05000528/LER-2005-005
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 08-26-2005 |
|---|---|
| Report date: | 10-24-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282005005R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
Arizona Public Service (APS) is reporting this condition pursuant to 10 CFR 50.73(a)(2)(iv)(A) for an automatic reactor trip caused from a Reactor Protection System (RPS) actuation and an Engineered Safety Feature (ESF) actuation from a Main Steam Isolation Signal.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
DFWCS Single Element Control Functional Description The Digital Feedwater Control System (DFWCS) is designed to provide automatic steam generator water level control from the time a main feedwater pump is placed into service at about 2% reactor power. In this low power mode the FWCS is in a Single Element Control Mode and uses Steam Generator (SG) narrow range level as the measurement for the developing the controller error relative to an Operator specified setpoint. To assist the level controller with anticipating load changes and the resulting shrink and swell of SG level, an output bias is added to the controller output processed from an impulse module signal developed from the Steam Bypass Master Controller Demand output and/or the Turbine Load Index output. To understand the responsiveness of the Single Element Control tuning, it is important to understand the impact of cold feedwater on the SG and that the controller is tuned adaptively from feedwater temperature. For a typical startup with feedwater temperature around 100°F, the responsiveness of the FWCS is tuned to be very slow. This is to compensate for the impact of the cold feedwater on the RCS and the amount of time it will take before the water will heat up and increase the SG level. As power is increased to about 11% and the main turbine is placed on line, feedwater temperature has increased to approximately 250°F and the tuning settings have become more responsive. At 14% reactor power, the FWCS shifts to Three Element Control.
To facilitate the Operators interface with the system, two methods of operating the system are provided. The Operator may chose from a graphical interface using a mouse for selecting and operating system features or the Operator may use an older style Auto/Manual Station interface. Both interfaces operate in parallel and reflect changes made on the other interface. Procedures 400P-9FT01, Feedwater Pump Turbine A, and 400P-9FT02, Feedwater Pump Turbine B, provide the guidance to smoothly transfer without system perturbation between manual and automatic by manually matching the controller output.
3. INITIAL PLANT CONDITIONS:
On August 26, 2005, at approximately 18:26 Mountain Standard Time (MST), Unit 1 was in Mode 2 in preparation to enter Mode 1. Reactor power was at approximately 3 percent power and at normal operating temperature (NOT) and normal operating pressure (NOP).
There were no other major structures, systems, or components that were inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
Following the transition from auxiliary feed to main feed in Unit 1 on 08/26/05, automatic steam generator feedwater level control was not smoothly established. The initial attempt at in a no feed condition. Recognizing this was not the proper result of this step, the secondary control room operator (SO) returned to manual valve control briefly to re-establish feedflow and again attempted to establish automatic control. The result of the second evolution was an underfeed condition that set a trend of gradually lowering SG levels. Uncomfortable being lower than the 30 to 40% narrow range level band prescribed by the procedure, several increases to the level setpoint were made in an attempt to restore sufficient feedflow to restore SG levels within band. Uncomfortable with the continuing slow SG level decrease, the SO again returned to manual valve control to increase feedflow until SG level was observed to be increasing. The DFWCS was again briefly returned to automatic operation when it appeared to the SO to be underfeeding again. After a setpoint adjustment in manual, the DFWCS was again returned to automatic operation where the level rapidly increased to about 76% NR level, when the DFWCS was taken to manual and valve setpoint reduced and then closed. The resultant expansion as the feedwater temperature increased from about 100°F to nearly 565°F caused the level to swell to the main steam isolation signal (MSIS) setpoint of 91.5% NR level initiating a reactor trip. The investigation has determined this event occurred because personnel failed to implement expected requirements and good practices prescribed by their training and delineated in procedures.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The event experienced by Unit 1 did not result in a transient more severe than those previously analyzed in the Palo Verde Updated Final Safety Analysis Report (UFSAR), Chapter 15. No Specified Acceptable Fuel Design Limit (SAFDL) violations occurred and transient peak pressures were less than 110 percent of design pressures. All equipment and systems assumed in UFSAR, Chapter 15 were functional and performed as required.
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The condition would not have prevented the fulfillment of any safety function and did not result in a safety system functional failure as defined by 10 CFR 50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
The Direct Causes were individual and crew failures to implement expected requirements and good practices prescribed by their training and delineated in procedures.
Root Cause # 1: Consistent standards of performance by Unit 1 Operations Crew C were not sufficiently anchored.
Root Cause # 2: Inconsistent feedwater control system performance at low power levels has not been effectively resolved since the digital upgrade. This has led to acceptance of operational strategies to cope with perceived system instability at low power levels.
Contributing Cause # 1: Procedures 40DP-9FT01 & (2), Feedwater Pump Turbine A(B), are not sufficiently human factored for all users, making them susceptible to performance error and resultant system perturbations.
Contributing Cause # 2: Common belief existed among licensed operators that the DFWCS can not reliably control SG level well at low power. This belief was not based on reviewing actual performance data and led to acceptance of the condition and mitigating strategies.
Contributing Cause # 3: Training was not commensurate with the difficulty of putting the DFWCS into auto operation given the current procedure detail and system design. This has resulted in improper operation of DFWCS components while in single element control.
7. CORRECTIVE ACTIONS:
An independent investigation of this event was conducted in accordance with Palo Verde's corrective action program. Listed below are some of the corrective actions from the investigation that were taken or are scheduled in the future.
- Operation's management removed affected personnel from shift duties until preliminary investigation determined extent of performance deficiencies.
- Operation's management coached those personnel removed from shift on personal accountability to ensure compliance with Operations Departments Principles and procedures prior to resumption of shift duties.
- Just-in-time (JIT) training was conducted for the licensed Operator placing the first Main Feed Pump in service including placing the DFWCS into automatic prior to the Unit 1 power ascension above 3%.
- Operation's management will evaluate crew performance and composition to ensure optimal matching of personal strengths necessary to sustain a culture committed to improving performance within the License and Operation's Department Practices and implementing procedures.
- Engineering will modify the DFWCS for all three Units to improve SG level control under single element control at low power.
- Revise Operating Procedures to provide better guidance and human factors when transferring feedwater control from manual to automatic.
8. PREVIOUS SIMILAR EVENTS:
No similar condition has been reported in the past three years.
9. ADDITIONAL INFORMATION:
None.