Information Notice No. 94-01: Turbine Blade Failures Caused by Torsional Excitation from Electrical System Disturbance

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON  D.C.  20555

                                January 7, 1994


NRC INFORMATION NOTICE 94-01:  TURBINE BLADE FAILURES CAUSED BY TORSIONAL
                               EXCITATION FROM ELECTRICAL SYSTEM DISTURBANCE


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential blade failures in low-pressure
turbines reportedly caused by torsional vibration resulting from disturbance
in the electrical power system.  It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems.  However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.

Description of Circumstances 

On July 12, 1993, at Susquehanna Unit 1, automatically shut down as a result
of high turbine vibration, which was felt in the control room (Reference 1). 
The maximum amplitude recorded was about 0.038 cm [15 mils] at bearing No. 8. 
Two blades from the L-1 row on the governor end of the low-pressure C turbine
separated from the rotor.  The failed blades gouged diaphragms (stationary
rotors) and damaged other blades in the L-0 and L-1 rows.  Cracks were found
at the roots of several blades in the L-1 row.  The fragments of the blades
damaged 50 to 100 condenser tubes.

The plant was operating at 100 percent power at the time of the event. 
Control room operators responded to the event by entering the appropriate
procedures.  The emergency core cooling and engineered safety feature systems
were not initiated.  The licensee concluded that primary containment integrity
was maintained.

Susquehanna Unit 1 uses General Electric low-pressure turbines, model number
TB 170X592.  Each turbine operates at 1800 revolutions per minute.  The L-1
blades are about 96 cm [38 inches] long.  The root (finger dovetails) of a
blade consists of six plates, each plate being about 6 cm [2 1/2 inches] wide,
10 cm [4 inches] long, and 1 cm [3/8 inch] thick.  Each plate has six pin
holes arranged in three rows of two holes each.  The blade is attached to the 

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rotor by pins staked in the holes.  The failed blades were severed at the top
rows of holes, as indicated in Figure 1. 

Previous Events

Between 1973 and 1976, at Prairie Island Units 1 and 2, the licensee, Northern
States Power Company, had sustained a total of five blade failures or cracking
events in its Westinghouse low-pressure turbines.  All failed blades were from
the L-2 or L-1 row of the low pressure rotor.  The licensee attributed these
failures to torsional vibration resulting from electrical system excitation. 
To resolve the problem, the licensee (1) modified the L-2 blades and the 
L-1 disc to be unresponsive to 120-Hz excitation, (2) performed a torsional
test before returning the system to service.

In 1985, eight blades from the last stage of a General Electric low-pressure
turbine failed and separated from the rotor at one of the units of a foreign
nuclear plant.  The blade failures were attributed to high cycle fatigue as a
result of the electrical disturbance which excited the 19th torsional mode of
rotor vibration at or near the resonant frequency of 120.5 Hz.  The failed
blades caused unbalanced mass and produced a large vibration on the turbine-
generator unit, which in turn failed the alternator shaft.  The failed
alternator shaft caused leakage of hydrogen and lubrication oil from the seal
system and led to a fire in the generator-alternator area.  A subsequent
torsional test at the other unit of that plant showed that a 0.05 Hz change in
the electrical frequency contributed to a three fold increase in stress
response in the L-0 row blades.

Discussion
  
Since the 1970's, some blade failures of low-pressure turbines at nuclear
power plants have been attributed to torsional excitation of the turbine-
generator shaft as a result of an electrical system disturbance.  When the
frequency of the excitation coincides with the natural torsional frequency of
the turbine rotor, the rotor and blade responses are highly magnified.  The
blade eventually fails by fatigue.  

The electrical system disturbances could be caused by (1) negative-phase
sequence current resulting from unbalanced load among the three electric
phases, caused by the load imbalance creating a current in the generator
stator opposite to the main current; (2) electric faults resulting from high
speed reclosing of field relay contacts; (3) load rejection; and (4) switching
operations on the transmission line (Reference 2).

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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                    ORIGINAL SIGNED BY


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  John Tsao, NRR
                    (301) 504-2702

References:
1.  Letter from E. C. Wenzinger of the 
      U.S. Nuclear Regulatory Commission to
      R.G. Byram of Pennsylvania Power & Light
      Company, Subject:  NRC Region I Combined 
      Inspection 50-387/93-11; 50-388/93-11,
      August 31, 1993.
2.  Technical Information Letter (TIL-1012-2), 
      "Effect of Electrical System Vibration
      on Turbine-Generator Torsional Response,"
      General Electric Company, Schenectady, 
      New York, September 8, 1987.

Attachments: (see file IN94001.WP1 for Figure 1)
1.  Figure 1.  The Finger Dovetail of a Typical Blade