05000530/LER-2011-001
| Palo Verde Nuclear Generating Station (Pvngs) Unit 3 | |
| Event date: | 01-19-2011 |
|---|---|
| Report date: | 03-21-2011 |
| Initial Reporting | |
| ENS 46556 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 5302011001R00 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to report an automatic actuation of the reactor protection system (RPS) and an auxiliary feedwater actuation signal (AFAS) associated with a failed open main feedwater pump (MFWP) minimum flow recirculation valve (mini-flow valve). This event was reported to the NRC on January 19, 2011, via the emergency notification system (ENS 46556).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The main feedwater system (see diagram) (EllS Code: SJ) consists of piping, main feedwater pumps (MFWP) (EllS Code: JK), high pressure heaters, valves, controls, instrumentation, and associated equipment which supply feedwater to the steam generators (SG) (EllS Code: AB).
Mini-Flow Valve Feedwater Simplified Diagram Each MFWP is a turbine-driven pump capable of supplying 65 percent of main feedwater system capacity. There are two MFWPs which serve both SGs. The MFWP mini-flow valves assure there is always adequate flow through the MFWPs to prevent pump damage.
Each mini-flow valve has the capability of providing a flow of approximately 7000 gpm to the main condenser (El IS Code: SG). The mini-flow valve is designed to fail open on loss of electrical signal or loss of instrument air to ensure that minimum flow is maintained through the MFWP to prevent pump damage on a loss of downstream flowpath during pump operation.
3. INITIAL PLANT CONDITIONS:
On January 19, 2011, Palo Verde Unit 3 was in Mode 1 (Power Operation) at 100 percent power at normal operating temperature and normal operating pressure. There were no inoperable structures, systems, or components at the time that contributed to this event.
4. EVENT DESCRIPTION:
On January 19, 2011, at 1840, Unit 3 experienced a reactor power cutback (RPCB) (El IS Code: JD) from 100 percent power to approximately 60 percent power due to MFWP B tripping on low suction pressure. The low suction pressure was experienced by both MFWPs. The MFWP trip logic is designed to trip MFWP B first to prevent simultaneous MFWP trips. The low suction pressure was caused by a failed diaphragm in a pneumatic 3-way precision relay (precision relay) for the mini-flow valve on MFWP A (see diagram) which caused the mini-flow valve to fail open and divert a percentage of feedwater flow from both SGs to the condenser.
The precision relay functions to cause full opening of the mini-flow valve when input control air pressure falls below 3 psig. Control air pressure is normally maintained between 3 to 15 psig.
The failed diaphragm caused air leakage within the precision relay which lowered the input control air pressure to less than 3 psig and actuated the precision relay to open the mini-flow valve.
During this transient, both SG levels decreased to the point that a reactor trip (El IS Code: JC) occurred at 1841. The SG levels continued to lower following the reactor trip resulting in an AFAS (EllS Code: JE). Both auxiliary feedwater pumps A and B started and fed SGs in conjunction with MFWP A. The plant was stabilized in Mode 3.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
There were no inoperable structures, systems, or components at the time that contributed to this event. The plant responded as designed for the RPCB, reactor trip and AFAS actuations. The conditional core damage probability for this event was calculated to be 1.26E-6. This event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. There were no actual safety consequences as a result of this event.
This event did not prevent the fulfillment of a safety function nor did it result in a safety system functional failure as described by 10 CFR 50.73 (a)(2)(v).
6. CAUSE OF THE EVENT:
The cause of the event was a failed diaphragm in the precision relay for the mini-flow valve control loop which resulted in the opening of the mini-flow valve on MFWP A. This allowed a percentage of feedwater flow to be diverted to the condenser, resulting in a MFWP B trip and a RPCB followed by a reactor trip and AFAS.
7. CORRECTIVE ACTIONS:
The following corrective actions were implemented:
1. The Unit 3 MFWP A mini-flow valve control system precision relay was immediately replaced.
2. The Unit 3 MFWP B mini-flow valve control system precision relay was replaced on February 18, 2011 as an interim action.
The extent of condition evaluation determined that the subject relays are not installed in the Units 1 and 2 MFWP mini-flow valve control systems. To improve maintenance reliability, a modification replaced these mini-flow valves and control systems during previous outages in Units 1 and 2. The same modification to replace the Unit 3 MFWP mini-flow valves and their control systems is planned during the next refueling outage.
The preventive maintenance process for these relays will be addressed as part of the corrective actions of this investigation. Any additional corrective actions taken as a result of the investigation of this event will be implemented in accordance with the requirements of the Palo Verde corrective action program. If information is subsequently developed which would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
8. PREVIOUS SIMILAR EVENTS:
No similar conditions have been reported by Palo Verde in the past three years.