05000251/LER-2010-001

LER-2010-001, Two Shutdown Bank Rods Were Dropped from Fully Withdrawn Position

Docket Number
Event date:	11-26-2009
Report date:	1-25-2010
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 45522	10 CFR 50.72(b)(3)(iv)(A), System Actuation
2512010001R00 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

DESCRIPTION OF THE EVENT

On November 26, 2009 at approximately 2018 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.67849e-4 months <br />, during performance of Rod Position Indication System Replacement Testing (Mode 3 tests) while withdrawing A and B control rod banks, the New Analog Rod Position Indication (NARPI) System indicated rod [EIIS: AA, ROD] H6 and H10 dropped from fully withdrawn Shutdown Bank B (Group 2). Annunciator B-9/3 "Shutdown Rod Off Top" annunciated and the associated "Rod Bottom Lights" illuminated. The NARPI System Replacement Testing was stopped.

Troubleshooting began to determine whether the problem was with NARPI or the Rod Control System.

Instrumentation and Control personnel checked voltages, recorded current traces, and reported that control room indication showed that both rods were dropped. ONOP-028.3, "Dropped RCC" was entered and the reactor was tripped at 2340 inserting all rods. At the time of the trip, the reactor was in mode 3 (Ken less than 0.99). A 10 CFR 50.72 8-hour report (EN 45522) was made to the NRC. The RCS boron concentration was maintained at or above the Minimum Shutdown Boron Concentration specified in the NARPI test procedure (4-PTP-028.2).

This event is considered to be reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), due to manual actuation of the Reactor Protection System to trip the reactor, inserting all rods on November 26, 2009.

CAUSE OF THE EVENT

Shortly after the event, troubleshooting was conducted on the rod control system, wherein the 2BD cabinet was instrumented with a Dewetron recorder, the reactor trip breakers were closed, and the shutdown rods were again stepped out of the core. The current traces taken were sent off to the vendor for their evaluation.

The vendor reported that they were uncertain due to the unusual characteristics of the traces. The plant personnel performed an inspection of the 2BD cabinet, where it was found that the neutral bus connection for the sampling resistor bank assembly A24 was slightly loose. Once this connection was disassembled, an inspection revealed a thin oxide coating between the lug and the copper bus bar. No indications of anomalies had been found during the recently performed preventive maintenance.

A loose connection being oxidized would result in an increased resistance, leading to a higher voltage being dropped for a lower current. The vendor determined that a current as low as 2.7 amps could lead to dropped rods due to insufficient holding power developed by the stationary grippers. The oxide layer observed combined with the loose connection was considered more than sufficient to provide the increased resistance that could lead to a reduced current. With the current reduced, the stationary gripper would not have been holding at the required tension, so that when subjected to the vibrations created by an adjacent lift mechanism, the control rod would be able to fall. The vendor recommended replacement of the circuit cards responsible for the regulation of current in this group of rods. The circuit cards (firing card, regulating card, and input/output AC card) were replaced as recommended. Electrical connections of all cabinets were inspected and loose connections were tightened (WO 39024029). Once the connections were cleaned and tightened and new cards installed, the system was re-energized and tested satisfactorily with acceptable current traces. The vendor concurred no further repairs were necessary. The apparent cause of the failure was a high resistance connection.

DOCKETFACILITY NAME (1) LER NUMBER (6) PAGE (3)NUMBER (2) 2010 - 001 -

Background

The Rod Control System [EIIS, JD] performs the following functions:

1. Controls the motion of the control rods within the core, in response to signals from the In-Hold-Out switch or the reactor control unit.

2. Maintains programmed reactor coolant system temperature, by regulating reactivity within the core, to within a programmed temperature range.

The regulation circuit for this operation is controlled from the sampling registers within the Rod Control System cabinets. The Rod Cluster Control Assembly (RCCA) drop event is an analyzed event in Section 14.1.4 of the Updated Final Safety Analysis Report.

Reportability The event was reported to the NRC in accordance with 10 CFR 50.72(b)(3) on November 26, 2009. A review of the reporting requirements of 10 CFR 50.72 and 10 CFR 50.73 and NRC guidance provided in NUREG-1022, Revision 2, Event Reporting Guidelines 10 CFR 50.72 and 10 CFR 50.73, was performed for the subject condition. As a result of this review, the condition was deemed reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), due to manual actuation of the Reactor Protection System to trip the reactor, inserting all rods November 26, 2009.

Analysis of Safety Significance At the time of the rod drop event, the reactor was in Mode 3 (Hot Standby) and the plant was performing post-modification testing of the NARPI system. The Technical Specifications for Rod Misalignment are applicable in Modes 1 and 2 and did not apply. Plant procedures directed the reactor_to be manually tripped due to the unexpected rod drop condition. At the time of the test, the boron concentration was high enough to maintain the shutdown margin with, all rods out. As the reactor was sub-critical with adequate shutdown margin, and since the reactor trip function was available, the safety significance of this event is considered to be minimal. Upon manually opening the reactor trip breakers, the rods that were withdrawn from the core were fully inserted.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

CORRECTIVE ACTIONS

Corrective actions include the following:

1. Corrective actions included cleaning and tightening of the Unit 4 affected connections followed by an inspection of all the connections of the neutral buses. Unit 3 uses the same model rod control cabinets and is subject to similar loose connections. Any additional troubleshooting on Unit 3 will be performed at the first available opportunity when the unit is offline.

refueling outage to include a more thorough inspection and cleaning of the feedback sensor resistor bank connections.

ADDITIONAL INFORMATION

component function identifier (if appropriate)]. Condition Report 2009-33606 was initiated due to this event.

FAILED COMPONENTS IDENTIFIED: None PREVIOUS SIMILAR EVENTS: None — However, rod H6 dropped again November 27, 2009 due to the conditions described in this event report, prior to the corrective actions being completed. Condition Report 2009-33607 was initiated due to this event.