Information Notice No. 95-16: Vibration Caused by Increased Recirculation Flow in a Boiling Water Reactor

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

                                 March 9, 1995


NRC INFORMATION NOTICE 95-16:  VIBRATION CAUSED BY INCREASED RECIRCULATION
                               FLOW IN A BOILING WATER REACTOR


Addressees

All holders of operating licenses or construction permits for boiling water
reactors (BWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for resonance vibration caused by
an increase in reactor coolant recirculation flow rates.  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 June 21, 1994, the operators at Susquehanna Steam Electric Station Unit 2
raised the reactor coolant recirculation flow rate from the normal rate of
about 45 Mkg/hr [100 Mlb/hr] to 49 Mkg/hr [108 Mlb/hr] as part of a testing
program for an approved uprate in reactor power.  The following day, with the
recirculation pump speed at 1570 to 1580 rpm (95 percent), the operators noted
a number of abnormal indications of significant vibration.  These included 
vibration of the containment instrument gas piping outside the containment and
vibration of the suppression pool hatch covers.  The operators in the control
room noted a low-frequency pulsating hum, which appeared to be coming from the
primary containment structure.  

The licensee attributed the excessive vibration to the increased speed of the
recirculation pumps.  Consequently, on June 23 the operators systematically
lowered the pump speed while monitoring the vibration.  When the speed of both
of the recirculation pumps was lowered to 1515 rpm, the vibration and noise
levels returned to normal.  The licensee estimated that the vibration had
persisted for approximately 18 hours, although it was not clear then at what
time and flow rate the vibration actually started.  The licensee performed a
thorough evaluation of the effects of the vibration on the plant and concluded
that it had not caused any damage.  The licensee also concluded that
maintaining the recirculation pumps at 1515 rpm, with a resulting flow rate of
46.7 Mkg/hr [103 Mlb/hr], was acceptable and administratively limited the
recirculation pumps to this speed. 



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                                                            March 9, 1995
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The licensee conducted a second increased reactor recirculation flow test
program on Unit 2 during the second week in December 1994, increasing the flow
rate in steps from 46.7 Mkg/hr [103 Mlb/hr] to 49 Mkg/hr [108 Mlb/hr].  At
each pump speed plateau, the licensee deliberately varied the speed difference
between the two pumps from 0 to 2 percent of the average pump speed.  When the
flow rate reached 47.6 Mkg/hr [105 Mlb/hr], the vibration noise phenomena
reappeared with the most pronounced effect occurring with a difference in
speed between the two pumps of 2 percent. 

Discussion

The licensee has concluded that the vibration was caused by pressure pulses
emitted from the recirculation pumps as the pump impeller vanes passed the
pump openings.  These pulses were amplified by an as yet undefined resonating
mechanism.  The pumps are manufactured by Byron Jackson.  They are of the
centrifugal type and have five vane impellers.  The 1570-rpm to 1580-rpm pump
speed when multiplied by the number of vanes corresponds to the measured    
131 Hz frequency of the vibration that was measured at the suppression pool
hatch covers.  This frequency also correlates well with the natural frequency
of the hatch covers, which was measured to be 133 Hz.  The low-frequency
modulation is caused by a slight difference in the frequency of the two
recirculation pumps, causing the two pumps to alternately go in and out of
synchronization with respect to vane passage of the respective pump openings. 
As this occurs, the vane-generated pressure pulses tend to alternately
reinforce and cancel each other, resulting in a modulation of the frequency.

The licensee has established a working group to identify the vibration-
amplifying mechanism.  It is coordinating this investigation with General
Electric, the reactor supplier.  The NRC staff will continue to monitor the
investigation.

Vibration attributable to high recirculation flow has been noted at several
nuclear power plants in the past.  Hope Creek has experienced excessive
vibration on several occasions when the reactor recirculation flow was above
47.4 Mkg/hr [104.5 Mlb/hr].  On September 18, 1987, excessive vibration caused
two leaks from the recirculation flow instruments.  The Hope Creek licensee
now monitors the recirculation system for excessive vibration during end-of-
cycle coastdown periods and, if excessive vibration is detected, reduces
recirculation flow.

In the fall of 1989, Quad Cities Unit 2 experienced a harmonic vibration
during reactor recirculation system coastdown tests when the pump speed passed
420 rpm.  The licensee attributed the vibration to the vane passing frequency
excitation phenomenon and established an administrative limit on recirculation
flow to minimize the possibility of a recurrence.

In September 1992, the Browns Ferry Unit 2 licensee increased the
recirculation pump speed in preparation for an end of cycle coastdown.  The
reactor experienced a resonant vibration when the recirculation pump speed was
near 1500 rpm.  The licensee administratively limited the pump speed to less
than this value.
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                                                            March 9, 1995
                                                            Page 3 of 3


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

                                    /S/'D BY BKGRIMES


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

Technical contacts:  P. Y. Chen, NRR      C. Poslusny, NRR
                     (301) 415-2789       (301) 415-1402

                     T. Greene, NRR
                     (301) 415-1175

Attachment:
List of Recently Issued NRC Information Notices