05000275/LER-2011-001
| Diablo Canyon Power Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2752011001R01 - NRC Website | |
I. Plant Conditions
When the event occurred, Unit 1 was transitioning from Mode 4 (Hot Shutdown) to Mode 3 (Hot Standby). The reactor coolant system (RCS) temperature and pressure at this time were approximately 350 degrees Fahrenheit and 1,000 pounds per square inch (psig) respectively, and steam generator (SG) pressure was approximately 115 psig. At the time of testing, the SG pressure was approximately 1,000 psig.
II. Description of Problem
A. Background
The Diablo Canyon Power Plant (DCPP) Unit 1 is a pressurized water reactor (PWR) with four reactor coolant loops (RCL)[AB] to circulate reactor coolant to each of the four SGs. Each SG is a vertical U-tube design provided by the Nuclear Steam Supply System (NSSS) vendor, Westinghouse. The auxiliary feedwater (AFW) system [BA] is a safety-related system that serves as a backup supply of feedwater to the secondary side of the SGs. The AFW system maintains the heat sink function of the SGs whenever the main feedwater (MFW) system is unavailable.
The AFW system consists of three AFW supply trains. One train employs a full capacity, approximately 780 gallons per minute steam turbine-driven AFW Pump (TD AFW PP) 1-1, aligned to all four SGs. The other two trains consist of half-capacity motor-driven AFW Pumps (MD AFW PP) 1-2 and 1-3, each supplying approximately 390 gpm to two of the four SGs, with the capability to be manually aligned to any of the four SGs. The normal operation of the AFW system, which is during unit startup and shutdown, is to supply the SGs with a secondary heat sink while MFW is unavailable.
Technical Specification Limiting Condition for Operation (TS LCO) 3.7.5, "Auxiliary Feedwater System," requires three AFW trains to be operable in Modes 1, 2, and 3. TS LCO 3.7.5 is modified by a note that prohibits mode transition with an inoperable AFW pump. Under Surveillance Requirement (SR) 3.7.5.2, testing of the TD AFW PP 1-1 is required to be performed within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after reaching 650 psig steam pressure in the SGs.
B. Event Description
On October 27, 2010, during the Unit 1 Sixteenth Refueling Outage (1R16), plant personnel completed replacement of the speed governor on the TD AFW PP 1-1. On November 6, 2010, at 0512 PST, Unit 1 entered Mode 3.
On November 6, 2010, at 2222 PST, DCPP commenced Surveillance Test Procedure (STP) P-AFW-11, "Routine Surveillance Test of Turbine-Driven Auxiliary Feedwater Pump 1-1." This STP was used in conjunction with a maintenance procedure to set the speed of the governor and also served as post-maintenance testing (PMT) for the TD AFW PP 1-1. Per the test procedure, the pump was declared inoperable prior to testing. When steam was admitted to the TD AFW PP 1-1 to raise the speed, test personnel observed speeds in excess of 4,300 rpm, at which point the governor had still not assumed control. Plant personnel interrupted the ramp and made internal adjustments to the governor in order to achieve a pump speed below 4,260 rpm (the maximum allowable recirculation speed stated in the STP). After the adjustments, the pump reached a recirculation speed of 4,210 rpm. The test was completed satisfactorily and the TD AFW PP 1-1 was secured on November 7, 2010, at 0026 PST.
OF
C. Other Systems or Secondary Functions Affected
No additional safety systems were adversely affected by this event.
D. Method of Discovery
The condition was discovered on January 3, 2011, when plant personnel stated that the TD AFW PP 1-1 had exceeded 4,300 rpm prior to governor adjustment.
E. Status of Inoperable Structures, Systems, or Components that Contributed to the Event When the pump recirculation speed exceeded 4,300 rpm without governor intervention, the TD AFW PP 1-1 was considered inoperable because it operated at a speed outside the band specified in the test procedure and in DCPP's Updated Final Safety Analysis Report (UFSAR). MD AFW PP 1-2 and 1-3 remained operable from entry into Mode 3 through completion of the TD AFW PP 1-1 governor adjustment and testing.
F. Operator Actions
No actions were required by plant operators.
G. Safety System Responses
Not applicable for this event.
III. Cause of the Problem
The apparent cause of this event was that the replacement TD AFW PP 1-1 speed governor was configured in accordance with the original speed governor build/testing specification. There was no documentation of internal adjustments made to the original speed governor; however, it was determined that the original speed governor had been adjusted in the past. As such, the refurbishment/replacement governor was improperly set to be a like-for-like replacement, resulting in a higher than acceptable startup speed. This condition resulted in the TD AFW PP 1-1 being inoperable during mode transition.
IV. Assessment of Safety Consequences
TD AFW PP flow increases with pump speed but is limited by the overspeed trip. Plant personnel worked with the speed governor vendor and found that the governor would have stabilized the pump at a speed below the overspeed trip setpoint. With the TD AFW PP inoperable, the two MD AFW PPs provide adequate AFW heat removal capability for the Mode 3 low decay-heat conditions.
A risk assessment was performed for the plant condition existing at the time of this event. The risk assessment assumed that the TD AFW PP 1-1 was in an overspeed condition and no other structures, systems, or components were out of service during the associated time period. This risk assessment concluded that the increased core damage probability and the increased large early release probability were approximately 3E-7 and 7E-8, respectively, well below the risk significance criteria. This low risk was mainly due to the short duration (20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />) of the TD AFW PP overspeed condition.
In summary, the actual Mode 3 conditions that existed during the period with the increased TD AFW PP 1-1 speed posed no significant threat to nuclear safety.
V. Corrective Actions
A. Immediate Corrective Actions
Plant personnel adjusted pump speed to an acceptable range.
B. Corrective Actions to Prevent Recurrence
1. Provided a build/testing specification to the vendor to ensure refurbished/replacement governors are properly set up.
2. Revised maintenance procedures and plans to update build/testing specifications when internal adjustments are performed.
3. Revised plant procedures for TDAFW governor replacement so that a preservice test is conducted following the replacement.
VI. Additional Information
A. Failed Components
When the TD AFW PP 1-1 was tested after transitioning into Mode 3, the pump speed governor did not perform its required fuction of controlling the speed within the acceptable band. Details on the governor are provide below:
Manufacturer: Woodward Governor Co.
Model Number: PG-A Manufacturer Serial Number: 1537799
B. Previous Similar Events
The replacement of the Unit 2 TDAFW PP 2-1 speed governor during the Unit 2 Fourteenth Refueling Outage (2R14) was reviewed and it was discovered that the turbine-driven pump exhibited similar issues. On April 10, 2008, after entry into Mode 3 from Mode 4, the TD AFW PP 2-1 as-found pump speed was lower than the minimum value specified in the STP; however, it would not have impacted the system's function, and thus was not a condition that could have prevented the fulfillment of a safety function. This event, which included a mode transition prohibited by TS, was uncovered more than three years after the incident and was therefore not reported in accordance with 10 CFR 50.72 or 50.73. The corrective actions detailed in this Licensee Event Report apply to, and will prevent recurrence of, this issue for both units at DCPP.