05000261/LER-2008-002

LER-2008-002, Manual Reactor Trip due to High Turbine Vibrations

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
2612008002R00 - NRC Website

I. DESCRIPTION OF EVENT

At 0551 hours0.00638 days <br />0.153 hours <br />9.11045e-4 weeks <br />2.096555e-4 months <br /> EST, on November 17, 2008, the H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, reactor was manually tripped from approximately 78% power due to high vibrations detected on the main turbine [TA:TRB]. The reactor trip was initiated in accordance with Abnormal Operating Procedure, AOP-006, "Turbine Eccentricity / Vibration.

At approximately 0230 hours0.00266 days <br />0.0639 hours <br />3.80291e-4 weeks <br />8.7515e-5 months <br /> EST, turbine vibration on the No. 9 bearing was approximately 10.9 mils and increasing. By 0516 hours0.00597 days <br />0.143 hours <br />8.531746e-4 weeks <br />1.96338e-4 months <br /> EST, the No. 9 bearing was at about 13.5 mils and still increasing.

A power reduction from 100% power was commenced in accordance with Operating Procedure, OP-105, "Maneuvering the Plant when Greater than 25% Power." At approximately 0551 hours0.00638 days <br />0.153 hours <br />9.11045e-4 weeks <br />2.096555e-4 months <br /> EST, with power level at approximately 78%, the No. 9 bearing vibrations reached the trip criterion of 14 mils specified in AOP-006 and the reactor was tripped.

The auxiliary feedwater system [BA] started automatically, as expected, in response to steam generator [AB:SG] level changes after the reactor trip. The 'B' Main Feedwater Pump [SJ:P] also tripped as typical following a reactor trip. The primary system and steam generator power operated relief valves [AB:RV] and safety valves did not actuate during this event. The main feedwater system [SJ], main steam system [SB], and condenser [SG:COND] remained available during the event.

II. CAUSE OF EVENT

The cause of the event was due to a manual reactor trip in accordance with procedure, AOP-006.

This guidance is conservative and in place to protect the turbine.

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 a manual or automatic actuation of any of the systems listed in 10 CFR 50.73 (a)(2)(iv)(B), including the reactor protection system and auxiliary feedwater system.

This event was investigated using the HBRSEP, Unit No. 2, Corrective Action Program and documented in Significant Adverse Nuclear Condition Report 306903. The investigation found that the root cause of the high turbine vibrations could not be narrowed down to one specific cause. The investigation determined that the most probable causes of the high turbine vibrations were: 1) The size and placement techniques for the shims installed under the exciter base were incorrect, 2) procedure, CM-024, "Exciter Maintenance and Inspection," utilized standard shim placement, which for the new exciter was not adequate, 3) incorrect clearances of the refurbished generator hydrogen seal resulted in a rub condition, and 4) a vendor fabrication error in the exciter gusset plate configuration could have resulted in a soft foot condition.

Additionally, a new procedure, CM-042, "Generator Maintenance and Inspection," was used to install the refurbished and machined generator hydrogen seals during RO-25. This procedure did not provide independent verification or a QC hold point for the critical dimensions. The lack of verification performed on these tight clearances is the most probable cause of the generator hydrogen seal rub.

The soft foot condition can be attributed to incorrect positioning of shim material.

IV. CORRECTIVE ACTIONS

Completed Corrective Actions:

• Gusset plates were added to provide more stability.

• Generator Hydrogen seal was replaced and critical dimensions verified.

• 15 inch shims were installed and were placed in different locations.

• Night Order 08-17 was issued on December 3, 2008, stating, "If turbine vibrations exceed 10 mils, a normal plant shutdown should be commenced in accordance with GP-006 [Normal Plant Shutdown From Power Operation To Hot Shutdown].

• Abnormal Operating Procedure, AOP-006, was revised to improve operator guidance for determining the need to trip the reactor, reduce the likelihood of reactor trips due to turbine vibrations, and has also been modified to add actions for operators to reduce turbine vibrations.

Planned Corrective Actions:

• Procedures CM-042, "Generator Maintenance and Inspection," and CM-024, "Exciter Maintenance and Inspection," are scheduled to be revised by March 20, 2009, to strengthen the detail concerning shim installation and to provide independent verification or QC hold points for all critical dimensions.

Previous Similar Events:

Recent LERs for HBRSEP, Unit No. 2, were reviewed. There were no similar events identified.