05000390/LER-2001-003
| Watts Bar Nuclear Plant (Wbn) Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 3902001003R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390
I. PLANT CONDITION(S)
The unit was in Mode 1 at 100 % power. Plant operating temperature at the time was 587.4 degrees F with reactor coolant system pressure at 2233 psig.
II. DESCRIPTION OF EVENT
A. Event:
On September 26, 2001, operations personnel noted that Pressure Indicator (PI) 1-PI-1-27A (EIIS code PI) was slowly increasing and swinging about 40 psig. The instrument loop involved in this event (1- LPP-1-27A) is required to perform measurement and processing of the Loop 4 Main Steam header pressure signal. Three operable channels on each steam header are provided to satisfy the protective requirements with a two out of three logic on each line for Safety Injection function "Steam Line Pressure - Low" and Steam Line Isolation functions of "Steam Line Pressure - Low" and "Steam Line Pressure Negative Rate - High.
Operability of the component is required by Technical Specification LCO 3.3.2, Table 3.3.2-1, items 1.e, 4.d.1, and 4.d.2.
Work Orders were generated to troubleshoot the instrumentation.
Recalibration of Pressure Transmitter (PT) 1-PT-1-27A as well as replacement of the amplifier board did not correct the problem. Following this, an attempt was made to backfill the sense line. During the backfill operation a blockage in the sense line was identified and cleared. The impact of the blockage was to slow the response time of the transmitter.
After previous maintenance the channel was considered to be operable based on checking the reading with other control room sources and although the time response impact on the channel was identified the channel was within its maximum channel deviation and it was thought the slow response was only in the upward directions. A check had shown the transmitter was responding quickly to a decrease in pressure in the sense line.
A review of the evolution found that the Safety Injection and Steam Line Isolation functions would have functioned as required with the remaining two channels operable.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 B. Inoperable Structures, Components, or Systems that Contributed to the
Event:
There were no other channels besides # 4 Steam Generator Pressure that were inoperable that impacted the event.
C. Dates and Approximate Times of Major Occurrences:
1 .. � ..
42, " �
- � , �
- .4....i.,.::. � . � :-.:3:;.:, 9/26/01/(1405) Noted that 1-PI-1-27A was slowly increasing and monitoring activities started.
9/28/01/(1425) 1502 1739 Troubleshooting conducted on 1-PT-1-27A, entered LCO 3.3.2.
Subsequent records review showed the transmitter was not reacting properly to system pressure.
Operations exited LCO 3.3.2, 1-LPP-1-27A removed from bypass believing that the transmitter was still operable, even though further troubleshooting was required.
9/29/01/(0159) TS LCO 3.3.2 Condition D entered, to backfill sense line.
9/29/01/(0313) 1-PT-1-27A sense line was unblocked, thus determined inoperable prior to this time.
9/29/01/(0542) As required by TS, the steam channel was placed in "TRIP.
D. Other Systems or Secondary Functions Affected:
None.
E. Method of Discovery:
During a computer channel trend review following maintenance activities which cleared a blocked sense line on September 29, 2001, engineers identified an abnormal channel response following the installation of a FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 pressure gauge connected to the instrument test tee. When the gauge was valved in it read approximately 880 psig which was about 80 psig below actual system pressure. The gauge pressure gradually rose to system pressure over a period of 45 to 60 minutes.
Review of Operations logs indicated that LCO 3.3.2 was entered and troubleshooting was conducted between 1425 and 1739 hours0.0201 days <br />0.483 hours <br />0.00288 weeks <br />6.616895e-4 months <br /> on September 28, 2001. The LCO was exited at 1739 hours0.0201 days <br />0.483 hours <br />0.00288 weeks <br />6.616895e-4 months <br /> believing that the pressure transmitter was operational. At this point the channel in question had normalized with other instruments. Although the slow upwards time response had been identified a separate check had shown a prompt response to decreasing pressure which was the safety function of the channel. It was subsequently learned the blockage made this check invalid.
LCO 3.3.2 was entered again on September 29, 2001, at 0159 hours0.00184 days <br />0.0442 hours <br />2.628968e-4 weeks <br />6.04995e-5 months <br /> with an entry that the line was unblocked at 0313. From a computer point review TVA has concluded that transmitter operation was degraded from September 12, 2001, until 1502 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.71511e-4 months <br /> on September 28, 2001 (the approximate time the Heise pressure gauge was installed). As the Maximum Channel Deviation (MCD) requirements were continuously met during this interval and no discrete event occurred that caused the condition that obstructed the sense line prior to September 28, 2001, the channel is considered functional but degraded until 1502 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.71511e-4 months <br /> on September 28, 2001. However, when the Heise pressure gauge and 1-PT- 1-27A response was observed to be much slower than the design sense line delay allowance when the instrument was returned to service. In hindsight this was objective evidence the impulse pressure input to the transmitter was not properly transmitting system pressure in the required response time.
Based on a review of the computer trend data for the other steam generator pressure channels the event was isolated to the single channel discussed above.
F. � Operator Actions:
Operators initiated a work order to investigate the problem with pressure swings.
Based on evaluation of the subject condition, the steam pressure sense line for 1- PT-1-27A was confirmed to be effectively blocked and thus incapable of FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390
II
performing its required protection function at approximately 0313 hours0.00362 days <br />0.0869 hours <br />5.175265e-4 weeks <br />1.190965e-4 months <br /> on September 29, 2001. At that time, appropriate Technical Specifications (TS) LCO Conditions (LCO 3.3.2 Condition D) had already been entered at 0159 hours0.00184 days <br />0.0442 hours <br />2.628968e-4 weeks <br />6.04995e-5 months <br /> on September 29, 2001, in order to backfill the instrument sense line and replace the pressure transmitter which was initially suspected as the problem. As required by TS, the steam channel was placed in "TRIP" at 0542 hours0.00627 days <br />0.151 hours <br />8.96164e-4 weeks <br />2.06231e-4 months <br /> on September 29, 2001.
The channel output had been undergoing monitoring and troubleshooting since September 26, 2001, due to Operations observations of unusual channel behavior (rising pressure on 1-PI-1-27A). Troubleshooting activities since September 26, 2001, included transmitter calibration, replacement of amplifier card, and temperature measurements. At 17:39 on September 28, 2001, Operations exited LCO 3.3.2 believing that the pressure channel was operational as discussed above.
The required TS Actions for an inoperable Steam Pressure Channel (3.3.2, Condition D) require that the channel be placed in TRIP within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> or enter Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. A subsequent review determined the channel was inoperable without being placed in TRIP for a period of approximately 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> 40 minutes commencing at 1502 on September 28, 2001, and ending at 0542 on September 29, 2001.
G. � Safety System Responses:
None
CAUSE OF THE EVENT
A. Immediate Cause:
Transmitter was not reacting properly to system pressure.
B. Root Cause:
The sense line for the transmitter was apparently blocked. The source of the blockage is unknown at this time.
C. Contributing Factor:
None.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 IV & V. A ANALYSIS OF THE EVENT - ASSESSMENT OF SAFETY CONSEQUENCES Each Main Steam header contains three pressure transmitters each of which provide input to the Engineered Safety Features Actuation System (ESFAS) as part of a two out of three logic for a Safety Injection or a Steam Line Isolation signal. The instrument loop involved in this event (1 - LPP-1-27A) is required to perform measurement and processing of the Loop 4 Main Steam header pressure signal. The instrument provides a comparator function for the Low Steam Line Pressure Safety Injection and Steam Line Isolation and for the High Negative Rate Steam Line Isolation.
This loop also provides input to the Plant Computer, Main Control Room (MCR) Annunciators, local and MCR indicators (PAM Category 1), a MCR recorder, and pressure compensation to the Loop 4 Steam Line flow loop (1-FT-1-28A).
Operability of the component is required by Technical Specification LCO 3.3.2, Table 3.3.2-1, items 1.e, 4.d.1, and 4.d.2.
Based on the problems encountered when attempting to backfill the transmitter sense line, the most probable cause for the erratic action of the pressure transmitter was a blockage of the sense line. Technical Specification Surveillance Requirement (SR) 3.3.2.10 and SR 3.3.2.1 ensure that the response times of ESF functions are less than or equal to the maximum values assumed in the accident analyses. Individual component response times are not modeled. The required response time for the Low Steamline Pressure SI and Steamline Isolation and High Negative Rate Steamline Isolation actuation signal is specified in the FSAR section 15.4.2.1.1 as less than or equal to 2 seconds. A partially blocked sense line could increase the delay associated with detection of steam pressure changes, and therefore could prevent the channel from performing required protective functions within the time allotted in the accident analyses.
Two operable channels on each steam header are sufficient to satisfy the protective requirements with a two out of three logic on each line for Safety Injection function "Steam Line Pressure - Low" and Steam Line Isolation functions of "Steam Line Pressure - Low" and "Steam Line Pressure Negative Rate - High." Trend graphs were generated for the remaining pressure transmitters of the loop 4 main steam header. The remaining two channels were functioning correctly and were not out of service during the time period that 1-LPP-1-27A was not functioning FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 correctly. Therefore, these safety functions could be performed by the two remaining channels.
VI. � CORRECTIVE ACTIONS A. � Immediate Corrective Actions:
1. Work Orders 01-014747-000 and 01-014747-001 were initiated to investigate problem with pressure swings.
2. Pressure Transmitter (PT) 1-PT-1-27A was calibrated.
3. The amplifier card for 1-PT-1-27A was replaced.
4. The common sensing line for 1-PT-1-27A and 1-PT-1-26C was backfilled. During the backfill an apparent obstruction in sensing line was removed by the backfill process.
5. 1-PT-1-27A was replaced.
B. � Corrective Actions to Prevent Recurrence:
The following actions are tracked under TVA's corrective action program and therefore, are not considered to be regulatory commitments.
1. Issue training memorandum to Operations, System Engineering and Maintenance Engineering describing this event and the symptoms of a sense line blockage.
2. Perform a review of industry information for similar types of events and based on this review, determine if additional corrective actions are warranted. (Completed) 3. Test the PT that was removed (refer to Immediate Corrective Action 5 above) to determine the effect the blockage of the sense line had on the component.
4. Backfill a sampling of the main steam pressure and flow transmitters during the upcoming refueling outage and including venting the subject transmitter through the condensing pot to observe any debris.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 VII. � ADDITIONAL INFORMATION
A. Failed Components:
None. The apparent cause was a blockage in the sense line. However, additional testing of the affected component 1-PT-1-27A will be performed (see corrective action 4 above).
B. Previous LERs on Similar Events:
A review of previous reportable events for the past six years was performed.
No similar events of sense line blocking were identified.
C. Additional Information:
None D. Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with NEI 99-02, Section 2.2.
E. Normal Heat Removal This event did not result in the loss of normal heat removal in accordance with NEI 99-02, Section 2.1.
VIII. COMMITMENTS
None.