05000286/LER-2003-001
| Indian Point Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2862003001R00 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within brackets ( }
DESCRIPTION OF EVENT
On January 13, at approximately 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br />, while at 100% steady state reactor power, Operations manually tripped {JC} the reactor (RCT) in accordance with Off Normal Operating Procedure ONOP-C-1 due to greater than three (3) inches differential pressure (dp) between sections of the main condenser (SG). The high dp was due to the trip of the 35 Circulating Water {KE} Pump {P) (CWP) while the 36 CWP was tagged out of service for planned maintenance. The 35 and 36 CWPs supply one section of the three-section condenser. The loss of both pumps in the same condenser section caused a partial loss of vacuum for that section resulting in a high dp between adjacent condenser sections.
On January 13, at approximately 0608 hours0.00704 days <br />0.169 hours <br />0.00101 weeks <br />2.31344e-4 months <br />, the 35 CWP tripped from its normal Load Commutated Inverter (LCI) drive and transferred to the standby LCI drive.
Within approximately 15 seconds of the transfer, the 35 CWP tripped from the standby drive. Central Control Room (CCR) {NA) operators observed an indicator light for the 35 CWP normal breaker open and then the 35 CWP standby light come on after a few seconds followed by a condenser low vacuum alarm. Operations dispatched operators and Instrument & Control (I&C) personnel to investigate the condition. Dispatched operators reported to the CCR that they reset all fault lights locally and attempts to restart CWP 35 were unsuccessful. At approximately 0615 hours0.00712 days <br />0.171 hours <br />0.00102 weeks <br />2.340075e-4 months <br />, CCR operators entered ONOP-C-1 and manually tripped the reactor at approximately 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br />, in accordance with the procedure, for exceeding three inches differential pressure between condenser sections.
CCR operators observed the rod bottom lights, Reactor Trip (RT) First Out Annunciator (Manual Trip), and Turbine Trip First Out Annunciator (Reactor Trip). CCR operators entered Emergency Operating Procedure (EOP) E-0, Reactor Trip or Safety Injection, then ES-0.1, Reactor Trip Response, and transitioned to Plant Operating Procedure (POP) 3.1, Plant Shutdown from 45% Power. The plant was stabilized in hot standby with decay heat being released to the main condenser via the steam dump valves {V) and the transient terminated. All control rods (AA) fully inserted. Station offsite power remained available and there was no automatic start of the emergency diesels {EK}. The Auxiliary Feed Water (AFW) system (BA) automatically started as expected due to changes in Steam Generator level from full power operation. The following systems failed to function properly; 1) the 32 Reactor Coolant (AB) Pump {P) tripped when the feed to its supply bus auto transferred from the Unit Auxiliary Transformer to the Station Auxiliary Transformer {XFMR}, 2) the 32 source range detector {IG) did not indicate as required after it energized (read low), 3) the Plant Vent Gas radiation monitor {IL) R-14 alarmed and spiked then returned to normal, and 4) CCR received an AFW low flow alarm requiring an operator to manually open the 31 AFW pump recirculation valve in accordance with Alarm Response Procedure (ARP-006).
At 0847 hours0.0098 days <br />0.235 hours <br />0.0014 weeks <br />3.222835e-4 months <br />, a four hour non-emergency notification (Incident Log No. 35506) was made to the NRC for a Reactor Protection System (RPS) actuation in accordance with 10 CFR 50.72(b)(2)(iv)(B). Operations recorded the event in the corrective action program (CAP) as Condition Report CR-IP3-2003-00160. A post transient evaluation (Report No. 03-01) was completed on January 14, 2003.
The CWPs are motor-driven variable speed type pumps {P} manufactured by Allis- Chalmers {A180}. Each CW pump drive unit consists of a variable speed synchronous motor {MO) powered from an adjustable-frequency power control system of the LCI {INVT) type. The motor rotor (field) is excited by an exciter (DC generator) coupled to the motor shaft. Excitation is controlled by the excitation voltage controller (EVC) {EC). Controlled excitation is necessary for load commutation. The exciter's rotor voltage is rectified by a 3 phase full wave rectifier on the rotor to supply DC field current to the motor. The CWP motors and LCI drives were manufactured by General Electric {G080). Six LCIs are dedicated to serving individual CW pump motors. A seventh LCI is a spare that is used as a standby drive and can replace any failed dedicated LCI.
A detailed engineering evaluation was performed to determine the cause of the 32 RCP trip. The RCP breaker trip was initiated by an overcurrent protection relay trip however no failure mechanism was identified. RCP motor testing and assessment of motor currents showed no anomalies. Motor feeder cable testing results were satisfactory. The motor protective relay was checked for damage or miscalibration and found satisfactory. The RCP bus tie breaker (UT4-ST6) contacts and arc chutes were checked and no indications of excessive currents were identified. Inspection of the 6.9 KV breaker for the 32 RCP did not show signs of arcing, overheating, or degradation. The evaluation determined that the 32 RCP and associated components are operating as designed and are ready for service.
Instrument & Control technicians performed troubleshooting on the 32 source range detector (N-32) {DET) of the excore nuclear instrumentation system (NIS) {IG} and could not identify the cause of the detector's improper readings. The NIS was manufactured by Westinghouse Electric Corporation (W121). The detector began tracking N-31 and was within its testing frequency and was returned to service.
CAUSE OF EVENT
The cause of the manual reactor scram was a high dp between condenser sections.
The high dp in one section of the three section condenser was due to the loss of both CWPs in one condenser section. The loss of two CWPs was due to the trip of the 35 CWP while the 36 CWP was tagged out for planned maintenance. The apparent cause of the 35 CWP trip was a failure of the DC exciter lead that connects the exciter rotor to the main rotor of the pump motor. The failure was a result of the positive DC lead rubbing the motor dust cover due to improper cable position during a previous maintenance activity. The lead was installed improperly during previous maintenance that installed a new CWP motor upper oil reservoir cooling coil that cools the upper motor bearing lubricating oil. The manufacturer, GE provided instructions on coil replacement but did not provide specific termination instructions and maintenance failed to request them. Subsequently, during coil replacement for the 35 CWP, inadequate clearance was provided for the DC lead that connects the exciter rotor to the main rotor. The DC lead rubbed during motor operation and vibrated the connection to the lug until the connection failed.
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CORRECTIVE ACTIONS
The following corrective actions have been or will be performed under the CAP to address the causes of this event and prevent recurrence.
1. Troubleshooting was performed on the CWP LCI Standby drive. One silicon controlled rectifier (SCR) was found shorted in the EVC cabinet and was replaced. The shorted SCR was attributed to the damaged motor exciter wire.
One power supply card (NPSE) in the standby drive ECV cabinet was replaced as a predictive maintenance action. Functional testing of the Standby drive was performed and test results were satisfactory. The 36 CWP was transferred to the standby drive and the standby drive was determined to be operational on January 14, 2003. The 36 CWP was transferred to its normal drive and returned to service.
2. Troubleshooting was performed on the 35 CWP motor exciter leads and associated lugs. The positive DC lead and associated lug was repaired. The CWP was tested and returned to service on January 14, 2003.
3. Immediately after discovery, a tailgate meeting was held for the Indian Point 3 Maintenance Department to discuss the event and reinforce management expectations on human performance.
4. A memorandum was issued to the Indian Point Energy Center (IPEC) population to describe the causes of the event and reinforce management's expectation for attention to detail.
5. Maintenance procedure MTR-004-CWP was revised to include a caution about motor exciter rotor leads and necessary assembly details to prevent recurrence.
6. Procedure PMP-052-CWS will be revised to include a requirement to inspect all CWP motors brought on site following any refurbishment completed by an outside vendor to ensure proper assembly of exciter leads.
EVENT REPORTING
The event is reportable under 10CFR50.73(a)(2)(iv)(A). The licensee shall report any event or condition that resulted in manual or automatic actuation of any of the systems listed under 10CFR50.73(a)(2)(iv)(B). Systems to which the requirements of 10CFR50.73(a)(2)(iv)(A) apply include the reactor protection system (RPS) including reactor scram or RT, and AFW.
This event meets the reporting criteria because the RPS was manually actuated and a RT occurred. In response to the RT, the AFWS actuated due to steam generator level changes, which occur after a RT from full power.
PAST SIMILAR EVENTS
A review of Licensee Event Reports (LERs) for the past two years did not identify any events that involved a RT caused by high differential pressure in the condenser.
SAFETY SIGNIFICANCE
This event had no effect on the health and safety of the public.
There were no actual safety consequences for the event because the event was an uncomplicated reactor trip with no other transients or accidents. Required safety systems performed as designed when the RT occurred. The AFWS actuation was expected due to steam generator level changes, which occur after a RT from high power levels.
There were no significant potential safety consequences of this event under reasonable and credible alternative conditions. A loss of a CWP (e.g., 35 CWP) and therefore cooling to a condenser section may result in a loss of condenser vacuum, loss of megawatts, or high turbine exhaust hood temperatures. Low condenser vacuum will result in a turbine trip. When the unit load is greater than the Permissive P-8 setpoint, a trip of the turbine generator initiates a RT. A loss of external electrical load/turbine trip is an analyzed event described in FSAR Chapter 14. The plant performed as expected and the event was bounded by the FSAR analysis. The trip of the 32 RCP, after transfer of the 32 RCP's normal 6.9 KV power source from Bus 4 to Bus 6, resulted in loss of forced flow in reactor coolant loop 32. The loss of forced RC flow caused by the loss of one out of four RCPs from full power is an analyzed event in FSAR Section 14.1.6. Protection from a partial loss of flow event is provided by a RT. Below the P-7 Permissive, natural circulation flow provides adequate cooling.
Following RT, the affected RCP will continue to coast down and a stable plant condition will be attained. The plant performed as expected and the 32 RCP trip event was bounded by the FSAR analysis. For this event rod control was in automatic and the reactor scrammed immediately upon a manual RT. RCS pressure remained below the set point for pressurizer PORV or code safety valve operation and above the set point for automatic safety injection actuation. � Following the RT, the plant was stabilized in hot standby.