05000498/LER-2023-003, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function
| ML24009A282 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 01/09/2024 |
| From: | Tomlinson J South Texas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NOC-AE-24004007, 35547939, G26 LER 2023-003-00 | |
| Download: ML24009A282 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4982023003R00 - NRC Website | |
text
.......
Nuclear Operating Company South li:xas Project Electric Generating Station P.O. Box 289 Wadsworth, Texas 77483 Attention: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 1 Docket No. STN 50-498 Licensee Event Report 2023-003-00 January 9, 2024 NOC-AE-24004007 10 CFR 50.73 STI: 3554 7939 File No. G26 Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function Pursuant to reporting requirements of 10 CFR 50.73(a)(2)(v)(D), STP Nuclear Operating Company (STPNOC) hereby submits the attached South Texas Project (STP) Unit 1 Licensee Event Report 2023-003-00.
The event did not have an adverse effect on the health and safety of the public.
There are no commitments in this letter.
If there are any questions on this submittal, please contact Derrick Johnson at 361-972-7088 or me at 361-972-8945.
A~~
ason Tomlinson Site Vice President
Attachment:
Licensee Event Report 2023-003-00, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function cc:
Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511
Attachment Licensee Event Report 2023-003-00 NOC-AE-24004007 Attachment Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function
Abstract
On 11/10/23 at 0642 Essential Chilled Water 'B' train and cascading equipment was declared inoperable due to chill water temperature exceeding limits. On 11/10/23 at 1413 Essential Chilled Water 'C' train and cascading equipment was declared inoperable due to discharge pressure exceeding limits. This condition resulted in an inoperable condition on two out of the three safety trains for the accident mitigating function. This condition was determined to be reportable as required by 10 CFR 50.73(a)(2)(v)(D). Essential Chilled Water *c* train inoperability was due to the Essential Chiller's compressor shaft mechanical seal becoming degraded to the point of losing seal integrity. Essential Chilled Water *s*
train inoperability cause was indeterminate. Corrective actions include a Performance Assessment to evaluate proper flatness check and proper installation of the compressor main shaft mechanical seal. The compressor shaft seal is being sent to an offsite vendor for failure analysis. Upon completion of the failure analysis, a supplement to this Licensee Event Report will be provided.
I. Description of Event
A. Reportable Event Classification
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00498 C::7 NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 003 1-0 This event is reportable per 50.73(a)(2)(v)(D) as a condition that could have prevented fulfillment of a safety function of structures or systems that are needed to: (D) Mitigate the consequences of an accident. This event resulted in an inoperable condition on two out of the three safety trains for the accident mitigating function including 'B' and 'C' train High Head Safety Injection, Low Head Safety Injection, Containment Spray, Control Room Envelope HVAC, and Essential Chilled Water.
B. Plant Operating Conditions Prior To Event
Prior to the event on November 10, 2023, Unit 1 was in MODE 1 at 100% power.
C. Status Of Structures, Systems, and Components That Were Inoperable At the Start of the Event and That Contributed To the Event There were no inoperable Structures, Systems, or Components at the start of the event that contributed to the event.
D. Narrative Summary of the Event
Timeline (Note: All times Are Central Standard Time) 11/10/23 (0642) - Essential Chilled Water 'B' train and cascading equipment was declared inoperable due to chill water temperature exceeding limits.
11/10/23 (1413) - Essential Chilled Water 'C' train declared inoperable due to exceeding the maximum compressor discharge pressure.
11/10/23 (2127) - Shift Manager determined that two inoperable Essential Chilled Water Trains resulted in a condition that could-have prevented the fulfillment of a safety function.
11/10/23 (2152) - Event Notification 56848 was made to the NRC Headquarters Operations Officer (HOO).
11 /11 /23 (1330) - Essential Chilled Water 'B' train declared OPERABLE which also restored the safety trains relied on for the accident mitigation function.
E. Method of Discovery
The Chill Water exceeding temperature limits in Essential Chilled Water Train 'B' and the discharge pressure exceeding limits in Essential Chilled Water Train 'C' were self-revealing.
II. Component Failures
A. Failure Mode, Mechanism, and Effects of Failed Component
- 2. DOCKET NUMBER I
00498
- 3. LER NUMBER c:J NUMBER I
~-I 003 REV NO.
1-0 On 11/10/23 at 1448, Essential Chiller 12C compressor discharge pressure was reading higher than allowable due to the Essential Chiller's Compressor Shaft Mechanical Seal becoming degraded to the point of losing seal integrity. The Motor Shaft Compressor Mechanical Seal was a significant source of air in-leakage due to being in a vacuum condition whether the chiller is running or idle. The degradation to failure created a gap which allowed air in-leakage to occur and consequently raised compressor discharge pressure above the allowable limit.
The Essential Chiller 128 temperature control loop controls the Leaving Water Temperature. On 11/10/23 at 0045, at least one component in the Essential Chiller 128 temperature control loop failed to operate properly which resulted in the inability to maintain proper refrigerant pressure and flow to maintain the chilled water discharge/outlet temperature at the pre-established setting.
B. Cause of Component or System Failure
Essential Chilled Water Train 'C' When the oil filters were replaced during the troubleshooting, the existing filters were discovered to have wear materials that appeared to be from the compressor mechanical seal. Maintenance removed the seal and found it was degraded.
After replacing the compressor mechanical seal, post-maintenance testing was satisfactory with normal compressor discharge pressure. Post-event examination (checking flatness via checking for light bands) of the removed shaft seal ring indicated wear cracks and chipping at the leading edge of the shaft seal ring. The side of the shaft seal ring also had an elongated gouge mark that appeared to indicate the spring that retains the shaft seal ring in place (so that it remains stationary during chiller operation), may have moved. This movement could have led to shaft seal ring torsion that could explain the wear cracks and chipping at the leading edge of the shaft seal ring.
There are only two possible causes for premature wear degradation to failure: a) the Shaft Seal Ring was not optically flat, orb) improper installation. The Preventative Maintenance work instructions have Maintenance check for optical flatness prior to installation and this step was recorded as "SAT." If the check for optical flatness was done correctly, then the most likely cause is improper installation. The compressor shaft seal is being sent to an offsite vendor for failure analysis in order to determine whether this could have been caused by a defective component.
Essential Chilled Water Train 'B' Neither the Hot Gas Bypass Motor Operated Valve Motor nor the Temperature Current Module were bench tested; therefore, cause determination is indeterminate.
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00498 r::J NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 003 1-0
C. Systems or Secondary Functions That Were Affected by the Failure of Components with Multiple Functions
Failure of the components in Essential Chilled Water 'B' and 'C' trains rendered the 'B' and 'C' trains of the following systems inoperable: High Head Safety Injection, Low Head Safety Injection, Containment Spray, Control Room Envelope HVAC, and Essential Chill Water.
D. Failed Component Information
System: Chilled Water System {CH}
Component: Compressor Shaft Seal {SEAL}
Manufacturer: Pure Carbon Company Model:
System: Chilled Water System {CH}
Component: Temperature Current Limit Control O Manufacture: Borg-Warner {B350}
Model: UCI-TCM-1A System: Chilled Water System {CH}
Component: Hot Gas Bypass Valve Motor Operated Valve{MO}
Manufacture: Barber-Colman{B066}
Model: MP-487-119-0-1 Ill. Analysis of Event A. Safety System Responses that Occurred No safety system responses occurred because of this event.
B. Duration of Safety System lnoperability Essential Chilled Water Trains 'B' and 'C' were INOPERABLE from 1413, November 10, 2023, to 1330, November 11, 2023, when Essential Chilled Water Train 'B' was declared OPERABLE. This was a total of 23 hours2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br /> and 17 minutes.
C. Safety Consequences and Implications
The accumulated incremental risk during the time period with two trains inoperable for Incremental Core Damage Probability (ICDP) and Incremental Large Early Release Probability (ILERP) are within normal work controls, but compensatory Risk Management Actions were implemented per the requirements of the STP Configuration Risk Management Program to offset the risk impacts of the configuration.
The event did not result in any offsite release of radioactivity or increase of offsite dose rates, and there were no personnel injuries or damage to any other safety-related equipment associated with this event.
Therefore, there was no adverse effect on the health and safety of the public.
IV. Cause of Event
Essential Chilled Water Train 'C'
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00498 r::J NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 003 1-0 There are only two possible causes for the Essential Chiller compressor shaft seal's premature wear degradation to failure: a) the shaft seal ring is not optically flat, orb) improper installation.
This compressor seal was installed on July 19, 2023. The work instructions have Maintenance check for optical flatness prior to installation and this step was recorded as "SAT." If the check for optical flatness was done correctly, then the most likely cause is improper installation. The compressor shaft seal is being sent to an offsite vendor for failure analysis in order to determine whether this could have been caused by a defective component.
Essential Chilled Water Train 'B' The cause of Essential Chilled Water *s* train inoperability was indeterminate.
V. Corrective Actions
Essential Chilled Water Train 'C' CR 23-10352 On 11/20/23, chiller technicians replaced the Compressor Shaft Mechanical Seal and the Lube Oil Pump Mechanical Seal. Post Maintenance testing was successful.
CR 23-10352 Performance Assessment on proper flatness check and proper installation of the compressor main shaft mechanical Seal.
CR 23-10352 The compressor shaft seal is being sent to an offsite vendor for failure analysis.
Essential Chilled Water Train 'B' CR 23-10345 On 11/11/13, Chiller technicians replaced the Hot Gas Bypass Valve MOV.
CR 23-6338-1 O - On 11 /11 /13, Chiller technicians replaced the existing Temperature and Current Limiter Controller.
VI. Previous Similar Events
No previous similar events within the last 10 years were identified. Page 6
of 6