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
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