IE Circular No. 79-07, Unexpected Speed Increase of Reactor Recirculation MG Set Resulted in Reactor Power Increase
CR79007
April 30, 1979
MEMORANDUM FOR: B. H. Grier, Director, Region I
J. P. O'Reilly, Director, Region II
J. G. Keppler, Director, Region III
K. V. Seyfrit, Director, Region IV
R. H. Engelken, Director, Region V
FROM: Norman C. Moseley, Director, Division of Reactor
Operations Inspection, Office of Inspection and
Enforcement
SUBJECT: IE CIRCULAR NO. 79-07, UNEXPECTED SPEED INCREASE OF
REACTOR RECIRCULATION MG SET RESULTED IN REACTOR POWER
INCREASE
The subject circular is transmitted for issuance no later than the
close of business on May 2, 1979. The circular should be issued to all
holders of BWR Operating Licenses or Construction Permits. Also enclosed is
a draft copy of the transmittal letter.
Norman C. Moseley, Director
Division of Reactor Operations
Inspection
Office of Inspection and Enforcement
Enclosures:
1. Draft Transmittal Letter
2. IE Circular No. 79-07
CONTACT: C. J. DeBevec, IE
49-28180
.
(Draft transmittal letter for IE Circular 79-07, to each holder of a BWR
Operating License or Construction Permit.)
IE Circular No. 79-07
Addressee:
The enclosed IE Circular No. 79-07, is forwarded to you for information. No
written response is required. Should you have any questions related to your
understanding of this matter, please contact this office.
Sincerely,
Signature
(Regional Director)
Enclosure:
IE Circular No. 79-07
.
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
May 2, 1979
IE Circular No. 79-07
UNEXPECTED SPEED INCREASE OF REACTOR RECIRCULATION MG SET RESULTED IN
REACTOR POWER INCREASE
Description of Circumstances:
On November 24, 1978, an unexpected rapid speed increase was experienced on
the `B' Reactor Recirculation MG set (RRMG) at the Cooper Nuclear Station.
Prior to the event, the RRMG set had been inadvertently tripped during the
performance of routine maintenance. Upon restart, the MG set would not
respond to a speed increase signal. Licensee personnel commenced trouble
shooting this problem in accordance with the instructions in the
manufacturer's technical manual.
One step of these instructions suggested removing the fuse from the Bailey
scoop tube actuator circuit (a blown fuse would de-energize the actuator and
not permit the scoop tube to move). When the terminal panel cover was
removed, licensee personnel noted two fuses instead of one as shown on the
vendor drawings. When licensee personnel removed the second fuse to verify
fuse integrity, a rapid speed increase of the MG set occurred. This increase
was terminated by replacement of the fuse or tripping of the MG set by the
control room operator (both actions took place almost simultaneously).
The input signal to the scoop tube actuator is received from the speed
controller via a function generator. This signal is a scoop tube position
demand signal. As the actuator drives the scoop tube in or out of the fluid
coupler, a position followup signal is generated to null the input signal
and stop scoop tube motion at the required position. The vendor's drawings
of the scoop tube actuator circuit showed only one fuse in the circuit. The
licensee contacted the vendor regarding the second fuse and was informed
that this fuse had been added as a change in the initial circuit design to
provide protection for certain electronic components in this circuit. This
additional fuse is located in the position followup circuit. Removal or loss
of the fuse would result in a loss of the nulling signal and would cause the
scoop tube to drive to the maximum speed position.
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IE Circular No. 79-07 May 2, 1979
The licensee examined the circuit for the 'a' RRMG set scoop tube actuator
and discovered that it contained only the single fuse as indicated on the
vendor's drawings.
At the time of the speed increase, the reactor was operating at 27 percent
power. The speed increase on the RRMG set caused power to increase rapidly
to 75 percent until terminated by the manual trip of the RRMg set. The short
term reactivity increase corresponded to a reactor period of less than 5
seconds.
A check of reactor parameters after this event verified that no core limits
were exceeded. The FSAR contains an analysis of a similar transient which is
failure of a speed controller causing one RRMG set to drive to the maximum
speed condition. This analyzed transient starts from a 100 percent power rod
pattern, whereas the observed transient commenced from a lower power rod
pattern. Therefore, this observed transient falls within the bounds of the
FSAR analysis.
As part of the corrective action for the event, the licensee has placed a
warning sign on the control panel cover to remind technicians of the effect
of removing the second fuse.
Recommended Actions for Licensee's Consideration:
All holders of operating licenses for BWR plants employing variable speed
Reactor Recirculation Motor Generator sets should be aware of the potential
for occurrences of the type indicated above. It is recommended that this
area be reviewed at your facility in the following respects:
1. A review of the scoop tube actuator circuit should be conducted to
verify that the circuit diagram indicates the actual number of fuses
installed.
2. Where the actual circuit configuration contains the second fuse (in the
scoop tube position follow-up circuit) a warning statement describing
the consequences of its removal when the unit is operating should be
incorporated into the troubleshooting guides. A permanent label to the
same effect should be placed on the control panel cover which houses
the fuse. In addition, operating personnel and maintenance technicians
should be apprised of the effect of removing the fuse from the circuit.
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IE Circular No. 79-07 May 2, 1979
3. Review procedural controls for troubleshooting systems and components
that have the potential to affect significant plant parameters such as
power level, water level pressure, etc. Ensure that when a vendor's
technical manual is being used in lieu of a detailed procedure, all
personnel are sensitive to differences between the actual equipment and
the manual description. When discrepancies are identified, resolution
should be obtained before proceeding.
This circular is being forwarded to all holders of BWR construction permits
for information.
No written response to this Circular is required. If you require additional
information regarding this matter, contact the Director of the appropriate
NRC Regional Office.
Enclosure:
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