Information Notice No. 85-38: Loose Parts Obstruct Control Rod Drive Mechanism
SSINS No.: 6835
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
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).
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.
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
Attachment: List of Recently Issued IE Information Notices
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