Information Notice No. 81-28: Failure of Rockwell-Edward Main Steam Isolation Valves
SSIN No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
September 3, 1981
Information Notice No. 81-28: FAILURE OF ROCKWELL-EDWARD MAIN STEAM
Description of Circumstances:
There have been several recent mechanical failures of the "Rockwell-Edward
Flite Flow Stop Valve," a "Y" pattern globe valve made by Rockwell
International, used for main steam isolation valves (MSIV) at some BWR
facilities. Operating BWR facilities using this valve include Brunswick Unit
1 and 2, Cooper, Duane Arnold, Fitzpatrick, Hatch Unit 2, and Vermont
Yankee. Of the seven total reported mechanical failures, five occurred at
Brunswick Unit 2 and one each occurred at Brunswick Unit 1 and Hatch Unit 2.
The valve components that have failed are shown in Figure 1. The piston
assembly is attached to the main disk (2) by thread engagement and then
restrained from unwinding by pin (4). The stem disk (1) is also attached to
the stem (6) by thread engagement tightened to 1050 ft-lb torque
specifications with an antirotation restraint provided by pin (3). The valve
is closed primarily by spring forces. The valve is installed so that reactor
steam pressure works to seat the main disk. The stem disk provides pressure
equalization action to reduce overseat load for opening the main disk
against system pressure.
The failures that have occurred involved a mechanical separation of valve
internals. This separation was either at the stem-to-stem-disk threaded
connection or at the main-disk-to-piston threaded connection. Either failure
permits the main disk to be free of the stem. The failure in either mode
results from failure of the threaded connection, which in some cases is
caused by vibration-induced rotation of the disk so that it becomes
disconnected from its threaded mate. Such rotation is believed to be caused
by or aided by the propensity of steam flow to produce vibration and to
create turning forces on valve internal components when antirotation
restraint is inadequate due to a failed or missing pin. Other contributing
causes are believed to result from reassembly of the valve after disassembly
at the site; such reassembly may have included inadequately torqued
connections and failure to properly install the pins. Also, an examination
of spare parts at the Brunswick site showed that the thread dimensions on
the stems and stem disks did not meet drawing tolerances.
The individual failures are described below in chronological order.
1. In January 1976, the Brunswick Unit 2 "D" steam line inboard valve main
disk separated from the piston. There was no evidence that pin (4) was
September 3, 1981
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2. On January 30, 1979, the Brunswick Unit 2 "A" steam line inboard valve
stem disk separated from the stem. A square pin was used in the round
hole at point (3). The corners of the pin experienced high stress,
thereby causing cracking of the pin.
3. On January 15, 1981, the Brunswick Unit 2 "C" steam line outboard valve
main disk separated from the piston. There was no evidence that pin (4)
was ever inserted. This was deduced from finding the hole not deformed
and no sign of a plug weld.
4. On March 5, 1981, the Hatch Unit 2 "A" steam line inboard valve stem
disk separated from the stem. Pin (3) was not fully inserted.
5. On March 30, 1981, the Brunswick Unit 1 "C" steam line outboard valve
stem disk separated from the stem. Pin (3) failed and was not
6. On July 2, 1981, the Brunswick Unit 2 "C" steam line inboard valve stem
disk separated from the stem. Pin (3) was not properly installed.
7. On July 18, 1981, the Brunswick Unit 2 "D" steam line inboard valve
main disk separated from the piston. Pin (4) was not fully inserted.
These failures have raised concerns regarding (1) the capability of the
valve to perform its required safety function and (2) increased challenges
to safety systems. However, it is noted that the failures to date have
resulted in the main disk going closed (i.e., not cocking open) with some
uncertainty only as to its leak tightness.
Detailed investigation of the July failures at Brunswick Unit 2 led to the
preliminary findings of possible excessive vibrations on valve internals
from steam flow turbulences created by the piping direction changes. In
addition, there was evidence of loose thread connections. Further
investigations and evaluations are currently in progress. Preliminary
corrective actions by the licensee include increasing the stem pin size from
5/16-inch to 3/8-inch, using three pins instead of one or two pins, and
increasing the hole depth 1/8-inch into the stem. In addition, corrective
actions for the main-disk-to-piston connection include adding an extra pin
(of the same 1/2-inch size) and increasing the hole depth by 1/8-inch.
This information is provided as notification of a potentially significant
matter that is still under review by the NRC staff. In case the continuing
NRC review finds that specific licensee actions would be appropriate, an IE
circular or bulletin may be issued. In the interim, we expect that licensees
will review this information for applicability to their facilities.
No written response to this information is required. If you need additional
information regarding this matter, please contact the Director of the
appropriate NRC Regional Office.
1. Figure 1
2. Recently issued IE Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015