05000261/LER-2010-007

LER-2010-007, Reactor Trip Due to a Degraded Connection on a Circuit Board in the Electro-Hydraulic Control Cabinet

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
2612010007R01 - NRC Website

I. DESCRIPTION OF EVENT

At 1437 hours0.0166 days <br />0.399 hours <br />0.00238 weeks <br />5.467785e-4 months <br /> EDT on September 9, 2010, with H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, in Mode 1 at approximately 100% power, an automatic reactor trip occurred. The reactor trip signal was based on the Overtemperature AT (OTAT) reactor protection function. Due to the plant transient condition, the steam generator Power Operated Relief Valves (PORVs) [AB:PCV] opened and one pressurizer PORV cycled momentarily in response to pressure changes in the steam generators [AB:SG] and pressurizer [AB:PZR]. The Auxiliary Feedwater System [BA] automatically actuated, as expected, and provided feedwater to the steam generators. The main steam safety valves [SB:PCV] did not open during the event.

II. CAUSE OF EVENT

This event is being investigated using the HBRSEP, Unit No. 2, Corrective Action Program (CAP) and is being documented in Nuclear Condition Report (NCR) 420936. During investigation of the cause of the transient, a degraded connection on a circuit board (1A08H2) in the electro-hydraulic (EH) control cabinet was found. It has been concluded that this condition caused the loss of the governor valve common signal to the four turbine governor valves. Closure of the governor valves caused pressure and temperature in the Reactor Coolant System (RCS) to change. This resulted in the opening of a pressurizer PORV and the OTAT reactor protection function initiating a reactor trip.

III. ANALYSIS OF EVENT

The condition described in this Licensee Event Report is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B).

IV. CORRECTIVE ACTIONS

Completed Corrective Actions:

• The defective circuit board (1A08H2) was replaced.

Planned Corrective Actions:

• Revise existing preventative maintenance model to require testing to validate proper circuit card seating of any EH System circuit cards that are replaced. This PM revision should include checking circuit boards for bent pins prior to installation as well as verifying all installed and surrounding circuit boards are seated properly.

Previous Similar Events:

Licensee Event Report 2006-001-00 At 0247 hours0.00286 days <br />0.0686 hours <br />4.083995e-4 weeks <br />9.39835e-5 months <br /> on October 25, 2006, with H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, in Mode 1 at 100% power, control room operators responded to alarms received for steam flow greater than feed flow on all three steam generators. The control room operators diagnosed the event as a 100% load rejection and initiated a manual reactor trip at 0248 hours0.00287 days <br />0.0689 hours <br />4.100529e-4 weeks <br />9.4364e-5 months <br />, 68.8 seconds following the start of the event. The root cause of this event was failure of a turbine governor valve electro-hydraulic control system card.

While equipment failures were identified as the cause of this event, no similar component failures were identified.