Information Notice No. 84-36: Loosening of Locking Nut on Limitorque Operator

                                                            SSINS No.:  6835
                                                            IN 84-36 

                               UNITED STATES 
                            WASHINGTON, DC 20545 

                                May 1, 1984 



All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP). 


This information notice is provided as notification of a potentially generic
problem with a Limitorque valve operator. The problem involves loosening of 
a set screw for the locking nut on the worm gear shaft of the operator lead-
ing to inoperability of the valve. Similar loosening problems with 
Limitorque operators were addressed in IE Circular 79-04. The concerns and 
recommended actions noted in that document are also applicable here. It is 
expected that recipients will 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 requirements and, therefore, no 
specific action or written response is required. 

Description of Circumstances: 

On August 3, 1983 (reported by LER No. 50-366/1983-076), while performing 
the residual heat removal (RHR) shutdown cooling mode test procedure, Hatch 
Unit 2 personnel discovered that the RHR inboard isolation injection valve, 
train A, would not open with the control switch. A similar event occurred on 
the train B valve on June 2, 1983. The cause of these events has been 
attributed to a defect in the Limitorque Model SMB-4 motor operators. The 
defect consists of a loose set screw on the locking nut for the worm gear 
shaft. Loosening of the set screw allowed the worm gear locking nut to back 
off, causing slippage of the worm gear from its intended position, which 
prevented the internal torque sensing switch from stopping the motor after 
the valve had fully closed. This resulted in the trip of the valve motor 
circuit breaker and the resultant failure of the valve to respond to an 
opening signal. 

The failure appears to be a common mode failure that could affect both 
trains of low-pressure core injection (LPCI) and render both trains 
inoperable. The safety concern is that the failure of these valves to open 
when needed following a design-basis accident, would prevent the LPCI system 
from injecting into the reactor vessel. Depending on the status and capacity 
of the core spray 


                                                            IN 84-36 
                                                            May 1, 1984 
                                                            Page 2 of 2 

system, there could be insufficient low pressure. emergency core cooling in 
the plant during a design-basis accident. 

During normal plant operation the inoperable valve could go undetected for 
an extended period of time. At this time it is not clear how long these 
valves were inoperable at Hatch; however, Technical Specifications for Hatch
require cycling of these valves on an interval of 18 months. Thus, there is 
a potential for having these valves in an inoperable state for a maximum 
period of 18 months. The valves were not inoperable for that long at Hatch 
because the operators for these valves were newly installed in February 

The operators in question were installed at Hatch as a result of the recent 
requirements of 10 CFR 50.49 (Equipment Qualification Program). However, 
Georgia Power Corporation (GPC) has other Limitorque SMB operators installed
in critical applications at the Hatch plant. 

The licensee has taken corrective action by tightening and staking the set 
screws in place as described in INPO Procedure No. PMP2-ZG-25, rev. OB and 
plans to perform an engineering evaluation of the remaining SMB operators 
for possible problems. GPC has issued a 10 CFR 21 notification to the NRC 
dated March 30, 1984, and has informed the vendor, Limitorque, of this 

Holders of construction permits and operating licenses should consider in-
spection of their Model SMB Limitorque operators for evidence of loose worm 
gear locking nut set screws. Where such set screws are found loose, 
appropriate measures (e.g. staking or as recommended by manufacturer) should 
be taken to secure set screws in place. No written response to this notice 
is required. If you have any questions regarding this matter, please contact 
the Regional Administrator of the appropriate NRC Regional Office or this 

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

Technical Contact:  J. J. Zudans 
                    (301) 492-8030 

List of Recently Issued IE Information Notices.

Page Last Reviewed/Updated Wednesday, March 24, 2021