05000528/LER-2004-003
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 5282004003R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
APS is reporting this condition pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an manual reactor trip that was directed by procedure.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The Control Element Drive Mechanism (CEDM) (EIIS Code: AA), also known as a magnetic jack, is an electro-mechanical device that used induced magnetic fields to operate a mechanism for moving a Control Element Assembly (CEA).
The CEDM Control System (CEDMCS) (EIIS Code: AA), provides power and control for the CEDM's. The CEDMCS controls the application of voltage to the CEDM coils. The coils produce magnetic fields that cause motion of the Upper Lift, Upper Gripper, Lower Lift, and Lower Gripper assemblies. The gripper assemblies are also referred to as latch assemblies.
By properly sequencing the motion of the gripper and lift assemblies, the CEDM's can withdraw or insert the CEA's.
The Upper Gripper holds the CEA in place when it is stationary. If the Upper Gripper does not re-engage, the CEA will fall or if it re-engages late, the CEA will slip.
The CEA Timer Card has selector switches that are used to control the timing during a CEA motion cycle. These switches are used to determine when a specific coil is energized, how long the coil is energized and whether high or low voltage is applied. Selector settings can be adjusted so the high voltage is applied for a longer duration to compensate for sluggishness in a CEDM.
3. INITIAL PLANT CONDITIONS:
On May 08, 2004, at approximately 20:39 Mountain Standard Time (MST), Palo Verde Unit 1 was at normal operating temperature (NOT) and normal operating pressure (NOP) in Mode 2 at 5.0 E-2 percent power. Low Power Physic Testing (LLPT) was in progress following the 11th 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. There were no failures that rendered a train of a safety system inoperable and no failures of components with multiple functions were involved.
4. EVENT DESCRIPTION:
On May 8, 2004, Unit 1 was in Mode 2 at 5.0 E-2 percent power and performing LPPT per procedure 72PY-9RX04, "Low Power Physics Tests Using RMAS". During the test, CEA #89 slipped to 129.6 inches withdrawn. At that time, Regulating Group (RG) 3 was at zero inches and RG 4 was at 136 inches. All other CEAs were fully withdrawn. LPPT was suspended and operators entered procedure 40A0-9ZZ11, "CEA Malfunctions". Procedure 40A0-9ZZ11 states:
3.0 Dropped or Slipped CEA Mode 1 or 2 2. If one CEA is deviating from its group by greater than 6.6 inches AND ANY of the following conditions exits:
- Reactor power is less than 1% Then perform the following:
a. Trip the Reactor b. Go To 40EP-9E001, Standard Post Trip Actions.
The operators obtained Control Element Assembly Computer (CEAC) printouts and determined that the CEA #89 was deviating 7.4 inches from the highest CEA in the subgroup and was deviating by 6.4 inches from the average of RG4. Procedure 40A0- 9ZZ11 instructs operators to trip the reactor if one CEA is deviating from its group by more than 6.6 inches. Unit 1 was manually tripped and 40EP-9E001 was performed.
No contingency actions were required so 40EP-9E002, "Reactor Trip" was performed.
This condition has been documented in the corrective action program. There were no other major structures, systems, or components that were inoperable at the start of the event that contributed to the event. There were no actual loss of safety function that rendered a train of a safety system inoperable, and no failures of components with multiple functions were involved. The event did not result in the release of radioactivity to the environment and did not adversely affect the safe operation of the plant or health and safety of the public.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The event experienced by Unit 1 on May 8, 2004 did not result in a transient more severe than those previously analyzed in the PVNGS UFSAR, Chapter 15. The Unit was in Mode 2 at approximately 0.05% power performing Low Power Physics Testing per 72PY-9RX04, Section 8.13. Operators manually tripped the reactor when CEA #89 slipped to 129.6 inches withdrawal resulting in a deviation of 7.4 inches from the highest CEA in its subgroup.
The event experienced by Unit 1 is 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. As expected for an uncomplicated reactor trip, No Specified Acceptable Fuel Design Limit (SAFDL) violation occurred and transient peak pressures were less than 110% of design pressure. A review of the trip data supports the conclusion that no SAFDL or peak pressure limits were violated.
Equipment and systems assumed in UFSAR Chapter 15 were functional and performed as required.
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 CFR50.73 (a) (2) (v).
6. CAUSE OF THE EVENT:
The cause of the event has been isolated to a sluggish gripper problem with CEA 89 that prevented the Upper Gripper to engage in proper sequence which caused the CEA to slip during a motion cycle.
Coil traces are taken at the end of each outage for all CEA's in the respective unit in the outage. The most recent coil traces for all CEA were re-evaluated since this event. The Unit 1 CEA 89 traces taken after U1R10 revealed a slight sluggishness, but it was not recognized during its review following that outage. The re-evaluation of all other CEAs for all three units revealed no other CEAs show signs of sluggish operation.
At this time, the problem appears to be an isolated failure. Coil traces are taken periodically that will provide continued monitoring of CEDM performance. Additionally, CEDMs are exercised quarterly by surveillance testing.
No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event.
7. CORRECTIVE ACTIONS:
An independent investigation of this event is being conducted in accordance with Palo Verde's corrective action program. Based on the preliminary results from the investigation the following corrective actions have been taken or are planned to prevent recurrence:
- Discuss the sluggish gripper problem with the CEDMCS technicians to review the Coil Traces for CEDM 89 type problems.
- Update maintenance procedure to reflect the timing changes for CEA 89 in Unit 1.
- Obtain another coil trace for CEA 89 in Unit 1 to verify adequacy of the timing change.
Any additional corrective actions taken as a result of the investigation of this event will be implemented in accordance with the APS corrective action program. If inform'ation is subsequently developed that 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 condition has been reported in the past three years.
9. ADDITIONAL INFORMATION:
None.