05000323/LER-2006-003
| Event date: | |
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| Report date: | |
| 3232006003R00 - NRC Website | |
EXPECTED
SUBMISSION DATE (15) 1 I 1
MON DAY YR
reactor while it was subcritical. An 8-hour nonemergency notification in accordance with 10 CFR 50.72(b)(3)(iv)(A) was made via the emergency notification system at 2231 PST.
On December 10, 2006, with Unit 2 operating in Mode 1, at 100 percent power, licensed plant operators initiated an unplanned reactor shutdown due to indications of increasing reactor coolant pump (RCP) 2-2 stator temperature. In accordance with plant procedures, operators manually tripped the reactor and shutdown RCP 2-2 when the temperature indication reached 300°F. At the time of the reactor trip, Unit 2 was subcritical in Mode 3, Hot Standby, with Control Rod Bank A at 106 steps withdrawn, all other control rods were fully inserted.
All rods fully inserted in response to the manual reactor trip, and all systems functioned as required. The auxiliary feedwater system was already in service as part of the shutdown.
The electrical transmission grid was stable. With the exception of RCP 2-2, all major equipment including the three emergency diesel generators remained operable. The other three RCPs maintained forced circulation of the reactor coolant system.
Investigations identified that a stator resistance temperature detector (RTD) had failed causing a false RCP 2-2 increasing temperature indication. The failed RTD input to the PPC monitoring was replaced with an installed spare RTD that was verified to be operating satisfactorily.� .
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
I. Plant Conditions
Diablo Canyon Power Plant Unit 2 was in Mode 1 (Power Operation) at approximately 100 percent reactor power prior to the event.
II. Description of Problem
A. Background
The Plant Process Computer (PPC)[ID] monitors one of the six original equipment manufacturer's (OEM) installed resistance temperature detector (RTD) temperature elements [TE] for each reactor coolant system (RCS)[AB] reactor coolant pump (RCP) motor. The motor stator temperature indication and alarm is provided to give plant operators an early warning of a potential RCP motor problem. The RTDs are 120 ohm nickel copper RTDs originally installed temperature elements provided by the OEM. While the stator has six stator RTDs installed, only one is connected to and monitored by the PPC.
PPC input to PK 05-02 alarms in the control room at approximately 248°F.
The annunciator alarm response procedure requires, in part; "IF at anv time ...the... temperature limits is met AND SFM concurs: RCP 2-2 stator at or above 300°F .... THEN ...TRIP the Reactor ... TRIP RCP 2-2.
B. Event Description
On December 10, 2006, at 1048 PST, plant operators received PK 05-02 alarm due to RCP 2-2 stator temp greater than the alarm setpoint. Plant operators reviewed the stator temperature history and identified that the temperature increase began approximately 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> prior and had been slowly increasing since.
On December 10, 2006, at 1320 PST, with Unit 2 operating in Mode 1, at 100 percent power, operators initiated an unplanned reactor shutdown due to indication of increasing RCP 2-2 stator temperature. In accordance with plant procedures, operators are required to manually trip the reactor and deenergize a RCP if the stator temperature reaches 300°F.
On December 10, 2006, at 16:08 PST, operators manually tripped the reactor while it was subcritical when the RCP stator temperature reached 300°F. Control Rod Bank A was at 106 steps withdrawn, all other control rods were fully inserted.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) All control rods fully inserted in response to the reactor trip, and all systems functioned as required. The auxiliary feedwater system was already in service as part of the shutdown. The electrical transmission grid was stable. With the exception of RCP 2-2, all major equipment including the three emergency diesel generators remained operable.
Unit 2 electrical loads were being supplied by the normal startup power.
The three unaffected RCPs maintained forced circulation of the RCS.
On December 10, 2006, at 2231 PST, an 8-hour notification in accordance with 10 CFR 50.72 (b)(3)(iv)(A) was made via the emergency notification system (ENS No. 43042).
Investigations identified that a RCP motor stator RTD had failed causing the false RCP 2-2 increasing temperature indication.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event The RCP stator temperature RTD failed, falsely indicating an increasing temperature. No actual RCP degradation occurred other than the RTD.
D. Other Systems or Secondary Functions Affected None.
E. Method of Discovery The condition was known to licensed plant operators due to alarms and indications provided in the control room.
F. Operator Actions Licensed plant operators responded to increasing temperature indication and alarm of RCP 2-2 stator temperature, reduced reactor power and placed Unit 2 in Mode 3, Hot Standby. Plant operators tripped the reactor and deenergized RCP 2-2 in accordance with approved plant procedures.
G. Safety System Responses The reactor trip breakers opened and the control rods fully inserted into the reactor as designed. Nonsafety control systems responded as designed throughout the plant shutdown activities.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) III.� Cause of the Problem
A. Immediate Cause
The stator RTD had failed causing a false RCP 2-2 increasing temperature indication for the single instrument indication.
B. Root Cause
Single random failure of the RTD such that plant staff were unable to confirm actual RCP motor temperature.
IV.� Assessment of Safety Consequences Licensed plant operators responded to increasing indication and alarm of RCP 2-2 stator temperature, entered approved plant procedures for power reduction, and manually tripped the reactor while it was subcritical in Mode 3.
The cause of the increasing temperature was identified as a failed RTD and not an actual RCP motor abnormal condition.
There were no actual safety consequences involved in this event since no safety related equipment was degraded and an orderly manual reactor shutdown is a normal operating practice previously evaluated in the Final Safety Analysis Report Update.
Also, the condition is not considered a Safety System Functional Failure.
Therefore, the event is not considered risk significant, and it did not adversely affect the health and safety of the public.
V.� Corrective Actions
A. Immediate Corrective Actions
The failed RTD input to the PPC monitoring was replaced with an installed spare RTD that was verified to be operating satisfactorily.
B. Corrective Actions
1. The RTD stator temperature alarm response procedure will be revised to provide additional guidance regarding information to be gathered for formal operational decision making prior to initiation of a reactor trip.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) 2. A program for RTD health monitoring and performance of periodic data assessment of critical RTDs identified as single point vulnerabilities will be developed to aid in identifying potentially failing RTDs.
3. The PPC alarm setpoint will be revised for the RCP stator RTDs to provide additional margin between alarm initiation and required operator action.
VI.�Additional Information A. Failed Component The RTDs are 120 ohm nickel copper RTDs originally installed temperature elements provided by the OEM.
B. Previous Similar Events
None.