05000457/LER-2011-001
| Braidwood Station, Unit 2 | |
| Event date: | 05-20-2011 |
|---|---|
| Report date: | 07-19-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| 4572011001R00 - NRC Website | |
A. Plant Operating Conditions Before the Event:
Event Date: May 20, 2011 Unit: 2 MODE: 1 Reactor Power: 100 percent Unit 2 Reactor Coolant System [AB]: Normal operating temperature and pressure Pertinent Auxiliary Feedwater (AF) [BA] System Details:
The AF system is designed to provide adequate water to the steam generators (SGs) to cool the unit down safely to 350 deg F at which time the residual heat removal (RH) [BP] system can be utilized. The AF system consists of two trains; Train A with a motor driven pump and Train B with a diesel engine driven pump. Each AF pump can deliver flow to all four SGs through air operated flow control valves.
The normal and preferred source for the AF system is the non-safety related condensate storage tank (CST). The safety related essential service water (SX) [BI] system provides a safety related and seismically qualified backup water source to the CSTs. Two motor-operated pump suction valves (2AF017A/B and 2AF006A/B) in series per pump are provided on the SX supply lines.
Historically, the suction piping between the 2AF017A/B and 2AF006A/B was maintained in a drained (dry-layup) condition. The original design intention of this arrangement was to prevent the possibility of raw water from contaminating the secondary side of the SGs during normal operation or transients not requiring the safety related make up source. The concern is that SG tube (primary reactor coolant pressure boundary) integrity could be compromised from exposure to raw water. The protection from SX intrusion was accomplished by the two normally closed cross-tie valves 2AF006A/B and 2AF017A/B. The space between the 2AF017A/B and 2AF006A/B valves was intentionally drained and monitored for leak-by as an added measure to ensure SX water did not intrude into the AF system and challenge the integrity of the SG tubes, a fission product barrier.
In March 2011, due to concerns identified with the drained section of piping between the cross-tie valves (this event is addressed under Unit 1 LER 2011-003-00), actions were initiated to install vent lines and fill the drained section of piping between the cross-tie valves.
B. Description of Event:
No structures, systems or components were inoperable at the start of this event that contributed to the event.
On March 30, 2011, during installation of a vent line between the 2AF006A and 2AF017A valves to address concerns with drained section of piping between the valves, Asiatic clam shells (approximately 8 square inches) were discovered and retrieved. On May 5, 2011, an evaluation for past operability concluded the 2A AF train was operable. The evaluation addressed the impact of the shells on the flow control valve disk stack of the 2AF005A/B/C/D valves, which is the most restrictive component in the AF flow path to the SGs.
On May 9, 2011, additional shells were found during actuation testing of the 2AF006A and 2AF017A valves.
Although these shells were similar in size to those found previously, there were more (approximately 41.25 square inches). Unit 2 was in Mode 4 at the time of the discovery of the shells, therefore AF was not required to be operable (AF is required to be operable in Modes 1, 2 and 3). An evaluation of past operability was initiated, the line was flushed, all required testing was completed and Unit 2A AF was determined to be operable.
On May 20, 2011, the evaluation of past operability concluded Unit 2 Train A of AF was not operable with this larger quantity of shells. The clam shells which were identified in the suction piping between the 2AF006A and 2AF017A valves had the potential to be transported through the 2A AF system and block flow through the AF flow control valves (2AF005A-D). The extent of condition has been evaluated for Unit 2 Train B of AF, and for Unit 1 Trains A and B, and it has been determined that the only affected train was 2A. This was determined to be an eight hour reportable event under 10 CFR 50.72(b)(3)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. An Event Notification Report was made at 2257 Eastern Daylight Time on May 20, 2011.
Technical Specifications Limiting Condition for Operation (LCO) 3.7.5, "Auxiliary Feedwater (AF) System," Condition A, "One AF train inoperable" requires restoring the AF train to OPERABLE status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. It is reasonable to conclude that this condition existed for a longer period of time than is allowed by LCO 3.7.5 Condition A. Therefore, this event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by Technical Specifications. As there were time periods during the past three years where the 2B AF train was coincidently out of service for testing or maintenance, this event is also being reported pursuant to 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety, and 10 CFR 50.73(a)(2)(v)(B) as a condition that could have prevented the fulfillment of a system's safety function.
On July 14, 2011, as a confirmatory action from the previous events, additional flushing of the line between the 2AF017A and 2AF006A valves resulted in additional shells being flushed out of the AF system. The amount of shells collected was greater than the amount collected from the May 9, 2011 flushing. Based on the amount of shells collected, operability of the 2A AF could not be supported. An investigation is ongoing, and either this Licensee Event Report (LER) will be supplemented, or a separate LER will be submitted.
Cause of Event
This investigation identified two root causes:
1. Historical inadequate chemical feed (CF) biocide treatment of SX prevented the termination of Asiatic clam larvae, allowing Asiatic clams to grow in the low flow section of the SX header to 2A AF suction.
- Prior to 1995, biocide feed to SX was inconsistent due to chemical feed system problems. However, after 1998 biocide feed became more consistent and then became very consistent with the installation of a new chemical feed system in 2000.
- It has been shown that continuous biocide injection is required to effectively terminate Asiatic clams and their larvae. The history of the chemical addition system can be used °to age the clams. An adult clam is killed by a consistent (uninterrupted) application of biocide for approximately 12 to 14 days. Therefore, the clams must have lived during a period when there was not continuous chlorination. The clams lived for about one year based on their size, so that the most recent time period when the clams could have lived would be before 2000, prior to the time when the new chemical feed system started to provide consistent chemical feed.
2. Historical ineffective Problem Identification and Resolution associated with discoveries of Asiatic clam shells in the 2A AF suction allowed the condition to remain unanalyzed and uncorrected.
- A review identified a number of historical issues involving clams and other debris clogging the Train 2A AF suction drain valve and sight glass. Actions taken focused on removing the debris. There where no attempts to perform a complete system flush to ensure all clam shells were removed from the system.
Safety Consequences:
There were no actual safety consequences impacting plant or public safety as a result of this event. Braidwood Station has not experienced an actual event that required the AF system to perform its safety function with flow supplied by the SX system. Flow to the AF system from the CSTs would not have been impacted by Asiatic clams in the SX system. Therefore, the existence of the clams in the SX supply line has not impacted AF flow to the steam generators.
In a design basis accident, an Engineered Safety Feature (ESF) signal will actuate the AF system, and the AF pumps will initially draw condensate from the CST. If the CST becomes depleted or unavailable coincident with AF ESF actuation, a low suction header pressure signal opens valves 2AF017A and 2AF006A, allowing pump supply from SX. Upon this action, the clam shells in the piping between 2AF017A and 2AF006A would pass through the 2A AF pump and on to the Train A AF control valves (2AF005A-D). The presence of the Asiatic clam shells in the 2A SX supply header had the potential to be transported through the 2A AF system and block flow through the AF control valves. While this condition affected the 2A AF train, there were time periods during the past three years where the 2B AF train was coincidently out of service for testing or maintenance. During these periods, the AF system safety function would have been unavailable. Therefore, this event represents a safety system functional failure.
From a risk perspective, the loss of function would only occur if, during a design basis accident, the CST were lost, 2B AF train fails or is unavailable due to maintenance, and the clam shells propagated through the system causing a loss of the 2A AF train. The probability of these three events occurring concurrently is considered unlikely. The potential impact of claim shells in the 2A AF train is considered as a low safety significance due to the low probability associated with events that would require use of SX as a suction source for the AF system.
E. Corrective Actions:
Corrective actions include:
- Flushing of the line between the 2AF006A and 2AF017A valves was performed to remove shells.
- Revising the service water heat exchanger inspection guide to incorporate additional guidance on actions to be taken upon the discovery of macrofouling and biological fouling of GL 89-13 systems. Guidance to ensure debris potentially affecting system operability is entered into the corrective action process for appropriate evaluation and resolution.
- Revising the AF 1/2AF006A/B and 1/2AF017A/B valve stroke surveillance to provide a location for documentation of the discovery of debris in the site glass or the drain, and instructions clarifying entering the discovery of debris into the corrective action process.
F. Previous Occurrences:
There has been one previous, similar Licensee Event Report identified at the Braidwood Station in the past three years:
- Licensee Event Report Number 2008-001 — 2A Essential Service Water Train Inoperable due to Strainer Fouling from Bryozoa Deposition and Growth
G. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A Braidwood Station, Unit 2 05000457