United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-68: Stuck Control Rod

                                                            SSINS No.: 6835 
                                                                   IN 86-68 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                               August 15, 1986

Information Notice No. 86-68:   STUCK CONTROL ROD 

Addressees: 

All boiling water reactor facilities holding an operating license or a 
construction permit. 

Purpose: 

This notice alerts addressees to the potential for a stuck control rod and 
damaged control rod drive as a result of closed manual isolation valves on 
the hydraulic control unit. 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: 

During a full scram preoperational test (before fuel loading) on April 29, 
1986, the Clinton nuclear power plant found that rod 08-25 was stuck at 
position 06. Attempts to move the rod by gradually increasing drive water 
pressure were unsuccessful. 

Investigation revealed that the 112 valve (scram discharge riser manual 
isolation) on the hydraulic control unit was in the closed position despite 
verification that the valve was in the proper (open) position. The closed 
112 valve caused a very high pressure to develop in the control rod drive, 
crushing the cylinder tube. Because the surface area above the piston is 
smaller than that below the piston, a closed 112 valve can cause pressures 
above the piston to be several times as large as the 1850 psi applied at the
bottom of the piston by the scram accumulators. The above-the-piston water 
surrounds the cylinder and can partially crush the cylinder. The crushed 
cylinder tube caused interference with the movement of the piston and thus 
the stuck rod. 

Subsequent unsuccessful attempts were made to insert the partially withdrawn
control rod into its drive by removing temporary blade guide assemblies and 
applying force from above with a specially constructed tool. The rod was 
ultimately removed by cutting the index tube into two pieces and removing it
from the drained reactor vessel. 
.

                                                           IN 86-68 
                                                           August 15, 1986 
                                                           Page 2 of 2 


The licensee has adopted a number of corrective measures. The manual 
isolation valves are locked open once they are verified to be in the proper 
position. The personnel access control to the hydraulic control units has 
been tightened. In addition to the normal independent verification of valve 
position, the startup organization has instituted a temporary measure of 
having management personnel make additional random checks. 

Discussion: 

Although not always causing damage to the control rod drive as occurred at 
Clinton, there have been several instances of manual isolation valves on the
hydraulic control units causing failure to scram of individual control rods.
Among the other instances when problems with the manual isolation valves on 
the hydraulic control unit have prevented a rod from scraming are: 

Date                Plant               Event-Failure to Scram One Rod 

October 20, 1984    Dresden Unit 3      Manual isolation valve disc 
                                        separated from valve stem 

October 24, 1984    Quad Cities Unit 2  Scram inlet isolation valve closed 

April 11, 1985      Perry               Inadvertently closed 112 valve - 
                                        CRDM damaged, index tube cut to 
                                        remove 

There are other valves on the hydraulic control unit that could prevent a 
scram or cause damage to the control rod drive mechanism. Some of these 
valves could be discovered by problems with normal rod movement before a 
scram. However, a closed 112 valve does not cause problems with normal rod 
movement. Because larger pressures are applied to the control rod drive 
during the scram, damage caused by closed valves on the hydraulic control 
unit is most likely to occur during the scram. 

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 
Page Last Reviewed/Updated Tuesday, November 12, 2013