05000456/LER-2005-003
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| Event date: | |
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| Initial Reporting | |
| 4562005003R00 - NRC Website | |
Plant Operating Conditions Before The Event:
Event Date: November 18, 2004� Event Time: 1119 Unit: 1 MODE: 1� Reactor Power: 99.9 percent Unit 1 Reactor Coolant System (RCS) [AB] Temperature: 587 degrees F, Pressure: 2238 psig
Description of Event:
There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.
On November 18, 2004, at 1119, Braidwood Unit 1 experienced an isolation of extraction steam to the 15A and 15B low pressure feedwater [SJ] heaters during maintenance on a level controller for the 15A emergency level control valve. This resulted in a heater drain system [SM] level control transient cascading from the 15A/B to the 16A/B to the 17A/B heaters. This loss of feedwater preheat allowed colder water to enter the steam generators and caused reactor power to increase due to the positive reactivity feedback. The plant responded as designed to the feedwater heater transient.
At 1121, Operations was preparing to ramp back the unit per procedures. Reactor power increased above the overpower delta-temperature runback set point on 2-of-4 channels, which initiated an auto rod-stop [AA] and a turbine runback [JJ]. At 1122, the turbine power runback/auto rod stop alarm cleared. At 1127, reactor power stabilized at 78% power.
An evaluation was performed to determine if reactor power exceeded reportable levels during the event. The investigation determined that the nuclear instrumentation and 10 minute average calorimetric indicated less than 102% reactor power. The reactor coolant loop delta-temperature indications exceeded 102% power during the event; however, the delta-temperature channels are less accurate during a transient than during steady state, and were not used to determine power level during this event. It was concluded that the power range nuclear instrumentation was the most reliable indication available. Since this indication did not exceed 102.0%, the event was determined not to be reportable.
Subsequent to the event, Nuclear Fuels and Westinghouse performed an additional evaluation. This evaluation focused on the effect of the RCS cold-leg temperature changes, causing non-conservative reading in the ex-core detectors, which resulted in reactivity and power changes greater than indicated by the uncorrected plant nuclear instrumentation. With corrections applied in this evaluation, it was determined that peak power likely exceeded 102% of rated thermal power. The value could have been as high as 103.3% for approximately one minute during the transient. There are no known current industry standard guidelines for correcting indicated plant nuclear instrumentation during transient conditions.
On December 2, 2005, at 1700, based on an independent review of this evaluation, the determination was made that Unit 1 reactor power likely exceeded the license limit.
License Condition 2.G, due to the violation of License Condition 2.C(1) "Maximum Power Level.
Cause of Event
The HI-2 switch on the 15A heater Magnetrol level control assembly was found partially actuated due to calibration issues. During the start of the normal calibration for the 15A level control assembly, the mechanical agitation due to the removal of a drain cap was sufficient to actuate the adjacent HI-2 switch contacts and isolate the extraction steam to the 15A and 15 B low pressure feedwater heaters.
The root cause for the initiating event was determined to be a lack of understanding for a potential adverse internal component interaction during the calibration of a level controller on the same level column as the Magnetrol level switches.
D. Safety Consequences:
There were no safety consequences impacting plant or public safety as a result of this event. This was an analyzed condition. The November 18, 2004 overpower transient caused by a feedwater temperature transient was bounded by the feedwater design transient described and analyzed in Braidwood UFSAR, Section 15.1.1, "Feedwater System Malfunctions Causing a Reduction in Feedwater Temperature." The calculated overpower condition existed only during the transient (i.e., approximately one minute). The plant responded to the transient as designed, with a turbine overpower delta-temperature runback.
This event did not result in a safety system functional failure.
Corrective Actions:
The corrective actions to prevent occurrence from the root cause evaluation completed in January 2005 included creation of a work package standard to ensure uniformity for alignment, adjustments and calibration of the tandem float Magnetrol level control assemblies.
Additional actions include developing and implementing a process to address, for overpower transient power changes such as feedwater temperature reductions, whether overpower conditions occurred, by correcting nuclear instrumentation system readings.
F. Previous Occurrences:
There have been no previous occurrences of overpower due to feedwater heater transients. There have been three overpower events in the past three years:
- Unit 1 Licensee Event Report 2004-001-00 — Licensed Maximum Power Level Exceeded Due to Inaccuracies in Feedwater Ultrasonic Flow Measurements
- Unit 2 Licensee Event Report 2003-002-00 — Licensed Maximum Power Level Exceeded Due to Inaccuracies in Feedwater Ultrasonic Flow Measurements Caused by Signal Noise Contamination
- Unit 1 Licensee Event Report 2003-002-00 — Licensed Maximum Power Level Exceeded Due To An Error In A Westinghouse Supplied Calorimetric Calculation Constant
G. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A