05000387/LER-2004-003

LER-2004-003, Manual Scram Following Main Turbine High Vibration

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3872004003R00 - NRC Website

EVENT DESCRIPTION

At 16:35 on April 21, 2004 with Unit.1 in Mode 1 at 17% power, Susquehanna Unit 1 was manually scrammed in response to main turbine (EIIS Code: TA) bearing high vibrations experienced during startup turbine testing. The turbine had been replaced during the recently completed Unit 1 refueling outage. The startup was being controlled via plant procedure and did include contingency plans to address expected "rub" induced vibration of the new turbine. Component rubs were anticipated because the improved efficiency of the new turbine is attributable, in part, to tighter clearances at the steam packing glands. The plans to address rubs included a reduction in turbine speed to 100 rpm if predetermined vibration levels were reached to allow the rub to work itself out (i.e. allow the components to wear themselves into an effective fit). The contingency plans also provided for controlled Unit shutdown and the addition of balance weights to the appropriate rotor if necessary. The turbine startup was uneventful through 1800 rpm. Generally, bearing vibration levels were between 2 and 3.5 mils although bearing #8 reached approximately 5 mils. After approximately 35 minutes of turbine operation at 1800 rpm and no load, a rub developed in the vicinity of bearing #2. Vibration levels increased from approximately 3 mils to 11.5 mils. At this point, control room operators (Licensed, utility) tripped the turbine as directed by operating procedure. The rub continued to cause localized heating thus resulting in a rotor.bow_condition and .a_steady-vibration level increase despite -- "

• -

• 7 the turbine trip. In order to reduce turbine speed and, subsequently, vibration levels more quickly, operating procedures direct the breaking of condenser vacuum (EIIS Code: SH) when extreme vibration levels are experienced. The rotor bow near bearing #2 caused increased vibration levels at bearings #1 and #3. When bearing #1 vibration levels increased rapidly to 25 mils, the Reactor Protection System (RPS; EIIS Code: JC) was manually initiated in preparation for breaking condenser vacuum. By 16:48 hours, Main Steam Line Isolation Valves (MSIV; EIIS Code: SB) were manually closed and condenser vacuum had been broken thus permitting turbine speed to decrease to less than 3 rpm. The turbine was then placed on the turning gear. While multiple speed decreases were anticipated during the test to allow turbine components to "rub" and ultimately create their own adequate operating clearances, it was not expected that a manual scram would be necessary to allow for a more rapid turbine speed reduction. As such, the manual RPS actuation and the subsequent manual initiation of the Reactor Core Isolation Cooling system (RCIC; ENS Code: BN) for level control purposes are not considered part of a pre-planned sequence and are, therefore, being reported in accordance with 10CRF50.73(a)(2)(iv)(A).

All control rods fully inserted on the scram. A Level 3 containment isolation signal was received, as expected. The signal did not, however, affect primary containment isolation valves (EIIS Code: JM) in any system as all potentially affected valves were already positioned to their safety state. RCIC was manually initiated to assist reactor water level control. The lowest reactor water level reached was approximately 3 inches. Post-scram conditions were maintained within procedural operating requirements. No radioactive releases resulted from this event.

CAUSE OF EVENT

Three root causes have been identified for this event:

1. The turbine trip limits were not set sufficiently low on startup to prevent high vibrations initiated by a rub event.

2. Vibration rates of increase were not monitored on startup and used as an input to identify impending high vibration.

3. Turbine Retrofit Project management did not establish contingency plans in sufficient depth to handle equipment failure, multiple vibration issues, and short time frames for analysis and action.

REPORTABILITY/SAFETY CONSEQUENCES ANALYSIS

Actual: This event was determined to be reportable under 10 CFR 50.73(a)(2)(iv)(A) in that unplanned system actuations occurred in response to main turbine bearing high vibrations experienced during startup turbine testing. The turbine vibration condition resulted in the manual actuation of RPS and the —manual initiation of the RCIC system.—The.RCIC.system.was conservatively initiated to assist with reactor level inventory control following the scram.

All safety systems functioned as designed. All control rods inserted and post scram reactor conditions were maintained within procedural requirements. There were no challenges to Primary or Secondary containment (EllS Code: NH) and integrity was maintained. Water level was readily restored to the nominal operating band. There were no in-plant or offsite radiological consequences that resulted from this event. There were no safety consequences or compromises to the health or safety of the public.

Potential: Manual reactor scrams can challenge the safety functions of plant structures, systems, and components (SSC). High turbine vibration levels can damage turbine components and auxiliary systems.

CORRECTIVE ACTIONS

The following corrective actions have been completed:

• A thorough inspection of plant components and an assessment of plant data were conducted to determine the potential for damage during the high vibration event. Results of this review supported turbine startup.

• Vibration amplitude and rate of change limits were procedurally established for the pending turbine startup.

• Closer observation of vibration amplitude and phase angle data on polar vibration plots was established for the pending startup.

• The turbine was successfully started and loaded in accordance with the established startup procedure.

The following corrective actions are planned:

vititieprocedurally established for future startups. During startup, vibration levels reaching conservatively established limits would require a reduction in turbine speed to 100 rpm.

• Startup vibration amplitude and rife of bhanbelifiiits7r­

• Sufficient and dedicated manpower support, with clearly defined responsibilities, will be procedurally established to enhance performance monitoring and to address any necessary contingency actions during future startups.

• Analytical tools will be utilized to monitor vibration amplitude, rate of change, and phase angle during future startups.

Past Similar Events:

Susquehanna Unit #2 underwent its turbine retrofit and startup without incident.