05000530/LER-2006-007
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 10-19-2006 |
|---|---|
| Report date: | 12-13-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5302006007R00 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER is being submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A) to report a manual actuation of the reactor protection system to trip the reactor.
Specifically, on October 19, 2006 at 1147 hours0.0133 days <br />0.319 hours <br />0.0019 weeks <br />4.364335e-4 months <br /> Palo Verde Unit 3 control room operators (licensed) manually tripped the reactor from approximately 100% rated thermal power.
At 1427 hours0.0165 days <br />0.396 hours <br />0.00236 weeks <br />5.429735e-4 months <br /> on October 19, an Event Notification System call was made to report the event (# 42920).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The condensate demineralizer system (SC) is used to maintain the purity and chemistry of the condensate, feedwater, and steam generator secondary side water. The SC system consists of the following major components:
- six condensate demineralizers (normally five in service and one standby)
- resin handling and regeneration equipment The SC system processes secondary plant condensate, when necessary for optimal water chemistry, by directing condensate flow from the discharge of the condensate pumps through an array of polishing demineralizers (ion exchangers). When secondary chemistry conditions permit, the system can be left in a standby mode with condensate flow bypassing the dernineralizers.
When an in-service demineralizer has been exhausted the standby demineralizer is placed into service and the exhausted demineralizer is removed from service and regenerated.
3. INITIAL PLANT CONDITIONS:
Palo Verde Unit 3 was operating in Mode 1, Power Operations, at approximately 100% rated thermal power at the time of the event. There was no inoperable equipment at the start of this event that contributed to the event.
4. EVENT DESCRIPTION:
On October 19, 2006 at 1147 hours0.0133 days <br />0.319 hours <br />0.0019 weeks <br />4.364335e-4 months <br />, with demineralizer regeneration activities in progress, condenser A-C and B-C high differential pressure alarms were received in the main control room coincident with lowering hotwell levels and degrading vacuum in the C shell. The lowering condenser hotwell level resulted in automatic trips (at 30 inches) of two (B & C) of the three operating condensate pumps. The loss of two condensate pumps with the unit operating at full power caused a reduction in main feedwater (MFW) pump suction pressure and the actuation of low suction pressure pre-trip alarms for both operating MFW pumps. The reactor was manually tripped by the control room operators from approximately 100% power and the Standard Post Trip Actions (SPTA) were performed. The plant was stabilized in Mode 3, Hot Standby, using steam bypass control system and main feedwater pump A. Offsite power remained available throughout the event.
Without operator intervention, this event would have resulted in an automatic reactor trip on low steam generator level following the trips of the MFW pumps after their respective low suction pressure trip timing circuits (MFW A 15 seconds; MFW B 10 seconds) had timed out. The control room staff action prevented the loss of the feedwater pumps which would have complicated event response and recovery.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The plant remained within safety limits throughout the event. The primary system and secondary pressure boundary limits were not approached and no violations of the specified acceptable fuel design limits (SAFDL) occurred. No ESF actuations occurred and none were required. There were no inoperable structures, systems, or components at the time of the event that contributed to this event. 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:
An investigation of this event was conducted in accordance with the PVNGS corrective action program. The direct cause of the event was determined to be that valve SCN UV-232 (air operated condensate demineralizer PRESERVICE RINSE valve) was failed open during vessel draining resulting in lowering hotwell level. The failed open SCN UV-232 valve created an opening from condenser shell C to atmosphere and caused vacuum degradation in the C shell with a lesser effect on the A and B shells. The resultant higher pressure in the C shell, caused condensate to be retained (held up) in the A and B condenser shell reheat trays and condensate flow quickly reduced the hotwell level to the condensate pump low level trip setpoint causing the B and C condensate pumps to trip.
The cause of the spurious opening of SCN-UV-232 was attributed to the preventive maintenance periodicity for the solenoid portion of the Bray actuator for valve SCN-UV 232 (J) was not adequate to identify and replace the worn parts prior to impact on function.
The root cause of the event was determined to be that the challenge to the unit posed by degraded condenser vacuum created by an open SCN-UV-232 valve concurrent with condensate demineralizer service vessel draining evolutions was not addressed through single point vulnerability reviews.
No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event.
7. CORRECTIVE ACTIONS:
Procedures 400P-9SC03, Operating the Condensate Demineralizer System and 400P 9SC06, Condensate Demineralizer Subsystems were revised requiring an in-line manual isolation valve to remain closed except during a pre-service rinse when the service vessel is pressurized. This manual valve had previously been installed as a means to isolate SCN-UV-232 from the condenser when maintenance was being performed on SCN-UV-232.
In addition, the preventative maintenance frequency for SCN-UV-232 will be changed to a frequency that prevents loss of valve function due to wear or age degradation.
8. PREVIOUS SIMILAR EVENTS:
There have been no similar events reported by APS in the last three years.