05000389/LER-2007-003

LER-2007-003, Manual Reactor Trip Following Misalignment of0Five (5) Control Element Assemblies (CEAs)0During Plant Startup

Docket Number
Event date:	12-29-2007
Report date:	02-27-2008
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3892007003R00 - NRC Website

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Description of the Event

On December 28, 2007, St. Lucie Unit 2 was approaching criticality after returning from a scheduled refueling outage, when control element assembly (CEA) #1 [EIIS:ZC] dropped to the bottom from the three inch withdrawn position. Troubleshooting revealed that the power switch assembly for CEA #1 was the probable cause of the fault. Maintenance replaced the power switch and post maintenance testing was performed. An additional CEA # 1 drop event occurred and maintenance subsequently replaced multiple components with the concurrence of the vendor. On December 29, 2007, plant startup resumed with CEA #1 fully withdrawn to the upper electrical limit (UEL) and Sub-Group 15 was placed on the Maintenance Hold Bus for voltage testing (Sub-Group 15 CEAs 60,62,64,66 and 1). The Sub-Group was in that position for approximately five (5) hours when Sub-Group # 15 slipped 20 inches into the core. At that time, in accordance with plant procedures, a manual reactor trip was initiated.

All control systems responded as required.

Cause of the Event

The most probable cause of the inability of the CEDM [EIIS: AA] System to maintain the CEAs in the fully withdrawn position was determined to be the aging Maintenance Hold Bus Power Supplies. A contributing factor was the failure of the voltage isolation card [EIIS:ET1 (Voltage Isolator card is unique to CEA # 1) for CEA # 1.

Extensive testing of the Maintenance Hold Bus Power Supplies was performed but the fault could not be reproduced. The voltage isolator board failure was located in an unused portion of the circuit card that provided a ground path resulting in the rod drop. The exact cause of the Maintenance Hold Bus power supply failure and the voltage isolation card failure is still under investigation. Should the results of this investigation affect this conclusion the results will be communicated in a revision to this LER.

Analysis of the Event

This event is reportable under 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in a manual or automatic reactor trip.

Analysis of Safety Significance The CEA drop event is a negative reactivity insertion Condition II event analyzed in the Unit 2 UFSAR in Section 15.4.3, which is assumed to be initiated by a single electrical or mechanical failure that causes any symmetrical configuration of CEAs to drop to the bottom of the core. Condition II occurrences are faults that may occur with moderate frequency during the life of the plant. They are mitigated, at most, by a reactor shutdown with the plant being capable of returning to operation after a corrective action. In addition, no Condition II occurrences cause consequential loss of function of fuel cladding and reactor coolant system barriers.

The resulting negative reactivity insertion in a single or sub-group CEA drop event causes nuclear power to rapidly decrease. An increase in the hot channel factor may occur due to the skewed power distribution representative of a CEA drop configuration. Since this is a Condition II event, the UFSAR analysis demonstrates that the departure from nucleate boiling (DNB)design basis is met for the combination FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) of power, hot channel factor, and other system conditions which exist following a CEA drop event. A spectrum of dropped CEA worths bounding the current Unit 2 reactor core was analyzed.

The UFSAR Chapter 15 bounding analysis for the CEA drop event accident includes full­ length single CEA drop and full-length CEA subgroup drops. When all five CEAs in subgroup 15 fully inserted into the core, Operations confirmed the rod drop indications and manually tripped the reactor in accordance with existing plant procedures. Also, existing plant procedures were followed for the single CEA drop events. No anomalies were observed after these CEA drop events took place, and Unit 2 operated in a normal and expected manner. Therefore, the conditions described above did not present a nuclear safety concern for St. Lucie Unit 2, and these events had no impact on the health and safety of the public.

Should a similar event occur, operational guidance is provided by plant off-normal operating procedures, using Unit 2 Technical Specification T.S. 3.1.3.1 as a basis.

Based on the above, the safety consequences of the event are judged to be low and there was no adverse impact on the health and safety of the public.

Corrective Actions

The corrective actions and supporting actions are entered into the site corrective action program. Any changes to the proposed actions will be managed under the commitment management change program.

Completed Corrective Actions:

1.Revised the Operating procedures adding precautions on the use of the Maintenance Hold Bus pending replacement of the power supplies.

2.Replaced voltage isolation card.

3.Measured input and output voltage of the voltage isolation card.

Actions to prevent recurrence:

1.Replacement of the Maintenance Hold Bus power supplies.

2. Complete the failure analysis of the failed voltage isolation card.

3. Develop a plan to improve the current maintenance practices of the CEDM system following assessment of available options.

Similar Events A search of the corrective action database of the previous 5 years for typical

• equipment failures relating to rod control related conditions, determined this not to be a repeat event. No industry operating experience was found following a search of the INPO database for events involving rod drops related to the Maintenance Hold Bus.

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Failed Components

Voltage Isolation Board (Combustion Engineering) Degraded Components Maintenance Hold Bus power supplies (Abbott Power Corp.)