05000333/LER-2012-003
| James A. Fitzpatrick Nuclear Power Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3332012003R00 - NRC Website | |
BACKGROUND
On August 30, 2012, the High Pressure Coolant Injection (HPCI) System [EllS System Identifier: BJ] was declared inoperable due to the failure of HPCI booster pump P-1B recirculation pressure control valve (23PCV 50) [EllS Component Identifier: PCV] (See JAF LER 2012-002-00). In order to complete repairs on 23PCV-50, the HPCI suction piping had to be drained. This uncovered and drained a small section of the sensing line in the vicinity of HPCI main pump discharge flow element (23FE-80) [EllS Component Identifier: PDT Following completion of these maintenance activities, the HPCI discharge piping was filled and vented in accordance with OP-15, "High Pressure Coolant Injection", section G.9, "Fill and Vent HPCI Suction Piping from Condensate Storage Tanks (CST)". In addition, 23FE-80 sensing lines were vented by instrumentation and control (I&C) technicians in accordance with IMP-G42, "Instrument Venting/Filling". This procedure vents in the downward direction from the higher elevation flow element to the lower elevation flow instrument, using the CSTs as the pressure source. No air was noted from the system vents, however, l&C technicians reported air venting from the instrument lines. At 1434 on September 2, 2012, ST-4N, "HPCI Quickstart, Inservice and Transient Monitoring Test (1ST)," was completed satisfactory thereby demonstrating HPCI operability.
EVENT DESCRIPTION & ANALYSIS At 2257, on September 2, 2012, with the HPCI system in standby, a step change of approximately 700 gallons per minute (gpm) was noted on HPCI flow indicating controller, 23FI-108-1 with a corresponding step change of 665 gpm noted on computer point EPIC-A-1257. Under these conditions, the HPCI system would not have achieved its design flow rate while in automatic and was therefore declared inoperable. Troubleshooting, interviews, and engineering evaluation determined that the step change on the flow indicating controller and EPIC computer point was due to air in the instrument sensing lines.
As previously discussed, the instrument sensing lines were vented by l&C personnel following maintenance of 23PCV-50. Since the instrument lines were not drained during this activity, they should have remained full of water. The instrument lines were designed to have a positive slope (upwards towards the pipe), any air in the instrument line would be dispersed back up into HPCI piping. However, it was noted during walkdowns that the instrument lines were not properly sloped in the positive direction.
Any air that was pushed down during instrument line venting could have worked its way past the negatively sloped areas in the low pressure instrument sensing line, causing a differential pressure (d/p). This d/p would have been sensed by the flow transmitter and converted into a flow signal, which then would have provided indication of HPCI flow to 23FI-108-1. The signal also provides input to EPIC-A-1257 computer point. The accuracy of this loop depends on the instrument lines (both the high and low pressure) being filled with water at all times. Any air trapped in a vertical leg will result in an inaccurate flow signal.
CAUSE OF EVENT
The false flow indication was the result of air in the instrument sensing line creating a d/p across the flow element. The apparent cause for the air void is the improper sloping of the instrument sensing line. This allowed water to drain from the line during maintenance and did not allow the line to self vent when the HPCI suction piping was filled.
A contributor to this event was inadequate procedural guidance for filling and venting the instrument sensing lines. IMP-G42 directs the technician to perform a forward flush by opening the lowest point drain valves, and then use CST water as the pressure source to vent the sensing line to the low point drain. This method of venting can push air bubbles down into the horizontal elevations of the sensing lines where it will not initially impact the instrument reading. After time, the air bubbles can migrate up into the vertical sensing line and become trapped. The trapped air would displace the water in this vertical section of piping creating a d/p across the flow element thus giving a false indication of flow.
EXTENT OF CONDITION
The extent of condition for this event reviewed CR-JAF-2001-00308 evaluation and corrective actions. It was determined that this evaluation was extensive and thorough. It was identified that the reactor core isolation cooling (RCIC) system [EIIS System Identifier: BN] instrumentation is most similar in configuration. Therefore, the extent of condition corrective action is limited to walking down the RCIC flow instrument tubing.
CORRECTIVE ACTIONS
Completed
- Revised IMP-G42 to provide instructions to perform a pressurized back flush.
- Performed multiple fill and vent cycles of the instrument sensing line.
- Operators periodically monitor the flow indicator-controller in the control room. No deviations have been identified.
Future Actions
- Perform an evaluation to determine best approach to correct this condition long term; options include relocating the instrument to be adjacent to the flow element or re-slope the instrument sensing lines in accordance with existing design requirements.
- Evaluate the effectiveness of the revised IMP-G42.
ASSESSMENT OF SAFETY CONSEQUENCES
Radiological & Industrial Safety There were no actual or potential radiological or industrial safety consequences as a result of this condition.
Nuclear Safety There was no actual nuclear safety consequences associated with this condition. The potential nuclear safety consequences are considered minimal because during this period of HPCI inoperability, the Automatic Depressurization System (ADS), CS, and Low Pressure Coolant Injection (LPCI) systems [EllS System Identifier: BO] were operable. The ADS in combination with the LPCI and CS systems would ensure adequate core cooling is maintained in the event of HPCI inoperability. Also, even though the RCIC system is not credited for accident analysis, it would have automatically provided makeup water at most reactor operating pressures.
Internal operating experience (OE) was reviewed for similar events relevant to the condition described in this LER. In 2010 during refueling outage 19, a similar event occurred where the HPCI flow indicator-controller indicated 1300 gpm with the system in standby. The corrective action was to vent the instrument and instrument sensing lines. In 2001 two events occurred in which the HPCI flow indicator-controller indicated flow with the system in standby.
The analysis performed in 2001 determined that the incorrect flow indication was caused by trapped air in the instrument sensing lines. Inconsistencies were discovered when the sensing lines were inspected for proper slope. Portions of the line were identified to have negative slope and air would be trapped in the line when the system was drained and refilled. Corrective actions were developed to re-slope the lines, but were ineffective.
External OE was reviewed on the Institute of Nuclear Power Operations (INPO) website. Two examples were found where plants identified incorrect slope on instrument tubing. This resulted in air causing an offset between the instrument readings and the actual parameter. In one example the offset was evaluated to have minimal impact on instrument setpoint or indications. In the second example, maintenance procedures were updated to vent the lines as they were identified.
REFERENCES
- JAF Condition Reports: CR-JAF-2012-05068, CR-JAF-2010-07095, CR-JAF-2001-00308, CR-JAF-2001-00328
- JAF FSAR 6.4.1 High Pressure Coolant Injection System