United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-43: Failure of the Bolt-Locking Device on the Reactor Coolant Pump Turning Vane

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                              September 28, 1995

                               REACTOR COOLANT PUMP TURNING VANE 


All holders of operating licenses or construction permits for nuclear power
reactors designed by Westinghouse Electric Corporation (W).


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent event involving problems that have
resulted in the loss of integrity of bolt-locking devices in the turning vane
diffuser attachment of Westinghouse reactor coolant pumps (RCPs).  It is
expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar
problems.  However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is

Description of Circumstances

On June 3, 1994, the licensee for the Seabrook Nuclear Power Station conducted
an underwater examination of reactor vessel internals.  Foreign material was
found on the reactor vessel internals lower core plate.  In a subsequent video
inspection, a bolt was found on the bottom of the reactor vessel and two bolt-
locking devices were found on the lower core plate.  One locking device was
intact, and the other was deformed and had portions missing.  The licensee
identified the bolt and locking devices as a cap screw and locking cups that
are used in the RCPs to attach and secure the turning vane diffuser to the
thermal barrier flange.


The Seabrook Plant is a four-loop Westinghouse pressurized-water reactor plant
with Westinghouse Model 93A-1 RCPs.  The RCP hydraulic section consists 
of a casing, an impeller, a diffuser adapter, a thermal barrier, and a turning
vane diffuser.  The turning vane diffuser is stationary and is attached to the
thermal barrier flange by 23 bolts.  The bolts are 3.8 centimeters         
[1.5 inches] in diameter and 25 centimeters [10 inches] long.  The ends of the
bolts are threaded.  The bolts are designed to secure the turning vane
diffuser to the thermal barrier flange with a preload of 2,712 newton meters
[2,000 ft-lbs] of torque.  The function of the bolt locking device is to
prevent bolt rotation and subsequent loss of bolt preload.  The locking
devices are fabricated from 304 stainless steel and are cylindrical members 

9509220300.                                                            IN 95-43
                                                            September 28, 1995
                                                            Page 2 of 2

with a single longitudinal split.  The locking devices are inserted into the   
annulus formed by the outer diameter of the bolt head and the counterbore for
the bolt head machined in the turning vane flange.  The spring action of the
locking device cylinder is designed to snap two bosses on the cylinder into
mating holes in the counterbore.  The anti-rotation function of the locking
device is accomplished by staking the top edge of the cylinder at two
locations into slots in the bolt head.  The bolts and locking devices are
exposed to the reactor coolant fluid within the pump but are not part of the
reactor coolant system pressure boundary.

During the 1994 Seabrook refueling outage, all four RCPs were inspected to
determine the origin of the loose parts.  The bolt and one of the locking
devices found in the vessel were traced to RCP "B," and the other locking
device was from RCP "D."  Various degrees of degradation were detected on some
locking devices in all four RCPs.  The licensee postulated that the bolt and
the locking devices dropped into the turning vane diffuser as a result of
degradation of the locking devices and the loss of the preload torque on the
bolt.  The bolt and locking devices were carried by the cold-leg flow stream
into the reactor vessel lower head and to the lower core support plate.  The
licensee replaced the bolts and locking devices on all four RCPs with a new

The degradation of the locking device and the release of the bolt was
evaluated by the licensee with assistance from Westinghouse.  The evaluation
is documented in Licensee Event Report No. 94-010-01, dated January 16, 1995.
The root cause of the release of the turning vane cap screw and locking cups
was attributed to the original design not adequately considering the affects
of flow-induced vibration on the locking cup and the turning vane cap screw. 
The licensee postulated that flow-induced vibration caused the locking cups to
erode and release from the turning vane.  The cap screw subsequently backed
out as a result of the loss of the preload torque and the effects of vibration
and gravity.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                    /S/'D BY DMCRUTCHFIELD

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contact:  Francis Grubelich, NRR
                    (301) 415-2784

Page Last Reviewed/Updated Monday, November 18, 2013