Information Notice No. 86-89: Uncontrolled Rod Withdrawal Because of a Single Failure
SSINS No.: 6835
IN 86-89
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
October 16, 1986
Information Notice No. 86-89: UNCONTROLLED ROD WITHDRAWAL BECAUSE OF A SINGLE FAILURE
Addressees:
All boiling water reactor (BWR) facilities holding an operating license or a
construction permit.
Purpose:
This notice is to alert recipients of a potentially generic problem with a
single failure that can cause both a single control rod drift to the
full-out position and then failure to insert on demand. Recipients are
expected to review the information for applicability to their facilities and
consider actions, if appropriate, to preclude similar problems occurring at
their facilities. However, suggestions contained,in this information notice
do not constitute NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances:
On July 30, 1986, the Grand Gulf nuclear station experienced an uncontrolled
single control rod withdrawal to the full-out position while at 69 percent
power. When attempting to notch rod 20-45 from position 08 to position 10,
the operator noticed "rod drift" and "rod block" alarms and observed
indications that the rod was at position 12 and continuing to withdraw. The
operator pressed the insert pushbutton several times and observed the
illumination of the "in" light and the "Settle" light. The repeated notch
insertion attempts slowed the rod outward movement, but the rod continued to
withdraw to full-out position 48. The control rod took about 3 minutes and
10 seconds to travel from position 08 to position 48 (which is about normal
settle speed).
The operator carried out the actions required by the Alarm Response
Instructions. As a conservative measure, reactor power was reduced to 60
percent for thermal limit concerns, and a coupling check was performed. Once
control of rod insertion was regained, the rod was placed at position 44 and
back to position 48 to test the rod withdrawal limiter. The rod was declared
inoperable, fully inserted, and hydraulically disarmed.
To determine the cause of the event, inspections and bench checks were
performed on withdraw control valve, C11-F422. (This valve is designated 122
at some older BWRs.) The valve demonstrated no sign of abnormal operation,
and no fouling of the valve seat was evident. The licensee concluded that
temporary particulate
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IN 86-89
October 16, 1986
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accumulation on the valve seating surface caused an incomplete closure of
the valve when the withdraw command was terminated, allowing drive water
pressure to leak past the valve and force the drive piston downward.
Subsequently, the withdraw control valve was replaced with a new valve and
the control rod restored to service. During the upcoming refueling outage,
the licensee plans to check the drive water filters and sample low Stagnant
points in the system for particulates. The licensee has provided additional
operator instructions for control rod/drive malfunctions based on General
Electric Service Information Letter (SIL) 292 that was issued in July 1979.
The procedure revisions include the following actions to be taken when
control rods drive out with no "withdraw command" present:
1. Application of continuous control rod insert signal until the rod
reaches zero. If the control rod continues to withdraw from zero,
reinsert the continuous control rod insert signal and hold until either
the control rod drive pressure can be reduced or someone can isolate
the affected hydraulic control unit (HCU) by closing the 103 drive
water riser isolation and 105 exhaust water riser isolation valves on
the HCU.
2. If the control rod drive does not respond to the insert signal, have
someone manually scram the rod from the HCU.
3. If the drive continues to demonstrate occurrences of inadvertent
withdrawal, the control rod should be inserted to position zero and the
drive should be valved out of service until the fault is located and
corrected. After the drive is valved out of service, the associated
valve no. 122 and control circuity should be checked and repaired if
necessary.
Discussion:
There have been other instances when the withdrawal directional control
valve caused uncontrolled rod withdrawal:
Plant name Date Cause
Pilgrim 4/8/78 122 Valve Corrosion Products
Browns Ferry 2 6/24/80 85-40C Valve Failed (85-40C is
TVA designation for 122 Valve)
In addition, there have been events when uncontrolled withdrawal has been
caused by unknown reasons or a collet finger problem.
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IN 86-89
October 16, 1986
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No specific action or written response is required by this information
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Eric Weiss, IE
(301) 492-9005
Attachment: List of Recently Issued IE Information Notices
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