, perform an evaluation of the potential for adverse effects of post-accident debris blockage.

Following the evaluation, and prior to September 1, 2005, licensees were to provide specific information from the results of the evaluation.

APS has completed the evaluation requested by GL 2004-02 for Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2, and 3. The evaluation was performed in accordance with the guidance in NEI 04-07, "Pressurized Water Reactor Sump Performance Evaluation Methodology," and the associated NRC safety evaluation. As a result of the evaluation, APS plans to increase the size of each of the two containment sump screens in each unit from the current area of approximately 210 ft2 to 2,000 ft2.

This increased sump screen size will provide sufficient area to assure emergency core cooling system (ECCS) and containment spray system (CSS) performance by accommodating any blockage that is postulated to occur following a loss of coolant accident (LOCA), and includes sufficient margin for chemical effects that have been estimated but not yet confirmed. Other corrective actions include removal of installed Fiberfrax insulation and changes to PVNGS programs and procedures to ensure and/or enhance the control of transportable debris in containment. All corrective actions required to assure conformance with the ECCS and CSS regulatory requirements specified in GL 2004-02 will be implemented by December 31, 2007, as directed in the GL.

I APS Response to Request No. 2 in GL 2004-02 NRC Generic Letter 2004-02 Information Request 2(a):

Addressees are requested to provide the following information no later than September 1, 2005:

Confirmation that the ECCS and CSS recirculation functions under debris loading conditions are or will be in compliance with the regulatory requirements listed in the Applicable Regulatory Requirements section of this generic letter. This submittal should address the configuration of the plant that will exist once all modifications required for regulatory compliance have been made and this licensing basis has been updated to reflect the results of the analysis described above.

APS Response to Request 2(a):

Activities are currently underway to ensure that the Emergency Core Cooling System (ECCS) and Containment Spray System (CSS) recirculation functions under debris loading conditions at Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2 and 3 will be in full compliance with the regulatory requirements listed in the Applicable Regulatory Requirements section of Generic Letter 2004-02 by December 31, 2007.

This full compliance will be achieved through analysis, mechanistic evaluations, modifications to increase the available sump screen area, changes to the plant to reduce the potential debris loading for the containment recirculation sump strainers, and programmatic and process controls to ensure continued compliance.

By December 31, 2007, installed Fiberfrax insulation will have been removed from PVNGS Units 1, 2, and 3, and new, larger sump strainers will have been installed. The new strainers will increase the available screen area from 210 ft2 in each of the two containment sumps to 2,000 ft2 per sump. The new strainers will occupy the same foot print as the existing strainers.

NRC Generic Letter 2004-02 Information Request 2(b):

A general description of and implementation schedule for all corrective actions, including any plant modifications, that you identified while responding to this generic letter. Efforts to implement the identified actions should be initiated no later than the first refueling outage starting after April 1, 2006. All actions should be completed by December 31, 2007. Provide justification for not implementing the identified actions during the first refueling outage starting after April 1, 2006.

If all corrective actions will not be completed by December 31, 2007, describe how the regulatory requirements discussed in the Applicable Regulatory Requirements section will be met until the corrective actions are completed.

APS Response to Request 2(b):

A description of and implementation schedule for all corrective actions that were identified while responding to Generic Letter 2004-02 are provided below. The 2

APS Response to Request No. 2 in GL 2004-02 corrective actions listed are more fully described in responses to items 2(c), 2(d),

and 2(f).

Description of and Implementation Schedule for Generic Lefter 2004-02 Corrective Actions

1. Evaluate the recommendations contained in the Westinghouse downstream effects evaluation for PVNGS and establish an implementation schedule for appropriate recommendations.

Completion: December 31, 2005

2. Perform confirmatory head-loss testing of new strainer with plant specific debris to ensure an adequate design.

Completion: March 31, 2006

3. Verify that a capture ratio of 97% or higher can be achieved in the final design of the new sump screen to ensure that the fuel evaluation contained in the Westinghouse downstream effects evaluation is bounding.

Completion: March 31, 2006

4. Perform sump strainer structural evaluation to ensure seismic and operational integrity.

Completion: March 31, 2006

5. Validate allocated margins for chemical effects in strainer head-loss to ensure an adequate design.

Completion: May 31, 2006

6. Perform a confirmatory containment latent debris walkdown of PVNGS Units 1 and 3.

Completion: June 30, 2006

7. Perform a confirmatory containment unqualified coating walkdown of PVNGS Units I and 3.

Completion: June 30, 2006

8. Review the existing programmatic controls for containment coatings identified in the response to GL 98-04, "Potential for Degradation of the Emergency Core Cooling System and the Containment Spray System after a Loss-of-Coolant Accident Because of Construction and Protective Coating Deficiencies and Foreign Material in Containment," for their adequacy.

Completion: November 30, 2006 3

APS Response to Request No. 2 in GL 2004-02

9. Review the existing programmatic and procedural controls in place to prevent potentially transportable debris (insulation, signs and foreign material) in the containment building to ensure that the bounding assumptions in the design of the new strainers will be maintained.

Completion: November 30, 2006

10. Implement in Unit 1 changes to programs and procedures to ensure and/or enhance the control of transportable debris in containment.

Completion: May 31, 2007 (1 RI 3 refueling outage)

11. Implement in Unit 2 changes to programs and procedures to ensure and/or enhance the control of transportable debris in containment.

Completion: November 30, 2006 (2R13 refueling outage)

12. Implement in Unit 3 changes to programs and procedures to ensure and/or enhance the control of transportable debris in containment.

Completion: December 31, 2007 (3R13 refueling outage)

13. Install larger sump strainers in PVNGS Unit 1.

Completion: May 31, 2007 (1R13 refueling outage)

14. Install larger sump strainers in PVNGS Unit 2.

Completion: November 30, 2006 (2R13 refueling outage)

15. Install larger sump strainers in PVNGS Unit 3.

Completion: December 31, 2007 (3R13 refueling outage)

16. Remove installed Fiberfrax insulation in PVNGS Unit 1.

Completion: May 31, 2007 (1R13 refueling outage)

17. Remove installed Fiberfrax insulation in PVNGS Unit 2.

Completion: November 30, 2006 (2R13 refueling outage)

18. Remove installed Fiberfrax insulation in PVNGS Unit 3.

Completion: December 31, 2007 (3R13 refueling outage)

By June 30, 2006, after plant specific strainer testing has been completed and the Westinghouse downstream effects evaluation for PVNGS has been evaluated, APS will submit an update to the NRC to report the validation of the allocated margins for chemical effects and identify any recommendations from the Westinghouse evaluation to be implemented.

Due to the time required to complete the baseline evaluation, select a strainer vendor, finalize a strainer installation design, fabricate, and deliver the strainer, all required activities will not be completed to support installation in the Unit 3 outage which is scheduled to begin on April 1, 2006. However, all activities will be complete to support installation in the subsequent PVNGS refueling outage (Unit 2, fall 2006), and since the 4

APS Response to Request No. 2 in GL 2004-02 three PVNGS units are on eighteen month refueling schedules, installation will be complete in all three units by December 31, 2007.

NRC Generic Letter 2004-02 Information Request 2(c):

A description of the methodology that was used to perform the analysis of the susceptibility of the ECCS and CSS recirculation functions to the adverse effects of post-accident debris blockage and operation with debris-laden fluids. The submittal may reference a guidance document (e.g., Regulatory Guide 1.82, Rev. 3, industry guidance) or other methodology previously submitted to the NRC. (The submittal may also reference the response to Item 1 of the Requested Information described above. The documents to be submitted or referenced should include the results of any supporting containment walkdown surveillance performed to identify potential debris sources and other pertinent containment characteristics.)

APS Response to Request 2(c):

The analyses relating to debris inventory, debris generation, debris transport and the resulting head loss were performed by a team from Sargent and Lundy. For the evaluation of downstream effects, a team from Westinghouse was utilized. Both contractors work under 10 CFR 50 Appendix B quality assurance programs.

The analysis method utilized for demonstrating compliance was the Baseline Evaluation Method described in NEI 04-07 and the associated NRC safety evaluation. Further information regarding this approach is provided in subsequent sections of this response.

For the downstream effects evaluation, Westinghouse utilized the methodology in WCAP-16406-P, June 2005, "Evaluation of Downstream Sump Debris Effects in Support of GSI-191." A list of deviations to NEI 04-07 and WCAP-16406-P is provided in Attachment 1.

The features of the PVNGS configuration, containment walkdown activities and the specific approaches used in the analyses performed are summarized below.

PVNGS Containment Sump Configuration Each PVNGS unit has two emergency recirculation sumps in the containment building.

The current sumps are protected with screens consisting of three elements: an outer grating, coarse flat screens and fine mesh screens. The surface area of the fine screen is 210 ft2. In addition each sump has a grating screen at the emergency core cooling system (ECCS) and containment spray system (CSS) pumps common suctions. The current screen hole size is 0.09".

New sump strainers will be installed in the same location as the existing strainers and located on the containment floor directly over the sump pits. The new strainers, to be supplied by Control Components, Inc. (CCI), will incorporate diverse geometry in their design which is less susceptible to thin bed effects than flat screens, will have modular 5

APS Response to Request No. 2 in GL 2004-02 construction which can be easily enlarged if needed, and will be constructed of perforated stainless steel plate. The new strainers are being designed to have approximately 2,000 ft2 of strainer surface area per sump with hole sizes less than 0.09". The new strainers will have no gaskets or soft sealants.

Containment Walkdown Activities APS performed a detailed walkdown for transportable debris in the Unit 2 containment building in spring 2005. The walkdown inventoried the amount and types of materials that could become transportable and could contribute to the sump blockage or cause detrimental effects if allowed to pass the sump strainer. Unit 2, which had replacement steam generators (SGs) installed in 2003, is assumed to be representative of the other units which will have their SG replacements completed by December 31, 2007.

Confirmatory latent debris walkdowns will be performed in Units 1 and 3 during their next refueling outages.

The Unit 2 walkdown identified 119 lbs of latent debris using the sampling methods described in NEI 04-07. It is expected that Units 1 and 3 will also be of that order of magnitude. The associated debris/head loss calculations conservatively use 200 lbs per unit. Other potentially transportable debris was also inventoried in Unit 2 such as fibrous materials insulation, equipment labels and miscellaneous stickers (paper/foil).

Upstream Effects Evaluation The walkdown in Unit 2 assessed the water flow path from potential break location and from containment spray back to the containment sump. The walkdown verified that a clear flow path exists to the sump such that injected water would not be held up and could flow freely back to the sump. During the walkdown no choke points were identified. Unit 2 is of the same design as the Units 1 and 3 and the Unit 2 results are applicable to all PVNGS units.

Break Selection / Debris Generation / Zone of influence / CFD The debris generation calculation considered four potentially limiting breaks inside containment. With only modest amounts of fiber at PVNGS along with the extensive use of reflective metal insulation (RMI) materials and an open area near the steam generators, the consideration of additional break locations was not warranted. For each of the scenarios the quantity of debris that could be generated using the guidance of the NEI 04-07 and the associated NRC safety evaluation was determined. The most limiting beak for debris generation is a hot leg break at the steam generator (SG) closest to the sumps. The debris generation calculation for coatings assumes the zone of influence (ZOI) as a cylindrical shape (which conservatively bounds a spherical shape) extending from the base of the area inside the bio-shield centered on the break and extends 10 diameters above the break location. The calculation assumes that all the RMI within the D-ring of the bio-wall becomes debris.

6 APS Response to Request No. 2 in GL 2004-02 The limiting break for debris transport is a cold leg break at a reactor coolant pump (RCP). The break at an RCP was selected because of the high potential for localized flow rates assumed to exit out of one pump bay. The water flow from the RCS back to the containment sump was modeled with the computational fluid dynamics program, "Fluent 6.1.22," under the Sargent and Lundy quality assurance program. Several flow cases were run and included in the evaluation. For all breaks that will result in a recirculation actuation signal (RAS) which requires the use of the sump, the water level in containment would be above the level of the existing strainers and will also completely cover the new strainers.

For debris transport, it was conservatively assumed that the run-out flow from the CS, LPSI and HPSI pumps for both trains is exiting one pump bay. This was done to maximize debris transport to the sump and thus provide a conservative head loss evaluation.

The debris generated by the limiting high energy line break consists of the materials shown in Table I below. The limiting break is a steam generator (S/G) no. 2 hot leg break at the S/G nozzle which generates the most debris.

Debris Transport The debris generation calculation is used as input for the debris transport evaluation.

The transport evaluation assumes that all of the fine material such as latent debris, affected coatings, and miscellaneous latent debris is transported to the screens.

Strainer Head-loss The evaluation calculated a minimum new strainer size. The minimum new strainer surface area was calculated to be 1,375 ft2 of flat screen for each sump, assuming the removal of installed Fiberfrax insulation. The evaluation was based on approximately 6 ft of head loss. The calculated minimum screen area does not include 250 ft2 of sacrificial screen that will be added to accommodate foreign material nor accounts for chemical effects that are discussed in response 2(d)(iii). As a conservative measure, the new strainers are being designed to have 2,000 ft2 of diverse geometry surface area per sump. The diverse geometry surface area has been shown to be less susceptible to thin bed effects than flat screens.

Downstream Effects Evaluation Westinghouse utilized the WCAP-16406-P methodology and its associated acceptance criteria to evaluate downstream ECCS and CSS components. Guidance from NEI 04-07 was used in conjunction with WCAP-16406-P to perform the downstream effects evaluation. The generic methodology provided in these documents is applicable to the PVNGS specific evaluation with the deviations noted in Attachment 1.

7 APS Response to Request No. 2 in GL 2004-02 Table I Generated and Transported to the Sump Bounding Debris Debris Type Units Fraction of Debris Debris Debris at Generated Quantity at Sump Screen Sump Insulation Within Break ZOI SS Mirror RMI Foil ft2 0.016 23,827 381 SS Transco RMI Foil ft2 0.750 30,338 22,754 Nukon ft3 0.650 11.5 7.5 Temp-Mat ft3 Not generated for Break S1 Thermo-lag 330 (fiber) f 3 1.00 0.25 0.25 Thermo-lag 330 ft3 1.00 4.75 4.75 (particulate)

CS Generated Debris l

Alpha-cloth ft3 1.00 0.1 0.1 Coatings

=

Qualified IOZ ft3 1.00 4.0 4.0 Qualified Epoxy ft3 1.00 7.0 7.0 Unqualified IOZ ft3 1.00 1.42 1.42 Unqualified Epoxy ft3 1.00 0.98 0.98 Unqualified Alkyds ft3 1.00 0.25 0.25 Damaged Qualified IOZ ft3 1.00 0.01 0.01 Foreign Materials Foreign Materials ft2 1.00 308.69 308.69 Light bulb particulate ft3 1.00 0.10 0.10 Latent Debris Latent Debris Ibm 1.00 200 200 1 19 Ibm identified in U2 NRC Generic Letter 2004-02 Information Request 2(d)(i):

The submittal should include, at a minimum, the following information:

The minimum available NPSH margin for the ECCS and CSS pumps with an unblocked sump screen.

8 APS Response to Request No. 2 in GL 2004-02 APS Response to Request 2(d)(i):

The NPSH margins for the existing design are shown in Table 2. The current design margins include a calculated head loss of 6.06 ft for a debris loaded strainer. The new design will allot approximately the same 6 ft of head loss for a debris loaded strainer and the new resulting NPSH margins will be greater than or equal to the existing design.

The supplier of the new strainers will confirm clean strainer head loss, which is expected to be very low, as part of the strainer design.

Table 2 LPSI, HPSI and CS Pumps NPSH Margin Pump Flow Rates NPSH NPSH NPSH (LPSIIHPSIICS)

Available1 Required Margin (gPm)

(ft)

(ft)_(ft)

LPSI Train A 5,000 /1,400 / 5,200 26.1 20 6.1 LPSI Train B 5,000 / 1,400 / 5,200 26.3 20 6.3 HPSI Train A 5,000 / 1,400 / 5,200 28.8 25 3.8 HPSI Train B 5,000 / 1,400 / 5,200 29.2 25 4.2 CS Train A 5,000 / 1,400 / 5,200 25.9 22 3.82 CS Train B 5,000 / 1,400 / 5,200 26.2 22 4._12 Note 1: Includes a head loss due to debris (screen blockage) of 6.06 ft.

Note 2: Discrepancies due to truncation.

NRC Generic Letter 2004-02 Information Request 2(d)(ii):

The submerged area of the sump screen at this time and the percent of submergence of the sump screen (i.e., partial or full) at the time of the switchover to sump recirculation.

APS Response to Request 2(d)(ii):

At the time of switchover to recirculation, 100% (2,000 ft2) of the sump screen will be submerged.

NRC Generic Letter 2004-02 Information Request 2(d)(iii):

The maximum head loss postulated from debris accumulation on the submerged sump screen, and a description of the primary constituents of the debris bed that result in this head loss. In addition to debris generated by jet forces from the pipe rupture, debris created by the resulting containment environment (thermal and chemical) and CSS washdown should be considered in the analyses. Examples of this type of debris are disbanded coatings in the form of chips and particulates and chemical precipitants caused by chemical reactions in the pool.

9 APS Response to Request No. 2 in GL 2004-02 APS Response to Request 2(d)(iii):

The ECCS pumps NPSH margins are listed in Table 2 in response 2(d)(i). The margins include a calculated head loss of 6.06 ft for a debris loaded strainer. The new design will allot approximately 6 ft of head loss for a debris loaded strainer and will preserve the existing margin. The primary constituents of the debris bed that result in this head loss include fiberglass fibers, coating particulates, latent debris particles and latent debris fibers. Note that the head-loss calculation takes into account only the fibrous and particulate debris. Appropriate additional screen area is being provided for foreign material such as labels/tags/cable ties/tape.

The primary constituents of the debris bed for the limiting break are outlined in Table 1 in response 2(c).

Chemical Effects Considerations PVNGS uses Trisodium Phosphate (TSP) as a buffering agent. Also the PVNGS units are predominantly RMI plants with some fibrous insulation. To address the chemical effects for the sump strainer design, available information from the integrated chemical effects test (ICET) no. 2 was utilized. The 2,000 ft2 surface area design of the new strainers provides an adequate amount of additional sump screen area to account for chemical effects. Additionally other margins that may be available have been identified and are provided in Table 3 below.

Table 3 Available Margins Analytical Margins Estimated Margin

(% of head-loss)

Margin retained in head-loss calculation 69%

Qualified coatings zone of influence (ZOI) 17%

reduction (i.e., 1 OD to 5D)

Operational Margin I

Reduced flow through the sump screen 43%

As shown in Table 3, significant margin is available in the existing head-loss calculation itself and is readily available (4 feet of available margin in NPSH which is not used in the strainer head-loss calculation). The other two margins can also be easily pursued if needed. Also, in addition to the above margins, some screen area is being allocated to account for chemical effects in the new strainer design as stated above. Finally, the new strainer design is modular in nature and provides the flexibility of easily adding screen area in the future if needed.

Based on the above, it is concluded that sufficient margin has been included in the containment sump evaluation and design to account for chemical effects. The allocated margins for chemical effects will be validated by June 30, 2006.

10 APS Response to Request No. 2 in GL 2004-02 NRC Generic Letter 2004-02 Information Request 2(d)(iv):

The basis for concluding that the water inventory required to ensure adequate ECCS or CSS recirculation would not be held up or diverted by debris blockage at choke-points in containment recirculation sump return flowpaths.

APS Response to Request 2(d)(iv):

The walkdown in Unit 2 assessed the water flow path from potential break location and from containment spray back to the containment sump. The walkdown verified that a clear flow path exists to the sump such that injected water would not be held up and could flow freely back to the sump. During the walkdown no choke points were identified. Unit 2 is of the same design as the Units I and 3 and the Unit 2 results are applicable to all PVNGS units.

NRC Generic Letter 2004-02 Information Request 2(d)(v):

The basis for concluding that inadequate core or containment cooling would not result due to debris blockage at flow restrictions in the ECCS and CSS flowpaths downstream of the sump screen, (e.g., a HPSI throttle valve, pump bearings and seals, fuel assembly inlet debris screen, or containment spray nozzles). The discussion should consider the adequacy of the sump screen's mesh spacing and state the basis for concluding that adverse gaps or breaches are not present on the screen surface.

APS Response to Request 2(d)(v):

An evaluation of the downstream impact of sump debris on the performance of the emergency core cooling system (ECCS) and containment spray system (CSS) following a loss of coolant accident (LOCA) has been completed. The evaluation considered the effect of debris ingested through the containment sump screen on the following components required to operate:

  • Piping and Instrument Tubing
  • Reactor Vessel Internals
  • Nuclear Fuel The PVNGS evaluation used a conservative evaluation approach which considered larger sized debris to pass through the sump screen than the actual size of the holes in the sump screen. This was done to maximize the adverse consequences of debris-laden fluid on ECCS and CSS components downstream of the sump. Deformable 11 APS Response to Request No. 2 in GL 2004-02 objects of up to two times a sump screen hole size of 0.09" are assumed to pass through the sump screen, and are assumed to deform to pass through any downstream clearance equal to or larger than the sump screen hole size. The new sump screen design will have holes less than 0.09".

The basis for concluding that inadequate core or containment cooling would not result from the debris-laden fluid effects is that the acceptance criteria of WCAP-1 6406-P are met by evaluation (verification as per 2.d.vi below) or by plant modifications (as per 2.b above).

The new strainers will be fabricated from perforated stainless steel plates that are shaped into strainer modules. The new modules are to be fabricated to close tolerances. The final installation on site will include appropriate inspection criteria to ensure no particulate bypass flow exists with dimensions greater than the screen perforation opening. The modules can be easily inspected visually both externally and internally since they can be easily removed by hand.

NRC Generic Letter 2004-02 Information Request 2(d)(vi):

Verification that close-tolerance subcomponents in pumps, valves and other ECCS and CSS components are not susceptible to plugging or excessive wear due to extended post-accident operation with debris-laden fluids.

APS Response to Request 2(d)(vi):

The PVNGS downstream effects evaluation supports the following conclusions for the components identified in response 2(d)(v) above.

Valves The valves were evaluated for plugging and erosion. The results of the evaluation confirmed that plugging and erosion will not be a concern for PVNGS ECCS and CSS valves. The evaluation included a review to assess the potential for sedimentation in the valves and no sedimentation concerns were identified.

The Westinghouse downstream evaluation contained a recommendation regarding potential emergency operating procedure changes. As stated in response 2(b), APS will evaluate the recommendations contained in the Westinghouse downstream effects evaluation and establish an implementation schedule for appropriate recommendations by December 31, 2005.

Pumps The results of the pump hydraulic wear evaluation show that neither the ECCS nor the CSS pumps' wear gaps increase to the point of causing a hydraulic performance concern.

12 APS Response to Request No. 2 in GL 2004-02 The pump mechanical evaluation identified that the wear ring gap increase due to the erosive and abrasive debris would be within the WCAP-16406-P acceptance criteria.

Therefore, there is no expected pump vibration concern due to the containment sump debris.

The mechanical shaft seal assembly performance evaluation resulted in one recommendation with regard to the replacement of the pumps' carbon/graphite seal backup bushings with a more wear resistant material, such as bronze. As stated in response 2(b), APS will evaluate the recommendations contained in the Westinghouse downstream effects evaluation and establish an implementation schedule for appropriate recommendations by December 31, 2005.

Heat Exchangers The Shutdown Cooling heat exchanger tube plugging evaluation demonstrated that the tube ID is larger than the anticipated debris particle size. Consequently, tube plugging will not occur. The heat exchanger wear evaluation demonstrated that, because the actual wall thickness minus the thickness lost to erosion is greater than the wall thickness required to retain system pressure, tube failure due to erosion is not a concern.

Nozzles and Orifices The spray nozzle plugging evaluation demonstrated that the orifice bore diameter is larger than the anticipated debris particle size. Consequently, plugging will not occur.

Also, for the spray nozzle wear evaluation, the increase in spray nozzle flow rate due to an increased orifice diameter is within the WCAP-16406-P acceptance criteria limits.

Consequently, the nozzles will perform their design basis functions.

The orifice plugging evaluation demonstrated that no orifice bore size is smaller than the largest particle that could pass through the sump strainer, therefore plugging is not a concern. The findings of the orifice wear evaluation concluded that the orifices were in compliance with the WCAP-16406-P acceptance criteria. Therefore, the orifices will perform their design basis functions.

Instrument Lines The instrumentation tubing evaluation demonstrated that the transverse ECCS recirculation flow velocity meets the WCAP-16406-P acceptance criteria to prevent debris settlement. Consequently, debris settlement does not occur and the instrumentation will perform its design basis functions.

Reactor Vessel Internals The smallest flow clearance found in the reactor vessel internals evaluation is 0.75",

which means that any replacement sump screen hole size smaller than 0.37" will not cause plugging by either deformable or non-deformable debris. The hole size in the new sump strainers is designed to be less than 0.09".

13 APS Response to Request No. 2 in GL 2004-02 Nuclear Fuel Analysis of several large-break LOCA scenarios has shown that the fiber concentration in the containment sump is sufficient to create a thin bed on the underside of the fuel bottom nozzle following a hot-leg break LOCA. Sensitivity studies showed that lowering the amount of bypass through the sump screen from 5% to 3% was sufficient to prevent a thin bed from occurring during a hot-leg break. As stated in response 2(b),

verification that a capture ratio of 97% or higher can be achieved in the final design of the new sump screen to ensure that the fuel evaluation contained in the Westinghouse downstream effects evaluation is bounding will be completed by March 31, 2006.

NRC Generic Letter 2004-02 Information Request 2(d)(vii):

Verification that the strength of the trash racks is adequate to protect the debris screens from missiles and other large debris. The submittal should also provide verification that the trash racks and sump screens are capable of withstanding the loads imposed by expanding jets, missiles, the accumulation of debris, and pressure differentials caused by post-LOCA blockage under predicted flow conditions.

APS Response to Request 2(d)(vii):

The new strainers will be installed in locations that are remote to high energy line breaks and are located outside of the bio-shield wall. The new strainers have an integral trash rack that provides additional module strength and durability. The detailed design will verify that the new strainer will not be subjected to high energy line breaks and the ability of the strainer to withstand high differential pressure under the most severe debris loading.

NRC Generic Letter 2004-02 Information Request 2(d)(viii):

If an active approach (e.g., backflushing, powered screens) is selected in lieu of or in addition to a passive approach to mitigate the effects of the debris blockage, describe the approach and associated analyses.

APS Response to Request 2(d)(viii):

An active approach will not be used at Palo Verde.

NRC Generic Letter 2004-02 Information Request 2(e):

A general description of and planned schedule for any changes to the plant licensing bases resulting from any analysis or plant modifications made to ensure compliance with the regulatory requirements listed in the Applicable Regulatory Requirements section of this generic letter. Any licensing actions or exemption requests needed to support changes to the plant licensing basis should be included.

14 APS Response to Request No. 2 in GL 2004-02 APS Response to Request 2(e):

The revised containment sump performance analysis and the containment sump screen design changes will be implemented under the provisions of 10 CFR 50.59. The associated Updated Final Safety Analysis Report (UFSAR) changes and any needed Technical Specification (TS) Bases changes and Technical Requirements Manual (TRM) changes will be identified as part of the design change preparations prior to implementation of the changes in the first PVNGS unit in fall 2006. The UFSAR will be updated with these changes in accordance with 10 CFR 50.71(e) following implementation of the design changes. Any needed TS Bases changes would be incorporated in accordance with the TS Bases control program (TS 5.5.14). Any needed TRM changes would be incorporated in accordance with the TRM control program (UFSAR 13.7).

At this time, no licensing actions or exemption requests requiring prior NRC approval are expected to be needed to support changes to the plant licensing bases.

NRC Generic Lefter 2004-02 Information Request 2(f0:

A description of the existing or planned programmatic controls that will ensure that potential sources of debris introduced into containment (e.g., insulations, signs, coatings, and foreign materials) will be assessed for potential adverse effects on the ECCS and CSS recirculation functions. Addressees may reference their responses to GL 98-04, "Potential for Degradation of the Emergency Core Cooling System and the Containment Spray System after a Loss-of-Coolant Accident Because of Construction and Protective Coating Deficiencies and Foreign Material in Containment," to the extent that their responses address these specific foreign material control issues.

APS Response to Request 2(f):

APS will review the existing programmatic controls for containment coatings identified in the response to GL 98-04, "Potential for Degradation of the Emergency Core Cooling System and the Containment Spray System after a Loss-of-Coolant Accident Because of Construction and Protective Coating Deficiencies and Foreign Material in Containment," for their adequacy.

The existing programmatic and procedural controls in place to prevent potentially transportable debris (insulation, signs and foreign material) in the containment building will be reviewed to ensure that the bounding assumptions in the design of the new strainers will be maintained.

15 Regulatory Commitments The following table identifies those new actions committed to by APS in this document.

Any other statements in this submittal are provided for information purposes and are not considered to be regulatory commitments. Please direct questions regarding these commitments to Thomas N. Weber at (623) 393-5764.

REGULATORY COMMITMENT DUE DATE

1. Evaluate the recommendations contained in the December 31, 2005 Westinghouse downstream effects evaluation for PVNGS and establish an implementation schedule for appropriate recommendations.
2. Perform confirmatory head-loss testing of new March 31, 2006 strainer with plant specific debris to ensure an adequate design.
3. Verify that a capture ratio of 97% or higher can be March 31, 2006 achieved in the final design of the new sump screen to ensure that the fuel evaluation contained in the Westinghouse downstream effects evaluation is bounding.
4. Perform sump strainer structural evaluation to ensure March 31, 2006 seismic and operational integrity.
5. Validate allocated margins for Chemical Effects in May 31, 2006 strainer head-loss to ensure an adequate design.
6. Perform a confirmatory containment latent debris June 30, 2006 walkdown of PVNGS Units 1 and 3.
7. Perform a confirmatory containment unqualified June 30, 2006 coating walkdown of PVNGS Units 1 and 3.
8. Review the existing programmatic controls for November 30, 2006 containment coatings identified in the response to GL 98-04, "Potential for Degradation of the Emergency Core Cooling System and the Containment Spray System after a Loss-of-Coolant Accident Because of Construction and Protective Coating Deficiencies and Foreign Material in Containment," for their adequacy.

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REGULATORY COMMITMENT DUE DATE

9. Review the existing programmatic and procedural November 30, 2006 controls in place to prevent potentially transportable debris (insulation, signs and foreign material) in'the containment building to ensure that the bounding assumptions in the design of the new strainers will be maintained.
10. Implement in Unit 1 changes to programs and May 31, 2007 procedures to ensure and/or enhance the control of (1 R13 refueling outage) transportable debris in containment.
11. Implement in Unit 2 changes to programs and November 30, 2006 procedures to ensure and/or enhance the control of (2R13 refueling outage) transportable debris in containment.
12. Implement in Unit 3 changes to programs and December 31, 2007 procedures to ensure and/or enhance the control of (3R13 refueling outage) transportable debris in containment.
13. Install larger sump strainers in PVNGS Unit 1.

May 31, 2007 (1R13 refueling outage)

14. Install larger sump strainers in PVNGS Unit 2.

November 30, 2006 (2R13 refueling outage)

15. Install larger sump strainers in PVNGS Unit 3.

December 31, 2007 (3R13 refueling outage)

16. Remove installed Fiberfrax insulation in PVNGS May 31, 2007 Unit 1.

(1 R1 3 refueling outage)

17. Remove installed Fiberfrax insulation in PVNGS November 30, 2006 Unit 2.

(2R13 refueling outage)

18. Remove installed Fiberfrax insulation in PVNGS December 31, 2007 Unit 3.

(3R13 refueling outage)

19. By June 30, 2006, after plant specific strainer testing June 30, 2006 has been completed and the Westinghouse downstream effects evaluation for PVNGS has been evaluated, APS will submit an update to the NRC to report the validation of the allocated margins for chemical effects and identify any recommendations from the Westinghouse evaluation to be implemented.

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Attachment I Deviations to NEI 04-07 and WCAP-16406-P The following deviations from the published guidance have been taken for the PVNGS evaluations:

1.

Deviation from the debris generation calculation (Deviation to NEI 04-07)

Section 3.4.2 of Volume 1 of NEI-04-07 and the associated NRC safety evaluation (SE)

(Volume 2 of NEI 04-07) specifies that the zone of influence (ZOI) should be spherical for the purposes of debris generation. For the PVNGS evaluations, a cylindrical ZOI was used that encompassed the spherical ZOI. This was done as a modeling aid and is conservative. This is not considered an exception to NEI 04-07.

2.

Deviation from the debris generation calculation (Deviation to NEI 04-07)

Section 3.3.5.2 of the NRC SE describes a systematic approach to break selection along individual piping runs that starts at an initial location along a pipe, generally a terminal end, and steps along in equal 5-ft. increments, placing breaks at each sequential location. The staff notes in the SE that the concept of equal increments is only a reminder to be systematic and thorough. Section 3.3.5.2 of the SE goes on to say that the key difference between many breaks (especially large breaks) will not be the exact location along the pipe, but rather the envelope of containment material targets that is affected. At PVNGS, the exact break location along the pipe is not critical. This is because the envelope of affected containment material targets is defined by the D-ring or pressurizer enclosure, and nearly all RMI insulation in the D-ring and Pressurizer enclosure will be within the ZOI for a break in the corresponding location based on the large ZOI radius for mirror insulation.

3.

Deviation from the Downstream Effects Evaluation (Deviation to NEI 04-07)

Section 3.4.2.1 of the NRC SE contains a requirement for licensees to assume that all unqualified coatings fail and become 10 pm particulate. Although this requirement is conservative when evaluating head loss across a sump screen for which a "thin bed" effect is possible, it is not conservative when evaluating wear on the downstream components and valves.

The Westinghouse wear evaluation of ECCS valves and components assumes an unqualified coating particulate size distribution that varies from 110% of the sump screen opening to 10 pm. This assumption is reasonable and conservative when evaluating the impact of unqualified coatings particulate on component and valve wear.

There is significant public-domain documentation that shows that coatings outside the conditions defined in the ZOI will tend to fail at sizes larger than their constituent pigment size.

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Attachment I Deviations to NEI 04-07 and WCAP-1 6406-P

4.

Deviation from the Downstream Effects Evaluation (Deviation to WCAP-16406-P)

The PVNGS evaluation included two areas where the application deviated from the guidance in WCAP-16406-P. The areas are as follows:

  • The concern for instrumentation tubing is the potential for debris collection in the instrument sensing lines. The instruments of concern are those which are connected to the recirculation flow path through the ECCS or CSS and which must function post-accident to support application of the emergency operating procedures (EOPs). For debris to enter the sensing lines, the flow in the process line must have insufficient velocity to maintain the debris in suspension.

The flow rates in the ECCS lines during the plant's recirculation mode are compared to the debris terminal settling velocity to evaluate the potential for debris settling. As long as the transverse velocity of the ECCS flow is 7 times that of the debris terminal settling velocity, the debris will be maintained in suspension and will not settle into the instrumentation tubing. WCAP-16406-P, Table 8.6-1 provides terminal settling velocities for various debris materials. The most limiting (highest) settling velocity in this table is 0.37 ft/sec for stainless steel. However, section 8.2 of WCAP-16406-P, which describes valve evaluations, states that the flow rate must be greater than 0.42 ft/sec to avoid debris settling. Given that this velocity is higher than any of those provided in WCAP-1 6406-P, Table 8.6-1, it is conservative to apply this as the acceptance criterion to instrument lines. Therefore, this more conservative acceptance criterion has been applied to the instrumentation line evaluation.

  • Important assumptions in the fuel evaluation are the amount of debris that is captured by the sump screen, and the amount of debris that is captured at the underside of the fuel bottom nozzle (core inlet). This amount is called the capture efficiency. The only data that exists to quantify these capture efficiencies is the Los Alamos Report LA-UR-04-5416, "Screen Penetration Test Report," dated November 2004. This report was used as the basis for the base case assumptions. However, data in the report can be used to justify other capture efficiencies. In order to understand the effects of different capture efficiencies, three sensitivity cases were run in addition to the base case. These sensitivity cases varied the capture efficiencies as shown in the table below.

Case Screen Capture Core Inlet Capture Efficiency (%)

Efficiency (%)

Base Case 95 95 Sensitivity Case 1 97 95 Sensitivity Case 2 95 50 Sensitivity Case 3 97 50 Note: It can be concluded from the table above that for capture efficiency greater than 97%, further evaluation is not required.

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