05000390/LER-2018-002, Regarding Shield Building Inoperability Due to Annulus Vacuum Transient
| ML18078B105 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 03/19/2018 |
| From: | Simmons P Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 2018-002-00 | |
| Download: ML18078B105 (8) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3902018002R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 March 19, 2018 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No. 50-390 10 CFR 50.73
Subject:
Licensee Event Report 390/2018-002-00, Shield Building lnoperability Due to Annulus Vacuum Transient This submittal provides Licensee Event Report (LER) 390/2018-002-00. This LER provides details concerning a condition where spurious equipment operation resulted in the shield building pressure limits exceeding Technical Specification allowed values. This condition is being reported as an event that could have prevented fulfillment of a safety function in accordance with 10 CFR 50. 73(a)(2)(v). A supplement to this LER will provide additional information related to the cause and corrective actions associated with this event, and is expected to be submitted by May 18, 2018.
There are no regulatory commitments contained in this letter. Please direct any questions concerning this matter to Kim Hulvey, WBN Licensing Manager, at (423) 365-7720.
Respec~lly, ~ ~
m~
Paul Simmons Site Vice President Watts Bar Nuclear Plant Enclosure cc: See Page 2
U.S. Nuclear Regulatory Commission Page 2 March 19, 2018 cc (Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020 (02-2018)
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
3.Page Watts Bar Nuclear Plant, Unit 1 05000390 1 OF 6
- 4. Title Shield Building lnoperability Due to Annulus Vacuum Transient
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Facilities Involved I
Sequential I Rev Facility Name Docket Number Month Day Year Year Month Day Year N/A 05000 Number No.
Facility Name Docket Number 01 17 2018 2018 - 002 00 03 19 2018 05000
- 9. Operating Mode
- 11. This Report is Submitted Pursuant to the Requirements of 10 CFR §: (Check all that apply)
D 20.2201 (b)
D 20.2203(a)(3)(i)
D 50. 73(a)(2)(ii)(A)
D 50. 73(a)(2)(viii)(A) 1 D 20.2201(d)
D 20.2203(a)(3)(ii)
D 50. 73(a)(2)(ii)(B)
D 50. 73(a)(2)(viii)(B)
D 20.2203(a)(1)
D 20.2203(a)(4)
D 50. 73(a)(2)(iii)
D 50. 73(a)(2)(ix)(A)
D 20.2203(a)(2)(i)
D 50.36(c)(1 )(i)(A)
D 50. 73(a)(2)(iv)(A)
D 50. 73( a)(2)(x)
- 10. Power Level D 20.2203(a)(2)(ii)
D 50.36(c)(1 )(ii)(A)
D 50.73(a)(2)(v)(A)
D 73.71 (a)(4)
D 20.2203(a)(2)(iii)
D 50.36(c)(2)
D 50. 73(a)(2)(v)(B)
D 73. 71 (a)(5)
D 20.2203(a)(2)(iv)
D 50.46(a)(3)(ii)
[g] 50. 73(a)(2)(v)(C)
D 73.77(a)(1) 100 D 20.2203(a)(2)(v)
D 50. 73(a)(2)(i)(A)
[g] 50. 73(a)(2)(v)(D)
D 73.77(a)(2)(i)
D 20.2203(a)(2)(vi)
D 50. 73(a)(2)(i)(B)
D 50. 73(a)(2)(vii)
D 73.77(a)(2)(ii)
D 50. 73(a)(2)(i)(C)
D OTHER Specify in Abstract below or in 2018 002 D. Manufacturer and Model Number of Components that Failed During the Event No failed components are associated with this event. Tuning of the control dampers was found not to be optimal.
E. Other Systems or Secondary Functions Affected
No other systems or secondary functions were affected.
F.
Method of discovery of each Component or System Failure or Procedural Error
Plant alarms indicated that shield building vacuum was less than required.
G. Failure Mode and Effect of Each Failed Component Not applicable.
H. Operator Actions
Upon receipt of the alarms, operations personnel responded in accordance with operating procedures.
I.
Automatically and Manually Initiated Safety System Responses
Not applicable. While operators swapped the annulus vacuum control dampers, this is not a safety related system.
00 Ill.
CAUSE OF THE EVENT
A The cause of each component or system failure or personnel error, if known.
No component failures were associated with this event. Control damper tuning was found not to be optimal.
B. The cause(s) and circumstances for each human performance related root cause.
Additional investigation is in progress related to this issue.
IV.
ANALYSIS OF THE EVENT
The WBN containment design includes a free standing steel pressure vessel surrounded by a reinforced concrete shield building. The shield building is maintained at a negative pressure of greater than -5 inches w.g. as required by TS during normal operation by the annulus vacuum control system, which is not safety-related. To address instrument uncertainty, the operations alarm for the shield building is set at -5.5 in.
w.g. The annulus vacuum control system consists, in part of redundant paths for pressure control, each of which include an isolation damper (1-PC0-65-93 and 1-PC0-65-94) and a control damper (1-PC0-65-48 and 1-PC0-65-49) to maintain the shield building vacuum at a setpoint of -6.2 in. w.g. (see simplified 2018 002 sketch). In the event of an accident, the safety-related Emergency Gas Treatment System (EGTS)
{EllS:BH} would filter the exhaust from the shield building, reducing the offsite dose to members of the public and to control room operators from postulated leakage of the containment pressure vessel.
00 The design of the EGTS is that it is capable of achieving an acceptable negative pressure in the shield building annulus assuming the annulus is not at a negative pressure. While the annulus is normally expected to be at a negative pressure relative to atmospheric, the dose analysis conservatively assumes the annulus is at atmospheric pressure at event initiation. After blowdown, the annulus pressure will increase rapidly due to expansion of the containment vessel as a result of primary containment atmosphere temperature and pressure increases. The annulus pressure will continue to rise due to heating of the annulus atmosphere by conduction through the containment vessel. After a delay, the EGTS operates to maintain the annulus pressure below atmospheric pressure. Review of data traces for this event indicate that the annulus pressure went as low as approximately -4.6 inches w.g which bounds the analysis limit of atmospheric pressure.
Accordingly, while the shield building was outside its normal pressure limits, the safety function of the shield building to limit dose to the public and to control room operations personnel was not lost.
On January 12, 2018, a surveillance test was performed on Train A EGTS. During this test, a pressure transient occurred that was allowed by the test which resulted in opening the isolation dampers of both annulus vacuum control paths, which is an abnormal configuration. This also resulted in both associated modulating dampers controlling annulus vacuum in a near fully closed position. This configuration was not recognized by the operating staff and was not addressed in plant procedures.
On January 17, 2018, a plant assistant unit operator recognized that both annulus vacuum control dampers were in service and closed isolation damper 1-PC0-65-94 to restore the annulus vacuum system to the normal operating alignment. The control room was not notified of this configuration change.
Because both trains of modulating dampers were fully closed at this time to maintain setpoint (-6.2 in.
w.g.), annulus pressure initially went more negative than normal and the modulating damper overcorrected, resulting in a reduction in annulus vacuum below that required by TS 3.6.15. When the assistant operator isolated the backup control damper, the plant response was not as expected, and the unit supervisor directed swapping to the backup control dampers to restore pressure. The slow response of the dampers contributed to this event.
V.
ASSESSMENT OF SAFETY CONSEQUENCES
As described in the previous section, the safety function of the shield building to mitigate a design basis accident was not lost. Therefore the consequences of this event are low.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Both trains of EGTS remained operable during this event.
B.
For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident 2018 002 Not applicable.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service The shield building was outside of its normal allowed negative pressure for approximately 22 minutes, however the shield building could still perform its safety function.
00 VI.
CORRECTIVE ACTIONS
This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Reports (CRs) 1378357, 1387151, and 1395914.
A
Immediate Corrective Actions
Operations personnel took prompt action to restore shield building vacuum to greater than 5 inches w.g. by swapping to the backup control damper. The dampers associated with the Unit 1 annulus vacuum control system were tuned to provide better response during a system transient.
- 8. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future Procedure changes related to swapping the annulus vacuum control dampers will be completed to address this issue. Preventative maintenance activities associated with the annulus vacuum control dampers will be revised to require field tuning to ensure proper damper response. A performance analysis will be completed respect to the assistant unit operator's action. Additional corrective actions are under evaluation and will be provided in a supplement to this report.
VII.
PREVIOUS SIMILAR EVENTS AT THE SAME SITE
LER 390/2017-008 reported an event where shield building vacuum was less than -5 inches *w.g. due to spurious equipment operation. As described above, no actual loss of safety function capability occurred because the safety function of the EGTS to reduce radiological dose is performed even if the shield building is not at a vacuum.
LER 390/2017-007 reported a number of events in the past three years where shield building vacuum was less than -5 inches w.g. These events were the result of either unexpected plant response during Unit 2 power ascension testing or were the result of equipment failures. As described above, no actual loss of safety function capability occurred because the safety function of the EGTS to reduce radiological dose is performed even if the shield building is not at a vacuum.
VIII.
ADDITIONAL INFORMATION
None.
IX.
COMMITMENTS
None.
MAINTAINS - 6.2" WC U1 ANNULUS YEAR 05000390 2018 SEQUENTIAL NUMBER 002 REV NO.
00 AUX BLOG VENT Annulus Vacuum Control Simplified Sketch Page _6_ of _6_