05000265/LER-2014-003
| Quad Cities Nuclear Power Station Unit 2 | |
| Event date: | 05-06-2014 |
|---|---|
| Report date: | 07-15-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2652014003R00 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
General Electric — Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
RPS Pressure Switch for Condenser Vacuum — Low Inoperable and Exceeded Technical Specifications
A. CONDITION PRIOR TO EVENT
Unit: 2 Reactor Mode: 1 Event Date: May 06, 2014 Event Time: 1820 Mode Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT
On May 16, 2014 at 0432, with Unit 2 operating at full power, a main condenser flow reversal was performed. During the flow reversal, the change in the 2C condenser hood backpressure indication responded slower than the 2A and 2B condenser hoods. The observed 2C condenser hood back pressure indication took approximately 30 minutes to reach the expected steady state operating pressure of the warm hood following the flow reversal. The 2A and 2B condenser hoods obtained a steady state backpressure approximately three minutes following the flow reversal, which is consistent with past history.
As a result of the slow response of the 2C condenser backpressure, the Low Pressure (LP) Turbine 2A Exhaust to Main Condenser pressure switch, PS 2-0503-B, was declared inoperable. This pressure switch inputs into the Reactor Protection System (RPS) as one of the four channels of Turbine Condenser Vacuum — Low function. TS 3.3.1.1 (RPS Instrumentation), Condition A, was entered at 0445. The required actions were to place the channel in trip in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> OR place associated trip system in trip in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
At 0550, Operations determined that the 2-3199-75C, the 'C' condenser pressure indicator root valve, was mostly closed. This valve isolated the RPS pressure switch PS 2-0503-B. The valve was opened and backseated. Mechanical Maintenance determined that one of the gland follower bolts and corresponding fastener were missing on the 2-3199-75C valve. In addition, one missing gland follower fastener was identified on the 2-3199-75B, and both of the gland follower fasteners on the 2-3199-75A valve were found loose. These discrepancies were corrected by Mechanical Maintenance. After the completion of these activities, the RPS pressure switch PS 2-0503-B was declared operable and TS 3.3.1.1, Condition A was exited at 1223 on May 16, 2014.
C. CAUSE OF EVENT
2014 - 003 - 0 The missing bolt from the 2-3199-75C gland follower was the most apparent cause leading to the valve being out of position. Preventative maintenance was not performed on the valve that would have ensured that the fasteners on the gland follower were tight. Over time, the fasteners on the gland follower are proven to vibrate loose, leaving the bolt with nothing but friction to hold it in place. Without fasteners the gland follower bolts will eventually fall out of place, leaving the valve position susceptible to normal system vibration.
On May 6, 2014, Unit 2 had returned to power after a refueling outage. A large amount of work took place in the steam side of the main condenser during the outage that involved large amounts of material and personnel. The 2-3199-75C valve is located approximately 6" above the personnel grating, which is the only path to gain access to the "C" hood of the main condenser. This valve was not likely repositioned by an inadvertent bumping, but was disturbed enough that the missing bolt held in place only by friction fell out. With only one bolt in place, the resistance is not enough to keep the valve from vibrating closed during operation at power.
The 2-3199-75A and 2-3199-75B, the other two RPS pressure switch isolation valves, are not located near their respective hood access ways. They are located 3 to 4 feet away from the personnel grating and are not easily accessible. The missing fasteners on these valves most likely vibrated loose during the most recent operating cycle.
The inadvertent closure of the 2-3199-75C valve resulted in an inoperable condenser low vacuum RPS channel. A review of recent condenser backpressure trends identified two different conditions prohibited by Technical Specifications. During the startup on May 6, 2014 an inflection point was identified for the 2C condenser backpressure indication at approximately 1820 on May 6, 2014. The review concluded this is the time when the isolation valve had vibrated mostly shut, while Unit 2 was in Mode 2. At 2252 on May 6, 2014, Unit 2 entered Mode 1 with an inoperable condenser low vacuum RPS channel. This condition did not meet TS 3.0.4 since the minimum number of required operable channels per TS 3.3.1.1 Function 10 was not met prior to entering Mode 1.
The second condition prohibited by TS was identified when the condition exceeded the allowable outage time of TS 3.3.1.1. The actions for Function 10 would be to place the channel in trip within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, and then be in Mode 2 in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. The RPS pressure switch was inoperable from 1820 May 6, 2014 until 1223 May 16, 2014. This exceeded the maximum allowed outage time of 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />.
D. SAFETY ANALYSIS
System Operation The Turbine Condenser Vacuum — Low Function is provided to shut down the reactor and reduce the energy input to the main condenser to prevent over-pressurization of the main condenser in the event of a loss of the main condenser vacuum. Loss of condenser vacuum occurs when the condenser can no longer handle the heat input (e.g., loss of heat transfer capability or excessive in-leakage.) This condition initiates a closure of the turbine stop valves and turbine bypass valves, which eliminates the reactor heat input to the condenser. Closure of the turbine stop and bypass valves causes a pressure 717:1131111 111INIM transient, neutron flux rise and an increase in fuel surface heat flux. To prevent the fuel cladding integrity Safety Limit from being exceeded if this occurs, a reactor scram occurs on turbine stop valve closure. The turbine stop valve closure scram function alone is adequate to prevent the fuel cladding integrity Safety Limit from being exceeded in the event of a turbine trip transient with bypass valve closure. The condenser low vacuum scram is anticipatory to the turbine stop valve closure scram.
Turbine condenser vacuum pressure signals are derived from four pressure switches that sense the pressure in the condenser. The Allowable Value is consistent with the main turbine trip on low main condenser vacuum set-point, and provides main condenser overpressure protection by shutting down the reactor; thereby, reducing energy into the main condenser. Four channels of Turbine Condenser Vacuum — Low Function, with two channels in each trip system arranged in a one-out-of-two-logic, are required to be OPERABLE to ensure that no single instrument failure will preclude a scram from this Function on a valid signal. The Function is required in MODE 1 since in this MODE there is a significant amount of core energy that can be rejected to the main condenser. During MODES 2, 3, 4, and 5, the core energy is significantly lower. This Function is automatically bypassed with the reactor mode switch in any position other than run.
Safety Impact This instrument inputs into RPS as one of the four channels of Turbine Condenser Vacuum — Low function, with two channels in each trip system arranged in a one-out-of-two logic. The function is designed with all instruments required to be operable to ensure that no single instrument failure will preclude a scram from this function on a valid signal. With the "C" Main Condenser switch inoperable, a "B" RPS channel would have occurred from the pressure switch PS 2-0503-D to allow a scram to occur on loss of vacuum. Although the pressure switch would have tripped at the proper set-point, the "C" pressure would have taken longer to trip than the other three unaffected RPS condenser backpressure pressure switches. There was no impact to the "A" RPS channel due to this event. Scram capability was maintained during this event.
The Main Condenser low vacuum pressure switches are required to be operable in MODE 1. At 2252 on May 6, 2014, Unit 2 entered Mode 1. The RPS pressure switch was inoperable from 1820 May 6, 2014 until 1223 May 16, 2014. Although the required instrument was not operable for a period of time, this did not create any actual plant or safety conseqt4ences since the Unit was not in an accident or transient condition requiring use of the RPS during this period of time.
In conclusion, this event resulted in a minimal , impact on plant risk, and the overall safety significance of this event was minimal.
E. CORRECTIVE ACTIONS
Immediate:
- Initiated a prompt investigation on the event.
- Valve 2-3199-75C was repaired and placed in the open position.
- Extent of Condition was performed for the other two isolation valves. Both valves were repaired and verified to be in the open position.
Follow-up:
- Evaluate the need for PMs on run-to-failure small manual globe valves which isolate instrumentation of the Reactor Protection System.
- Implement new PMs to replace/adjust gland follower hardware as required.
- Revise the Startup Checklist to add a prerequisite to verify 1(2)-3199-75A(B)(C) are open.
F. PREVIOUS OCCURRENCES
The station events database, LERs, and INPO Consolidated Event System (ICES) were reviewed for similar events at QUad Cities Nuclear Power Station. This event was caused by a valve repositioning due to vibration because no fasteners was holding the gland follower in place.
- Station Event Database — Quad Cities IR 1056375 (4/14/10) — During Startup of Unit 2, it was identified that the 2-3199-75C valve, "C" Condenser Pressure Transmitter Root Valve was partially closed, rather than full open as required. As a result, "C" condenser pressure (vacuum) indication was responding slower than normal as vacuum was established.
Because there is an RPS signal from this instrument line (which was not determined until after entry into MODE 1), this event was reportable as LER 50-265/2010-001-00.
An Appatent CauSe Evaluation determined that the April 14, 2010 event was due to bumping.
The valve is located in a walkway that gets a significant amount of traffic from multiple work groups during an outage.
The cause of this event was due to inadvertent bumping and is not directly applicable to the current LER.
G. COMPONENT FAILURE DATA
This event does not involve the failure of any equipment. Therefore, an ICES report has not been generated. -
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LAID
- Quad Cities Nuclear Power Station Unit 2 I 05000265