United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #:  9705300009

Donald C. Cook Plant, Units 1 and 2

Docket Numbers 50-315 and 50-316

Operating Licenses DPR-58 and DPR-74

This fax serves as a report of a 10 CFR Part 21 defect, reported under 10

CFR 21.21.d.3.i.

Description of Event

On January 8, 1997, while installing a new torque switch assembly for

motor operated valve 2-MMO-421, it was noted that a spacer appeared to be

missing from the torque switch.  An inspection of all the standard SMB-00

torque switches assemblies remaining in stock determined that they were

all missing a spacer.  These torque switch assemblies were part of a

group of 7 received from Limitorque for general use at DC Cook under ASP

18787.  A condition report was written to investigate if this was a new


On January 12, 1997, 2-WMO-728, West Essential Service Water (ESW) Supply

to the 2CD Emergency Diesel Generator (EDG) Heat Exchanger Shutoff Valve,

failed to fully close during performance of routine surveillance **2 OHP

4030.STP-027CD, "CD Diesel Generator Operability Test".  Investigation of

the failure on January 14, 1997 discovered that the torque switch

assembly installed on the valve was also missing a spacer, The valve was

declared inoperable, however the valve was still capable of performing

its design basis function of opening, therefore, the operability of the

2CD Diesel Generator was not affected.

On January 14,1997, in evaluating the failure of 2-WMO-728 and the

effects of the missing spacer on the torque switch assembly, it was

determined that the lack of the spacer allows the torque switch shaft to

rotate slightly along the shaft length.  This shaft rotation allowed the

torque switch contacts to open.  With the closed torque switch contacts

open, the actuator control circuit will not allow the actuator to run in

the closed direction and therefore will not allow the valve to fully

close.  The actuator control circuit uses only the closed set of contacts

and therefore the open operation is not affected.

In the investigation, it was determined that 2-CMO-415, the Component

Cooling Water (CCW) to Miscellaneous Header Shutoff Valve for Train A,

had received one of the torque switch assemblies from the lot of 7 which

Part 21 Report

DC Cook Plant

Page 2

were missing the spacer.  Because the operability of the valve could not

be assured, it was declared inoperable.  As the defective assembly had

been installed in September of 1996, it was determined that a

10CFR50.72(b)(1)(ii)(B) ENS call for a condition outside the design basis

was appropriate.  The ENS call was made on January 14, 1997 at 1650


The fourth defective torque switch assembly was found to have been

installed in 1-WMO-754.  With the exception of the 2-CMO-415, all the

valves in which the defective assemblies had been installed had either an

"open" safety function (2-WMO-728 and 1-WMO-724) or were a BOP valve (2-

MMO-421), and thus their operability was not in question.

During the investigation of this event, an operability evaluation was

performed for 2-CMO-415.  The valve was found to have been successfully

tested under dynamic conditions during the September 1996 to January 1997

interval and that the defective torque switch assembly would not have

kept the valve from performing its design basis function of closing.

Therefore, the valve had never been inoperable.  The ENS notification was

retracted on February 22, 1997.

Cause of Event

The defective lot of 7 received from Limitorque apparently resulted from

a manufacturing/assembly error at the Limitorque facility.

Effect of Failure on Plant Operations

The double pole torque switch is a standard configuration for American

Electric power and is used extensively at the DC Cook Plant in SMB-00

actuators.  The 7 assemblies supplied under ASP 18787 were placed in open

stock for use in the next valve requiring one.  Therefore, the 3 torque

Part 21 Report

DC Cook Plant

Page 3

switch assemblies in open stock could have been used in any number of

safety related applications with unknown results.  The failures in the 4

valves in which they were installed did not result in any safety impact,

however, this could not be guaranteed for all other SMB-00 valve

actuators in the plant due to the large variety of applications for the

assembly.  Because it is not possible to definitively evaluate all

possible applications for significant safety hazard, it was

conservatively decided to submit this event as a Part 21.

Corrective Action

All 4 of the defective assemblies which were installed have been either

replaced, or repaired by addition of a spacer.  All 7 assemblies were

returned to Limitorque for evaluation on March 10, 1997.  Limitorque has

not been able to predict when their evaluation might be complete.


Page Last Reviewed/Updated Thursday, March 29, 2012