05000237/LER-2004-005
| Docket Number | |
| Event date: | 05-16-2004 |
|---|---|
| Report date: | 08-30-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2372004005R00 - NRC Website | |
Dresden Nuclear Power Station Units 2 and 3 are General Electric Company Boiling Water Reactors with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 02� Event Date: 5-16-2004 Reactor Mode: 1� Mode Name: Power Operation � Power Level: 100 percent Reactor Coolant System Pressure: 1000 psig
B. Description of Event:
On May 16, 2004, with Unit 2 at approximately 100 percent power in Mode 1, the 2A Turbine Condenser Vacuum - Low switch for the Reactor Protection System (RPS) became inoperable due to the collection of water in its vacuum sensing line. The inoperable switch was not immediately detected and the condition existed for approximately 1.5 days, which exceeded the Completion Times associated with Required Actions of Technical Specification (TS) 3.3.1.1, "Reactor Protection System (RPS) Instrumentation.
Additionally, on May 1, 2004, with Unit 3 at approximately 100 percent power in Mode 1, the 3C Turbine Condenser Vacuum - Low switch for the RPS became inoperable due to the collection of water in its vacuum sensing line. The inoperable switch was not immediately detected and the condition existed for approximately 3 days, which exceeded the Completion Times associated with Required Actions of TS 3.3.1.1.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition which was prohibited by the plant's Technical Specifications." TS 3.3.1.1 Condition A requires that the inoperable Turbine Condenser Vacuum - Low channel or associated trip system be placed in the tripped condition within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
This action was not accomplished, as it was not recognized at the time that the Turbine Condenser Vacuum - Low switch was inoperable.
C. Cause of Event:
The root cause of the event was that previous efforts to correct the vacuum sensing line errors were too narrowly focused and did not inspect or repair all internal and external sensing lines to all turbine hoods. The corrective action to prevent recurrence is to perform internal and external condenser walk downs to determine Turbine Condenser Vacuum - Low switch sensing line slope and repair or modify as necessary.
Under certain plant conditions, main condenser vacuum indications were in error due to the collection of water within the vacuum sensing lines. Water in the sensing lines caused instrumentation to sense vacuum at higher levels than actually existed in the main condenser shell. Water collects as a result of condensation within the sensing line, and typically occurs when there is a significant reduction in condenser vacuum. Both the magnitude and duration of indication error for each of these occurrences were variable. Both Dresden units were affected and at least three separate sensing lines were affected.
In January 2004, Dresden Plant Engineering initiated improved trending and monitoring plans to identify degrading equipment prior to failure. The effort resulted in increased attention to the performance of the Turbine Condenser Vacuum - Low switches. As a result of this effort, in late May 2004 temporary plant modifications were installed in the 3C and 2A Turbine Condenser Vacuum - Low switch sensing lines to continuously purge and clear condensation from the line.
NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (7-2001) Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER On July 1, 2004, an NRC Resident Inspector questioned the historical reportability of the Turbine Condenser Vacuum - Low switches. Dresden initiated an evaluation that identified that the Turbine Condenser Vacuum - Low switches had been historically inoperable for periods of time that exceeded the TS Allowed Outage Time.
Engineering identified the most recent occurrence on each unit and that this situation had occurred at least 4 times in the past 2 years. These events were not reported in accordance with the requirements of 10 CFR 50.73.
Additionally, the evaluation also concluded that the failure to report the Turbine Condenser Vacuum - Low switch events was due to the inability of operations personnel to recognize that the Turbine Condenser Vacuum - Low switch was degraded past the point of being operable due to the lack of adequate acceptance criteria in shift logs and operating procedures.
D. Safety Analysis:
The safety significance of the event is minimal. Each Dresden unit has four channels of the Turbine Condenser Vacuum - Low Function that are arranged in a one-out-of-two trip logic to ensure that no single instrument failure will preclude a scram from this Function on a valid signal. The evaluation of past inoperabilities of the Turbine Condenser Vacuum 7 Low switches did not identify any exceedance of main condenser vacuum trip setpoint limits and no cases were identified when two Turbine Condenser Vacuum - Low switches were inoperable on the same unit at the same time due to condensation in the sensing lines. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
Temporary vent valves were installed on Turbine Condenser Vacuum - Low switches 2A, 2C, 3A and 3C to continuously purge and clear condensation from the line.
Internal and external condenser walk downs for Units 2 and 3, will be performed during the next refueling outage on each unit, to determine Turbine Condenser Vacuum - Low switch sensing line slope and potential changes to the penetration at the condenser wall. The Turbine Condenser Vacuum - Low switch sensing lines will be repaired or modified as necessary.
Operations personnel will be provided with enhanced guidance to verify the operability of the Turbine Condenser Vacuum - Low switches by September 1, 2004.
The operating procedures related to condenser waterbox flow reversal will be revised to provide condenser vacuum monitoring guidance by September 1, 2004.
Operator round logs will be revised to include appropriate acceptance criteria for the Turbine Condenser Vacuum - Low switches by September 1, 2004.
NRC FORM 366A � U.S. NUCLEAR REGULATORY COMMISSION (7-200t) Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER F.�Previous Occurrences:
A review of Dresden Nuclear Power Station Licensee Event Reports (LERs) and operating experience over the previous six years identified the following similar occurrence associated with a failed Turbine Condenser Vacuum - Low switch.
- LER 98-006-00, "RPS Condenser Vacuum Switch Potentially Inoperable Due To Improper Instrument Sensing Line Slope," dated July 15, 1998. The LER discussed an event in which a Turbine Condenser Vacuum - Low switch was determined to be inoperable due to water collecting in the sensing line. The corrective action to correct the slope of the 3C Turbine Condenser Vacuum - Low switch sensing line was not successful in preventing this event.
G.�Component Failure Data:
N.A.