05000390/LER-2011-001
| Watts Bar Nuclear Plant | |
| Event date: | 05-09-2011 |
|---|---|
| Report date: | 07-08-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3902011001R01 - NRC Website | |
A. Event
On May 9, 2011 at 15:17 Eastern Standard Time (EST) with Watts Bar Nuclear Plant, Unit 1 in Mode 5, and the Reactor Coolant System (RCS) [EIIS1 Code BQ] in a near water solid condition, a licensed operator started Safety Injection Pump 1A-A (SIP 1A-A) [EllS Code AB] to fill and vent the Cold Leg Accumulators (CLAs) [EIIS Code ACC] in accordance with System Operating Instruction SOI-63.01. Following startup of SIP 1A-A, RCS pressure immediately began to rise, reaching a maximum pressure of 328 psig before the operators secured the pump. The RCS pressure transient during this event did not exceed the Cold Overpressure Mitigation System (COMS) setpoint. The COMS protects RCS pressure boundary integrity by ensuring that the plant operates within the pressure/temperature (P/T) limits specified in the Pressure Temperature Limits Report (PTLR). The COMS configuration for Mode 5 operation consists of two operable pressurizer power operated relief valves (PORVs) with reduced lift settings, or one operable PORV and one operable Residual Heat Removal (RHR) suction relief valve.
B. Inoperable Structures, Components, or Systems that Contributed to the Event.
The RCS pressure transient was due to improper alignment of the Safety Injection System (SIS) for filling and venting the CLAs. Specifically, SIP 1A-A Crosstie Valve (1-FCV-63-152) was open when it should have been closed to isolate the SIS from the RCS.
C. Dates and Approximate Times of Major Occurrences Date Time (EST) Event 05/01/2011 Temp Lift 2011-0811 was created/issued to allow Full Flow Testing to be performed; left 1-FCV-63-152A in open position.
05/02/2011 Completion of 1-SI-63-906 (SI CHV Full Flow Test) did not return 1- FCV-63-152A to closed as required for current plant conditions.
05/09/2011 15:18 Started SIP 1A-A to fill and vent CLAs per S01-63-01. RCS pressure increased rapidly to 328 psig. Approximately 29 seconds later, Operator secured SIP 1A-A.
05/09/2011 15:19 Entered LCO 3.4.12, Condition A, to verify no SIPs are capable of injecting into the RCS.
05/09/2011 15:21 Racked-out SIP 1A-A, SIP B-B, and Centrifugal Charging Pump CCP-A-A to render pumps inoperable.
05/09/2011 16:12 Performed temporary clearance lift on associated breakers to validate positions of valves 1-FCV-63-152 & 1-FCV-63-156. Found -152 open, and -156 closed. Closed 1-FCV-63-152 to restore to required configuration.
05/09/2011 16:35 Exited LCO 3.4.12 and resumed filling and venting of CLAs in accordance with SO1-63.01.
D. Other Systems or Secondary Functions Affected
No other system or secondary functions were affected by this event. The pressure transient did not challenge the Cold Overpressure Mitigation System (COMS) or the reactor coolant system pressure boundary integrity.
E. Method of Discovery
Control room indication of abnormal RCS pressure transient following startup of SIP 1A-A.
F. Operator Actions
Shortly after starting SIP 1A-A, multiple main control room (MCR) indications of abnormal RCS pressure prompted the operators to secure SIP 1A-A and terminate filling of the CLAs. RCS pressure immediately began to decrease and returned to normal operating pressure within approximately 12 minutes. There was no MCR indication that the PORVs had lifted. Personnel were immediately dispatched to the Boron Injection Tank room to verify the position of the SIP 1A-A Crosstie Valve (1-FCV-63-152) and Hot Leg Injection Valve (1-FCV-63-156). While this action was being performed, the breaker for SIP 1A-A was racked-out to ensure COMS compliance. It was soon reported that 1-FCV-63-152 was OPEN. A temporary clearance lift was obtained for 1-BKR-63-152 and 1-HS-63-152A to allow the MCR to close 1-FCV-63-152. After assuring 1-FCV-63-152 was closed, the breaker for SIP 1A-A was racked-in and the pump restored to OPERABLE status.
G. Safety System Responses
Inadvertent injection from the SIP into the RCS in a near water solid condition resulted in a mass input transient that caused the RCS pressure to increase. The abnormal pressure increase did not challenge the COMS or the RCS pressure boundary integrity.
III. CAUSE OF EVENT
One SIP in Mode 5 is normally used to fill and vent the CLAs in accordance with SOI-63.01. During performance of this procedure, SIP 1A-A Crosstie valve 1-FCV-63-152 is closed to preclude injecting directly into the reactor coolant system. However, previously in Mode 6 a temporary clearance lift was issued to open 1-FCV-63-152 to perform Safety Injection full flow testing in accordance with 1-SI-63-906, "Safety Injection Check Valve Full-Flow Testing During Refueling Outages." Following completion of this surveillance SIP 1A-A was secured, but the crosstie valve 1-FCV-63-152 was not closed. Not realizing that the crosstie valve was open, the licensed operators resumed filling and venting the CLAs in accordance with SOI-63.01. SIP 1A-A was started and RCS pressure immediately began to rise and reached a maximum pressure of 328 psig before the operators secured the pump. Failure to close 1-FCV-63-152 was attributed to failure to comply with plant clearance procedures for system alignment, in that the clearance was restored without verifying valve position.
IV. ANALYSIS OF THE EVENT
Prior to startup of SIP 1A-A in Mode 5, RCS pressure was approximately 118 psig with the minimum reactor cold leg temperature at 125 °F. At the time of the event, the plant was in a near water solid condition with the pressurizer PORV block valves and the RHR suction valves open and the CLA isolation valves closed.
Immediately following startup of SIP 1A-A, RCS pressure remained relatively flat at 118 psig for approximately 5 seconds and then ramped up to 328 psig in approximately 24 seconds. The RCS was not completely water solid at the time of the incident because the reactor coolant pumps were not previously started to sweep out the gas voids in the Steam Generator U-Tubes. As a consequence, the pressure rise was initially dampened by the gas voids and ramp-up as the pump accelerated to full speed (approximately 5 seconds). The COMS did not actuate because the operators quickly secured SIP 1A-A before RCS pressure exceeded the maximum pressure for the existing temperature, as defined by the PTLR.
The safety significance of this event is low because COMS and the integrity of the reactor coolant system pressure boundary were not challenged.
The COMS configuration in Mode 5 consisted of two OPERABLE PORVs and one OPERABLE RHR relief valve.
The COMS actuation logic monitors RCS pressure and temperature to determine if operating condition is approaching the limits specified in the PTLR. The reactor coolant cold leg temperature indications are auctioneered to select the lowest cold leg temperature signal. The lowest temperature signal is processed through a function generator that calculates a pressure limit for that temperature. The calculated pressure limit is then compared with the indicated RCS pressure from a wide range pressure channel. If the indicated pressure meets or exceeds the calculated value, a PORV is signaled to open. Based on fracture mechanics analyses the reactor vessel is protected when the PORVs are set to open at or below the P/T limit as specified in the PTLR.
The P/T limits are periodically modified as reactor vessel material toughness decreases due to neutron embrittlement. The revised limits are determined using neutron fluence projections and the results of examinations of the reactor vessel material irradiation surveillance specimens. According to Site Engineering Setpoint and Scaling Documents, the COMS pressure setpoint corresponding to a RCS temperature of 125 °F with no RCPs running is 377 psig. Since the maximum RCS pressure during this transient was 328 psig, the safety margin was equal to 49 psig.
Since RCS pressure never exceeded the COMS setpoints for actuating the pressurizer PORVs or the RHR suction relief valves, the integrity of the reactor coolant system pressure boundary was not challenged, as the P/T limits specified in the PTLR were not exceeded.
VI. CORRECTIVE ACTIONS
This event was documented within TVA's Corrective Action Program as PER 366905.
A. Immediate Corrective Actions
Immediate Corrective Actions were taken to:
1. Trip SIP 1A-A 2. Enter TS LCO 3.4.12, Condition A, which requires initiating action to verify no safety injection pumps are capable of injecting into the RCS.
3. Rack-out SIP 1A-A, SIP-1B and CCP 1B-B circuit breakers.
4. Dispatch AUO to verify position of SIP 1A-A Cold Leg Injection Valve 1-FCV-63-152 and Hot Leg Injection Valve 1-FCV-63-156. Found 1-FCV-63-152 open. Closed 1-FCV-63-152.
5. Exit TS LCO 3.4.12.
B. Corrective Actions to Prevent Recurrence
Corrective Actions to prevent recurrence include the following:
1. Revise surveillance instruction 1-SI-63-906 to specify that 1-FCV-63-152 be closed following testing.
2. Revise Nuclear Power Group Procedure NPG-SPP-10.2 to provide more specific instructions on how to perform a temporary Hold Order Lift.
3. Add to the Just-in-Time (JIT) Training agenda for the next refueling outage an internal Operating Experience (OE) and lessons learned from this event.
A. Failed Components
None B. Previous LERs on Similar Events A search of LERs and PERS documenting COMS or solid plant operations at Watts Bar Unit 1 found no previous failures similar to that which occurred on 5/9/2011 i.e., inadvertent injection, as a result of valve misposition, from a safety injection pump into the RCS with the plant water solid.
NRC White Finding VIO 05000390/2006007-01, "Failure to Implement Shutdown Procedures which Resulted in Pressurizer PORV Actuations," identified a situation in which personnel lack of sensitivity to and failure to recognize the hazards associated with the approach to solid water operations, and long standing equipment performance problems led to a situation in which pressurizer PORVs actuated several times. TVA does not consider that 2005 event similar to the current event, where actuation of COMS was averted as a result of operator sensitivity to solid water conditions and quick operator action when RCS pressure increased due to a valve not restored to its proper position following a temporary lift of a clearance boundary.
C. Additional Information:
None.
D. Safety System Functional Failure This event did not involve a safety system functional failure as defined in NEI 99-02, Revision 5.
E. Loss of Normal Heat Removal Consideration None.
VIII.COMMITMENTS None.