05000423/LER-2005-005

LER-2005-005,

Docket Number
Event date:	12-01-2005
Report date:	05-18-2006
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4232005005R01 - NRC Website

1. Event Description At approximately 14:45 on December 1, 2005, with Millstone Unit 3 (MP3) in Mode 1 at an indicated power level of 38%, an automatic reactor trip occurred on 'C' Steam Generator Low-Low level (S/G Low-Low level). At the time of the reactor trip, a power reduction to 30% was in progress to facilitate a containment entry to locate and repair a Reactor Coolant System (RCS) [AB] leak. As turbine [TA] load and reactor power were reduced, main turbine vibrations increased above allowable values and the main turbine was manually tripped in accordance with procedure. After the turbine was manually tripped, the reactor automatically tripped on 'C' S/G Low-Low level. All safety systems performed as designed, including auto actuation of the auxiliary feedwater system. The balance of the plant shutdown was uncomplicated.

The automatic reactor trip is reportable as a reactor protection system [JC] actuation in accordance with 10CFR50.73(a)(2)(iv). The actuation of the auxiliary feedwater system [BA] [JD] is also reportable in accordance with 10CFR50.73(a)(2)(iv)A.

2. Cause The direct cause of the reactor trip was a Low-Low level in the 'C' S/G that resulted from the shrink in S/G water level created by the manual turbine trip. The plant is designed to maintain the reactor critical following a turbine trip below 45% power (P-9 permissive setpoint). At the time of the trip, the Reactor Protection System (RPS) S/G Low-Low Level Trip setpoints were being maintained at 27% vice the nominal value of 18% to address Westinghouse identified S/G level inaccuracies associated with the S/G mid-deck plate differential pressure.

Operation at the increased set point reduced the margin available to accommodate S/G level transients that occur following a turbine trip at power. During normal plant shutdowns, the Low-Low S/G Level setpoint is manually reduced to 18% at power levels less than 40% rated thermal power. This restores the margin to trip to the design value at low power levels. During rapid power reductions there is insufficient time to change the narrow range level setpoint.

3. Assessment of Safety Conseauences The reactor trip occurred at a conservatively high S/G level, was uncomplicated, and all safety functions were accomplished per design. A reactor trip on Low-Low S/G level protects the reactor from a loss of heat sink.

4. Corrective Action An engineering analysis to determine the new S/G Low-Low setpoints to provide additional margin is being evaluated. Additional corrective actions are being taken in accordance with the station's corrective action program.

5. Previous Occurrences There have been no occurrences of a reactor trip following a turbine trip below the P-9 permissive setpoint in the previous 3 years.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].