05000461/LER-2013-002
| Clinton Power Station, Unit 1 | |
| Event date: | 03-07-2013 |
|---|---|
| Report date: | 05-02-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 48812 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 4612013002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric -- Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EllS) codes are identified in test as [XX].
EVENT IDENTIFICATION
Deficient Fuse Causes Main Generator Trip, Turbine Trip and Reactor SCRAM
A. Plant Operating Conditions Before the Event
Unit: 1 Event Date: 3/7/2013 Event Time: 0758 hours0.00877 days <br />0.211 hours <br />0.00125 weeks <br />2.88419e-4 months <br /> CST Mode: 1 Mode Name: Power Operation Reactor Power: 96.9 percent
B. DESCRIPTION OF EVENT
At 0642 on 3/7/13, the plant was in Mode 1 (Power Operation) at 96.9 percent power. The Main Control Room (MCR) received a main generator [TG] [TB] trouble alarm [ALM] for the Automatic Voltage Regulator (AVR) [EC] automatically transferring from channel 2 to channel 1 due to a fault. Operators verified main generator parameters were normal and dispatched Equipment Operators and Electrical Maintenance technicians to investigate the reason for the fault. The MCR completed pre-emptive briefs to discuss the alarms received and a Reactor Operator was assigned to focus on the main generator and exciter [EXC].
Contingency actions for a turbine trip and reactor SCRAM were also briefed.
At 0756 hours0.00875 days <br />0.21 hours <br />0.00125 weeks <br />2.87658e-4 months <br />, the main generator tripped. At 0758 hours0.00877 days <br />0.211 hours <br />0.00125 weeks <br />2.88419e-4 months <br />, Operators in the MCR received numerous alarms for a main turbine [TRB] trip and reactor [RCT] SCRAM. Subsequently, the Reactor Operator placed the reactor mode switch [HS] into the shutdown position. Operators entered the Reactor Scram Off- Normal Procedure and subsequently entered Emergency Operating Procedure (EOP)-1, "Reactor Pressure Vessel Level Control," due to an expected low reactor water level 3 trip signal. All control rods fully inserted and all plant equipment responded as expected to the SCRAM. At 0928 hours0.0107 days <br />0.258 hours <br />0.00153 weeks <br />3.53104e-4 months <br />, Operators established a reactor coolant pressure of 500 to 600 pounds per square inch gage, using Turbine Bypass Valves, a reactor pressure vessel water level of 30 to 39 inches, and exited EOP-1 in accordance with normal plant procedures.
A troubleshooting team was formed to investigate the cause of the trip. The team determined that the AVR initially transferred from channel 2 to channel 1 due to a degrading potential transformer [XPT] (PT) C phase regulating fuse [FU]. The AVR responded to the fuse degradation by sensing an increase in voltage without sensing the corresponding change in current during the same time period, and the AVR acted as designed in its internal algorithm by automatically switching to the backup channel. However, the PT fuse protecting the C phase to the Main Generator did not function as designed. This fuse is designed to "fast blow" and a voltage balance relay [60] is installed to sense a "fast blow" failure and prevent a generator trip. The troubleshooting team identified that the fuse did not fast blow, and instead degraded gradually which did not allow the voltage balance relay to detect the problem and prevent the main generator trip and subsequent reactor SCRAM.
As expected during the event, the low reactor water level 3 trip signal caused primary containment isolation valves [ISV] in Group 2 (Residual Heat Removal (RHR) [BC]), Group 3 (RHR), and Group 20 (miscellaneous systems) to receive signals to shut; operators verified that the valves properly responded to the trip signal.
This event was determined to be reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), as a event or condition that resulted in automatic actuation of the reactor protection system (RPS). At 1052 hours0.0122 days <br />0.292 hours <br />0.00174 weeks <br />4.00286e-4 months <br />, the station notified the NRC of this event via Emergency Notification 48812. Issue Report 1484624 was initiated to investigate this event.
C. CAUSE OF EVENT
A failure analysis of the fuse identified that the cause of the C phase PT fuse failure was a manufacturing defect in a solder connection in the fuse. As the solder joint failed, the increasing resistance caused a false overvoltage signal, which resulted in the main generator trip followed by a main turbine trip and reactor SCRAM. The deficient solder connection was found to have poor coverage and wetting, which would have resulted in a mechanically weak solder connection. The poor solder connection was caused by deficiencies in Eaton Cutler-Hammer's manufacturing process at the Dominican Republic production facility. A contributing cause for the fuse failure was a misalignment of the stack assembly that increased forces on the solder connection. The internal boric acid block stack assembly end cap was found displaced and angled due to a wedging of the heat shrink wrap under one end of the cap, which resulted in some misalignment of the spring end and element within the assembly.
D. SAFETY CONSEQUENCES
The actuation of the RPS placed the plant in a safe and stable condition. There were no plant safety limits exceeded, and no other Engineered Safety Feature (ESF) actuations, and risk significance was low. Safety related systems functioned correctly in response to this event with critical plant parameters remaining within the bounds of plant design, Technical Specifications, Updated Safety Analysis Report, Offsite Dose Calculation Manual, and Core Operating Limits Report. The affected system (TG system) is non-safety related.
No loss of safety function occurred during this event.
E. CORRECTIVE ACTIONS
The fuse that failed was replaced with a fuse that was not manufactured at the Dominican Republic production facility.
An extent of condition review has been performed to identify other potentially affected installed fuses and a plan is being developed to inspect and replace the affected fuses.
F. PREVIOUS OCCURRENCES
A review for previous occurrences did not identify similar events at Clinton Power Station.
G. COMPONENT FAILURE DATA
Component Description: C Phase Regulating Potential Transformer Fuse Manufacturer: Eaton Cutler-Hammer Nomenclature: Potential Transformer Fuse Model: CLPT Manufacturer Part Number: 25CLPT-.5E Date of Manufacture Code: 9/2009