ML073380152

Generic Letter 2004-02 Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized Water Reactors Extension Request Evaluation

ML073380152
Person / Time
Site:	Watts Bar
Issue date:	12/06/2007
From:	Boyce T NRC/NRR/ADRO/DORL/LPLII-2
To:	Campbell W Tennessee Valley Authority
Bowen, Jeremy NRR/ADRO/DORL/LPL2-2
References
GL-04-002, TAC MC4730
Download: ML073380152 (11)

Text

December 6, 2007 Mr. William R. Campbell, Jr.

Chief Nuclear Officer and Executive Vice President Tennessee Valley Authority 6A Lookout Place 1101 Market Street Chattanooga, TN 37402-2801

SUBJECT:

WATTS BAR NUCLEAR PLANT, UNIT 1 - GENERIC LETTER 2004-02, POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED WATER REACTORS, EXTENSION REQUEST EVALUATION (TAC NO. MC4730)

Dear Mr. Campbell:

By letter dated August 1, 2007, revised by letter dated October 2, 2007, Tennessee Valley Authority (TVA, the licensee) submitted a request for an extension of the completion date for Generic Letter (GL) 2004-02 corrective actions at the Watts Bar Nuclear Plant, Unit 1 (WBN-1).

The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the licensees request and concludes that it is acceptable to extend the due date for completion of thermal insulation modifications until the completion of the WBN-1 spring 2008 refueling outage, scheduled to begin February 10, 2008. Should the licensee elect to begin the outage more than 30 days after February 10, 2008, TVA will need to provide the NRC additional justification for further delay in completing corrective actions for GL 2004-02.

Further details on the bases for the NRC staffs conclusions are contained in the enclosed evaluation. If you have any questions regarding this issue, please feel free to contact the WBN-1 Project Manager, Brendan Moroney, at (301) 415-3974.

Sincerely,

/RA/

Thomas H. Boyce, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-390

Enclosure:

Evaluation cc w/enclosure: See next page

ML073380152 NRR-106 OFFICE LPL2-2/PM LPL2-2/PM LPL2-2/LA SSIB/BC LPL2-2/BC NAME JBowen BMoroney RSola MScott by TBoyce memo dated DATE 12/05/07 12/05/07 12/05/07 11/21/07 12/06/07 William R. Campbell, Jr.

Tennessee Valley Authority WATTS BAR NUCLEAR PLANT cc:

Mr. Gordon P. Arent Ms. Beth A. Wetzel, Manager New Generation Licensing Manager Corporate Nuclear Licensing and Tennessee Valley Authority Industry Affairs 5A Lookout Place Tennessee Valley Authority 1101 Market Street 4X Blue Ridge Chattanooga, TN 37402-2801 1101 Market Street Chattanooga, TN 37402-2801 Mr. Ashok S. Bhatnagar Senior Vice President Mr. Masoud Bajestani, Vice President Nuclear Generation Development Watts Bar Unit 2 and Construction Watts Bar Nuclear Plant Tennessee Valley Authority Tennessee Valley Authority 6A Lookout Place P.O. Box 2000, EQB 1B 1101 Market Street Spring City, TN 37381 Chattanooga, TN 37402-2801 Mr. Michael K. Brandon, Manager Mr. James R. Douet Licensing and Industry Affairs Vice President Watts Bar Nuclear Plant Nuclear Support Tennessee Valley Authority Tennessee Valley Authority P.O. Box 2000 3R Lookout Place Spring City, TN 37381 1101 Market Street Chattanooga, TN 37402-2801 Mr. Michael J. Lorek, Plant Manager Watts Bar Nuclear Plant Mr. H. Rick Rogers Tennessee Valley Authority Vice President P.O. Box 2000 Nuclear Engineering & Technical Services Spring City, TN 37381 Tennessee Valley Authority 3R Lookout Place Senior Resident Inspector 1101 Market Street Watts Bar Nuclear Plant Chattanooga, TN 37402-2801 U.S. Nuclear Regulatory Commission 1260 Nuclear Plant Road Mr. Michael D. Skaggs, Site Vice President Spring City, TN 37381 Watts Bar Nuclear Plant Tennessee Valley Authority County Executive P.O. Box 2000 375 Church Street Spring City, TN 37381 Suite 215 Dayton, TN 37321 General Counsel Tennessee Valley Authority County Mayor 6A West Tower P. O. Box 156 400 West Summit Hill Drive Decatur, TN 37322 Knoxville, TN 37902 Mr. Lawrence E. Nanney, Director Mr. John C. Fornicola, Manager Division of Radiological Health Nuclear Assurance Dept. of Environment & Conservation Tennessee Valley Authority Third Floor, L and C Annex 3R Lookout Place 401 Church Street 1101 Market Street Nashville, TN 37243-1532 Chattanooga, TN 37402-2801 Mr. Larry E. Nicholson, General Manager Performance Improvement Tennessee Valley Authority 4X Blue Ridge 1101 Market Street Chattanooga, TN 37402-2801

EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION GSI-191/GL2004-02 EXTENSION REQUEST TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 1 DOCKET NO. 50-390 In a letter dated August 1, 2007, revised by letter dated October 2, 2007, Tennessee Valley Authority (TVA) requested an extension to the corrective action due date of December 31, 2007, stated in NRC Generic Letter 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation During Design-Basis Accidents at Pressurized Water Reactors" (GL 2004-02), for the Watts Bar Nuclear Plant, Unit 1 (WBN-1). In the letter, TVA stated that it had partially completed the corrective actions associated with GL 2004-02 during the WBN-1 fall 2006 refueling outage. However, as a result of a revised debris generation analysis, the inventory of fiber Min-K insulation and 3M fire-wrap had increased with respect to the quantity represented in the WBN-1 strainer head loss testing program. Therefore, TVA has determined that Min-K insulation on certain components needs to be replaced with reflective metal insulation (RMI), and that some Min-K needs to be secured with additional bands to prevent damage. TVA plans to perform this work during the refueling outage scheduled to start February 10, 2008.

As stated in SECY-06-0078, proposed extensions to permit changes at the next outage of opportunity after December 2007 may be granted if, based on the licensees request, the staff determines that:

1. The licensee has a plant-specific technical/experimental plan with milestones and schedules to address outstanding technical issues with enough margin to account for uncertainties, and

2. The licensee identifies mitigative measures to be put in place prior to December 31, 2007, and adequately describes how these mitigative measures will minimize the risk of degraded emergency core cooling system (ECCS) and containment spray system (CSS) functions during the extension period.

SECY-06-0078 also states that for proposed extensions beyond several months, a licensees request will more likely be accepted if the proposed mitigative measures include temporary physical improvements to the ECCS sump or materials inside containment to better ensure a high level of ECCS sump performance.

In regard to the first criterion for approving an extension, WBN-1 has a plant-specific plan, with milestones and schedules, to complete the GL 2004-02 required corrective actions and modifications by February 2008. Specifically, during the refueling outage scheduled to start on February 10, 2008, TVA plans to replace some Min-K insulation with RMI insulation and add restraint bands to Min-K insulation that cannot be replaced.

Enclosure

In regard to the second criterion for approving an extension, TVA has stated that the following modifications, mitigating measures, compensatory measures, and/or favorable conditions are in effect at WBN-1, minimizing the risk of degraded ECCS and CSS functions during the extension period:

1. All planned modifications (except for the modifications to the Min-K insulation described herein) required to comply with the requirements of GL 2004-02 have been completed.

The major modifications are as follows:

• A new strainer with smaller opening size and larger flow area has been installed to accommodate the loss-of-coolant accident (LOCA) generated debris load. The new strainer has 0.085-inch diameter perforations and a flow area of approximately 4600 square feet (ft2) whereas the original screen had 1/4-inch rectangular mesh and 200 ft2 flow area. The small size selected for the perforations in the new strainer is to prevent large material with a potential to block [coolant flow paths through the] fuel, ECCS injection pathways, or the containment spray nozzles from bypassing the strainer.

• The size of each orifice in the high head injection flow paths has been reduced to allow for increased seat openings in the throttle valves. As a result, the potential for throttle valve clogging is reduced. The orifice sizes and valve openings are, at minimum, 50 percent larger than the openings in the sump strainer.

• The LOCA-generated coatings debris source term has been reduced as a result of the installation of uncoated replacement steam generators.

2. An emergency operating procedure has been prepared to manage ECCS sump performance during a LOCA. The procedure provides guidance to the operators for monitoring the containment sump and ECCS and CSS pumps for sump blockage and for the ability to deliver the necessary flow. To reduce the rate of debris transport to the sump strainer, the procedure provides guidance on reducing containment spray and ECCS flow to single-train operation. The reduction in flow and accumulated debris results in a reduced pressure drop across the sump strainer. The procedure also provides for refill of the refueling water storage tank to make additional water available for core cooling in the event of sump blockage.

3. A formal program for inspecting and cleaning areas inside containment has been implemented to control the quantity of latent debris. To ensure that no loose debris is left in containment, containment access and housekeeping procedures provide guidelines for entering/exiting containment and specify acceptance criteria for cleanliness.

4. Design features that help minimize the possibility of sump strainer and fuel channel blockage have been incorporated into WBN-1. Specifically:

• The containment sump is located in the containment floor below the refueling canal, shielded from high-energy pipe failures.

• The lower containment is an open, one-level area, with features that channel recirculation flow in a manner that minimizes the transport of debris to the ECCS sump strainer. The only drains which are used to route water to the sump from upper containment are the two large refueling cavity drains and the twenty ice condenser floor drains. These drains route water away from the sump strainers, thereby enhancing the settling of debris.

• The new ECCS sump strainers are located in the annular space between the reactor biological shield wall and the divider wall located beneath the refueling canal. There are only two water entry points in lower containment leading to the sump strainers, one on each side of the strainer. Therefore, if a break were to occur near one of the sump entry points, water would have to travel around in this annulus area to the opposite entry point, allowing the bulk of the debris to settle and not reach the strainer.

• The high containment pool water level in this ice condenser containment provides large margins in available net positive suction head (NPSH). This feature, coupled with the low head losses established during strainer testing, indicates that the ECCS pumps have a large safety margin.

• Sodium tetraborate is used as the buffering agent, sump water temperature is relatively low, and a relatively neutral sump pH is maintained throughout the accident period. These attributes limit the corrosion of light metals and, therefore, limit chemical effects. Testing performed to date supports the conclusion that chemical effects are not a significant issue at WBN-1.

• Surveillance Instruction 1-SI-304-2, 18 Month ECCS Containment Sump Inspection, is performed during each refueling outage to ensure that the containment sump suction pit is free of debris and that the sump components, including strainers, show no evidence of degradation.

• Surveillance Instruction 1-SI-72-3, Containment Refueling Canal Drains, is performed every 92 days to ensure that the refueling canal drains that direct the flow of containment spray water from the upper compartment to the lower compartment (accessible during plant operation) are not blocked.

• Surveillance Instruction 1-SI-61-9, 18 Months Ice Condenser Floor Drains Visual Inspection, is performed during each refueling outage to ensure that the twenty ice-condenser floor drains to lower containment and associated piping and valves are free of ice, frost, or debris and that each valve seat is free of corrosion, pitting or cracking.

• Procedure 1-TRI-40-901, Accumulator Rooms 3 and 4 Open Ended Crane Wall Drains ASME Section XI Unimpaired Flow Test, is performed every other outage to verify that the two floor drains located in accumulator Rooms 3 and 4 are clear and free-flowing. These two floor drains return water to the area inside the polar-crane wall.

Also, Surveillance Instruction 1-SI-304-2, 18 Month ECCS Containment Sump Inspection, in conjunction with the sump screen inspection, requires inspection of these drains for no blockage each refueling outage.

TVA performed a risk evaluation and concluded that the probability of a Large Break LOCA (LBLOCA) resulting in core damage during a two-month extension is less than 1E-06. The risk evaluation conservatively assumed that fibrous material, released from the existing Min-K insulation inside containment, resulted in a debris layer on the containment sump screen sufficient to cause the loss of NPSH to the ECCS pumps, and subsequent core damage.

Because an LBLOCA is assumed to result in core damage, the probability of core damage is assumed to be equal to the probability of an LBLOCA -- 2.67E-06 per year (calculated in the WBN-1 PRA model) times the fraction of a year the extension is in effect (planned for less than two months). Therefore, the probability of core damage during the extension period is less than 1E-06.

TVA stated that the assumption that the LBLOCA results in sump blockage and core damage is conservative because mitigative measures to deal with sump blockage were not credited. The staff also notes that the presence of additional insulation in the debris inventory does not necessarily mean that the WBN-1 strainer will not perform acceptably during an LBLOCA. Rather, the licensee has not tested the strainers performance with this inventory. The licensees decision to remove or remediate the unrepresented quantity of insulation, rather than retesting, is an acceptable (and conservative over the long term) method for addressing the discrepancy between the quantity of fiber represented in the strainer head-loss test and the quantity of debris estimated to be generated during a LOCA.

The NRC has confidence that TVAs plan, as described herein, will result in the installation of final GSI-191 modifications that provide acceptable strainer function with adequate margin for uncertainties. Further, the NRC has concluded that TVA has put mitigation measures in place to adequately reduce risk for the approximate 2-month extension period, Therefore, it is acceptable to extend the completion date for the corrective actions for certain issues discussed in GL 2004-02 (specifically, replacement of some Min-K insulation with RMI and the installation of additional restraint banding to some Min-K insulation) until the completion of the WBN-1 spring 2008 refueling outage, currently scheduled to begin February 10, 2008. Should TVA elect to begin the outage more than 30 days after February 10, 2008, TVA will need to provide the NRC additional justification for further delay in completing corrective actions for GL 2004-02.