05000260/LER-2005-003
| Browns Ferry Unit 2 | |
| Event date: | 04-13-2005 |
|---|---|
| Report date: | 06-09-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2602005003R00 - NRC Website | |
I. PLANT CONDITION(S)
At the time of this event, Unit 2 was in Mode 4 during its Cycle 13 refueling outage. Unit 1 was shutdown and defueled and was unaffected by the event. Unit 3 was in Mode 1 at approximately 3458 megawatts thermal (100 percent power) and was also unaffected by this event.
II. DESCRIPTION OF EVENT
A. Event:
On April 13, 2005, Unit 2 was in cold shutdown (Mode 4) during the latter stages of refueling outage U2C13, with surveillance testing being performed in support of returning the unit to power operations.
At approximately 0954 hours0.011 days <br />0.265 hours <br />0.00158 weeks <br />3.62997e-4 months <br /> central daylight time (CDT), an unplanned actuation of one channel of the anticipated transient without scram/alternate rod insertion (ATWS/ARI) logic occurred. The actuation was caused by an interaction between a surveillance test being actively performed and another surveillance test which had been in conduct, but which was halted for troubleshooting an equipment issue. The plant equipment response to this logic actuation isolated and vented the control rod drive (CRD) [AA] scram air header, causing the scram inlet and outlet valves on each CRD hydraulic control unit (HCU) to open and the scram discharge volume (SDV) vent and drain valves to close. With the scram outlet valves open, per plant design a flow path exists from the reactor vessel, through the 185 individual control rod drives and the open scram outlet valve on each drive's HCU, to the associated SDV (east or west), and each SDV began filling. Each SDV (east/west) has an associated scram discharge instrument volume (SDIV) physically located beneath it, and each SDIV is instrumented to initiate a reactor scram upon high level, thereby ensuring all control rods are fully inserted prior to the filling of the associated SDV itself. This action occurred in accordance with the plant design, and at approximately 0955 hours0.0111 days <br />0.265 hours <br />0.00158 weeks <br />3.633775e-4 months <br /> CDT, Unit 2 received a reactor scram from high water level in both the east and west SDIV's. All control rods were already fully inserted prior to the scram. There was no impact to plant operations as a result of the scram.
This event resulted in filling of the SDIV's and thereby the initiation of a valid, automatic actuation of the reactor protection system (RPS) [JC] on SDIV high level. The scram was not part of a pre planned sequence, therefore this event is reportable in accordance with 10 CFR 50.73 (a) (2) (iv) (A).
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Major Occurrences:
April 12, 2005 1555 hours0.018 days <br />0.432 hours <br />0.00257 weeks <br />5.916775e-4 months <br /> CDT Instrument Maintenance (IM) technicians commenced performance of calibration testing on reactor water level instrument loop 2-L-3-58B 1835 hours0.0212 days <br />0.51 hours <br />0.00303 weeks <br />6.982175e-4 months <br /> CDT Instrument loop calibration activities halted for troubleshooting an equipment issue April 13, 2005 0815 hours0.00943 days <br />0.226 hours <br />0.00135 weeks <br />3.101075e-4 months <br /> CDT Operations commenced performance of surveillance testing on reactor core isolation cooling (RCIC) initiation logic 0954 hours0.011 days <br />0.265 hours <br />0.00158 weeks <br />3.62997e-4 months <br /> CDT ATWS/ARI logic initiation occurred with resulting reactor scram on high SDIV level closely following. With Unit 2 in cold shutdown, there was no significant plant impact from the scram logic actuation.
D. Other Systems or Secondary Functions Affected None E. Method of Discovery Operations personnel received control room annunciation of the automatic reactor scram.
F. Operator Actions Operator action in response to this event was appropriate. Since Unit 2 was shutdown and in Mode 4 at the time of the event, the occurrence of the scram did not result in a plant transient. The control room crew suspended surveillance testing activities and took the necessary actions to verify the source of the scram.
G. Safety System Responses The only safety systems and/or components required to respond to the event were the RPS and portions of the CRD system. The switches monitoring the east and west SDIV levels properly sensed the fill event, and the RPS logic properly responded to the level switch operation by initiating a reactor scram. The CRD HCU's and SDV vent and drain valves operated in accordance with the plant design during this event.
III. CAUSE OF THE EVENT
A. Immediate Cause The immediate cause of this event was the inadvertent actuation of the ATWS/ARI logic which led to the filling of the east and west SDIV's beyond the high level reactor scram setpoint.
B. Root Cause The root cause of this event was inadequate communication between the Operations and IM testing groups. It was not clearly understood by the Operations test director that, even though surveillance testing had been halted, test equipment was still connected in the field while the related troubleshooting was being conducted.
C. Contributing Factors None
IV. ANALYSIS OF THE EVENT
IM personnel commenced testing of a reactor vessel water level instrument loop on April 12, 2005.
This testing included the connection of a volt-ohm meter (VOM) across a set of logic relay contacts for the purpose of verifying contact operation. During the test conduct, an incorrect equipment response was identified, and testing was halted to perform troubleshooting. The VOM connection, which was properly documented by sign-offs in the procedure, was left in place while this troubleshooting was performed.
Operations personnel were responsible for performing a separate surveillance test on reactor core isolation cooling (RCIC) initiation logic, and it was understood by both Operations and IM personnel that the water level instrument loop testing and the RCIC logic testing could not be conducted simultaneously. However, with the water level instrument loop surveillance testing temporarily halted for the previously mentioned troubleshooting, the decision was made to proceed with the RCIC logic testing. The status of the water level surveillance troubleshooting and what test equipment was still installed in the field (i.e, VOM connected) was not clearly communicated between the two testing groups.
The VOM installed by the water level instrument loop test procedure was configured to monitor relay contact resistance, and in such a configuration, the meter itself acted as a closed logic contact to the associated logic circuitry. During performance of the RCIC logic testing, a separate, series relay was actuated. The actuation of this relay, together with the VOM placed across the series contacts in the other logic channel, resulted in completion of Channel A of the ATWS/ARI initiation logic. The ATWS/ARI logic isolated and vented the CRD scram air header, and this action resulted in opening the HCU scram inlet and outlet valves and filling the SDIV's. The reportable RPS logic actuation then occurred.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The ATWS/ARI logic, the actuated equipment, and the RPS all operated in accordance with the plant design. At the time of this event, Unit 2 was shutdown in Mode 4, and all control rods were already fully inserted into the core. The reactor scram from high SDIV level is part of the BFN design, and the occurrence of this event from at-power conditions has been analyzed. It should be noted that the testing environment during cold shutdown conditions is quite different from that with the reactor at power, and the conduct of testing similar to that which led to this event would undergo greater scrutiny prior to authorization for at-power performance, particularly if there were any possibility of conflict with other plant activities.
Based on the above discussion, it is apparent there was no adverse safety impact of this event, either as it occurred or if it had hypothetically occurred with the reactor at power. There was no effect on the health and safety of the public.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
Surveillance testing was halted and the interaction between the two tests identified. The VOM was removed from the circuit and the RCIC logic testing satisfactorily completed.
B. Corrective Actions to Prevent Recurrence)
- The essential nature of clear, unambiguous communication will be reinforced to site personnel involved in testing activities (1) TVA does not consider these corrective actions regulatory commitments. The completion of these actions will be tracked in TVA's Corrective Action Program.
VII. ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs on Similar Events None
C. Additional Information
Browns Ferry corrective action document PER 80721 D. Safety System Functional Failure Consideration:
This event does not involve a safety system functional failure which would be reported in accordance with NEI 99-02. The RPS responded properly to the SDIV high level condition.
E. Loss of Normal Heat Removal Consideration:
This event occurred with the reactor in Mode 4 (cold shutdown), and the main condenser was not in service, nor was it needed, at the time of this event.
VIII. COMMITMENTS
None