United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-28: Failure of Rockwell-Edward Main Steam Isolation Valves

                                                             SSIN No.: 6835 
                                                             Accession No.: 
                                                             8103300413    
                                                             IN 81-28      

                                UNITED STATES
                        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 
                                   ISOLATION VALVES 

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 
     installed. 
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                                                          IN 81-28         
                                                          September 3, 1981 
                                                          Page 2 of 2      

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 
     recovered. 

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. 

Attachments:
1.   Figure 1 
2.   Recently issued IE Information Notices

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