Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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IN 84-36
May 1, 1984
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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.
