Home > NRC Library > Document Collections > General Communications > Information Notices > 1984 > IN 84-36
SSINS No.: 6835 IN 84-36 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, DC 20545 May 1, 1984 Information Notice No. 84-36: LOOSENING OF LOCKING NUT ON LIMITORQUE OPERATOR Addressees: All holders of a nuclear power reactor operating license (OL) or construction permit (CP). Purpose: 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 8404230132 . 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 1983. 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 occurrence. 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 office. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: J. J. Zudans (301) 492-8030 Attachment: List of Recently Issued IE Information Notices.
Page Last Reviewed/Updated Thursday, March 29, 2012