05000458/LER-2007-005

LER-2007-005, River Bend Station
5485 U.S. Highway 61N
St. Francisville, LA 70775
Tel 225 381 4157Entergy Fax 225 635 5068
dlonin@entergy.com
David N. Lorfing
Manager-Licensing
November 14, 2007
U. S. Nuclear Regulatory Commission
ATTN: Document Control Desk
Washington, DC 20555
Subject:G Licensee Event Report 50-458 / 07-005-00
River Bend Station — Unit 1
Docket No. 50-458
License No. NPF-47
File Nos.G G9.5, G9.25.1.3
RBG-46760
RBF1-07-0208
Ladies and Gentlemen:
In accordance with 10CFR50.73, enclosed is the subject Licensee Event Report.
This document contains no commitments.
Sincerely,
David N. Lorfing
Manager — Licensing
DNL/dhw
Enclosure
Licensee Event Report 50-458 / 07-005-00
November 14, 2007
RBG-46760
RBF1-07-0208
Page 2 of 2
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NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION
(9-2007)
LICENSEE EVENT REPORT (LER)
(See reverse for required number of
digits/characters for each block)
1. FACILITY NAME
River Bend Station - Unit 1
4. TITLE
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2. DOCKET NUMBER 3. PAGE
05000-458 1 of 4
Unplanned Reactor Scram During Surveillance Testing Due to Damaged Terminal Board

River Bend Station - Unit 1
Event date:	09-26-2007
Report date:	11-14-2007
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
4582007005R00 - NRC Website

in response to this event. Following the initial transient, the operators promptly stabilized reactor pressure and water level.

INVESTIGATION and CAUSAL ANALYSIS At the time of this event, all emergency core cooling systems were in their normal standby configuration. The Division 1 diesel generator (DG) was running for a scheduled monthly surveillance test, and the Division 2 and 3 DGs were in standby.

The APRM surveillance test contains steps where an actuation signal of the Division 1 reactor protection system is intentionally generated in order to test the circuitry. This "half-scram" actuation does not cause any actual control rod motion, as both divisions of the RPS must be tripped to accomplish a reactor scram. In order to verify that the RPS system is properly aligned for the test and that no trip signals are already active, the technician is required by the procedure to verify that status lights for the individual RPS channels are energized. This step was properly performed, and the half-scram signal was subsequently actuated. At this point, the Group 2 control rods inserted.

An inspection of components in the affected circuits was performed to verify electrical continuity and proper operation. Engineering and maintenance personnel found that wiring and a terminal board in an RPS pilot scram solenoid circuit had sustained severe thermal damage. This failure had interrupted power to the Division 2 coils on the Group 2 pilot scram solenoid valves, in effect causing an undetected Division 2 half-scram signal for the Group 2 control rods. When the surveillance test inserted the half-scram signal in Division 1, the logic for the Group 2 control rods was completed, and the rods inserted as designed. The circuit failure was downstream of the RPS status lights on the reactor

• screws in the terminal boards in the same circuit location were physically verified to be tight, and thermographic readings taken on these terminal boards.

• resistance and voltage measurements were taken on the failed circuit and comparisons made with a known normal circuit to verify that no downstream problem had caused the terminal board overheating.

• thermographic readings were taken on the RPS Group 2 pilot solenoids to confirm that no damage had happened to the solenoids involved.

Plant modifications are being developed to reduce the vulnerability to similar "hidden" conditions of de-energized scram pilot valves. This action is being tracked in the station's - corrective action program.

PREVIOUS EVENT EVALUATION

No previous reactor scrams occurring at. River Bend Station in the last ten years have been caused by a similar sequence of events.

SAFETY SIGNIFICANCE

The insertion of Group 2 control rods caused a power reduction, which in turn, caused a decrease in steam production which depressed reactor water level. Reactor water level reached the Level 3 scram setpoint approximately six seconds after the Group 2 control rods inserted. A review of the core responses during the event confirmed that neither