05000390/LER-2003-004
| Docket Numbersequential. Revmo Day Year Year Mo Day Yearnumber No 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3902003004R02 - NRC Website | |
I. PLANT CONDITIONS:
Watts Bar Nuclear Plant Unit 1 was in Mode 1 operating at approximately 100 percent reactor power when this event occurred.
II. DESCRIPTION OF EVENT
A. Event
During preparation for the Unit 1 Cycle 5 refueling outage, work orders were being prepared to perform ultrasonic testing (UT) on the emergency core cooling system (ECCS) safety injection (EIIS BO) "piggy back" supply piping to the safety injection pump (EIIS P) 1B-B. The "piggyback" piping connects the ECCS residual heat removal (RHR) system (EIIS BP) pump discharge piping to the suction of safety injection pump 1B-B. It is also possible for this line to supply the suction of safety injection pump 1A-A and both centrifugal charging pumps (CCP) (EIIS CB). A history review of that system was being performed during the work order preparation. It was discovered during that review, that on January 14- 15, 2003, the safety injection pump was drained to support maintenance activities. It appeared that the draining and subsequent refilling of the safety injection pump 1B-B may have been inadequate because the associated work orders did not contain instructions for draining and refilling, and because there was no evidence of special precautions to assure the "piggyback" supply piping to the safety injection pump 1B-B was filled with water when restoring the equipment to service. The operator log entry on January 15, 2003, indicated that fill and vent of the safety injection pump 1B-B was performed in accordance with the System Operating Instruction (SOI)-63.01, "Safety Injection System." However, the method in the procedure was for filling and venting the entire system piping rather than the small portion that was actually drained. The procedure involved valve alignments outside the clearance boundary and therefore, did not adequately address the "piggy back" line. On August 28, 2003, due to the suspicion that gas may be in the piping from the inadequate filling and venting, a UT of the suction line to the pump was performed and verified that gas was in the line. This is contrary to Technical Specification 3.5.2, Emergency Core Cooling System, (ECCS) — Operating, Surveillance Requirement (SR) 3.5.2.3 which is to verify the ECCS piping is full of water.
Problem Evaluation Report (PER) 03-014922-000 was initiated to document this event in the TVA Corrective Action Program.
B. Inoperable Structures, Components, or Systems that Contributed to the Event There were no structures, components or systems inoperable at the start of the event that contributed to the event.
II.�DESCRIPTION OF EVENT (continued) C. Dates and Approximate Times of Major Occurrences Time Occurrences t-� Entered Limiting Condition for Operation (LCO)January 14, 2003�2300 Eastern Standard Time 3.5.2 for safety injection pump 1B-B to remove(EST) pump from service for maintenance January 15, 2003�1930 EST Held pretest briefing for filling and venting of safety injection pump 1B-B using SOI-63.01.
January 15, 2003�2235 EST Exited LCO 3.5.2 for safety injection pump 1B-B due to all post modification testing being complete August 28, 2003�1025 Eastern Daylight Time Entered LCO 3.5.2 Action A and LCO 3.6.6, (EDT) Containment Spray System, Action B to allow UT inspection of the ECCS "piggyback" suction piping for the safety injection pump 1B-B due to suspicion that gas may be in the line.
August 28, 2003�1245 EDT UT confirmed that the horizontal run of piping at Elevation 692 in the Auxiliary Building in the pipe chase, was approximately 7/8 full of water and that the 14-foot vertical run of piping above this was empty of water.
August 28, 2003�1810 EDT Plant Operations Review Committee approved a one-time only fill and vent plan as this affected piping was not in the procedure to verify the ECCS piping was full of water.
August 28, 2003�1850 EDT Completed filling and venting and achieved a solid stream of water for one minute. UT confirmed pipe was full of water.
D. Other Systems or Secondary Functions Affected
No other systems or secondary functions were affected by this event.
E. Method of Discovery
As discussed above, during preparation of Unit 1 Cycle 5 refueling outage, work orders were being prepared to perform UT on the ECCS safety injection system "piggy back" supply piping to the safety injection pump 1B-B. A history review of that system for the work order preparation discovered that on January 14-15, 2003, the safety injection pump was drained to support maintenance activities. It appeared that the draining and subsequent refilling of the safety injection pump 1 B-B may have been inadequate because the associated work orders did not contain instructions for draining and refilling, and because there was no evidence of special precautions to assure the "piggyback" supply piping to the safety injection pump 1B-B was filled with water when restoring the equipment to service.
II. DESCRIPTION OF EVENT (continued)
F. Operator Actions
Upon discovery of the potential for gas to be in the piggyback supply piping to the safety injection pump 1B-B, LCO, Action A of Technical Specification 3.5.2 and LCO, Action B of Technical Specification 3.6.6 were entered. The piggyback line was isolated by a tagout to ensure that the potential gas pocket could not be transported into any other section of ECCS piping if an ECCS pump started unexpectedly.
Following venting and UT verification that the piping was full of water, the Technical Specifications LCO actions were exited.
G. Safety System Responses
There were no automatic or manual safety system responses and none were necessary.
III. CAUSE OF EVENT
A. Immediate Cause
The immediate cause appeared to be inadequate procedural guidance for filling and venting of the safety injection pump 1B-B suction line during the maintenance activities on January 14-15, 2003. In addition, the operators failed to recognize the affected portions of ECCS piping were required to be verified full of water by Technical Specification SR 3.5.2.3.
B. Root Cause
The safety injection pump 1B-B suction line was vented at the accessible point instead of the high point of the system. The cause is due to work practices by the TVA Licensed Operators involved with the work order and hold order review. The operators did not identify the potential impact for draining activities on a portion of the ECCS piping during the maintenance of the pump.
IV. ANALYSIS OF EVENT
On August 28, 2003, a gas bubble with a volume of 5.5 cubic feet was discovered below the Unit 1 flow control valve, 1-FCV-63-11-B. This valve is in a line in the ECCS piping on the discharge side of the RHR pump 1B-B.
The normal position for 1-FCV-63-11-B is closed and the valve is in a vertical configuration. The line connects the RHR pump discharge to the suction of the safety injection pump 1B-B and to the centrifugal charging pumps (CCPs) and the safety injection pimp 1A-A via an alternate flow path. Valve FCV-63-11-B is opened after the injection phase of an event when the ECCS pump suctions are transferred from the refueling water storage tank to the containment sump at the start of long term recirculation.
VII. ADDITIONAL INFORMATION (continued) B. Previous LERs on Similar Events (continued) CLA had not been factored into this performance of the 1-SI-63-10-A and the venting requirements had been waived. The cause for this event was determined to be that no formal process exists to document system status issues such as the leakage through the check valve. This event was documented as LER 390/1996-019.
C. Additional Information:
None.
D. Safety System Functional Failure This event did not involve a safety system functional failure as defined in NEI-99-02, Revision 0.
E. Loss of Normal Heat Removal Consideration This event is not considered a scram with loss of normal heat removal.
VIII. COMMITMENTS
None IV. ANALYSIS OF EVENT (continued) TVA's Nuclear Steam Supply System (NSSS) vendor for TVA's pressurized water reactors (PWR) has previously evaluated gas pockets for a TVA nuclear plant in similar locations to the pocket found at Watts Bar Nuclear Plant. The safety injection pumps' and CCPs' line sizes, pump flows, and general layout is essentially the same for both of TVA's PWR plants. Therefore, the NSSS vendor's evaluation is equally applicable to both plants. The amount of gas evaluated by the NSSS vendor was 6.0 cubic feet.
That evaluation considered piping length to the pumps, number of elbows and degree of the elbows, pump design and manufacturer, and the flow rates. That evaluation concluded that the gas in the piping would be moved through the piping and be mixed by the elbows. When compressed by discharge pressure from an RHR pump, the initial 6.0 cubic feet of gas would be reduced to approximately 1.71 cubic feet, which would mix and pass through the CCP with a void fraction no greater than five percent by volume. The evaluation concluded that catastrophic pump failure would be unlikely for a total initial gas accumulation of less than 6.0 cubic feet when compressed to 1.71 cubic feet and mixed with the flow, resulting in a void fraction at the pump suction of no greater than five percent by volume during accident conditions.
The piping containing the gas at Watts Bar is routed to both safety injection pumps and both CCPs. The previous evaluation addressed the CCPs. However, the evaluation included a statement that the gas in the lines could travel to the safety injection pumps, but CCPs would be evaluated since the piping to the safety injection pumps is longer with more elbows than that for the CCPs. Consequently, it is considered the acceptance criteria established for gas located in the piping applies to both the CCPs and the safety injection pumps.
Watts Bar compared the piping length to the pumps, number of elbows and degree of the elbows, pump design and manufacturer, and the flow rate to that in the previous NSSS vendor evaluation. In addition, four cases which covered various combinations of ECCS equipment being out of service were evaluated. In the worse case, the RHR pump discharge pressure computed for Watts Bar would compress the initial 5.5 cubic feet of gas to 1.63 cubic feet, or less, to any one safety injection pump or CCP. Since the lines to the Watts Bar pumps have similar fluid velocities and a similar geometry, as compared to TVA's other PWR plant and the resulting gas volume ingested by any single pump is less than TVA's other PWR plant, it is concluded that the previous evaluation bounds the condition for Watts Bar Nuclear Plant. Therefore, it can be concluded that the overall volume fraction of gas ingested by a safety injection pump or CCP would have been less than five percent if a LOCA had occurred during the time period that the condition existed. Similar to the conclusion reached in the NSSS vendor evaluation for TVA's other PWR plant, catastrophic safety injection pump failure would have also been unlikely for the total initial Watts Bar gas volume of 5.5 cubic feet.
V. ASSESSMENT OF SAFETY CONSEQUENCES
Based on the discussion in Section IV above, the condition described in this event does not result in a loss of functional capability for the safety function provided by the ECCS. Therefore there was no safety significance to this event.
Initiated work order to perform UT on the pipe and check for the presence of gas. Technical Specification LCO 3.5.2, Action A and LCO 3.6.6, Action B were entered to check for gas. The UT verified that gas was present in the piggyback supply piping to the Safety Injection pump 1B-B. A work order was initiated to vent the piping. The piping was subsequently vented and a follow-up UT verified the piping was full of water. The Technical Specification LCOs were exited.
B. Corrective Actions to Prevent Recurrence - (TVA does not consider these items to constitute regulatory commitments. TVA's corrective action program tracks completion of these actions.) 1. The plant design was modified during the Unit Cycle 5 Refueling Outage to provide additional vent points throughout the ECCS including upstream and downstream of Valve 1-FCV-63-11-B on the "piggyback" line.
2. The system operating instructions for the ECCS Systems and the procedure for draining and filling operations have been revised to provide for performance of Surveillance Instruction (SI) 1-SI-63-10-A to ensure the piping is full of water.
3. The work order planners guide has been revised to provide guidance on the need to ensure that ECCS components are full of water following maintenance activities.
4. The individuals involved with the work order preparation and hold order review have been coached on the importance of verifying the ECCS piping is full of water.
VII. ADDITIONAL INFORMATION
A. Failed Components
There were no failed components involved in this LER.
B. Previous LERs on Similar Events On June 26, 1996, SR 3.5.2.3 was implemented in 1-SI-63-10-A, to ensure the piping from the ECCS pumps to the reactor coolant system was full of water. However, 1-SI-63-10-A allowed for the venting requirements to be waived for the vent paths inside containment if it could be verified through administrative means that no periodic filling of a cold leg accumulator (CLA) had occurred in conjunction with leakage through a cold leg injection line check valve. At 1545 hours0.0179 days <br />0.429 hours <br />0.00255 weeks <br />5.878725e-4 months <br /> (EST), licensed personnel initiated actions to perform 1-SI-63-10-A. During this activity it was noted that refilling of a CLA had occurred during the month of May and that action had been taken to back-seat a check valve to resolve the need to refill the CLA. A review of the package for 1-SI-63-10-A dated May 28, 1996, identified that the refilling of the CLA had not been factored into this performance of the SI and the venting requirements had been waived. The cause for this event was determined to be that no formal process exists to document system status issues such as the leakage through the check valve. This event was documented as LER 390/1996-019.