Information Notice No. 84-36, Supplement 1: Loosening of Locking Nut on Limitorque Operator
SSINS No. 6835
IN 84-36 SUPP. 1
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
SEPTEMBER 11, 1984
INFORMATION NOTICE NO. 84-36 SUPPLEMENT 1: LOOSENING OF LOCKING NUT ON
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
This information notice is being provided to supplement Information Notice
84-36 for the purpose of assuring that loosening of locking nuts does not
occur in Limitorque valve operators. The NRC expects that recipients will
review this notice for applicability to their facilities. Suggestions
contained in this notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
Information Notice 84-36 described events at the Hatch Nuclear Power Plant
where loosening of a set screw on a locking nut on the worm gear shaft of a
Limitorque operator led to backing out of a locknut and subsequent
inoperability of a safety-related valve. Recent information received from
the vendor indicates that the specific part affected on the SMB-4 valve
actuator was Part #83 (bearing locknut, see Figure 1), which is located on
the actuator worm shaft--not the worm gear shaft. An enlarged drawing
showing more details of the bearing lock nut is shown in Fig. 2. The
bearing lock nut performs the function of transmitting loads from the worm
gear (Part #16) to the worm shaft causing axial movement of the worm shaft
followed by compression of the belleville spring and activation of the
torque switch. The vendor indicates that the operators at Hatch contain a
unique feature that is not found in most other valve applications that
resulted in the originally noted defect. These units use a ball screw valve
stem instead of the valve manufacturer's conventional Acme threaded valve
stem. The ball screw has the opposite hand (right-hand) thread to the
standard Acme (left-hand) thread.
The significance of this difference is that unthreading of the bearing
locknut could affect either the closing torque switch function when the
valve is equipped with a right-hand stem thread or the opening torque switch
IN 84-36 SUPP. 1
September 11, 1984
Page 2 of 2
(with obstructed or overloaded disk) when the valve is equipped with a
standard Acme left-hand stem thread. This is because the torque control
mechanism is activated as a result of axial movement of the worm shaft due
to forces generated as the valve comes to a full-close position, full-open
position, or some intermediate obstructed position. Figure 2 shows the
direction of movement of the worm shaft at the Hatch facility when the valve
comes to a closed position.
It is recommended that holders of CPs and OLs consider this additional
information for applicability in their ongoing efforts to resolve the
concerns addressed in Information Notice 84-36.
No written response to this information notice is required. If there are
any questions regarding 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: John J. Zudans, IE
1. Figure 1, "SMB-0 to SMB-4 & SMB-4T"
2. Figure 2, "Worm & Torque Spring Assembly"
3. List of Recently Issued IE Information Notices
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