ML20143A272

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

ML20143A272
Person / Time
Site:	Sequoyah
Issue date:	05/28/2020
From:	Michael Wentzel Plant Licensing Branch II
To:	Jim Barstow Tennessee Valley Authority
Wentzel M
References
Download: ML20143A272 (5)

Text

May 28, 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:

SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 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 November 17, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14283A526), the U.S. Nuclear Regulatory Commission (NRC) staff documented that the Tennessee Valley Authority (TVA) 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 Sequoyah Nuclear Plant, Units 1 and 2 (Sequoyah). For closure of the in-vessel portion of GL 2004-02, the TVA demonstrated that the requirements of Topical Report (TR)

WCAP-16793-NP-A, Revision 2, and the associated NRC staff safety evaluation (ADAMS Accession No. ML13239A114) were met for Sequoyah. In its safety evaluation, the NRC staff concluded that plants with relatively low fiber amounts reaching the core could use the methodology in TR 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 (BAP) had not been evaluated. This left the issue of BAP unresolved.

In its response to GL 2004-02, the TVA demonstrated for Sequoyah 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 BAP timing and that no changes

are needed to the licensing basis for Sequoyah to address BAP. The NRC staff has no further questions related to potential effects of post-accident debris on the reactor vessel at Sequoyah.

If you have any questions, please contact me at 301-415-6459 or Michael.Wentzel@nrc.gov.

Sincerely, Michael Wentzel, Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-327 and 50-328

Enclosure:

As stated cc: Listserv Michael J.

Wentzel Digitally signed by Michael J. Wentzel Date: 2020.05.28 08:40:41 -04'00'

Enclosure 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 SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328 In September 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 (10 CFR) 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 letter dated November 17, 2014 (ADAMS Accession No. ML14283A526), the NRC staff documented that Tennessee Valley Authority (TVA) had provided sufficient information to close GL 2004-02 for Sequoyah Nuclear Plant, Units 1 and 2 (Sequoyah). TVA provided documentation for closure of the in-vessel portion of GL 2004-02 demonstrating that the requirements of Topical Report (TR) WCAP-16793-NP-A, Revision 2, and the associated NRC staff safety evaluation (ADAMS Accession No. ML13239A114) were satisfied for Sequoyah. In its safety evaluation for TR WCAP-16793-NP-A, Revision 2, 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 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 (BAP) had not been evaluated. This left the question of the effects of debris on the plant licensing basis related to 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.

However, the NRC staff found that the TR methodology provided meaningful safety and regulatory insights regarding the treatment of BAP. The NRC staffs technical review of the methodology is documented in its technical evaluation report dated June 13, 2019 (ADAMS Accession No. ML19178A252). TR 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 staff performed sensitivity studies during its review of TR 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 TR WCAP-16793-NP-A, Revision 2.

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

ML20143A272 OFFICE DORL/LPL2-2/PM DORL/LPL2-2/LA DSS/STSB/BC DORL/LPL2-2/BC DORL/LPL2-2/PM*

NAME MWentzel BAbeywickrama VCusumano UShoop MWentzel DATE 05/27/2020 05/27/2020 05/19/2020 05/27/2020 05/28/2020