United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-14: Failure of a Heavy Control Rod (B4C) Drive Assembly to Insert on a Trip Signal

                                                          SSINS No.:  6835 
                                                          IN 85-14 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C. 20555 

                             February 22, 1985 

Information Notice No. 85-14: FAILURE OF A HEAVY CONTROL ROD (B4C) DRIVE 
                                 ASSEMBLY TO INSERT ON A TRIP SIGNAL 

Addressees: 
  
All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 
  
Purpose: 

This information notice is to alert recipients of a potentially significant 
event pertaining to a stuck drive rod assembly of a control rod drive 
mechanism (CRDM). This event occurred while performing hot rod drop tests at
a foreign pressurized water reactor designed by Westinghouse Electric 
Corporation (Westinghouse). This event was caused by a breech guide screw 
that became disengaged from the external breech of a drive rod assembly and 
fell on top of the CRDM latch assembly where it became lodged and prevented 
driveline motion. It is expected that recipients will review the information
contained in this notice for applicability to their facilities and consider 
actions, if appropriate, to preclude similar problems from occurring at 
their facilities. However, suggestions contained in this notice do not 
constitute Nuclear Regulatory Commission (NRC) requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

On the afternoon of December 5, 1984, Westinghouse notified Duke Power 
Company (Duke) of an event that occurred on November 19, 1984, at a foreign 
reactor concerning the CRDM design similar to the one installed at McGuire 2 
(the McGuire design is completely different). Based on initial information, 
Westinghouse considered the event to be an isolated case. However, on the 
afternoon of December 6, 1984, Westinghouse notified Duke of unfavorable 
inspection results on similar CRDMs at several plants and of the impact on 
the operation of McGuire Unit 2. Based on this new information, Duke orally 
informed NRC Region II of the event and began a safety assessment of 
continued operation 4 of McGuire 2, the results of which were provided to 
NRC personnel by conference telephone call on the afternoon of December 6, 
1984, and subsequently documented in a letter dated December 12, 1984, from 
H. B. Tucker to J. P. O'Reilly. 

By letter dated December 7, 1984, E. P, Rahe, Jr. to R. DeYoung, 
Westinghouse documented Mr. Rahe's telephone call of December 7, 19 with Mr. 
C. E. Rossi reporting the event associated with CRDM heavy drive rod 
assemblies under 10 CFR 21, identified as a Potential Substantial Safety 
Hazard, for one 


8502190265 
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                                                          IN 85-14  
                                                          February 22, 1985 
                                                          Page 2 of 3 

operating plant (McGuire 2) and for six construction plants (Catawba 1 and 
2, Seabrook 1 and 2, Watts Bar 1 and 2). (Note: At that time, Catawba 1 had 
been issued a low power testing license but had not yet attained initial 
criticality.) The drive rod became stuck at a foreign reactor during 
downward stepping while performing hot rod drop tests as part of 
preoperational testing prior to achieving initial criticality. Subsequent 
on-site investigations revealed that a breech guide screw from a CRDM heavy 
drive rod assembly had rotate,d. out of position and fell on top of the CRDM 
latch assembly where it became lodged and, prevented drivel ine motion. 

The function of the breech guide screw is to provide alignment and guidance 
during coupling and uncoupling of the drive rod from the rod cluster control
assembly during refueling. The breech guide screw is 0.52 inch long and has 
a 0.433 inch diameter. If a breech guide screw were to rotate out of the 
drive rod assembly, it would fall into the annulus between the external 
breech and the rod travel housing. Although this annulus is nominally 3/8 
inch wide, it is sufficiently flexible to allow a loose breech guide screw 
to travel downward during rod stepping. The loose breech guide screw would 
then lodge on top of the CRDM latch assembly, potentially causing misstep-
ping, intermittent sticking of the driveline or a totally stuck driveline. 
Such a loose breech guide screw would not be able to pass below the above 
position because the clearance between the guide tube and the drive rod 
assembly is only 0.055 inch.. Thus, if a breech guide screw should become 
loose, it would not be able to migrate into the reactor upperhead region 
during plant operation; therefore, it would not subsequently be expected to 
become a loose part in the reactor coolant system. 

The breech guide screw of a heavy drive rod assembly is designed to be held 
in position by a locking pin. The locking pin is inserted in a drilled hole 
that intersects the mating threads of the breech guide screw and those of a 
threaded hole in the external breech of the drive rod assembly. The locking 
pin, in turn, is welded to the head of the breech guide screw to ensure that
it stays in place. (See Attachment 1, Drive Rod Assembly.) However, in the 
foreign plant, the breech guide screw was drilled at an angle such that the 
locking pin did not intersect the threads. Thus, the locking pin was 
ineffectual in that the breech guide screw was not actually locked in place 
but was free to unscrew from the external breech. 

Subsequent to the drive rod becoming stuck at a foreign facility, Westing-
house recommended that a reverse torque test be conducted on the plant's 
remaining 51 breech guide screws. This reverse torque test consists of 
applying twice the installation torque on the breech guide screw but in the 
reverse direction, with a test failure being either the complete unscrewing 
of the breech guide screw or a rotation of 15 or more of the screw. 
Three breech guide screws at the affected foreign plant became unscrewed 
when tested. Afterwards, the breech guide screws were similarly tested at 
another foreign reactor using similar CRDM heavy drive rods to those at the 
plant that experienced a stuck drive rod. These tests revealed several 
(exact number unknown) breech guide screws that were classified as being 
either finger tight or loose. In addition, similar tests were conducted at 
Catawba 2, Seabrook 1, and Watts Bar 1 and 2 with the  

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                                                          IN 85-14  
                                                          February 22, 1985 
                                                          Page 3 of 3 

following results: (1) Catawba 2 - of the 57 rod drive assemblies tested, 
five breech guide screws were found to be finger tight and were completely 
unscrewed, and 20 breech guide screws rotated when reverse torque tested; 
(2) Seabrook 1 of the 57 rod drive assemblies tested, 17 breech guide screws
became unscrewed and one rotated but did not become unscrewed when reverse 
torque tested; and (3) Watts Bar 1 and 2 - of the 144 rod drive assemblies 
tested, two breech guide screws were found to be finger tight and were 
completely unscrewed and 20 became unscrewed when reverse torque tested. 
Finally, the breech guide screws at Catawba 1 were reverse torque tested, at
which time two breech guide screws became unscrewed and three breech guide 
screws were found with disfigured heads that blocked the attaching of the 
torque wrench to the screws. Following these tests, Duke replaced 14 CRDMs 
at Catawba 1. 

Westinghouse has advised its affected utility customers of the event and has
provided them with recommendations for operating plants and plants under 
construction. Westinghouse recommends that potentially affected operating 
plants take the following actions: 

1.   Increase the frequency of control rod stepping tests from once every 31
     days to once every 7 days. 

2.   If rod stepping anomalies of a mechanical nature occur during these 
     stepping tests or during any normal rod stepping, the plant should be 
     shut down and the drive rod assemblies inspected. 

3.   If no rod stepping anomalies of a mechanical nature occur, the drive 
     rod assemblies should be inspected at the next scheduled outage. 

Westinghouse is inspecting the CRDMs of all affected plants under construc-
tion and will repair those assemblies with loose breech guide screws. West-
inghouse has developed a repair procedure for any breech guide screw that 
fails the inspection. The procedure consists of drilling another hole in the
breech guide screw at least 90 degrees from the existing hole, inserting a 
locking pin in the new hole and welding the locking pin in place. 

No specific action or written response is required by this information 
notice; however, if you have any question regarding this notice, please 
contact the Regional Administrator of the appropriate NRC regional office or
the technical contact listed below. 



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

Technical Contact:  I. Villalva, IE 
                    (301) 492-9007 

Attachments: 
1.   Drive Rod Assembly 
2.   List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013