05000498/LER-2024-002-01, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function

Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function

ML24179A341
Person / Time
Site:	South Texas
Issue date:	06/27/2024
From:	Tomlinson J South Texas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-24004047 LER 2024-002-01
Download: ML24179A341 (1)

LER-2024-002, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v), Loss of Safety Function
4982024002R01 - NRC Website

text

June 27, 2024 NOC-AE-24004047 10 CFR 50.73 STI: 35609717 ATTN: 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 2024-002-01 Supplement to Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function

Reference:

Letter; J. Tomlinson (STP) to Document Control Desk (NRC); Licensee Event Report 2024-002-00 Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function; May 9, 2024; (NOC-AE-24004040) (ML24130A271).

On May 9, 2024, STP Nuclear Operating Company (STPNOC) submitted the referenced Licensee Event Report. This letter is a supplement to the report to provide the updates results of the causal evaluation. The updated information is denoted by revision bars located in the right-hand margin. The report is submitted in accordance with 10 CFR 50.73.

The event did not have an adverse effect on the health and safety of the public.

There are no commitments in this submittal.

If there are any questions regarding this submittal, please contact Zachary Dibbern at 361-972-4336 or me at 361-972-8945.

Jason R. Tomlinson Site Vice President Attachment: Unit 1 LER 2024-002-01, 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 Jason R Tomlinson Digitally signed by Jason R Tomlinson Date: 2024.06.27 17:04:28 -05'00'

NOC-AE-24004047 Attachment Attachment Unit 1 LER 2024-002-01 Unit 1 LER 2024-002-01, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function

Abstract

South Texas Unit 1 I

00498 5

Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function 03 10 2024 2024 002 01 06 27 2024 1

100



Zachary Dibbern / Licensing Engineer 361-972-4336 B

BI PSP S280 Y

X KM CHU Y018 Y



On 03/09/24 at 2126 CST train 'C' Essential Cooling Water and cascading equipment were declared inoperable due to a through-wall leak on Essential Cooling Water Pump 1C discharge vent line. On 03/10/24 at 0353 CDT, train 'B' Essential Chilled Water and cascading equipment were declared inoperable due to chilled water outlet temperature exceeding limits. This condition resulted in an inoperable condition on two out of the three safety trains for the accident mitigation function. This condition was determined reportable per 10 CFR 50.73(a)(2)(v)(D). The Essential Cooling Water 'C' train inoperability cause was due to erosion of the pipe material resulting from aggressive flow due to pipe geometry. The Essential Chilled Water 'B' train inoperability cause was indeterminate. Corrective and preventative actions include (1) Essential Cooling Water 'C' was repaired, (2) all other Essential Cooling Water vent lines were replaced like-for-like, (3) a modification for the Essential Cooling Water vent lines is being evaluated and (4) temporary additional monitoring capabilities were implemented for Essential Chiller 12B in case the event repeats itself.

Summary of the Event Timeline (Note: All times have been converted to Central Standard Time) 03/09/24 (2126) - Train 'C' Essential Cooling Water and supported equipment was declared inoperable due to discovery of through-wall leakage on Essential Cooling Water Pump 1C discharge vent line.

03/09/24 (2151) - Trains 'A' and 'B' Essential Cooling Water Screen Wash Booster Pumps and Essential Cooling Water Pump 1A and 1B Traveling Screens were secured due to cross train operability.

03/09/24 (2253) - Started 'A' train CRE HVAC and Secured 'C' train CRE HVAC for chiller swaps.

03/10/24 (0221) - Started Essential Chill Water Pump 11B for equipment rotation.

03/10/24 (0223) - Started Essential Chiller 12B for equipment rotation.

03/10/24 (0253) - Train 'B' Essential Chiller and cascading equipment was declared inoperable due to outlet temperature exceeding limits.

03/10/24 (0340) - Train 'B' Essential Chiller and cascading equipment was declared operable following temperature adjustment performed per reference CR 24-2328.

03/10/24 (1005) - This event resulted in an inoperable condition on two out of the three safety trains for the accident mitigating function which could have prevented the fulfillment of a safety function, and was reported to NRC Headquarters Operations Officer. (Event Notification 57019) 2 5

South Texas Unit 1 00498 2024 002 01 IPage of

3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.

052 050 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 Cooling Water Train C The original design allowed pipe geometry that resulted in two shortcomings when combined with the aluminum-bronze pipe material. Specifically, the design involved a sharp 90-degree turn from the incoming 1.5-inch vertical diameter vent pipe. This geometry led to high flow acceleration and turbulence in the vicinity of the turn and direct flow impingement on the inner surface of the 3-inch pipe across from the vent pipe entrance. Additionally, the small vent pipe diameters caused higher flow velocities. These coupled together led to higher erosion rates and eventually a pinhole through-wall leak.

Essential Chilled Water Train 'B' The cause of the Essential Chilled Water 'B' train inoperability was indeterminate.

V. Corrective Actions

Essential Cooling Water Train C On 03/11/24, the Essential Cooling Water Pump 1C Discharge Vent Line was repaired. All other Essential Cooling Water pump vent lines were replaced with like-for-like pipe spools as an extent of condition action. A modification to the design is being evaluated to modify the pipe geometry and consider other pipe materials.

Essential Chilled Water Train 'B' On 03/15/24, a temporary modification was developed and installed on Essential Chiller 12B to provide additional monitoring to aid in determining cause of failure in case the high outlet temperature event repeats.

VI. Previous Similar Events

LER 2023-001-00, "Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function; dated January 15, 2024.

LER 2023-003-00, "Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function;" dated January 9, 2024.

LER 2023-004-00, "Condition Prohibited by Technical Specifications Due to Inoperable Train of Essential Chilled Water;"

dated February 5, 2024.

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South Texas Unit 1 00498 2024 002 01 I