ML20262H109

Resolution of Issues Pertaining to Boric Acid Precipitation Related to the Closeout of Generic Letter, 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurize

ML20262H109
Person / Time
Site:	Watts Bar
Issue date:	10/16/2020
From:	Kimberly Green Plant Licensing Branch II
To:	Jim Barstow Tennessee Valley Authority
Green K - NRR/DORL 301-415-1627
References
GL 2004-02
Download: ML20262H109 (5)

Text

October 16, 2020 Mr. James Barstow Vice President, Nuclear Regulatory Affairs and Support Services Tennessee Valley Authority 1101 Market Street, LP 4A-C Chattanooga, TN 37402-2801

SUBJECT:

WATTS BAR NUCLEAR PLANT, UNIT 2 - RESOLUTION OF ISSUES PERTAINING TO BORIC ACID PRECIPITATION RELATED TO THE CLOSEOUT OF GENERIC LETTER, 2004-02, POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED-WATER REACTORS

Dear Mr. Barstow:

By letter dated September 18, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14163A658), the U.S. Nuclear Regulatory Commission (NRC) staff documented that the Tennessee Valley Authority had provided sufficient information in its responses to Generic Letter (GL) 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors, (ADAMS Accession No. ML042360586), to close out the matter for Watts Bar Nuclear Plant (WBN), Unit 2. For closure of the in-vessel portion of GL 2004-02, the Tennessee Valley Authority demonstrated that the requirements of topical report WCAP-16793-NP-A, Revision 2, and the associated NRC staff safety evaluation (ADAMS Accession No. ML13239A114) were met for WBN, Unit 2. In its safety evaluation, the NRC staff concluded that plants with relatively low fiber amounts reaching the core could use the methodology in topical report WCAP-16793-NP-A, Revision 2, to show that core cooling would not be adversely affected by debris. However, the safety evaluation stated that the potential for debris to change flow patterns or inhibit the mixing of boric acid in the core that might result in earlier boric acid precipitation had not been evaluated. This left the issue of boric acid precipitation unresolved.

In its response to GL 2004-02, the Tennessee Valley Authority demonstrated for WBN, Unit 2 that a very small amount of fiber may arrive at the core inlet. As discussed in the enclosure to this letter, the NRC staff has determined that this amount of debris will not adversely affect boric acid precipitation timing and that no changes are needed to the licensing basis for WBN, Unit 2

J. Barstow to address boric acid precipitation. The NRC staff has no further questions related to potential effects of post-accident debris on the reactor vessel at WBN, Unit 2.

If you have any questions, please contact me at 301-415-1627 or Kimberly.Green@nrc.gov.

Sincerely,

/RA/

Kimberly J. Green, Senior Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-390

Enclosures:

As stated cc: Listserv

U.S. NUCLEAR REGULATORY COMMISSION STAFF DOCUMENTATION OF BORIC ACID PRECIPITATION CONCERNING GENERIC LETTER 2004-02 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED-WATER REACTORS TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 2 DOCKET NO. 50-391 In September 13, 2004, the U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized Water Reactors (Agencywide Documents Access and Management System (ADAMS) Accession No. ML042360586), to holders of operating licenses for pressurized-water reactors. In GL 2004-02, the NRC staff requested that licensees perform an evaluation of their emergency core cooling system (ECCS) and containment spray system (CSS) recirculation functions, considering the potential for debris-laden coolant to be circulated by the ECCS and the CSS after a loss-of-coolant accident or high-energy line break inside containment, and, if appropriate, take additional action to ensure system function. GL 2004 02 required, per Title 10 of the Code of Federal Regulations, paragraph 50.54(f), that licensees provide the NRC a written response describing the results of their evaluation and any modifications made, or planned, to ensure ECCS and CSS system function during recirculation following a design-basis event, or any alternate action proposed and the basis for its acceptability.

In a letter dated September 18, 2014 (ADAMS Accession No. ML14163A658), the NRC documented that Watts Bar Nuclear Plant (WBN), Unit 2 had provided all information required to close GL 2004-02. Documentation for closure of the in-vessel portion of GL 2004-02 was performed by demonstrating that WBN, Unit 2 met the requirements of topical report (TR)

WCAP-16793-NP-A, Revision 2, and the associated NRC staff safety evaluation (SE) (ADAMS Accession No. ML13239A114). In its SE, the NRC staff concluded that plants with relatively low fiber amounts reaching the core could use the TR methodology to show that core cooling would not be adversely affected by debris. However, the SE stated that the potential for debris to change flow patterns or inhibit the mixing of boric acid in the core that might result in earlier boric acid precipitation (BAP) had not been evaluated. This left the question of the effects of debris on the plant licensing basis for BAP unresolved.

The Pressurized Water Reactors Owners Group continued to evaluate the effects of larger amounts of debris on long-term core cooling. This work is documented in TR WCAP-17788, Revision 1, Comprehensive Analysis and Test Program for GSI-191 Closure (PA-SEE-1090)

(ADAMS Package Accession No. ML20010F181). The NRC staff performed a thorough review of this TR but was unable to conclude that the methodology used for evaluating reactor core debris limits was acceptable for licensing basis calculations.

Enclosure

However, the NRC staff found that the TR methodology provided meaningful safety and regulatory insights regarding the treatment of BAP. The staffs technical review of the methodology is documented in its technical evaluation report dated June 13, 2019 (ADAMS Accession No. ML19178A252). WCAP-17788, Revision 1 evaluated the potential for debris to affect current BAP analyses and found that BAP timing would not be adversely affected. The NRC performed sensitivity studies during its review of WCAP-17788, Revision 1. These analyses explicitly modeled the physical phenomena that affect the potential for BAP and were conducted for reactor designs considered to be the most limiting with respect to BAP. The analyses found that debris collecting at the core inlet would not adversely affect BAP timing under conditions that conservatively modeled the effects of debris. As discussed in NRC staff guidance dated September 4, 2019 (ADAMS Accession No. ML19228A011), the NRC staff determined that licensees that demonstrate that their plants fall within specific bounds can maintain their current licensing basis as it relates to BAP, even with debris amounts greater than those approved in WCAP-16793-NP-A, Revision 2.

In its response to GL 2004-02, WBN, Unit 2 demonstrated that it has a very small amount of fiber that may arrive at the core inlet. The NRC staff has determined that this amount of debris will not adversely affect BAP timing and WBN, Unit 2 can maintain its current licensing basis for BAP. The NRC staff has no further questions related to potential effects of post-accident debris on the reactor vessel.

ML20262H109 *by e-mail OFFICE NRR/DORL/LPL2-2/PM NRR/DORL/LPL2-2/LA* NRR/DSS/STSB/BC*

NAME KGreen BAbeywickrama VCusumano DATE 09/28/2020 09/24/2020 05/19/2020 OFFICE NRR/DORL/LPL2-2/BC* NRR/DORL/LPL2-2/PM*

NAME UShoop KGreen DATE 10/15/2020 10/16/2020