05000454/LER-2007-003
| Byron Station | |
| Event date: | |
|---|---|
| Report date: | |
| 4542007003R00 - NRC Website | |
A. Plant Condition Prior to Event:
Event Date: October 5, 2007 Discovery Date: January 4, 2008 Unit 1 experienced a forced outage and was in Mode 5, "Cold Shutdown" from October 19, 2007 until October 31, 2007. Otherwise Unit 1 was Mode 1, "Power Operations" with Reactor Power nominally at 100%.
Reactor Coolant System (RC) [AB]: Normal operating temperature and pressure.
No structures, systems, or components were inoperable at the start of this event that contributed to the initiation or mitigation of this event.
Background:
Two of the functions of Technical Specification (TS) 3.3.1, "Reactor Trip System (RTS) Instrumentation" are the RC Overtemperature Delta Temperature (OTDT) (JC) and the Overpower Delta Temperature (OPDT) (JC) functions. The OTDT reactor trip function is designed to prevent departure from nucleate boiling in the reactor core and the OPDT function is designed to ensure integrity of the fuel cladding.
The Limiting Condition for Operation (LCO) for these two functions requires four Delta T/T-Average channels to be operable (i.e., 1A, 1B, 1C, and 1D). Two of four channel logic coincidence is needed to generate a reactor trip signal. Condition E of TS 3.3.1 allows for one channel to be inoperable indefinitely provided it is placed in the tripped condition within six hours. This reduces the logic coincidence to one of three channels.
The RC Delta T/T-Average channel processes the instrument signals that perform the OTDT and OPDT functions stated on TS Table 3.3.1-1, Note 1 and Note 2, respectively. The subject 1B Delta T signal is dynamically compensated (via a lead/lag circuit card) to account for the response time delay of the RC temperature detectors versus actual RC temperature during transient conditions.
� E. ^ E‘,:cnt On January 4, 2008, during the performance of the Channel Operational Test (COT) for the 1B Reactor Coolant (RC) Delta T/T-Average protection channel, an Instrument Maintenance Department technician (non-licensed) questioned and subsequently validated that the COT procedure contained an incorrect jumper configuration on a lead/lag circuit card that provides dynamic compensation to the Delta T signal. An ensuing investigation revealed this incorrect configuration was installed on the � lead/lag circuit card using this incorrect procedure during the previous execution of the 1B Delta T/T- Average channel COT on October 5, 2007.
The lead/lag circuit card in its correct configuration dynamically compensates the 1B Delta T signal with an eight second lead and a 3 second lag time constant, such that the card output leads the input.
The incorrect card configuration resulted in an 8 second lag-only time constant, such that the card output lagged the input. Therefore, the 1B Delta T signal response would have been delayed/slowed, resulting in a longer time to trip on the OTDT and/or OPDT setpoints. As a consequence of this delay in trip, the 1B Delta Tit-Average channel OTDT and OPDT functions were unknowingly inoperable from October 5, 2007 until October 19, 2007 and from October 31, 2007 until January 4, 2008. From October 19, 2007 to October 31, 2007, Unit 1 was in Mode 5, which is a non-applicable mode for TS 3.3.1.
Having an OTDT and OPDT channel inoperable and not complying with condition E of TS 3.3.1 is a condition prohibited by TS and consequently reportable to the NRC in accordance with 10 CFR 50.73 (a)(2)(i)(b).
The remaining seven Delta Tit-Average COT procedures (three channels for Unit 1 and four channels for Unit 2) were reviewed and determined to contain the correct jumper configurations.
C. Cause of the Event
The eight Delta TfT-Average COT procedures were significantly revised recently to improve the method used to control the jumper position on the circuit cards. The incorrect jumper positions for the lead/lag card in the 1B Delta TfT-Average COT procedure were the same as the correct jumper positions for another card in the next step of the procedure. During the procedure revision development process for the eight Delta TfT-Average COT procedures, the jumper positions for the lead/lag card for the 1B Delta channel were inadvertently transposed from another card. The subsequent procedure review and approval process did not identify this error. This is considered a human performance deficiency.
D. Safety Analysis
There was minimal safety siymilualice to this condition. The other three available Delta TiT-Aveld channels were operable to provide input into the OTDT and OPDT reactor trip functions. Considering a single failure of another Delta T/T-Average channel in response to an overtemperature or overpower .condition, two channels remained operable to trip the reactor. Other Delta T/T-Average channel COTs on Unit 1 were performed during this time period. In this condition the channel undergoing the COT is placed in the tripped condition, thus leaving two operable channels to provide the remaining single trip signal.
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E. Corrective Actions
The correct jumper configuration was determined and the 1B Delta T/T-Average procedure revised to restore the circuit card to the proper configuration.
An extent of condition review was conducted with other procedure revisions involving the personnel involved in preparation and review. No similar discrepancies were discovered.
The investigation report and associated lessons learned have been communicated with Maintenance procedure writers and reviewers.
F. Previous Occurrences
There have been no Maintenance procedure errors leading to a significant event, such as a Technical Specifications non-compliance in the past two years.