WBL-20-007, 10 CFR 50.46 - 30-Day and Annual Report for Watts Bar Nuclear Plant Units 1 and 2

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10 CFR 50.46 - 30-Day and Annual Report for Watts Bar Nuclear Plant Units 1 and 2
ML20065F821
Person / Time
Site: Watts Bar  Tennessee Valley Authority icon.png
Issue date: 03/05/2020
From: Anthony Williams
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
WBL-20-007
Download: ML20065F821 (15)
v • d • e


Text

Tennessee Valley Authority , 1101 Market Street, Chattanooga, TN 37 402 WBL-20-007 March 5, 2020 10 CFR 50.4 10 CFR 50.46 ATTN : Document Control Desk U.S. Nuclear Regulatory Commission Washington , D.C. 20555-0001 Watts Bar Nuclear Plant, Units 1 and 2 Facility Operating License NPF-90 and NPF-96 NRC Docket Nos. 50-390 and 50-391

Subject:

10 CFR 50.46 Day and Annual Report for Watts Bar Nuclear Plant Units 1 and 2 Reference : TVA Letter to NRC, WBL-19-019 , "10 CFR 50.46- 30-Day and Annual Report for Watts Bar Nuclear Plant Units 1 and 2," dated March 7, 2019 (ML19066A046)

The purpose of this letter is to provide the annual report of changes and errors to the calculated peak cladding temperature (PCT) for the Watts Bar Nuclear Plant (WBN) Units 1 and 2, Emergency Core Cooling System (ECCS) evaluation model. This report is required in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.46, "Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors ,"

paragraph (a)(3)(ii) .

Tennessee Valley Authority (TVA) submitted the last 10 CFR 50.46 annual report for WBN Units 1 and 2 to the Nuclear Regulatory Commission (NRC) in the referenced letter. As indicated in the enclosure, there are no changes to the current updated (net) licensing basis PCT for the WBN Unit 1 large break loss of coolant accident (LBLOCA) and the updated (net) licensing basis PCT for the WBN Unit 1 small break loss of coolant accident (SBLOCA).

Similarly, there are no changes to the current updated (net) licensing basis PCT for the WBN Unit 2 LBLOCA and the updated (net) licensing basis PCT for the WBN Unit 2 SBLOCA.

The enclosure also serves as the 30-day report in accordance with 10 CFR 50.46(a)(3)(ii),

which states that a holder of an operating license or construction permit is required to report changes and errors affecting an ECCS evaluation model to the NRC within 30 days when the cumulative sum of the absolute magnitudes of resulting PCT changes exceeds 50°F. The licensee is also required to include with the report, a proposed schedule for providing a

U.S. Nuclear Regulatory Commission WBL-20-007 Page 2 March 5, 2020 reanalysis or taking other action as may be needed to show compl iance with the 10 CFR 50.46 requirements. Westinghouse performs the LOCA analysis for WBN and Westinghouse submitted a list of changes and errors to the LOCA analysis to TVA on February 11 , 2020. All the submitted changes and errors result in a 0°F change in PCT, but because the accumulated changes and errors from previous years are more than 50°F (with the exception of the WBN Units 1 and 2 SBLOCA analysis) , a 30-day report is required .

As presented in this report, compliance with 10 CFR 50.46 requirements is demonstrated by the calculated PCT for both WBN units remaining below the 2200°F limit. Therefore , TVA has concluded that no proposed schedule for providing a reanalysis or other action is required . No further actions are needed to show compliance with 10 CFR 50.46 requirements .

There are no regulatory commitments in this letter. Please address any questions regard ing this response to Michael A. Brown at (423) 365-7720.

Enclosure:

Watts Bar Nuclear Plant, 10 CFR 50.46 30-Day and Annual Report for 2019 cc (Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant Division of Radiolog ical Health - State of Tennessee

ENCLOSURE Watts Bar Nuclear Plant, 10 CFR 50.46 30-Day and Annual Report for 2019 In accordance with the reporting requirements of Title 10 of the Code of Federal Regulations (10 CFR) 50.46(a)(3)(ii), the Tennessee Valley Authority (TVA) is providing the following summary of the limiting design basis loss of coolant accident (LOCA) analysis results established using the current Watts Bar Nuclear Plant (WBN) Emergency Core Cooling System (ECCS) evaluation models for WBN Units 1 and 2. This report describes the changes and errors affecting the calculated peak cladding temperatures (PCTs} since the last analysis of record (AOR) was submitted to the Nuclear Regulatory Commission (NRC).

TVA submitted the last 10 CFR 50.46 annual report for WBN Units 1 and 2 to the NRC in Reference 1.

Table 1 lists the changes and errors in the large break LOCA (LBLOCA) analysis for WBN Unit 1 since the analysis of record (AOR) and the associated effect on PCT. Table 2 lists the changes and errors in the small break LOCA (SBLOCA) analyses for WBN Unit 1 since the AOR and the associated effect on PCT. The changes that were not previously identified in Reference 1 are described in the notes to Tables 1 and 2.

Table 3 lists the changes and errors in the LBLOCA analysis for WBN Unit 2 since the AOR and the associated effect on PCT. Table 4 lists the changes and errors in the SBLOCA analyses for WBN Unit 2 since the AOR and the associated effect on PCT. The changes that were not previously identified in Reference 1 are described in the notes to Tables 3 and 4.

The updated (net) licensing basis PCT for the LBLOCA and SBLOCA remain unchanged for WBN Units 1 and 2 from the last annual report.

This report also serves as the 30-day report in accordance with 10 CFR 50.46(a)(3)(ii), which states that a holder of an operating license or construction permit is required to report changes and errors affecting an ECCS evaluation model to the NRC within 30 days when the cumulative sum of the absolute magnitudes of resulting PCT changes exceeds 50°F. The licensee is also required to include with the report, a proposed schedule for providing a reanalysis or taking other action as may be needed to show compliance with the 10 CFR 50.46 requirements. Westinghouse performs the LOCA analysis for WBN; Westinghouse submitted a list of changes and errors to the LOCA analysis to TVA on February 11, 2020. All the submitted changes and errors result in a 0°F change in PCT, but because the accumulated changes and errors from previous years are more than 50°F, a 30-day report is required.

As presented in this report, compliance with 10 CFR 50.46 requirements is demonstrated by the calculated PCT for both WBN units remaining below the 2200°F limit. Therefore, TVA has concluded that no proposed schedule for providing a reanalysis or other action is required.

WBL-20-007 E 1 of 13

ENCLOSURE Table 1 Watts Bar Unit 1 LBLOCA Reflood 1 Reflood 2 dPCT jdPCTI dPCT ldPCTI Note Reference Year Description (OF) (OF) (°F) (OF) 1998 BE LBLOCA AOR PCT 1656 --- 1892 - -- --- 3 1998 Vessel Channel DX Error 56 56 -4 4 --- 4 1999 Accumulator Line/Pressurizer Surge -37 37 -131 131 --- 4 Line Data Evaluation 2000 Increased Accumulator Room Temperature 4 4 4 4 --- 4 Evaluation 2000 1.4% Uprate Evaluation 12 12 12 12 --- 4 2000 MONTECF Decay Heat Uncertainty Error 4 4 4 4 --- 5 2001 WBN Specific LBLOCA Vessel Geometry Input 0 0 0 0 --- 6 Errors 2003 Input Error Resulting in Incomplete Solution 60 60 0 0 - -- 7 Matrix 2003 TavQ Bias Error 8 8 8 8 -- - 7 2004 Increased Stroke Time for 0 0 0 0 --- 8 ECCS Valves 2004 Revised Slowdown Heatup Uncertainty 5 5 5 5 --- 8 Distribution 2006 Replacement Steam Generators (D3 to -50 50 -10 10 - -- 9 68AXP) 2006 HOTSPOT' Fuel Relocation Error 0 0 65 65 --- 9 2012 PMID/PBOT Violation Evaluation 20 20 20 20 --- 10, 11 2012 TCD and Peaking Factor Burndown 114 114 15 15 --- 10, 11 WBL-20-007 E 2 of 13

ENCLOSURE Table 1 Watts Bar Unit 1 LBLOCA Reflood 1 Reflood 2

~PCT ,~PCTI ~PCT ,~PCTI Note Reference Year Description (Of) (Of) (Of) (Of) 2013 WCOBRA/TRACTM History File Dimension 0 0 0 0 - -- 12 Error 2013 General Code Maintenance 0 0 0 0 -- - 13 2013 HOTSPOT' Burst Temperature Calculation 0 0 0 0 --- 13 for ZIRLO' Claddina 2013 HOTSPOT' Iteration Algorithm for Calculation Initial Fuel Pellet Average 0 0 0 0 - -- 13 Temperature 2013 WCOBRA/TRAC' Automated Restart 0 0 0 0 --- 13 Process Loaic Error 2013 Rod Internal Pressure Calculation Error 0 0 0 0 - -- 13 2013 Elevations for Heat Slab Temperature Initialization 0 0 0 0 -- - 14 2013 Heat Transfer Model Error Corrections 0 0 0 0 --- 14 2013 Correction to Heat Transfer Node 0 0 0 0 --- 14 Initialization 2013 Mass Conservation Error Fix 0 0 0 0 --- 14 2013 Correction to Split Channel Momentum 0 0 0 0 -- - 14 Equation Heat Transfer Logic 2013 Correction for Rod Burst 0 0 0 0 --- 14 Calculation Changes to Vessel 2013 Superheated Steam 0 0 0 0 - -- 14 Properties WBL-20-007 E 3 of 13

ENCLOSURE Table 1 Watts Bar Unit 1 LBLOCA Reflood 1 Reflood 2

~PCT ,~PCTI ~PCT j~PCTj Note Reference Year Description (OF) (OF) (OF) (OF)

Update to Metal Density 2013 Reference Temperatures 0 0 0 0 --- 14 Decay Heat Model Error 2013 Corrections 0 0 0 0 --- 14 Correction to the Pipe 2013 0 0 0 0 --- 14 Exit Pressure Drop Error WCOBRA/TRAC File 2013 Dimension Error 0 0 0 0 - -- 14 Correction Revised Heat Transfer 2013 -40 40 -85 85 - -- 14 Multiplier Distributions HOTSPOT Burst Strain 2013 Error 20 20 70 70 -- - 15 General Computer Code 2014 Maintenance 0 0 0 0 --- 20 Revised Uncertainty in 2014 LBLOCA Monte Carl 0 0 0 0 - -- 20 Simulations General Code 2016 Maintenance 0 0 0 0 -- - 22 Clad Oxidation 2016 0 0 0 0 --- 22 Calculation LOTIC2 Net Free Volume 2016 Direction of Conservatism 0 0 0 0 --- 22 and Ice Melt Condition LOTIC2 Calculation of the 2016 Thermodynamic 0 0 0 0 --- 22 Properties of Air Increased Safety Injection 2016 Time Delay with Offsite 0 0 0 0 --- 22 Power Available Increase in ECCS pump 2017 0 0 0 0 --- 23 injection delay time General Code 2017 0 0 0 0 --- 24 Maintenance Entrained Liquid / Vapor lnterfacial Drag 2017 0 0 0 0 --- 24 Coefficient Calculation Inconsistency WBL-20-007 E 4 of 13

ENCLOSURE Table 1 Watts Bar Unit 1 LBLOCA Reflood 1 Reflood 2 i:\PCT I i:\PCTI i:\PCT I i:\PCTI Note Reference Year Description (Of) (Of) (Of) (Of)

Resetting of the 2017 Transverse Liquid Mass 0 0 0 0 --- 24 Flow Vapor Temperature 2018 0 0 0 0 --- 1 Resettino General Code 2019 0 0 0 0 1 ---

Maintenance Removal of the Vessel 2019 lnterfacial Heat Transfer 0 0 0 0 2 ---

Limit Updated (net) licensing

- -- basis PCT 1832 -- 1865 -- - --- -- -

AOR PCT + I" i:\PCT Cumulative sum of PCT

--- changes --- 430 --- 433 --- ---

L I i:\PCTI Notes:

1. Various changes have been made to enhance the usability of codes and to streamline future analyses. Examples of these changes include improving the input diagnostic checks; enhancing the code output; optimizing active coding; and eliminating inactive coding. The nature of these changes leads to an estimated peak cladding temperature impact of 0°F.
2. The WCOBRA/TRAC code included a limit on the vessel interfacial heat transfer to prevent extreme conditions, which are detrimental to the robustness of the numerical method. An error was found in the implementation of the limit, which effectively negates the application of the limit.

The error was corrected by removing the limit from the WCOBRA/TRAC code. Removal of the limit was found to have a negligible impact of the code validation results, resulting in a peak cladding temperature impact of 0°F.

WBL-20-007 E 5 of 13

ENCLOSURE Table 2 Watts Bar Unit 1 SBLOCA SBLOCA SBLOCA Year Description APCT IAPCTI Note Reference (Of) (Of) 2006 SBLOCA AOR PCT 1132 --- --- 16 NOTRUMP-EM' Evaluation of Fuel 2013 Pellet TCD 0 0 - -- 13 General Computer Code 2014 0 0 - -- 20 Maintenance 2014 Fuel Rod Gap Conductance Error 0 0 - -- 20 2014 Radiation Heat Transfer Model Error 0 0 --- 20 SBLOCTA Pre-DNB Cladding 2014 Surface Heat Transfer Coefficient 0 0 --- 20 Calculation 2014 Evaluation of 8-inch Axial Blankets 0 0 - -- 20 General Computer Code 2015 Maintenance 0 0 --- 21 Auxiliary Feedwater (AFW) 2015 Temperature Increase During Cold 0 0 -- - 21 Leg Recirculation Plant-Specific Upper Head 2016 0 0 --- 22 Geometric Data Increased Containment Spray Flow 2016 Rate and Reduced RWST Useable 0 0 --- 22 Volume 2017 General Code Maintenance 0 0 --- 24 Upper Plenum Fluid Volume 2017 Reduction 0 0 --- 24 WBL-20-007 E 6 of 13

ENCLOSURE Table 2 Watts Bar Unit 1 SBLOCA SBLOCA SBLOCA Year Description L\PCT IAPCTI Note Reference (OF) (OF) 2018 U02 Fuel Pellet Heat Capacity 0 0 --- 1 Updated (net) licensing basis PCT

- -- AOR PCT + L L\PCT 1132 --- -- - ---

Cumulative sum of PCT changes

- -- LI L\PCTI

-- - 0 --- ---

WBL-20-007 E 7 of 13

ENCLOSURE Table 3 Watts Bar Unit 2 LBLOCA Year LBLOCA LBLOCA Description Note Reference

~PCT (°F) I~PCTI (°F) 2013 LBLOCA AOR PCT 1766 -- - --- 17 Elevations for Heat Slab 2013 Temperature Initialization 0 0 --- 19 Heat Transfer Model Error 2013 Corrections 0 0 --- 19 Correction to Heat Transfer Node 2013 Initialization 0 0 --- 19 2013 Mass Conservation Error Fix 0 0 --- 19 Correction to Split Channel 2013 Momentum Equation 0 0 --- 19 Heat Transfer Logic Correction for 2013 Rod Burst Calculation 0 0 --- 19 Changes to Vessel Superheated 2013 Steam Properties 0 0 --- 19 Update to Metal Density Reference 2013 Temperatures 0 0 --- 19 Decay Heat Model Error 2013 Corrections 0 0 --- 19 Correction to the Pipe Exit 2013 Pressure Drop Error 0 0 --- 19 WCOBRA/TRAC File Dimension 2013 Error Correction 0 0 --- 19 Revised Heat Transfer Multiplier 2013 Distributions

-55 55 --- 19 Initial Fuel Pellet Average 2013 Temperature Uncertainty Calculation 0 0 --- 19 2013 HOTSPOT Burst Strain Error 0 0 --- 15 Cold Leg Accumulator Injection 2014 Lines Hydraulic Resistance 0 0 --- 2 Changes General Computer Code 2014 Maintenance 0 0 --- 20 Errors in Decay Group Uncertainty 2014 Factors 0 0 --- 20 WBL-20-007 E 8 of 13

ENCLOSURE Table 3 Watts Bar Unit 2 LBLOCA Year LBLOCA LBLOCA Description Note Reference t\PCT (°F) I t\PCT I (°F)

Treatment of Burnup Effects on 2014 Thermal Conductivity Degradation 0 0 --- 20 2016 General Code Maintenance 0 0 --- 22 2016 Clad Oxidation Calculation 0 0 --- 22 Use of Steam Tables in Upper 2016 Head Fluid Temperature 22 0 0 ---

Calculations Lower Support Plate Heat 2016 0 0 --- 22 Conductor Surface Area LOTIC2 Net Free Volume Direction 2016 of Conservatism and Ice Melt 22 0 0 - --

Condition LOTIC2 Calculation of the 2016 Thermodynamic Properties of Air 0 0 --- 22 2017 General Code Maintenance 0 0 --- 24 Entrained Liquid / Vapor lnterfacial 2017 Drag Coefficient Calculation 0 0 --- 24 Inconsistency Resetting of Transverse Liquid 2017 Mass Flow 0 0 --- 24 Steady State Fuel Temperature 2017 Calibration non-Conservatisms 0 0 --- 24 2018 Vapor Temperature Resetting 0 0 --- 1 2019 Core Barrel Heat Slab Error 0 0 1 -- -

2019 General Code Maintenance 0 0 2 ---

Removal of the Vessel lnterfacial 2019 0 0 3 ---

Heat Transfer Limit Updated (net) licensing basis PCT

--- 1711 --- --- ---

(AOR PCT + r t\PCT)

Cumulative sum of PCT changes

--- --- 55 --- ---

r I t\PCTI Notes:

1. In the LBLOCA analysis, a modeling error was discovered in heat slab connection inputs, which resulted in erroneous wetted perimeters in the vessel component. An evaluation of the impact determined the difference in the wetted perimeter due to the error is very small and WBL-20-007 E 9 of 13

ENCLOSURE would be expected to produce a negligible effect on the large break LOCA analysis results, leading to an estimated PCT impact of 0°F.

2. Various changes have been made to enhance the usability of codes and to streamline future analyses. Examples of these changes include improving the input diagnostic checks; enhancing the code output; optimizing active coding; and eliminating inactive coding. The nature of these changes leads to an estimated peak cladding temperature impact of 0°F.
3. The WCOBRA/TRAC code included a limit on the vessel interfacial heat transfer to prevent extreme conditions, which are detrimental to the robustness of the numerical method. An error was found in the implementation of the limit, which effectively negates the application of the limit. The error was corrected by removing the limit from the WCOBRA/TRAC code. Removal of the limit was found to have a negligible impact of the code validation results, resulting in a peak cladding temperature impact of 0°F.

WBL-20-007 E10of13

ENCLOSURE Table 4 Watts Bar Unit 2 SBLOCA Year Description SBLOCA SBLOCA Note Reference

~PCT (°F) I ~PCTI (°F) 2010 SBLOCA AOR PCT 1184 --- --- 18 Cold Leg Accumulator Injection 2014 Lines Hydraulic Resistance 0 0 --- 2 Chanoes General Computer Code 2014 Maintenance 0 0 --- 20 2014 Fuel Rod Gap Conductance Error 0 0 - -- 20 Radiation Heat Transfer Model 2014 Error 0 0 --- 20 SBLOCTA Pre-DNB Cladding 2014 Surface Heat Transfer Coefficient 0 0 --- 20 Calculation General Computer Code 2015 Maintenance 0 0 --- 21 AFW Temperature Increase During 2015 Cold Leg Recirculation 0 0 --- 21 Plant-Specific Upper Head 2016 0 0 --- 22 Geometric Data 2016 Reduced RWST Useable Volume 0 0 - -- 22 2017 Increased SI injection time delay 0 0 --- 24 2017 General Code Maintenance 0 0 --- 24 Upper Plenum Fluid Volume 2017 Reduction 0 0 --- 24 2018 U02 Fuel Pellet Heat Capacity 0 0 --- 1 Updated (net) licensing basis PCT

--- 1184 --- --- ---

(AOR PCT + 2 ~PCT)

Cumulative sum of PCT changes

- -- L l~PCTI --- 0 -- - - --

WBL-20-007 E11of13

ENCLOSURE

References:

1. TVA Letter to NRC, WBL-19-019, "10 CFR 50.46-30-Day and Annual Report for Watts Bar Nuclear Plant Units 1 and 2," dated March 7, 2019 (ML19066A046)
2. TVA letter to NRC, CNL-15-034, "10 CFR 50.46- 30-Day Report for Watts Bar, Unit 2" dated February 6, 2015 (ML15037A725)
3. WCAP-14839, Revision 1, "Best Estimate Analysis of the Large Break Loss of Coolant Accident for the Watts Bar Nuclear Plant," September 1998
4. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes Day Report and Annual Notification and Reporting for 2000," dated October 26, 2000 (ML003764646)
5. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes Day Report and Revised Annual Notification Report for 2000," dated September 7, 2001 (ML012570290)
6. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes - Annual Notification and Reporting for 2001," dated April 3, 2002 (ML021070404)
7. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes Day Report and Revised Annual Notification and Reporting for 2003," dated April 19, 2004 (ML041130196)
8. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes -Annual Notification and Reporting for 2004," dated April 19, 2005 (ML051120164)
9. TVA letter to NRC, "Watts Bar Nuclear Plant (WBN) Unit 1 - Emergency Core Cooling System (ECCS) Evaluation Model Changes Day Report and Annual Notification and Reporting for 2006," dated July 3, 2007 (ML071860388)
10. TVA letter to NRC, "Supplement to 10 CFR 50.46 Day Special Report," dated February 13, 2013 {ML13046A002)
11. TVA letter to NRC, "10 CFR 50.46 Day Special Report," dated October 18, 2012 (ML12296A254)
12. TVA letter to NRC, "10 CFR 50.46 Day Report for Watts Bar Unit 1," dated March 19, 2013 (ML13080A405)
13. TVA letter to NRC, "10 CFR 50.46 Day and Annual Report for 2012," dated April 25, 2013 (ML13120A005)
14. TVA letter to NRC, "10 CFR 50.46 - 30 day Report for Watts Bar, Unit 1," dated August 28, 2013 (ML13267A034)
15. TVA letter to NRC, CNL-14-035, "10 CFR 50.46 - 30 day Report for Watts Bar, Units 1 and 2,"

dated February 28, 2014 (ML14064A431)

16. WTV-RSG-06-015, "LOCA & Non-LOCA Analysis Summary for Replacement Steam Generator,"

February 2006

17. WCAP-17093-P, Revision 1, "Best Estimate Analysis of the Large Break Loss of Coolant Accident for the Watts Bar Unit 2 Nuclear Plant Using the ASTRUM Methodology," June 2013
18. WBT-D-1460, "Final Small Break LOCA Summary Report forWBN Unit 2," January 2010 WBL-20-007 E12of13

ENCLOSURE

19. TVA letter to NRC, "Watts Bar Nuclear Plant, Unit 2 - Emergency Core Cooling System Evaluation Model Changes - 30 Day Report - 10 CFR 50.46 Notification," dated August 28, 2013 (ML13246A076)
20. TVA letter to NRC, CNL-15-053, "10 CFR 50.46 Day and Annual Report for Watts Bar, Units 1 and 2," dated March 30, 2015 (ML15098A124)
21. TVA letter to NRC, CNL-16-057, "10 CFR 50.46-Annual Report for Watts Bar, Units 1 and 2,"

dated March 18, 2016 (ML16081A248)

22. TVA Letter to NRC, "10 CFR 50.46 Day and Annual Report for Watts Bar, Units 1 and 2,"

dated March 8, 2017 (ML17067A079)

23. TVA letter to NRC, 10 CFR 50.46 Day Report for Watts Bar Nuclear Plant Unit 1," dated August 21, 2017 (ML17235A905)
24. TVA Letter to NRC, CNL-18-040, "10 CFR 50.46 Day and Annual Report for Watts Bar, Units 1 and 2," dated March 7, 2018 (ML18066A730)

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