05000260/LER-2010-005
| Browns Ferry Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2602010005R00 - NRC Website | |
I. PLANT CONDITION(S)
Browns Ferry Nuclear Plant (BFN) Units 2 and 3 were operating in Mode 1 at 100 percent thermal power. BFN Unit 1 was operating in Mode 1 at approximately 80 percent power.
II. DESCRIPTION OF EVENT
A. Event:
On July 12, 2010, at approximately 1230 Central Daylight Time (CDT), BFN maintenance personnel commenced a scheduled performance of Surveillance Procedure, 2-SR-3.3.6.1.2(3B), High Pressure Coolant Injection (HPCI) [BJ] Steam Line Supply Low Pressure Functional Test on the Unit 2 HPCI system. At 1310 CDT, the Unit 2 HPCI [BJ] system unexpectedly received a Unit 2 Group 4 Primary Containment Isolation Signal (PCIS) [JE]. Group 4 isolation provides for isolation of the HPCI system, and includes the HPCI steam supply isolation valve, and the HPCI pump torus suction isolation valves. Also, at 1310 hours0.0152 days <br />0.364 hours <br />0.00217 weeks <br />4.98455e-4 months <br /> CDT, Operations personnel declared the HPCI system inoperable and entered Technical Specification (TS) Limiting Condition for Operation 3.5.1 Action C, which requires verifying by administrative means that the Reactor Core Isolation Cooling (RCIC) [BN] system is operable, and restoring the HPCI system to operable status within 14 days. Operations verified that the RCIC system was operable.
Operations entered Abnormal Operating Instruction, 2-A0I-64-2B, Group 4 High Pressure Coolant Isolation.
By 1325 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.041625e-4 months <br /> CDT, Operations personnel reset the Unit 2 HPCI system isolation and placed the HPCI system in standby readiness, exiting the TS Action.
The Tennessee Valley Authority (TVA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(v)(D), as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Major Occurrences:
July 12, 2010, at 1310 hours0.0152 days <br />0.364 hours <br />0.00217 weeks <br />4.98455e-4 months <br /> CDT O Inadvertent isolation of the Unit 2 HPCI system during performance of 2-SR-3.3.6.1.2(3B).
July 12, 2010, at 1325 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.041625e-4 months <br /> CDTO Operations returned the Unit 2 HPCI system to standby readiness.
July 12, 2010, at 1731 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.586455e-4 months <br /> CDT O Operations made an Non-Emergency Notification System report to the NRC in accordance with 10 CFR 50.72(b)(2)(v).
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
Unit 2 main control room Operations personnel noted the Unit 2 HPCI system had isolated. The surveillance performer also realized that there was an unexpected response to the procedure.
F. Operator Actions
None
G. Safety System Responses
None
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause for the HPCI System isolation was the installation of multiple volt ohm meters (VOMs) in the associated pressure switch circuits during performance of the surveillance. The VOMs were placed on terminals specified by the surveillance procedure to determine the presence of voltage on the pressure switch circuits to be tested. TVA's evaluation concluded that multiple VOMs installed across multiple circuits simultaneously, and set to the procedure specified range or scale, caused the PCIS Group 4 isolation signal.
B. Root Cause
The cause of the event is a lack of clear procedural guidance that allowed the performer to interpret the requirements in the surveillance procedure.
The steps in the procedure did not clearly indicate that the intent was to only momentarily place a VOM on terminals and then remove the VOM once the required checks were completed. It was not understood by the performer that placing multiple VOMs across the terminals would allow current to flow across the terminals and cause the Group 4 isolation when simulating the HPCI system low steam supply pressure test.
C. Contributing Factors
None
IV. ANALYSIS OF THE EVENT
The PCIS Group 4 isolation was received during the performance of Section 7.2, HPCI Low Pressure Steam Supply (2-PS-073-0001A) of the surveillance procedure. The investigation concluded the maintenance personnel involved in performance of the surveillance performed the procedure as written.
The procedural steps for placement of the VOMs are as follows:
7.2 HPCI Low Press Steam Supply (2-PS-073-0001A) [2] PLACE VOM set on 300 VDC Scale between 2-TB-073-0111/JJ-13 and 2-TB-073-0111/JJ-14 for 2-PS-073-0001A and VERIFY no voltage is present if voltage is present, THEN PERFORM the following; otherwise, NA Step 7.2[3] and CONTINUE.
[3] CHANGE voltage scale of VOM and VERIFY voltage is induced. IF voltage is not induced, THEN STOP work and notify the Instrument Foreman.
The above steps were repeated during the performance of the surveillance until an individual VOM was installed on each of four sets of terminals. The personnel involved misinterpreted the procedural steps to require installing a separate VOM on each set of terminals assuming that each VOM would be removed at a later step.
During the progression of the procedural steps which immediately followed the installation of the VOMs, the performers were instructed, if voltage is present, change the voltage scale of the VOM down to the 30 volts direct current (VDC) range and verify that voltage present is induced. Each of the VOMs had been ranged down to the 30 VDC range. The intent of these procedural requirements is to check for stray or induced voltage across the pressure switch circuits to be tested.
Subsequently, the procedure required that (1) a VOM set on the resistance (i.e., ohm) scale be connected across terminals 2-TB-073-0111/KK-18 and 2-TB-073-0111/KK-19 to verify the operability of the HPCI Steam Line Pressure - Low function of pressure switch (PS) 2-PS-073-1A and (2), a drain valve momentarily opened to simulate low pressure in the HPCI steam supply line. When the drain valve was opened, the Group 4 isolation occurred. The performer, realizing that there was an unexpected response to the surveillance steps, stopped the performance of the surveillance and notified the main control room.
At the time the unexpected PCIS Group 4 isolation signal was received, there were four VOMs in the PS circuit. Three of the VOMs were set on 30 VDC and the fourth VOM was on the ohm scale as directed by the surveillance procedure.
Following the initial event, troubleshooting activities confirmed that when multiple VOMs are installed with the range setting on the lower ranges approximately 2.88 milliamps was passed through the circuit. This amount is enough current to complete the 1 out of 2 taken twice logic, causing the PCIS Group 4 isolation signal.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of the event were not significant. BFN TSs allows continued power operation for up to 14 days with the HPCI system inoperable provided the RCIC system is operable.
The RCIC system was verified by administrative means to be operable during the time HPCI was inoperable. Therefore, TVA concludes that there was no significant reduction in the protection of the public by this event.
VI. CORRECTIVE ACTIONS
A.�Immediate Corrective Actions Unit 2 Operations personnel terminated the performance of 2-SR-3.3.6.1.2(3). Operations personnel subsequently entered 2-AOI-64-2B and returned the Unit 2 HPCI to standby readiness.
B. Corrective Actions to Prevent Recurrence
The corrective actions to prevent recurrence are being managed by TVA's Corrective Action Program.
During the investigation, maintenance personnel recognized that similar step guidance was contained within the surveillance procedures for the Units 1 and 3 HPCI Low Steam Header Isolation Switch Functional Test, and the Units 1, 2, and 3 RCIC Low Steam Header Isolation Switch Functional Test. The surveillance procedures for Units 1 and 3 HPCI Low Steam Header Isolation Switch Functional Test and the surveillance procedures for the Units 1, 2, and 3 RCIC Low Steam Header Isolation Switch Functional Test were revised to eliminate the need for performer interpretation. The procedure revisions included steps to connect and disconnect each VOM during checks for voltage.
This event will be discussed in the Continuing Training program and the Initial Training programs.
VII. ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs on Similar Events None
C. Additional Information
Corrective action document for this report is Problem Evaluation Report - 239313.
D. Safety System Functional Failure Consideration:
This event is a safety system functional failure in accordance with NEI 99-02.
E. Scram With Complications:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None