United States Nuclear Regulatory Commission - Protecting People and the Environment

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 


8610100105 
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                                                       IN 86-89 
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                                                       Page 2 of 3 

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 
                                                       Page 3 of 3 

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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