05000454/LER-2006-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. Byron Unit 2 0500455 | |
| Event date: | 04-11-2006 |
|---|---|
| Report date: | 06-12-2006 |
| 4542006002R00 - NRC Website | |
Background:
Each unit at Byron Station contains one Refueling Water Storage Tank (RWST) [BQ] that supplies borated water to the Chemical and Volume Control System (CV) [CB] during abnormal operating conditions and to the Emergency Core Cooling System (ECCS) [BQ] and Containment Spray (CS) [BE] during accident conditions.
Each RWST has four level instrumentation channels. The primary purpose of these level channels ensures switchover of the suction source for the ECCS pumps from the RWST to the containment sump occurs at the correct RWST low level.
In 1989, a modification was installed on each RWST to allow the reference leg of the four RWST level instruments (i.e., Level Transmitter (LT)-930, 931, 932 and 933) to communicate with the RWST vapor space rather than the auxiliary building atmosphere. This was intended to ensure RWST level would be accurately reflected by the instrumentation. During drawdown of the RWST during an ECCS injection, vacuum pressure could exist in an RWST. With the reference leg vented to the auxiliary building atmosphere instead of the RWST vapor space, an error could have been introduced in the indicated tank level. The modification corrected this condition by attaching the instruments' common reference leg to the overflow line of the RWST (i.e., communicating with the vapor space).
B. Description of Event:
On March 15, 2006, during the 18-month calibration of Unit 1 LT-933, a licensed control room operator noticed a minor deviation in level indication on Unit 1 LT-930. This anomaly was documented in the corrective action program. The ensuing investigation concluded on April 11, 2006, that the 1989 modification to the reference leg was flawed in that it did not provide for a single channel calibration activity without impacting the other three channels. Disconnecting an instrument's reference leg for the calibration activity exposes the common reference line of the other channels to the auxiliary building atmosphere.
The condition renders the four safety related channels of RWST level indication inoperable when one of the four is being calibrated. Technical Specification (TS) 3.3.2, "Engineered Safety Feature Actuation System (ESFAS) Instrumentation, condition K, allows one instrument channel to be inoperable for 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. With all four channels inoperable, Limiting Condition for Operations (LCO) 3.0.3 becomes applicable. LCO 3.0.3 requires actions to be initiated within one hour to place the unit in Mode 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />, Mode 4 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> and Mode 5 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />. A review of all RWST level instrumentation channel calibration activities for the past three years indicated the calibration timeframe was always less than an hour. Consequently, the conditions of LCO 3.0.3 were not violated. However, this condition is reportable to the NRC under 10 CFR 50.73 (a)(2)(v), as an event or condition that could have prevented fulfillment of a safety function.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) (If more space is requ ed, use additional copies of NRC Form 366A)(17)
C. Cause of the Event
The exact cause could not be determined. The modification was designed in 1989 timeframe by the Station's engineering vendor. The modification was developed by the vendor's Mechanical Design and Drafting Division instead of their Control and Instrument Division (C&ID). In addition, the C&ID did not perform a cross disciplinary review of the proposed design. It could not be determined why the task was assigned in this manner or why a review was not performed by C&ID. It is speculated that the engineer involved from the mechanical division, though very experienced in mechanical design, was unaware of the C&ID design criteria to provide isolation valves for sensing lines to allow for testing without taking the complete line and other connecting instruments out of service.
A contributing cause includes the design change process in the 1989 timeframe did not utilize a design consideration checklist as used in today's process which would have triggered an Instrument Maintenance review for impact on calibration activities. An Instrument Maintenance review would have likely caught this design flaw.
Another contributing cause involved the instrument mechanics not questioning this non-typical calibration method in that it required a fitting to be disconnected versus a more typical calibration where a five-valve manifold is provided.
D. Safety Analysis
This condition had minimal safety consequences. During the injection phase of a design basis loss of coolant accident (LOCA), the RWST is the source of water for all ECCS pumps. A low level in the RWST coincident with a Safety Injection signal provides protection against a loss of water for the ECCS pumps and indicates the end of the injection phase of the LOCA. The RWST is equipped with four level transmitters which are calibrated on an 18 month frequency. A two-out-of-four logic initiates the protection function actuation.
This condition could have caused a premature end of the injection phase resulting in possible inadequate source of water for the ECCS pumps. However, this condition existed only during past transmitter calibrations between the time of disconnecting and reconnecting the tubing for the reference leg. The typical duration of this activity was less than an hour. Therefore, the time exposure to this condition was relatively small. Since the exact magnitude of level error during Safety Injection could not be quantified, the volume of RWST water injected and available to the recirculation sumps may have been less than assumed in design analysis.
Operators have emergency procedural guidance to respond to inadequate water inventory in the containment sump. A bounding risk analysis indicates that the condition of having all four RWST channels inoperable for approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> four times every 18 months is not risk significant.
E. Corrective Actions
A design change to the RWST level instrumentation will be installed to allow for single channel calibration that will not impact the remaining channels.
The current design change process was reviewed and determined to have sufficient controls in place to ensure similar issues would not occur in current design changes.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron Station 0500454 2006 002 00 (If ore space is required, use additional copies of NRC Form 366A)(17) Since this is a legacy issue involving the design change process 17 years ago, an extent of condition review of mechanical modifications that impact instrumentation will be conducted. This review will cover the timeframe since initial startup of the units until present.
A review of calibration practices for other Technical Specification level instruments will be conducted to identify and correct similar issues.
F. Previous Occurrences
There have been no LER occurrences of this nature at Byron in the previous two years.