05000529/LER-2008-002
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 05-28-2008 |
|---|---|
| Report date: | 07-28-2008 |
| 5292008002R00 - 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 (EIIS: JC).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The control element assemblies (CEAs) (EIIS Code: AA) provide a mechanical means to shutdown the reactor and regulate reactor power. The CEAs are withdrawn from the reactor core to achieve the critical condition and they are inserted into the core when a reactor shutdown is initiated. There are a total of 89 CEAs as part of the Combustion Engineering (CE) System 80 reactor design.
The Control Element Drive Mechanism (CEDM) (EllS Code: AA), also known as a magnetic jack, is an electro-mechanical device that uses induced magnetic fields to operate a mechanism for moving a CEA.
The control element drive mechanism control system (CEDMCS) (EIIS Code: AA), provides drive signals to the coils of the magnetic-jack control element drive mechanisms (CEDMs) which position and hold the reactor control element assemblies (CEAs). CEDMCS controls full strength CEAs either automatically or manually, and power shaping (part strength) CEAs manually, as required.
Two motor/generator sets connected in parallel to a common bus, supply 240 VAC, 3(p power through two parallel sets of two reactor trip circuit breakers. These four breakers and their controls make up the reactor trip switchgear (RTSG). The output from the RTSG is directed through circuit breakers for subgroups of CEAs and downstream individual CEA circuit breakers (EIIS: 52) and then through power switch assemblies containing silicon controlled rectifiers (SCRs) converting the 3cp, AC power to a smoothed DC output. This smoothed DC output is then divided and distributed throughout the CEDMCS cabinets for ultimate delivery to the CEDM coils for 89 CEAs.
3.. INITIAL PLANT CONDITIONS:
On May 28, 2008, at 21:31, Palo Verde Unit 2 was at normal operating temperature (NOT) and normal operating pressure (NOP) in Mode 2 at 1.6E-2 percent power. Low Power Physic Testing (LLPT) was in progress following the 14th refueling outage.
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:
At 21:31 on May 28, 2008, Unit 2 was performing low power physics rod worth measurements and CEA shutdown group A, subgroups 3 and 4, were being inserted into the core. During low power physics testing, CEA shutdown group A, subgroup 3 (CEAs 7, 9, 11 and 13), dropped from approximately 120 inches withdrawn to fully inserted. Testing was suspended and the control room supervisor (utility — licensed) entered the "CEA Malfunctions" procedure, 40A0- 9ZZ11, which directs tripping the reactor if one CEA is deviating from its group by more than 6.6 inches. The reactor operator (utility — licensed) manually tripped the reactor and the plant was stabilized in Mode 3. No abnormalities were noted following the reactor trip.
CEA subgroup 3 circuit breaker (CB3003), on the back of the CEDMCS cabinet power switch assembly, was found in the open position.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The event did not result in a transient more severe than those previously analyzed in the PVNGS UFSAR, Chapter 15. The event was considered an uncomplicated reactor trip.
The reactor trip was not automatically initiated as a result of any of the categories defined in UFSAR Section 15.0.1.2. The Specified Acceptable Fuel Design Limits and reactor coolant system pressure limit were not exceeded. Equipment and systems assumed in UFSAR Chapter 15 were functional and performed as required.
A The safety function, to shut down the reactor and maintain it in a safe shutdown condition, remained fulfilled. There are no actual safety consequences as a result of this condition; the condition would not have prevented the fulfillment of the safety function; and, the condition 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 CEAs dropped because the associated CEA subgroup 3 circuit breaker CB3003 opened.
The root cause was indeterminate. The probable cause was high contact resistance on the "A" phase of circuit breaker CB3003 which led to the circuit breaker tripping on overcurrent and the subsequent manual reactor trip due to 4 dropped CEAs.
7. CORRECTIVE ACTIONS:
The circuit breaker CB3003 was replaced. To minimize the resistance buildup on CEA circuit breakers, the remaining Unit 2 CEA subgroup circuit breakers were cycled twice (opened twice and closed twice) to clean the circuit breaker contacts. A repetitive task will be added to Operations procedures to cycle CEA subgroup breakers each refueling outage to minimize the likelihood of similar occurrences. These breakers have not been routinely cycled in the past.
8. PREVIOUS SIMILAR EVENTS:
Four events occurred in the recent history in which CEAs slipped several inches related to CEDMCS problems, one of which resulted in a manual reactor trip (LER 50-528/2004 003-00, May 8, 2004). The May 2004 event and two other events involved electronic problems with card connectors or card failures. One additional event involved sluggish operation of a CEA magnetic jack. None of the four events were related to subgroup or individual CEA circuit breaker problems and none involved dropping CEAs to the fully inserted position.
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