IR 05000390/2018050
| ML18172A015 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 06/21/2018 |
| From: | Joel Munday Division Reactor Projects II |
| To: | James Shea Tennessee Valley Authority |
| References | |
| IR 2018050 | |
| Download: ML18172A015 (25) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION une 21, 2018
SUBJECT:
WATTS BAR NUCLEAR PLANT - NUCLEAR REGULATORY COMMISSION SPECIAL INSPECTION REPORT 05000390/2018050 AND 05000391/2018050
Dear Mr. Shea:
On May 14, 2018, the U.S. Nuclear Regulatory Commission (NRC) completed a reactive inspection pursuant to Inspection Procedure 93812, Special Inspection, at your Watts Bar Nuclear Plant, Units 1 and 2. The enclosed inspection report documents the inspection results which were discussed on May 14, 2018, with Mr. P. Simmons, and other members of your staff.
The special inspection began on April 30, 2018, in accordance with NRC Management Directive 8.3, NRC Incident Investigation Program, and Inspection Manual Chapter 0309, Reactive Inspection Decisions Basis for Reactors, based on the initial risk and deterministic criteria evaluation performed by the NRC. The special inspection reviewed the circumstances surrounding the emergency core cooling system gas accumulation acceptance criteria being non-conservative. Conservative acceptance criteria were established; however on April 21, 2018, a gas void determination for the Unit 1 residual heat removal (RHR) system exceeded this criteria, resulting in the inoperability of both trains of RHR and required entry into Technical Specification (TS) 3.0.3 until the RHR system was vented. On April 22, 2018, a similar condition was found for the Unit 2 RHR system that resulted in a TS 3.0.3 entry until the Unit 2 RHR system was vented. Based on this initial assessment, the NRC sent an inspection team to your site on April 30, 2018.
The report documents one licensee-identified violation that was determined to be of very low safety significance. The NRC is treating this violation as a non-cited violation (NCV) consistent with Section 2.3.2 of the Enforcement Policy.
If you contest the violation or significance of the NCV, you should provide a response within 30 days of the date of this inspection report, with the basis for your denial, to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, DC 20555-0001; with copies to the Regional Administrator, Region II; the Director, Office of Enforcement, U.S.
Nuclear Regulatory Commission, Washington, DC 20555-0001; and the NRC Resident Inspector at Watts Bar Nuclear Plant. This letter, its enclosure, and your response (if any) will be made available for public inspection and copying at http://www.nrc.gov/reading-rm/adams.html and at the NRC Public Document Room in accordance with 10 CFR 2.390, Public Inspections, Exemptions, Requests for Withholding.
Sincerely,
/RA/
Joel T. Munday, Director Division of Reactor Projects Docket Nos.: 05000390, 05000391 License Nos.: NPF-90, NPF-96
Enclosure:
IR 05000390/2018050 and 05000391/2018050
REGION II==
Docket Numbers: 50-390, 50-391 License Numbers: NPF-90, NPF-96 Report Numbers: 05000390/2018050, 05000391/2018050 Enterprise Identifier: I-2018-050-0001 Facility: Watts Bar Nuclear Plant, Units 1 and 2 Location: Spring City, TN 37381 Dates: April 30, 2018 - May 14, 2018 Team Leader: T. Morrissey, Senior Resident Inspector Inspectors: J. Jandovitz, Resident Inspector J. Eargle, Senior Project Manager Approved By: Anthony D. Masters, Chief Reactor Projects Branch 5 Division of Reactor Projects Enclosure
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) monitored the licensees performance by conducting a special inspection at Watts Bar Nuclear Plant, Units 1 and 2, in accordance with the Reactor Oversight Process. The Reactor Oversight Process is the NRCs program for overseeing the safe operation of commercial nuclear power reactors. Refer to https://www.nrc.gov/reactors/operating/oversight.html for more information. A licensee-identified non-cited violation is documented in the inspection results section of the report.
List of Findings and Violations No NRC findings were identified
REPORT DETAILS
Event Description Since initial operation of Unit 1 in 1996 and Unit 2 in 2016, Tennessee Valley Authority has not had a complete and adequate design basis that considered the total effects of gas intrusion for the SI and RHR systems. The water hammer effects in the original basis were not accurate, resulting in non-conservative voiding design acceptance criteria for the RHR and SI systems of both units.
On April 19, 2018, the Watts Bar Nuclear Plant (WBN) Operations Superintendent called the WBN NRC resident to report that WBN was notified by their contractor of errors in calculations relating to the emergency core cooling system (ECCS) void size. The calculations were being reviewed as part of an extent of condition for an auxiliary feedwater system (AFW) piping stress analysis. The initial acceptance criteria was based on an acceptable delay in injecting water into the reactor coolant system (RCS). On April 19, 2018, interim void volume acceptance criteria were established that were based on stresses from a water hammer event when ECCS is initiated. These reduced values were provided to WBN by Westinghouse and were based on Westinghouses experience with similar plants, and were not specific to either unit at WBN.
This new interim criteria was significantly lower than the previous acceptance criteria.
Specifically, the residual heat removal (RHR), or low head injection system, void limit was reduced from 40 cubic feet (ft3) to 1.3 ft3, and the safety injection (SI), or high head injection system, void limit was reduced from 27.6 ft3 to 5 ft3.
An immediate determination of operability was completed on April 20, 2018, and it was determined that the affected ECCS on Units 1 and 2 were operable but degraded with interim compensatory measures established. The interim compensatory measures were increased frequency of void measurement and venting of ECCS. The frequency was increased from monthly to daily.
On April 21, 2018, WBN notified the NRC that the accumulated gas in the Unit 1 RHR system was found to be greater than the 1.3 ft3 limit (approximately 5 ft3). Unit 1 then entered Technical Specification (TS) 3.0.3 and exited the TS once the RHR system was vented. Entry into TS 3.0.3 requires a unit shutdown in six hours unless the condition for the entry is corrected and the TS is exited.
On April 22, 2018, WBN notified the NRC that the accumulated gas in the Unit 2 RHR system was found to be greater than the 1.3 ft3 limit (approximately 1.367 ft3). Unit 2 then entered TS 3.0.3 and exited the TS once the RHR system was vented.
Special Inspection Team Charter A Special Inspection Charter was established on April 26, 2018, containing the scope of the special inspection. The Charter is found in Attachment
INSPECTION SCOPE
Inspections were conducted using the appropriate portions of inspection procedure (IP) 93812, Special Inspection, in effect at the beginning of the inspection. Currently approved IPs with their attached revision histories are located on the public website at http://www.nrc.gov/reading-rm/doc-collections/insp-manual/inspection-procedure/index.html. The inspectors reviewed selected procedures and records, observed activities, and interviewed personnel to assess licensee performance and compliance with Commission rules and regulations, license conditions, site procedures, and standards.
OTHER ACTIVITIES
- TEMPORARY INSTRUCTIONS, INFREQUENT AND ABNORMAL
IP 93812 - Special Inspection (1 Sample)
the acceptance criteria for gas accumulation in the WBN Unit 1 and Unit 2 SI system and RHR system discharge piping may be non-conservative, which resulted in both units RHR system being inoperable and follow-up actions taken by the licensee.
See Attachment 1
.2 Charter Item 2: Determine if the licensees acceptance criteria for surveillance
requirements to keep ECCS full of water are consistent with the plant licensing basis and as-built design.
Since initial operation of Unit 1 in 1996 and Unit 2 in 2016, Tennessee Valley Authority has not had a complete and adequate design basis that considered the total effects of gas intrusion for the SI and RHR systems. The water hammer effects in the original basis were not accurate, resulting in non-conservative voiding design acceptance criteria for the RHR and SI systems of both units.
Upon discovery, the licensee, assisted by Westinghouse, established interim ECCS voiding acceptance criteria for WBN Unit 1 and Unit2 SI and RHR systems. The interim acceptance criteria was conservative and developed only for the purpose ensuring operability while further, more accurate analysis is developed. These criteria established the justification for the Prompt Determination of Operability (PDO, condition report (CR)1407257) and are as follows:
Alert Limit ft3 Upper limit ft3 Unit 1 RHR 3.5 27.1 Unit 2 RHR 4.0 27.5 Unit 1 SI 2.5 16.6 Unit 2 SI 1.0 27.5 The licensee established an operational decision-making issue (ODMI) evaluation document (CR 1407257) that provided trigger values to adjust venting frequencies based on void trend data. Changes to the venting frequencies required concurrence from Operations and Engineering. The venting frequencies are based on maintaining voids at or below the Alert limits established.
The licensee determined that the ECCS were considered operable but non-conforming when operating with ECCS voiding within the above established limits. The systems are considered non-conforming since design margins are conservatively reduced and credit overpressure protection by system relief valves that may transiently lift and reseat. At the conclusion of the inspection, the licensee had plans to determine ECCS design basis voiding limits, utilizing a RELAP5 or similar thermo-hydraulic computer model with plant-specific inputs.
The inspectors concluded that there was reasonable assurance that the ECCS would perform their safety functions as long as gas voiding was maintained within the operability limits established by the licensee. Additional details associated with the inspectors review of the calculations associated with the above established limits is documented under Charter Item 7.
.3 Charter Item 3: Review drawings, walk down portions of the RHR and SI systems, and
determine if the vent locations enable system venting to meet void size acceptance criteria.
The team reviewed applicable ECCS piping and instrumentation diagrams (P&IDs) and isometric drawings and walked down accessible portions of the RHR and SI systems to verify vent locations enabled proper venting of the systems.
The team interviewed licensee personnel involved with ultra sonic testing (UT) of horizontal RHR pipe locations that were not in the vicinity of high point vents. The licensee selected pipe locations to UT that may be at or slightly above the elevation of the vent valves to verify if any voiding was present. These locations were selected based on pipe slope data obtained during the NRC Generic Letter (GL) 2008-01, Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal and Containment Spray Systems, walkdowns. All selected pipe locations were determined to be full of water by UT.
The team determined that the vent valve locations were acceptable. This conclusion was based on inspector review of isometric drawings, review of Unit 1 and Unit 2 GL 2008-01 walkdown reports, limited inspector walkdowns of accessible piping, and reviews of locations selected and UT data obtained as noted above.
.4 Charter Item 4: Determine how the licensee is identifying, quantifying, trending, and
venting the gas.
The team reviewed the licensees methodology associated with the UT testing of ECCS piping to determine the amount of pipe voiding. The team observed the performance of procedure 1-SI-63-10.1-A, ECCS Discharge Pipes Venting - Train A Inside Containment, and found the measurement of voids and the venting methodology to be acceptable. The methodology of determining gas volume by UT is an acceptable method as described in GL 2008-01. The team also reviewed Unit 1 and Unit 2 void trending data obtained since March 2016 for the ECCS. Accurate trending data prior to March 2016 was not available. NRC Integrated Inspection Report 05000390/2016002, 05000391/2016002 (ML16228A005) documents a non-cited violation associated with this issue.
Overall, the team determined that the licensee was appropriately identifying, quantifying, trending and venting gas from the ECCS.
.5 Charter Item 5: Evaluate the licensees venting process activities to ensure that there is
no adverse impact the plant operations. Review the configuration control and circumstances regarding the overpressure of the Unit 1 RHR train event that occurred on April 24, 2018.
The team reviewed the licensees venting process activities and venting procedures.
The team determined that the venting process would have no impact on plant operations. On April 24, 2018, Unit 1 Operations allowed ECCS venting to be performed at the same time the cold leg accumulator #3 was being filled. Operations did not recognize the conflict in performing these procedures at the same time as performing other operations of ECCS. The parallel performance resulted in pressurizing the standby RHR train and lifting of its relief valve. The regulatory significance of this event will be documented in the NRC quarterly integrated inspection report.
.6 Charter Item 6: Review licensees compliance with the guidance and commitments in
the licensees responses to GL 2008-01. Compare operating experience involving gas voiding of emergency core cooling system discharge piping to actions implemented at Watts Bar. Determine if there are any generic issues related to the design and operation practices that resulted in the voiding of the ECCS discharge piping beyond those already described in GL 2008-01. Promptly communicate these issues to NRC regional management.
The team reviewed selected licensee commitments to GL 2008-01 to verify compliance with the GL. The team reviewed system isometric drawings as well as the GL 2008-01 walkdown reports for both units to verify vent locations were appropriate. The licensees GL 2008-01 walkdowns utilized a ZiplevelTM to obtain horizontal ECCS piping slopes.
The data recorded was the relative position on a horizontal pipe location referenced to a fixed location. This data was used to determine the high point on horizontal ECCS piping. During the special inspection, the licensee selected several RHR pipe locations for both units based on the walkdown report data to confirm the piping was full of water.
The team reviewed the selected locations on the isometric drawings and based on the locations being full of water determined that the vent valves were appropriately positioned.
Although Unit 2 was not subject to GL 2008-01 since it was still in the construction phase, the inspectors verified that Unit 2 did comply with the requested actions in the GL and installed vents similar to Unit 1.
The team did not identify any operating experience involving ECCS voiding issues that was not already addressed by WBNs response to the GL. The operating experience associated with the calculation errors that resulted in non-conservative void acceptance criteria at WBN has been placed in the corrective action program for TVAs Sequoyah (CR 1392429) and Browns Ferry (CR 1392435) nuclear plants.
The team did not identify any generic issues related to design or operational practices that resulted in ECCS pipe voiding that was beyond what was covered in the GL. No generic issues were communicated to regional management.
.7 Charter Item 7: Review the licensees calculations relating to ECCS void size. Review
key assumptions and facts. Determine if the licensees root cause analysis and corrective actions have addressed the extent of condition for gas accumulation of ECCS.
The licensees root cause analysis was not completed in time for this inspection. At the close of the inspection, the licensee approved a root cause evaluation charter. The team reviewed the root cause charter and determined that it appropriately included items that needed to be addressed. The charter included: review of ECCS vent acceptance criteria shortfalls including the non-conservative calculation error, review of NRC GL 2008-01 implementation at WBN, review of ECCS venting methodology, and a review of the licensees thoroughness in addressing plant issues. The charter also included reviewing the extent of condition to those systems addressed by GL 2008-01.
The inspectors reviewed the licensees methodology in determining the system operability and operability limits for gas voids in ECCS piping to ensure there was reasonable assurance that the ECCS could perform their safety function if required. The calculations reviewed were associated with Unit 1 and Unit 2 RHR and SI systems.
The inspectors reviewed:
- System drawings and flow diagrams to ensure that the licensee properly established the boundary for the water hammer evaluation;
- Licensee calculations to ensure that the licensees methodology for determining potential water hammer pressures/forces for various gas void sizes was sound and that inputs and assumptions were reasonable and reliable;
- Licensee calculations to ensure that pipe stresses were analyzed using the peak loads to evaluate the bounding void sizes on the piping system; and
- Design criteria documents to ensure that the calculated pipe and pipe support forces were within the licensees operability limits.
In addition, the inspectors:
- Conducted interviews with licensee personnel and reviewed supporting documents to ensure that the licensees engineering judgment used to screen out pipe supports with load increases of 10 percent or less from further evaluation based on operability allowable limits established in design criteria documents was reasonable;
- Conducted interviews with licensee personnel and reviewed the licensees evaluation of valves that are required to operate (active) to ensure that they would remain functional after a system transient due to gas voiding; and
- Reviewed a valve vendor manual to ensure that the vendor recommendations and/or operational limits were not exceeded by the calculated pressure transients.
The inspectors concluded that there was reasonable assurance that the ECCS would have performed their design basis functions as long as any gas voiding was maintained within the operability limits established by the licensee.
.8 Charter Item 8: Assess licensees actions to capture and evaluate the differences
between Unit 1 and Unit 2 as they related to conclusions of the root cause evaluation.
The root cause evaluation was not completed in time for this inspection. At the close of the inspection, the licensee approved a root cause evaluation charter.
The team noted that the licensee was aware and had taken into account unit differences in their ECCS venting methodology that has been incorporated in unit-specific venting procedures. The team also verified that unit differences were addressed in the calculations associated with the interim acceptance criteria established.
.9 Charter Item 9: Assess the licensees operability evaluation, including compensatory
measures. Review actions against applicable procedures and 10 CFR 50.59 if applicable.
The team reviewed the licensees prompt operability determination (CR 1407257) that provided interim limits for voiding of both units ECCS. The limits established are listed under Charter Item 2. The team reviewed the calculations associated with these limits and determined that there was reasonable assurance for operability. Additional details associated with the inspectors review of these calculations is documented under Charter Item 7. The team reviewed trend data and determined that the frequency for UT and venting of the ECCS systems was conservative. Initially, UT and venting was completed each shift. The licensee appropriately changed the frequency based on trend data.
.10 Charter Item 10: Collect data necessary to support a risk analysis. Specifically, obtain
information associated with the degree to which the SI and RHR systems were affected, extent of damage from water hammer, and the ability to recover from damage.
The team determined, based on review of the operability acceptance criteria calculations for RHR and SI voiding, that the NRC had reasonable assurance that the systems remained operable but non-conforming, and that no damage would be expected. The team reviewed historical voiding data documented since March 2016 for both units ECCS systems to verify the voiding measured was bounded by the interim acceptance criteria.
INSPECTION RESULTS
Licensee-Identified Non-Cited Violation IP 93812 Special Inspection This violation of very low safety significance (Green) was identified by the licensee and has been entered into the licensees corrective action program and is being treated as a Non-Cited Violation, consistent with Section 2.3.2 of the Enforcement Policy.
Violation: Title 10 of the Code of Federal Regulations (10 CFR) Part 50 (10 CFR 50),
Appendix B, Criterion III, Design Control, requires the licensee to effectively implement design control measures for piping analysis calculations* associated with the Unit 1 and Unit 2 emergency core cooling systems (ECCS).
Contrary to the above, since initial operation of Unit 1 in 1996 and Unit 2 in 2016, Tennessee Valley Authority failed to ensure the proper hydraulic time history was utilized in TVAs TPIPE special purpose computer program used to determine static and dynamic linear elastic analyses for the ECCS including the effects of pipe voiding. This resulted in non-conservative voiding design acceptance criteria for the RHR and SI systems of both units.
This performance deficiency was more than minor because it was associated with the Design Control attribute of the Mitigating Systems Cornerstone and affected the cornerstone objective of ensuring the availability, reliability, and capability of systems that respond to initiating events to prevent undesirable consequences. Specifically, the failure to utilize proper hydraulic time history in the licensees TPIPE computer model resulted in developing non-conservative voiding acceptance criteria that was used during operation that could challenge ECCS functionality. The finding was determined to be of very low safety significance since additional analysis determined with reasonable assurance that the systems remained operable but non-conforming and would have performed their safety function.
Significance/Severity Level: Green. Using Inspection Manual Chapter 0609, Appendix A, The Significance Determination Process (SDP) for Findings At-Power, dated June 19, 2012, the inspectors determined that this finding was of very low safety significance (Green)because the finding affected the design or qualification of mitigating systems; however, the mitigating systems maintained their operability.
Corrective Action Reference: CR 1407257
- TPIPE Piping analysis calculations impacted Unit 1 Analysis Number Description Current Revision N36303A SI pumps to shield building 19 penetrations N37401A RHR pumps to shield 35 building penetrations 060002000901 SI/RHR piping accumulators 31 1 and 4 to cold legs 060002000902 SI/RHR piping accumulators 29 2 and 3 to RCS cold legs 060002000903 SI piping cold leg injection 17 piping 060002000904 SI piping SI pumps to RCS 16 loop Unit 2 N36304A SI pumps to penetrations 14 N37404A RHR pumps to penetrations 7 060002500901 SI/RHR piping accumulators 9 to cold legs 060002500902 SI/RHR piping penetration to 14 loop 2 and 3 cold legs and accumulators 060002500903 RHR piping penetration to 1 inline anchors N36301R SI piping between two in line 0 anchors
EXIT MEETINGS AND DEBRIEFS
The inspectors confirmed that proprietary information was controlled to protect from public disclosure.
On May 14, 2018, the team presented the special inspection results to Mr. P. Simmons, Site Vice President, and other members of the licensees staff.
DOCUMENTS REVIEWED
Corrective Action Documents Written as a Result of the Inspection
CR 02192942, Pre-Fire Plan Enhancements
CR 1411405; Evaluate Westinghouse Recommendation of ECCS Venting
CR 1411408; Door/Hatch Configuration Control During Containment Entry
Procedures
NPG Standard Department Procedure NEDP-28 Emergency Core Cooling System Gas
Management Guidance. Revision 0
N-UT-11, Ultrasonic Examination for Detecting and Measuring Fluid Levels in Austenitic and
Ferritic Systems, Revision 7
Drawings
Unit 1, 1-47W810-1, Flow Diagram Residual Heat Removal System, Revision 20
Unit 2, 2-47W810-1, Flow Diagram Residual Heat Removal System, Revision 26
Unit 1, 1-47W811-1, Flow Diagram Safety Injection System, Revision 64
Unit 2, 2-47W811-1, Flow Diagram Safety Injection System, Revision 54
Unit 1, H-435-007-026, Design Change Authorization (DCA) 39382-12, Prob. N3-63-R45R, Anal.
Isometric for SIS Vent (1-VTV-63-654) Revision 1
Unit 1, 47W435-219, Problem 0600200-09-03 PENET X-33, Isometric-Static, Thermal & Dynamic
Analysis of SIS System, 4 & 2 Cold Leg Injection from PEN X-33 to Break PTANC on 09-01 &
09-04, Revision 4
Unit 1, 1-47W435-260B, Problem 0600200-09-01, Analysis Isometric of SIS Piping, Revision 2
Unit 1, 1-47W435-280, Problem 0600200-09-04, Analysis Isometric of SIS Piping, Revision 2
Unit 1, 1-47W435-283, Problem 0600200-09-07, Analysis Isometric of SIS Piping, Revision 2
Unit 1, 1-47W435-284, Problem 0600200-09-09, Analysis Isometric of SIS Piping, Revision 2
Unit 1, 1-47W435-284A, Problem 200-09-09, Analysis Isometric of SIS Piping, Revision 0
Unit 1, 47W435-223, Prob. 0600200-09-10, 6 x 2 Safety Inj from Hot Leg #2 to 09-09 Bk Pt,
Revision 5
Unit 1, 1-47W435-220A, Problem 0600200-09-02, Analysis Isometric of SIS Piping, Revision 0
Unit 1, H-435-008-008, DCA 39382-13, Prob. N3-63-R45R, Anal. Isometric for SIS Vent
(1-VTV-63-660) Revision 1
Unit 1, H-435-009-017, DCA 39382-16, Prob. N3-63-R45R, Anal. Isometric for SIS Vent (1-VTV-
63-659) Revision 1
Unit 1, H-435-007-027, DCA 39382-14, Prob. N3-63-R45R, Anal. Isometric for SIS Vent (1-VTV-
63-661) Revision 1
Unit 2, 2-47W435-267, Prob. 0600250-09-02 063 Safety Inj From SCV PEN X-20B to Loops 2& 3
Cold Legs and 2 & 3 Cold Leg Accumulators, Revision 3
115E005, Motor Op Gate Valve 08000GM84FEB01B 8-300 ASME CL.1 GPO Assy, Revision 0
Calculations
CEB850223958, Calculation Of Pipe Support No. 2-63-064, Revision 3
Design Criteria Documents
WB-DC-20-33, Piping And Pipe Support Operability Criteria, Revision 2
Engineering Work Requests
EWR18CIV074252, Civil Evaluation For PDO 1407257, 04/30/2018
EWR18MEC074254, IDO Support For U2 RHR Gas Voids, 04/23/2018
EWR18CIV074251, Civil Evaluation For PDO, 04/23/2018
EWR18CIV074250, Civil Evaluation For PDO 1407257, 04/28/2018
EWR18MEC074258, IDO Support for U1 RHR Gas Voids, on CR 1407257
EWR18CIV074253, Civil Evaluation for PDO 1407257
EWR18MEC063262, Support for U1 SI Gas Voids, dated May 11, 2018
EWR18CIV063264, Civil Evaluation for PDO 1407257 for U1 Safety Injection; dated
May 11, 2018
Vendor Manual
WBN-VTD-W120-2958, Westinghouse Motor Operated Gate Valves, Manually Operated Gate
Valves, Swing Check Valves, Revision 13
Miscellaneous Documents
NRC Letter, August 23, 2011, Watts Bar Nuclear Plant, UNIT 2 - Closeout of Generic Letter
Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and
Containment spray Systems (TAC N
NRC Letter, June 3, 2011, Watts Bar Nuclear Plant, UNIT 1 - Closeout of Generic Letter
Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and
Containment spray Systems (TAC N
March 11, 2011, Watts Bar Nuclear Plant (WBN) Unit 2 -Response after Completion of the
Engineering for the, RHR, AND CSS SYSTEMS in UNIT 2 to NRC Generic Letter (GL)
Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and
Containment spray Systems
October 11, 2008, Browns Ferry Nuclear Plant (BFN) Units 1, 2 AND 3, Sequoyah Nuclear Plant
(SQN) Units 1 AND 2, AND Watts Bar Nuclear Plant (WBN) Unit 1 - 9 Month Response to
NRC Generic Letter (GL) 2008-01: Managing Gas Accumulation in Emergency Core
Cooling, Decay Heat Removal, and Containment spray Systems, dated January 11, 2008
WBNENGF10006, Self-Assessment: Readiness for NRC inspection of WBN response to
Generic Letter (GL) 200801, August 2010
April 30, 2018, Westinghouse letter: Watts Bar Nuclear Plant Units 1 & 2
Gas Voids Operating Experience Summary Information
April 20, 2018 Westinghouse letter (Proprietary) Ref: WAT-D-12424: TENNESSEE VALLEY
AUTHORITY WATTS BAR NUCLEAR PLANT UNITS 1 & 2 Gas Intrusion Assessment on
the ECCS Pump Discharge Side Piping
MEMORANDUM TO: Thomas Morrissey, Senior Resident Inspector, FROM: Catherine Haney,
Regional Administrator SUBJECT: Special Inspection Charter to Evaluate the Watts Bar
Units 1 AND 2 Emergency Core Cooling System (ECCS) Void Issues
NRC Inspection Report, Watts Bar Nuclear Plant Unit 2 Construction-NRC Integrated Inspection
Report 05000391/2013605, Section OA.1.25, ML13220A640
Report of Calibration, Asset ID E53749, Ultrasonic Flaw Detector, dated 2/13/18
NRC Integrated Inspection Report 05000390/2011005, ML120310076
Certificate of Method Qualification Tennessee Valley Authority dated 3/17/17
Visual Acuity Examination Record for NDE Personnel, EIN 1C2IUW35L, dated 3/15/18
Digital Ultrasonic Calibration Data Sheet, Report Number:BOP-5098, dated 4/23/18
Log Entries Report, Unit 1, for 4/19/18, 4/20/18, and 4/21/18
Log Entries Report, Unit 2, for 4/18/18, 4/21/18, and 4/22/18
Standing Order 18-10; ODMI for ECCS Venting, May 4, 2018
Work Order 119549615; ECCS Ultrasonic Water Level Measurements
WAT-D-12429, Transmittal Of RHR System Discharge Side Non-Condensable Gas Water
Hammer Evaluation, 04/27/2018
WAT-D-12424, Gas Intrusion Assessment on the ECCS Pump Discharge piping, April 20, 2018
Pipe Support Load Capacity Data Sheet Manual, Revision 0
Component Evaluation For Gas Void Transport, Revision 1
Corrective Action Documents
CR 1408251; WBN-1-VTV-063-0101-A, Cold Leg 1 RHR Inj Line Vent; April 24, 2018
CR 1408589; WBN-1-063-0010-A Safety Injection Pump 1A-A; April 24, 2018
CR 1408923; WO Acceptance Criteria Incorrectly Calculated, April 25, 2018
CR 1408948; This CR is to Document NRC Resident Question Regarding ECCS Operability for
Condition in CR 1408573; April 25, 2018
CR 1409089; Completed Urgent Change to 2-SI-63-10.1-A, April 26, 2018
CR 1411256; WBN-1-FCV-063-0111, Cold Leg 2 and 3 RHR Check Valve Leak Test Isolation
CR 1411262; WBN-1-FCV-063-0112, Cold Leg 1 and 4 RHR Check Valve Leak Test Isolation
Prompt Determination of Operability Documentation for CR 1135820, March 18, 2016
Prompt Determination of Operability Documentation for CR 1407257, Revision 1 through 6
CR 1378163, Snubber Support Design Legacy Issue, 01/17/2018
ATTACHMENT 1
TIMELINE OF EVENTS
Date Time Documentation Description
2003 Unit 1: Twelve ECCS piping vents were installed
June 3, 2011 NRC Correspondence NRC Letter issued for close out of Generic Letter 2008-01 for Unit 1
August 23, 2011 NRC Correspondence NRC Letter issued for close out of GL 2008-01 for Unit 2 containing 7
ML111520233 commitments
November 7, 2011 EWR Gas void Acceptance Criteria (AC) set at: RHR 22 ft3; SI 16 ft3;
MDQ0016320110182 performed as a result of CRs 950029, 962314, and 80602.
January 31, 2012 NRC Integrated NRC closeout of Unit 1 Instruction (TI) 2515/177, Managing Gas Accumulation
Inspection Report in Emergency Core Cooling, Decay Heat Removal, and Containment Spray
05000390/2011005 Systems (GL 2008-01), and Violation 2011005-04 for failure to quantify
ML120310076 accumulated gas in ECCS
February 12, 2016 CR 1136791 Unit 1 Cold Leg accumulators 2,3,4 are losing 36 gallons per day. This is
significant since a source of gas voids is leakage through accumulator isolation
valves into the ECCS systems
February 24, 2016 1-SI-63-10.1-A Started taking Ultrasonic data for ECCS gas void measurement on Unit 1
March 18, 2016 PDO 1134820 Unit 1 ECCS gas void AC: RHR 40 ft3 SI 40 ft3; Reference Letter LTR-LIS-08-
27
March 31, 2016 2-SI-63-10.1-A Started taking Ultrasonic data for void measurement on Unit 2
April 1, 2016 PDO 1156054 Unit 2 ECCS void AC: RHR 40 ft3 SI 40 ft3 Reference Letter LTR-LIS-08-627
May 10, 2016 LER 390/2016-003-00 Technical Specification Surveillance Requirement Not Met During Emergency
Core Cooling System Venting
June 5, 2016 CR 1178400 Unit 2 Safety Injection (SI) during reactor trip due to failure of the linkage in the
linear variable differential transmitter in the Electro-Hydraulic Control System.
This event and its effect on the plant was used as part of the operability
determination on April 19, 2018 for CR 1407257.
August 12, 2016 NRC Integrated Green NCV 2016002-10; Failure to Maintain an Adequate Surveillance
Inspection Report Procedure for Emergency Core Cooling System Venting.
August 2, 2017 Design Change Notice Unit 1, Vendor supplied detailed analysis for loads in ECCS. Design criteria for
(DCN) 66453 RHR was 40.0 ft3 and for SI 27.6 ft3; Reference Calculation
MDQ001106320110182
ATTACHMENT 1
TIMELINE OF EVENTS
April 28, 2017 Plant Process Accumulator #3 leakage after outage repairs is minimal
Computer data
June, 2017 Plant Process Accumulator #3 leakage starts increasing to about 20 gallons per day
Computer data
October 3, 2017 Check Valve 63-555, Accumulator #3 valve found to be leaking into ECCS,
scoped into next outage
2018
February 8 CR 1127959 An error was discovered during reviews for a DCN to the AFW system on Unit 2.
Specifically, it was planned to replace both motor driven AFW pump discharge
pressure control valves with cavitating flow venturis to provide more reliable
pressure control. The DCN required a review of the design bases piping stress
analyses for the purposes of assessing the impact of this change. During these
reviews, a CR was written to capture errors discovered in the way the analyses
were used by the vendor during the original development of the Watts Bar
design bases. An extent of condition (EOC) action was created in order to
evaluate if the errors also resulted in a challenge to the maximum allowable pipe
stresses in other systems across both units.
April 19 1000 As part of the EOC evaluation from the 2016 AFW issue, a request made to
Fauske Associates, Inc (FAI) to provide an assessment by the end of the day of
what the acceptable void sizes could be for both units at WBN, to support an
Immediate Determination of Operability (IDO).
April 19 1822 Draft assessment is provided by FAI that concluded the following based on
comparisons between Watts Bar plant specific parameters and a Westinghouse
4loop PWR:
- RHR system for each unit can tolerate up to 1.3 ft3 of total non
condensable gas. Any void size larger than 1.3 ft3 may potentially cause
a relief valve lift.
- SI system for each unit can tolerate up to 5 ft3 of total noncondensable
gas. Five cubic feet is the maximum void size that is allowed to be
injected into the primary system for Westinghouse plants.
due to the hydrodynamic loads generated for the circumstance listed
above. This is based on the experience with the operating fleet and the
analysis performed for the sample Westinghouse 4loop plant.
It was noted that the conclusions above were to be considered as professional
opinion based on experience to support a short term decision. However, it was
ATTACHMENT 1
TIMELINE OF EVENTS
recommended that Watts Bar should establish their discharge side non-
condensable gas acceptance criteria based on engineering analysis, which
should be acceptable to US NRC rules and regulations.
April 19 1944 CR 1407257 A condition report was generated that identified the current acceptance criteria
for gas accumulation in the WBN Unit 1 and Unit 2 SI system and RHR system
discharge piping may have been non-conservative. A sensitivity run of the
piping analyses was performed by Engineering, and for the worst case piping
analysis, the loads provided by the vendor would have to be reduced by up to
65% in order for the piping and supports to pass the operability allowable limits.
The IDO concluded the ECCS for both units were operable based in part on the
draft assessment from FA
- I.
April 19 2216 Operating Logs Unit 2 entered TS 3.0.3 due to low head RHR voiding measured at 1.302,
exceeding the acceptance criteria of 1.300
April 20 0018 Operating Logs Unit 2 retracted LCO 3.0.3 based on Engineering Work Request (EWR)
18MEC063243 provided by Engineering.
April 20 0055 EN 53349 On April 19 at 1944, WBN determined that a preliminary analysis shows current
acceptance criteria for gas accumulation in the WBN Unit 1 and Unit 2 SI and
RHR discharge piping may be non-conservative. The surveillances that check
void values and allow venting of the systems are to be performed utilizing
conservative criteria at more frequent intervals to ensure gas void volumes
remain under acceptable limits. Additional analysis is being performed to
determine final actions
April 20 1513 WAT-D-12424 Westinghouse submitted the formal Gas Intrusion Assessment on the ECCS
Pump Discharge Side Piping. The three draft conclusions remained the same
but an additional conclusion was added that these limits will still be applicable
under increased pressures in RHR (greater than the hydrostatic pressure from
Refueling Water Storage Tank).
April 21 0105 Operating Logs The PDO was issued by Engineering that determined the Unit 1 and Unit 2
PDO Rev 0 ECCS were operable but non-conforming. Compensatory measures were made
to increase the frequency of the ECCS void surveillance as necessary.
April 21 2152 Operating Logs Unit 1 entered Technical Specification (TS) Limiting Condition for Operation
CR 1407731 (LCO) 3.0.3 due to void size of 5.632 ft3 in the low pressure portion of ECCS
(RHR) on piping common to both trains of ECCS exceeding the allowable void
size of 1.302 ft3.
April 21 2222 Operating Logs Unit 1 exited LCO 3.0.3 after venting to an acceptable void volume of the
affected piping.
ATTACHMENT 1
TIMELINE OF EVENTS
April 22 0222 Operating Logs Unit 2 entered TS 3.0.3 due to void size of 1.367 ft3 in the low pressure portion of
ECCS (RHR) on piping common to both trains of ECCS exceeding the allowable
void size of 1.302 ft3.
April 22 0234 EN 53355 On April 21 at 2152, WBN Unit 1 entered TS LCO 3.0.3 due to both trains of the
RHR becoming inoperable. During surveillance testing, the gas void values on
ECCS piping common to both trains did not meet acceptance criteria. This
caused both RHR trains to become inoperable. Operations subsequently vented
the RHR to meet the acceptance criteria and exited TS LCO 3.0.3 at 2222 EDT.
More frequent surveillances will be conducted to monitor gas void volumes while
additional analysis is being performed to determine corrective actions.
April 22 0428 EN 53356 On April 22 at 0222, WBN Unit 2 entered TS LCO 3.0.3 due to both trains of the
RHR becoming inoperable. During surveillance testing, the gas void values on
ECCS piping common to both trains did not meet acceptance criteria. This
caused both RHR trains to become inoperable. Operations subsequently vented
the RHR to meet the acceptance criteria and exited TS LCO 3.0.3 at 0227 EDT.
More frequent surveillances will be conducted to monitor gas void volumes while
additional analysis is being performed to determine corrective actions
April 24 PDO Rev 1 Revision 1 to the PDO was issued containing a recommended change to the
CR 1407257 frequency of the SI performances for both Unit 1 and Unit 2, to once per 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />
shift as driven by CR 1407801 and as a result of unexpected TS 3.0.3 entry on
April 4, 2018, when following the guidance provided in Revision 0 of this PDO.
April 25 Procedure 1-SI-63- Change venting process to static and dynamic venting for Unit 1 in 1-SI-63-10.1-
10.1-A; Urgent A ECCS Discharge Pipes Venting - Train A Inside Containment
Change #1
April 25 1538 Chemistry data sheet Chemistry sampling determines void does not contain Hydrogen which is an
indication that RCS fluid is not back-leaking into ECCS systems as a source of
the voids.
April 26 0849 Chemistry data sheet Chemistry sampling results confirm ECCS gas voids do not contain Hydrogen
April 27 1113 Chemistry data sheet Chemistry sampling results confirm ECCS gas voids do not contain Hydrogen
April 27 Pressurized the Unit 1 Cold Leg accumulator with Nitrogen (N2), no significant
void growth observed.
April 27 PDO Rev 2 CR This revision changed the acceptance criteria for U2 RHR gas void acceptance
1407257 criteria to 4 ft3
April 28 EWR Changed Unit 2 venting to once per day
18MEC0635256,
April 28 Pressurized Unit 1 Cold Leg accumulator with N2, no significant void growth
ATTACHMENT 1
TIMELINE OF EVENTS
April 30 1900 PDO Rev 3 This revision changed the Unit 1 RHR gas void acceptance criteria to 3.5 ft3 and
CR 1407257 clarified that the Unit 2 limit of 4.0 ft3 was an alert criteria limit only.
May 1 1030 EWR 18MEC063256 Changed Unit 2 RHR venting frequency to 1 time every two days.
May 3 1700 PDO Rev 4 This revision clarified that the Unit 1 RHR void volume criteria of 3.5 ft3 was an
alert level only. Also changed Unit 1 venting frequency to once a day
May 4 EWR Unit 2 changed venting frequency to once every three days as allowed by the
18MEC0635256, rev 1 EWR.
May 4 1455 EN 53356 This event was retracted. The initial report was based on a conservative
acceptance criteria for gas accumulation adopted on April 19, 2018, when it was
determined that the previously used acceptance criteria for gas accumulation in
the ECCS was non-conservative. Additional analysis has subsequently been
performed and determined that a higher gas accumulation acceptance criteria
does not challenge operability. With a void of less than the acceptance criteria, in
the event of ECCS actuation, the system piping support loads will remain within
structural limits and the piping system will remain operable. Therefore, both
trains of Unit 2 RHR were operable and the previously reported 10 CFR 50.72(b)(3)(v)(B) event is being retracted.
May 7 1600 EWR 18MEC0635256 Unit 1: Changed to daily monitoring of voids with a venting frequency change to
vent once every 2 days
May 7 CR 1407257 Root Cause Charter Issued
May 9 PDO Rev 5 This revision changed the gas void volume criteria for Unit 2 SI to 1 ft3 for an
alert limit and 27.5 ft3 as the operability limit
May 9 1310 EN 53355 This event was retracted. The initial report was based on a conservative
acceptance criteria for gas accumulation adopted on April 19, 2018, when it was
determined that the previously used acceptance criteria for gas accumulation in
the ECCS was non-conservative. Additional analysis has subsequently been
performed and determined that a higher gas accumulation acceptance criteria
does not challenge operability. With a void of less than the acceptance criteria, in
the event of ECCS actuation, the system piping support loads will remain within
structural limits and the piping system will remain operable. Therefore, both
trains of Unit 1 RHR were operable and the previously reported 10 CFR 50.72(b)(3)(v)(B) event is being retracted.
May 10 EWR 18MEC063268 Recommended weekly venting on unit 2 ECCS based on the trend results of
2-SI-63.10.1-
ATTACHMENT 1
TIMELINE OF EVENTS
May 11 PDO Rev 6 This revision changed the gas void volume criteria for Unit 1 SI to 2.5 ft3 for an
alert limit and 16.6 ft3 as the operability limit alert limit and 16.6 ft3 as the
operability limit
ATTACHMENT 2
Charter
UNITED STATES
NUCLEAR REGULATORY COMMISSION
REGION II
245 PEACHTREE CENTER AVENUE NE, SUITE
200 ATLANTA, GEORGIA 30303-1257
April 26, 2018
MEMORANDUM TO: Thomas Morrissey
Senior Resident Inspector
Division of Reactor Projects
FROM: Catherine Haney /RA Laura A. Dudes Acting for/
Regional Administrator
SUBJECT: SPECIAL INSPECTION CHARTER TO EVALUATE THE WATTS
BAR UNITS 1 AND 2 EMERGENCY CORE COOLING SYSTEM
(ECCS) VOID ISSUES
You have been selected to lead a Special Inspection to assess the circumstances surrounding
ECCS void issues which resulted in both residual heat removal (RHR) trains being inoperable
for Unit 1 on April 21, and Unit 2 on April 22, 2018. Your onsite inspection should begin on
April 30, 2018. John Jandovitz will be assisting you in this inspection.
A. Basis
On April 19, 2018, the Watts Bar Nuclear Plant (WBN) Operations Superintendent called the
WBN NRC resident inspector to report that WBN was notified by their contractor of errors in
calculations relating to the acceptable ECCS void size. The calculations were being
reviewed as part of an extent of condition for an auxiliary feedwater stress analysis. The
initial acceptance criteria was based on an acceptable delay, due to voiding, in injecting
water into the reactor coolant system (RCS). The new void volumes are based on stresses
from a water hammer event when ECCS is initiated with voids present. This new criteria is
significantly lower than the previous acceptance criteria. Specifically, the RHR void limit was
reduced from 40 ft3 to 1.3 ft3 and the safety injection (SI) void limit was reduced from 27.6 ft3
to 5 ft3. These reduced values were provided to WBN by Westinghouse on April 19, 2018,
and were based on their experience with similar plants and were not specific to either unit at
WB
- N.
An immediate determination of operability was completed on April 20, 2018, and determined
that the affected ECCS systems on Units 1 and 2 were operable but degraded with interim
compensatory measures established. The interim compensatory measures included
increased frequency of void measurement and increased venting of ECCS from monthly to
daily.
On April 21, 2018, WBN contacted the resident inspector to inform him that, for the most
recent void determination on Unit 1, the accumulated gas in RHR was found to be greater
ATTACHMENT 2
Charter
than the 1.3 ft3 limit (approximately 5 ft3). Unit 1 then entered Technical Specifications (TS)
Limiting Condition for Operation (LCO) 3.0.3 and exited once RHR was vented.
On April 22, 2018, WBN contacted the resident inspector to inform him that, for the most
recent void determination on Unit 2, the accumulated gas in RHR was found to be greater
than the 1.3 ft3 limit (approximately 1.367 ft3). Unit 2 then entered TS LCO 3.0.3 and exited
once RHR was vented.
In accordance with Management Directive (MD) 8.3, NRC Incident Investigation Program,
deterministic and conditional risk criteria were used to evaluate the level of NRC response
for this operational event. Through review of the MD 8.3 deterministic criteria, the staff
concluded that this event involved technically complex void issues in the ECCS systems
with both RHR trains inoperable for both units. The conditional core damage probability
(CCDP) for this event was in the overlap region between a Special Inspection and No
Additional Inspection. Region II determined that the appropriate level of NRC response is a
Special Inspection.
This Special Inspection is chartered to identify the circumstances surrounding ECCS void
issues and assess the adequacy of the licensees actions to address the causes of these
issues.
B. Scope
The inspection is expected to perform data gathering and fact-finding in order to address the
following:
1. Develop a complete sequence of events related to the discovery that the acceptance
criteria for gas accumulation in the WBN Unit 1 and Unit 2 SI system and RHR
system discharge piping may be non-conservative, which resulted in both Units RHR
system being inoperable as well as follow-up actions taken by the licensee.
2. Determine if the licensees acceptance criteria for surveillance requirements to
minimize voids in the ECCS are consistent with the plant licensing basis and as-built
design.
3. Review drawings, walkdown portions of the RHR and SI systems, and determine if
the vent locations enable system venting to meet void size acceptance criteria.
4. Review licensee procedures and processes for identifying, quantifying, trending, and
venting the gas.
5. Evaluate the licensees venting process activities to ensure that there is no adverse
impact to plant operations. Review the configuration control and circumstances
surrounding the Unit 1 RHR over-pressurization event that occurred on April 24,
2018.
6. Review the licensees adherence to commitments and guidance associated with
Generic Letter (GL) 2008-01. Compare operating experience involving gas voiding of
ECCS discharge piping to actions implemented at WBN. Determine if there are any
generic issues related to the design and operation practices that resulted in the
voiding of the ECCS discharge piping beyond those already described in GL 2008-
01. Promptly communicate these issues to NRC regional management.
ATTACHMENT 2
Charter
7. Review the licensees calculations relating to ECCS void size. Review key
assumptions and facts. Determine if the licensees root cause analysis and corrective
actions have addressed the extent of condition for gas accumulation in ECCS
systems.
8. Assess licensees actions to capture and evaluate the differences between Unit 1
and Unit 2 as they relate to conclusions of the root cause evaluation.
9. Assess the licensees operability evaluation, including compensatory measures.
Review actions against applicable procedures and 10 CFR 50.59, if applicable.
10. Collect data necessary to support a risk analysis. Specifically obtain information
associated with the degree to which the SI and RHR systems were affected, extent
of damage from a potential water hammer, and the ability to recover from damage.
C. Guidance
Inspection Procedure (IP) 93812, "Special Inspection," provides additional guidance to be
used during the conduct of the Special Inspection. Your duties will be as described in IP 93812. The inspection should emphasize fact-finding in its review of the circumstances
surrounding the event. Safety or security concerns identified that are not directly related to
the event should be reported to the Region II office for appropriate action.
You will report to the site, conduct an entrance, and begin inspection no later than April 30,
2018. A daily status briefing of Region II management will be provided beginning the first
day onsite at approximately 3:00 p.m., Eastern Daylight Time (EDT).
In accordance with IP 93812, you should promptly recommend a change in inspection scope
or escalation if information indicates that the assumptions utilized in the MD 8.3 risk analysis
were not accurate. A report documenting the results of the inspection should be issued
within 45 days of the completion of the inspection. The report should address all applicable
areas specified in Section 3.02 of Inspection Procedure 93812. At the completion of the
inspection, you should provide recommendations for improving the reactor oversight
process baseline inspection procedures and the Special Inspection process based on any
lessons learned.
This charter may be modified should you develop significant new information that warrants
review. Should you have any questions concerning this charter, contact Anthony Masters at
404-997-4465.
Docket Nos.: 50-390, 50-391
License No.: NPF-90, 96
ADAMS Accession No. ML18116A422
3