ML20174A482

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

ML20174A482
Person / Time
Site:	Byron, Braidwood
Issue date:	08/12/2020
From:	Joel Wiebe Plant Licensing Branch III
To:	Bryan Hanson Exelon Generation Co, Exelon Nuclear
Wiebe J
References
GL-04-002
Download: ML20174A482 (5)

Text

August 12, 2020 Mr. Bryan C. Hanson Senior Vice President Exelon Generation Company, LLC President and Chief Nuclear Officer (CNO)

Exelon Nuclear Braidwood and Byron Station 4300 Winfield Road Warrenville, IL 60555

SUBJECT:

BRAIDWOOD STATION, UNITS 1 AND 2 AND BYRON STATION, UNIT NOS. 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. Hanson:

In letter dated May 19, 2016 Agencywide Documents Access and Management System (ADAMS) Accession No. ML15296A358), the U.S. Nuclear Regulatory Commission (NRC) documented that Braidwood Station, Units 1 and 2, (Braidwood), and Byron Station, Units 1 and 2, (Byron), had provided all information required to close Generic Letter (GL) 2004 02.

Documentation for closure of the in-vessel portion of GL 2004 02 was performed by demonstrating that Braidwood and Byron 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 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 open.

In its response to GL 2004 02, Braidwood and Byron demonstrated that they have 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 Braidwood and Byron can maintain their current licensing basis. (See staff technical evaluation report (ADAMS Accession No. ML19178A252) and staff guidance (ADAMS Accession No. ML19228A011) for more information.) The staff has no further questions related to potential effects of post-accident debris on the reactor vessel.

If you have any questions, please contact me at 301-415-6606 or via e-mail at Joel.Wiebe@nrc.gov Sincerely,

/RA/

Joel S. Wiebe, Senior Project Manager Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. STN 50-456 and STN 50-457 STN 50-454 and STN 50-455

Enclosure:

Staff Documentation of issues related to Generic Letter 2004-02 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 EXELON GENERATION COMPANY, LLC BRAIDWOOD STATION, UNITS 1 AND 2, AND BYRON STATION, UNIT NOS. 1 AND 2 DOCKET NOS. STN 50-456 AND STN 50-457; STN 50-454 AND STN 50-455 In September 2004, the U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter 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 Generic Letter (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) Section 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 May 19, 2016 (ADAMS Accession No. ML15296A358), the NRC documented that Braidwood Station, Units 1 and 2, (Braidwood), and Byron Station, Units 1 and 2, (Byron),

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 Braidwood and Byron 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 open.

Enclosure

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 (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 staffs technical review of the TR methodology is documented in its technical evaluation report (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. The NRC staff determined that licensees that demonstrate that their plants fall within specific bounds can maintain their current licensing basis for BAP (Refer to Staff Guidance in ADAMS Accession No. ML19228A011), even with debris amounts greater than those approved in WCAP-16793-NP-A, Revision 2.

In its response to GL 2004-02, Braidwood and Byron demonstrated that they have 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 Braidwood and Byron can maintain their current licensing basis for BAP. The staff has no further questions related to potential effects of post-accident debris on the reactor vessel.

ML20174A482 via e-mail OFFICE NRR/DORL/LPL3/PM* NRR/DORL/LPL3/LA* NRR/DSS/STSB/BC*

NAME JWiebe SRohrer VCusumano DATE 07/07/2020 06/23/2020 05/22/2020 OFFICE NRR/DORL/LPL3/BC* NRR/DORL/LPL3/PM*

NAME NSalgado JWiebe DATE 08/12/2020 08/12/2020