ML050970315
| ML050970315 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 03/31/2005 |
| From: | Rosalyn Jones Duke Power Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| O-05-1401, O-05-1567 | |
| Download: ML050970315 (8) | |
Text
Duke RON A. JONES Ut Powere Vice President A Duke Energy Company Oconee Nuclear Duke Power ON0I VP / 7800 Rochester Highway Seneca, SC 29672 864 885 3158 864 885 3564 fax March 31, 2005 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555
Subject:
Oconee Nuclear Station Docket Nos. 50-287 Special Report per Technical Specification 5.6.6 Problem Investigation Process No.: 0-05-1567 0-05-1401 Gentlemen:
Pursuant to Oconee Nuclear Station Technical Specification 5.6.6 and Selected Licensee Commitment (SLC) 16.7.2, attached is a special report regarding the inoperability of the Unit 3, Train "A" Reactor Coolant System Wide Range Pressure and Hot Leg Level instruments. These instruments perform post-accident monitoring functions per Duke Power Company's response to Regulatory Guide 1.97 and provide an input for an Anticipated Transient Without Scram (ATWS) mitigation function.
The apparent cause is a loss of capillary line fill inside containment. This line is inaccessible during normal operations so repairs will be made during the next reasonable opportunity, no later than the next Unit 3 refueling outage, projected for spring 2006. Redundant components in Train "B" remain operable.
Redundancy/diversity is also provided by other instruments and indications. This event is considered to be of no significance with respect to the health and safety of the public.
If there are any questions you may contact B. G. Davenport at (864) 885-3044 or R. P. Todd at (864) 885-3418.
Very truly yours, Attachment www. dukepower. corn
Document Control Desk Date: March 31, 2005 Page 2 cc: Mr. William D. Travers Administrator, Region II U.S. Nuclear Regulatory Commission 61 Forsyth Street, S. W., Suite 23T85 Atlanta, GA 30303 Mr. L. N. Olshan Project Manager U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, D.C. 20555 Mr. M. C. Shannon NRC Senior Resident Inspector Oconee Nuclear Station INPO (via E-mail)
Oconee Nuclear Station Special Report per TS 5.6.6 Inoperability of the Unit 3, Train "A",
Reactor Coolant System Wide Range Pressure and Hot leg Level Instruments March 31, 2005 Abstract:
At 03:38 on 2-26-05, Unit 3 control room received a "Level High/Low" statalarm on Pressurizer Level. Operators referred to the Alarm Response Guide and followed Plant Transient Response administrative guidance. They observed some divergence between different channels of Pressurizer Level indication, but there was no significant level transient. Some other instruments also indicated changes. Operators concluded an instrument error existed and, at 04:00, transferred Pressurizer level control to an unaffected channel. Pressurizer level stabilized at the normal level of approximately 220 inches.
The Apparent Cause is a loss of capillary line fill inside containment on the impulse line to instruments 3RC PT0244 Reactor Coolant System (RCS) Wide Range (WR) Pressure Train A and 3RC LT0123 Hot Leg Level Transmitter. The pressure instrument provides an input to a number of calculated variables, including corrected pressurizer level, which were affected as a result.
Operations entered Technical Specification (TS) 3.3.8 Condition A for functions 4 (RCS WR Pressure), 11 (Pressurizer Level) and 17 (Sub Cooling Monitor (SCM)) and Selected Licensee Commitment (SLC) 17.7.2 Condition B for Diverse Scram System (DSS) channel 1.
Due to the location of the tubing inside containment, access is limited while the unit is in Mode 1. Investigation/repair activities will be conducted during the next reasonable opportunity, no later than the next Unit 3 scheduled outage, projected for spring 2006. The redundant train ("B") and other diverse indications remain Operable. A temporary design change is being developed to connect an alternate RCS pressure indication to allow DSS to be returned to service.
This event is considered to be of no significance with respect to the health and safety of the public.
Oconee Nuclear Station Page 2 of 6 Special Report March 31, 2005
Background:
Instruments 3RC PT0244 RCS WR Pressure Train A and 3RC LT0123 Hot Leg Level Transmitter are Regulatory Guide (RG) 1.97 Post Accident Monitoring (PAM) instrumentation. The pressure indication also provides an input to several calculated variables which also provide PAM functions. TS 3.3.8 defines the following PAM functions which were affected by this event:
Function Description RG 1.97 UFSAR Number Type Category Reference 3 RCS Hot Leg Level B 1 7.5.2.2.3 4 RCS WR Pressure A 1 7.5.2.1 11 Pressurizer Level A 1 7.5.2.3 17 Sub Cooling Monitor A 1 7.5.2.2.2 TS require two trains of instruments for each function. If one train is inoperable, TS allow 30 days for restoration or a report must be submitted within the following 14 days. The report must outline the preplanned alternate method of monitoring (PAM only), the cause of the inoperability, and the plans and schedule for restoring the instrumentation channels of the Function to operable status.
The Train A Inadequate Core Cooling Monitor (ICCM) provides a graphic display of several PAM parameters including SCM, hot leg level, RCS pressure, and several different displays of RCS Pressurizer level.
In addition, RCS WR Pressure is an input for DSS, which performs an Anticipated Transient Without Scram (ATWS) mitigation function, to provide an alternate method of tripping the reactor during an ATWS event. DSS requires both trains to indicate high RCS pressure in order to trip. Associated with DSS is the ATWS Mitigation Systems Actuation Circuitry (AMSAC) which provides alternate signals for emergency feedwater actuation. AMSAC/DSS system design provides two trains, using two of two logic, to limit spurious actuation. Therefore the AMSAC/DSS system design is such that when any one channel is bypassed, the output function of the entire system is disabled.
Oconee Nuclear Station Page 3 of 6 Special Report March 31, 2005 AMSAC/DSS operability is addressed by SLC 16.7.2, which allows a 7 day completion time for restoration when one or both trains of AMSAC and/or DSS is inoperable, with submission of a written report to NRC within 30 days if this time is not met.
Instruments 3RC PT0244 and 3RC LT0123 share a common impulse line. For various reasons, the line is made of capillary tubing filled with water and separated from direct contact with the RCS fluid by a bellows assembly. The particular capillary tubing in question is designed to transfer RCS pressure from the top of the "A" Hot Leg to a hydraulic isolator (PS453) located outside of the containment in the East Penetration Room. From the hydraulic isolator, additional capillary tubing transfers the pressure to the WR Pressure Transmitter (PT244) and Hot Leg Level Transmitter (LT123) located in the Cable Room.
Prior to this event Unit 3 was operating in Mode 1 at 100%
power with no safety systems or components out of service that would have contributed to this event.
Event
Description:
At 03:38 on 2-26-05, Unit 3 control room received a "Level High/Low" statalarm on Pressurizer Level. This alarm was generated based on ICCM "pressure compensated" Pressurizer level. The level prior to the event had been 220 inches and the alarm was received at 200 inches. Operators referred to the Alarm Response Guide and followed Plant Transient Response administrative guidance. They observed some divergence between different channels of Pressurizer Level.
Specifically "pressure compensated" Pressurizer levels showed significant variation between channels. "Temperature compensated" level channels showed minor variations, and "Uncompensated" channels showed little or no variation.
Some other instruments also indicated changes.
The differences between "pressure compensated" Pressurizer level channel indications continued to increase slowly. Indicated level on the controlling channel decreased to 155 inches while actual level increased as the controls called for additional make-up. Operators concluded there was an instrument error and, at 04:00, transferred Pressurizer level control to an unaffected
Oconee Nuclear Station Page 4 of 6 Special Report March 31, 2005 channel. Pressurizer level stabilized at approximately 220 inches, the normal control setpoint.
After some troubleshooting, Operations concluded that instruments 3RC PT0244 RCS WR Pressure Train "A" and 3RC LT0123 "Hot Leg Level Transmitter" had failed. The pressure instrument provides an input to a number of calculated variables which were affected as a result.
Operations entered TS 3.3.8 Condition A for functions 4 (RCS WR Pressure), 11 (Pressurizer Level) and 17 (SCM) and SLC 17.7.2 Condition B for DSS channel 1 as of 03:38.
The RCS WR Train A pressure signal had failed low. Because DSS uses two of two logic and actuates on high RCS pressure, this made DSS inoperable. In this state, procedural guidance in place at the time directed Operations to bypass both channels of AMSAC/DSS. The system design is such that bypassing a channel of one sub-system (e.g. DSS) also bypasses the associated channel of the other subsystem (e.g. AMSAC) At 05:30 Operations also entered SLC 16.7.2 Condition A on AMSAC due to the decision to bypass AMSAC/DSS.
The procedural guidance has since been changed, such that AMSAC/DSS was taken out of bypass on 3-3-2005 at 11:15. This returned AMSAC to service, but DSS remains inoperable due to the inoperable RCS pressure input.
A work request was initiated to investigate and repair the Train "A" instrument string. Troubleshooting indicated that the fault was a loss of pressure in the section of capillary tubing running between the isolation bellows connected to RCS piping inside containment and the hydraulic isolator located in the penetration room outside containment.
Due to the location of the tubing inside containment, access is limited for investigation and trouble shooting while the unit is in Mode 1. Therefore, although the Apparent Cause is a loss of capillary line fill inside containment, the exact failed component, mode of failure, and root cause cannot be determined at this time.
Investigation/repair activities will be conducted during the next reasonable opportunity, no later than the next Unit 3 scheduled outage, projected for spring 2006. The Work Order for investigation/repair has been placed on the "hot list" to be
Oconee Nuclear Station Page 5 of 6 Special Report March 31, 2005 worked during any unscheduled outage of sufficient duration which will allow access into that area of the building. The primary troubleshooting method will be pressurizing the capillary line and looking for leaks. Once the leak is identified, the affected part will be corrected and the capillary line re-filled. Spare parts have been confirmed to be available.
The effects of losing these two transmitters are indicated below.
Loss of 3RC LT0123 (RCS Hot Leg Level Train A) (failed high due to loss of reference leg) results in:
Loss of indication of Train "A" hot leg level via:
ICCM Train A Plasma Display, Operator Aid Computer (OAC) Point (RC Hot Leg A Level),
Chart recorder.
Note:
Reactor Vessel Head Level Train A was not affected by this failure and remains operable.
Train B indications are still available for all of the above except no alternate chart recorder indication is available. Only Train A supplies inputs to the chart recorders.
Loss of 3RC PT0244 (RCS WR Pressure Train A) (failed low) results in:
- Loss of indication of Train "A" RCS pressure via:
ICCM Train A Plasma Display, OAC Point (RCS WR Pressure Loop A),
Control board 3UB1 RCS pressure indicator, ATWS cabinet RCS pressure indicator, Chart recorder.
- Loss of ICCM Train A SCM Monitor
- Inaccurate ICCM Train A density compensated Pressurizer Levels (Channels 1 and 2). The maximum effect on level indication is approximately four inches.
- Loss of RCS pressure signal to Channel A of DSS. Since this is a 2 out of 2 logic system, this renders DSS inoperable.
Note:
The OAC SCM monitor is available but is not qualified.
Train B indications are still available for all of the above except no alternate chart recorder indication is available.
Only Train A supplies inputs to the chart recorders.
Oconee Nuclear Station Page 6 of 6 Special Report March 31, 2005 Several interim corrective actions have been or will be taken to minimize the impact of this failure:
- 1. Operations reviewed the Emergency Operating Procedure to determine if the failed hot leg level indication impacted event mitigation strategies. They concluded that there was no impact. This is primarily due to the fact that no design basis event requires use of the high point vents in response to hot leg level indications.
- 2. Operations developed Operations Guide 5-07 to provide information to unit 3 control room operators about valid and invalid indications resulting from this condition.
- 3. A temporary design change is being prepared to provide an alternate signal to DSS in order to return DSS to service.
- 4. A temporary change is being prepared to provide a fixed pressure input signal to the ICCM Train A. This will allow the Stat-alarm to be reset and enable it to respond to changes in other parameters.