United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-20: ITT Grinnell Figure 306/307 Mechanical Snubber Attachment Interference

                                                            SSINS No.: 6835 
                                                            IN 83-20 

                               UNITED STATES 
                         WASHINGTON, D. C.  20555  

                               April 13, 1983 

Information Notice No. 83-20:   ITT GRINNELL FIGURE 306/307 MECHANICAL 
                                   SNUBBER ATTACHMENT INTERFERENCE 


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


This information notice is provided as a notification of a potentially 
significant interference problem pertaining to ITT Grinnell Figure 306/307 
mechanical snubber assemblies. An assembly consists of ITT Grinnell 
attachment clamps and a Pacific Scientific mechanical snubber. No specific 
action is required in response to this information notice, but it is 
expected that recipients will review the information for applicability to 
their facilities. 

Description of Circumstances: 

On January 19, 1983, Arizona Public Service Company reported (DER 82-82) an 
attachment interference between the ITT Grinnell attachment clamps and their
associated Pacific Scientific mechanical snubbers that preventeo the 
required lateral pipe movement in either direction. Prevention of binding is 
obtained by designing the snubber attachments to allow for angular movement 
at each end of the snubber. Typically, an angular cone of five to six 
degrees from the axis is called for in the design specifications. The 
utility considered this condition to be reportable because, if left 
uncorrected, the piping could be restricted from lateral movement, and the 
resulting reactive forces could cause binding of the snubber, thereby 
impairing its safety function. 

The utility's report, which is based on a November 16, 1982 letter from ITT 
Grinnell, does not highlight the affect that restricted lateral movement 
could have on the piping systems. Restricted motion could lead to cyclic 
loadings that were not part of the original design stress analysis. The 
magnitude of this loading depends on the amount of binding caused by the 
lateral loading on the snubber. Should it become great enough, the snubber 
would be damaged permanently and, quite likely, "freeze" (lock permanently 
in one position). Depending on the snubber position when it "freezes," the 
piping system could experience a stress reversal when the thermal loading is
removed. (It should be noted that the responses to IE Bulletin 81-01, 
"Surveillance of Mechanical Snubbers", indicate that the Pacific Scientific 
mechanical snubbers are vulnerable to "freezing" when subjected to lateral 
loads. This is particularly true with the smaller size snubbers.) 


                                                           IN 83-20 
                                                           April 13, 1983 
                                                           Page 2 of 2 

In their November 16, 1982, letter to the utility, ITT Grinnell stated: 

"Not all Figyre 306/307 mechanical snubbers are affected. Based on our 
engineering evaluation, we have determined that:  

1.   All mechanical snubbers produced and shipped by ITT Grinnell prior to 
     October, 1978 should be reviewed to assure sufficient clearance. 

2.   All mechanical snubbers produced and shipped by ITT Grinnell after 
     April 1980 are acceptable, based on design changes incorporated at that 

3.   Mechanical snubbers produced and shipped by ITT Grinnell between 
     October 1978 and April 1980 may exhibit a reduction of the included 
     angle cone of action. Particularly: 

     a.    All mechanical snubbers that incorporate welded rear brackets; 
     b.   Size 3 and 35 clamps. 

Both of the conditions identified above can be alleviated by slight 
grinding. The ends of the clamps may be ground to the ITT Grinnell specified 
minimum edge distance. The problems with the rear brackets/rear pivot fit 
can be resolved by grinding the welds, in the area of interference, to the 
ITT Grinnell cified minimum weld sizes. The only grinding required is that 
which will avoid interface. Both of these problems can be resolved readily 
without replacing material." 

On January 10, 1983, Commonwealth Edison reported (deficiency number 82-09) 
a similar problem at their Byron Station Units 1 and 2 and Braidwood Station
Units 1 and 2. In addition, it is known that modifications have been 
performed at both the Susquehanna and the McGuire sites. 

No written response to this notice is required. If you have any questions 
regarding this matter, please contact the Regional Administrator of the 
appropriate NRC Regional Office, or this office. 

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

Technical Contact: Richard J. Kiessel 
                   (301) 492-8119 

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