05000278/LER-2005-002
| Docket Numbersequential Revmonth Day Year Year Month Day Year 05000Number No. | |
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| ENS 41510 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2782005002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 3 was in Mode 1 and operating at approximately 100% rated thermal power when the event occurred. There were no structures, systems or components out of service that contributed to this event. The Unit 3 High Pressure Coolant Injection system (EIIS:BJ) was considered inoperable due to preparations for routine surveillance testing on the instrumentation associated with the HPCI suction source transfer function. The HPCI system was not operating at the time of discovery.
Description of the Event
On 3/21/05, at approximately 1640 hours0.019 days <br />0.456 hours <br />0.00271 weeks <br />6.2402e-4 months <br />, during operational preparations for the performance of a routine surveillance test for the Unit 3 High Pressure Coolant Injection (HPCI) system, Operations personnel discovered that the Condensate Storage Tank (CST) (EIIS: TK) suction source Motor Operated (MO) Valve (M0-17) (EIIS:
V) did not automatically close when the Suppression Pool Suction source valves (M0-57 & MO-58) were opened. The HPCI suction source was being transferred from the CST to the Suppression pool in preparation for routine surveillance testing on the instrumentation associated with the HPCI suction source transfer function. In accordance with the Bases for Technical Specification (TS) 3.3.5.1, if CST water level falls below a preselected level or if the Suppression Pool level raises above a preselected value, the MO-57 & MO-58 valves first open and then the MO-17 valve automatically closes. Contrary to this requirement, the MO-17 did not automatically close during the preparation activities. Operations personnel promptly detected the condition and declared the HPCI system inoperable in accordance with TS 3.3.5.1.
The MO-17 was closed by 1826 hours0.0211 days <br />0.507 hours <br />0.00302 weeks <br />6.94793e-4 months <br /> and the MO-57 & MO-58 valves were maintained open. HPCI was returned to its normal CST suction path line-up (i.e. MO-17 open, MO-57 & MO-58 closed) by 2310 hours0.0267 days <br />0.642 hours <br />0.00382 weeks <br />8.78955e-4 months <br />.
Further troubleshooting by Maintenance personnel determined that the automatic closure of the MO-17 did not occur due to a limit switch (EIIS: 33) being out of adjustment. The limit switch is within the motor operator housing for the MO-58 valve and provides an auto-close input into the MO-17 valve logic. This limit switch being out of adjustment prevented the auto close function of the MO-17 when the MO-58 valve opened.
The MO-58 limit switch was adjusted by 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on 3/22/05 and the HPCI system was declared operable by 2045 hours0.0237 days <br />0.568 hours <br />0.00338 weeks <br />7.781225e-4 months <br /> on 3/22/05.
This condition was reported to the NRC on 3/21/05 at approximately 2234 hours0.0259 days <br />0.621 hours <br />0.00369 weeks <br />8.50037e-4 months <br /> pursuant to the requirements of 10CFR 50.72(b)(3)(v)(D) (Event Notification # 41510). It was subsequently determined that HPCI was capable of performing its safety function due to the availability of an adequate HPCI system suction supply. However, a condition prohibited by Technical Specifications is judged to have existed. This was due to the Technical Specification 3.3.5.1 function of transferring the HPCI suction from the CST to Suppression Pool being considered to be inoperable fora time period longer than allowed in Technical Specification 3.3.5.1. Technical Specification 3.3.5.1 requires that the CST Level —Low and Suppression Pool Water Level =High functions be capable of automatically operating the MO-57, MO-58 and MO-17 valves. If this function is not met, Technical Specifications actions include aligning the HPCI suction to the Suppression Pool (i.e. MO-57 & MO-58 open, MO-17 closed) within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. It is judged that the condition of the deficient limit switch existed for a time period greater than the Technical Specification Required Action time. Therefore, this event is being reported as a condition prohibited by Technical Specifications.
Analysis of the Event
There were no actual safety consequences associated with this event.
The HPCI system is part of the Emergency Core Cooling System (ECCS) network that provides cooling water to the reactor vessel for certain design basis events. The HPCI system is designed to provide core cooling for a wide range of reactor pressures. Upon receipt of an initiation signal, the HPCI system valves align as necessary, the turbine accelerates, and the appropriate flow rate is achieved. The HPCI system is designed with two suction flow paths: the CST and the Suppression Pool flow paths. Normally, HPCI is aligned in the standby mode to the CST during plant operations. Transfer to the Suppression Pool suction source occurs if there is a low CST low water level or Suppression Pool high water level. In accordance with the Bases for Technical Specification (TS) 3.3.5.1, if CST water level falls below a preselected level or if the Suppression Pool raises above a preselected value, the MO-57 & MO-58 valves first open and then the MO-17 valve automatically closes. The MO-17 closure is initiated by the closure of two limit switches (one in each of the two motor operator housing for the Suppression Pool suction valves MO-57 & MO-58). Both limit switches need to close to provide the signal to close the MO-17 valve.
While normally aligned to the CST, the suction flow path from the Suppression Pool is isolated using the MO-57 and MO-58 valves. Therefore, for design basis events with HPC1 aligned from the normal CST suction source, there would be no impact as a result of the mis-adjusted limit switch associated with the MO 17 valve logic relay. However, for other design events involving either a CST low water level condition or a high Suppression Pool high water level condition, HPCI suction would need to transfer from the normal CST source to the Suppression Pool water source. This prevents an excessively high Suppression Pool water level which could result in loads on the Suppression Pool exceeding their design values if there would be a blow down of the Reactor Vessel pressure through the safety / relief valves.
If a design basis event occurred while the limit switch deficiency existed, the Suppression Pool suction valves (MO-57 & MO-58) would have opened as designed. However, the CST suction source isolation valve (MO 17) would not have closed. However, engineering analyses exist that conclude that HPCI would have been capable of performing its design function while aligned to both suction sources. Therefore, the HPCI safety function was not affected by the limit switch deficiency.
The surveillance performed to check operability of the Suppression Pool high water level instruments is performed quarterly. The previous performance of the surveillance occurred on 12/15/04 and was successful.
Also, the MO-57 and MO-58 valves were opened (and the MO-17 automatically closed) on 12/20/04 as part of the in-service testing program.
Cause of the Event
Troubleshooting of the HPCI logic associated with the MO-17 valve determined that the limit switch (Limitorque Corporation, 2 Train Geared Limit Switch, Part 10106) housed in the motor operator for the MO 58 valve was slightly out of adjustment. The MO-17 closure is initiated by the closure of two limit switches (one in each of the two motor operator housings for the Suppression Pool suction valves MO-57 & MO-58).
Both limit switches need to close to provide the signal to close the MO-17 valve. Troubleshooting performed just subsequent to the event determined that the MO-57 limit switch operated properly; however, the MO-58 limit switch (LS-2) did not fully make up.
Further investigation determined that the limit switch that stops the MO-58 valve in the open direction (LS-4) and the limit switch that provides input into the MO-17 valve logic circuit (LS-2) had slightly low finger preload adjustments. The LS-4 limit switch opens when the MO-58 valve is in the full open position. At the same time, LS-2 (located on the same limit switch rotor) closes providing an input to close the MO-17 valve.
Had the preload been slightly higher, the LS-4 limit switch would have opened slightly later which would have ensured that the LS-2 contacts would have been made up. Also, had the LS-2 contact finger preload been slightly higher, the LS-2 contact would have closed slightly earlier ensuring proper logic input to close the MO-17 valve.
Preventive maintenance was performed on the motor operator for the MO-58 valve in May 2003. There were no adjustments made to the limit switches and the limit switches were ensured to be functional.
The underlying cause of the low limit switch finger preload settings was insufficient procedural control of limit switch adjustments. Although maintenance procedures provide significant detail concerning the repair and adjustment of components within valve motor operators, there was insufficient detail concerning the adjustment of limit switch settings when using multiple limit switch contacts for complex logic driven from the same limit switch rotor. It was determined that additional procedural direction was required concerning the verification of the proper adjustment of the limit switch rotors and contacts to ensure adequate limit switch contact engagement.
Corrective Actions
The limit switches (LS-2 and LS-4) in the MO-58 valve motor operator were adjusted to provide appropriate limit switch operation in the MO-17 and MO-58 valve logics. The MO-57 valve motor operator limit switches were inspected with no significant concerns noted. Post-maintenance testing of the HPCI suction valve logic was successfully performed by 1630 hours0.0189 days <br />0.453 hours <br />0.0027 weeks <br />6.20215e-4 months <br /> on 3/22/05.
An extent of condition review was performed to identify similar valves that have limit switches that provide interlocks to other plant equipment such as pumps, relays or other valves. Appropriate evaluations and inspections of these valve limit switches will be performed in accordance with the Corrective Action Program.
Programmatic documents involving valve motor operator limit switch adjustments will be revised to provide additional guidance on the adjustment of limit switches.
Previous Similar Occurrences An event occurred in October 2004 (LER 3-04-03) concerning a failure of the HPCI suction transfer logic to properly operate. In the October 2004 event, the MO-17 did not close when the MO-57 and MO-58 opened.
Corrective actions performed as part of LER 3-04-03 included the repair of a loose wire connection on a logic relay located in the Cable Spreading Room associated with the suction transfer and therefore, would not have been expected to have prevented this event. Subsequent to the repair of the loose wire, post maintenance testing and subsequent routine surveillance testing was successfully performed. However, the possibility that the limit switch was the cause of the October 2004 event can not be ruled out.