United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-38: Loose Parts Obstruct Control Rod Drive Mechanism

                                                        SSINS No.: 6835  
                                                          IN 85-38         

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

                                May 21, 1985 

Information Notice No. 85-38:   LOOSE PARTS OBSTRUCT CONTROL ROD DRIVE 
                                   MECHANISM 

Addressees: 

All utilities with pressurized water reactor (PWR) nuclear power facilities 
designed by Babcock & Wilcox (B&W) and holding an operating license (OL) or 
a construction permit (CP). 

Purpose: 

This information notice is provided to alert recipients of a potentially 
significant problem pertaining to loose parts that can obstruct and prevent 
motion inside of control rod drive mechanisms. It is expected that 
recipients will review the information for applicability to their facilities 
and consider actions, if appropriate, to preclude a similar problem 
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: 

In 1981 at the Davis-Besse Nuclear Plant, a locking spring broke in a 
control rod drive mechanism (CRDM) in core location C-7 and became wedged 
against the control rod, preventing it from operating. The cause was not 
determined at that time. During the 1984 outage, the locking springs were 
inspected and none appeared broken or in the wrong position. On March 16, 
1985, the control rod in core location E-3 would not drop into the core on 
demand and had to be driven downward by control room personnel. This control 
rod was tested during an outage on March 21, 1985, and jammed after three 
cycles of operation. 

Inside of this CRDM were foreign objects and a broken locking spring. The 
foreign objects were pieces of a set screw from the handling tool used 
during the 1984 outage that had lodged in the CRDM and prevented the drive 
screws from being disengaged. The locking spring (a flat tee-shaped device 
with a riveted tab) was broken at the tee and the upper rivet hole. The 
upper portion of the broken spring was not found and is believed to be 
inside of the CRDM. 

Further examination of all of the mechanisms revealed a broken locking 
spring in core location M-5. In addition, four springs were not in their 
normal locking position. During operation there is no means of detecting 
broken springs or foreign objects in the CRDM. Exercising the control rod 
will allow loose pieces to move. 


8505200168 
.

                                                             IN 85-38      
                                                             May 21, 1985 
                                                             Page 2 of 2   

The most likely cause of the spring failures is that the unit went into 
service with some of the locking springs not in their correct position. The 
present assembly procedure has the maintenance technician determining that 
the spring is in the correct position by "feel" through a long handling 
tool. If this process is not successful, the reactor will be placed in 
operation with the spring out of position. When the control rod is fully 
withdrawn, an out-of-position spring will hit the inside of the torque tube 
cap and snap when sufficiently loaded. This was confirmed by the appearance 
of the spring failures which were brittle, intergranular fractures. In 
addition, examination of the inside of the cap showed a gouge in the tapered 
portion near the bottom and an indentation on the bottom surface. 

Spring failures are considered to be a potential common mode failure that 
could affect the reactor trip function because (1) four springs at Davis 
Besse were found not to be in their normal position and two others were 
broken; (2) the likelihood for out-of-position springs to be broken when the
control rod is fully withdrawn; and (3) the potential for a broken spring to
cause the control rod to jam. 

The corrective action by Davis-Besse Nuclear Plant was to replace all of the
out-of-position spring assemblies and to verify by visual examination that 
the springs were in their correct positions. Consideration is being given to
adding this verification as a regular part of the maintenance procedures. 
The B&W Owners Group has notified their members that there were failed 
locking springs at Davis-Besse in the control rod drive mechanisms. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 


                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  P. Cortland, IE 
                    301-492-4175 

Attachment:    List of Recently Issued IE Information Notices 
Page Last Reviewed/Updated Tuesday, November 12, 2013