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HomeNRC LibraryDocument CollectionsGeneric CommunicationsInformation Notices1979

# Information Notice No. 79-11 - Lower Reactor Vessel Head Insulation Support Problem

```
IN79011 

                                May 4, 1979 

MEMORANDUM FOR:     B. H. Grier, Director, Region I 
                    J. P. O'Reilly, Director, Region II 
                    J. G. Keppler, Director, Region III 
                    K. V. Seyfrit, Director, Region IV 
                    R. H. Engelken, Director, Region V 

FROM:               Harold D. Thornburg, Director, RCI 

SUBJECT:            Information Notice No. 79-11 - LOWER REACTOR VESSEL 
                    HEAD INSULATION SUPPORT PROBLEM 

The subject Information Notice is transmitted for issuance on May 7, 1979. 
The Information Notice should be issued to all holders of Reactor Operating 
Licenses and Construction Permits.  Also enclosed is a draft copy of the 
transmittal letter for this purpose. 


                                        Harold D. Thornburg, Director 
                                        Division of Reactor Construction 
                                          Inspection 
                                        Office of Inspection and Enforcement

Enclosures:
1.   IE Information Notice
       No. 79-11
2.   Draft Transmittal Letter

CONTACT:  W. Laudan, IE 
          49-27551 
.

(Draft letter to all holders of Reactor Operating Licenses and Construction 
Permits) 

                                            Information Notice No. 79-11 

Addressee: 

This Information Notice is provided as an early notification of a possibly 
significant matter.  It is expected that recipients will review the 
information for possible applicability to their facilities.  No specific 
action or response is requested at this time.  If further NRC evaluations so
indicate, an IE Circular or Bulletin will be issued to recommend or request 
specific licensee actions.  If you have questions regarding this matter, 
please contact the Director of the appropriate NRC Regional Office. 


                                        Signature 
                                        (Regional Director) 

Enclosure:
IE Information Notice
  No. 79-11
.

                             UNITED STATES 
                      NUCLEAR REGULATORY COMMISSION 
                   OFFICE OF INSPECTION AND ENFORCEMENT 
                          WASHINGTON, D.C.  20555 
                                     
                                May 7, 1979 

                                            Information Notice No. 79-11 

LOWER REACTOR VESSEL HEAD INSULATION SUPPORT PROBLEM 

Virginia Electric and Power Company, North Anna Power Station NO. 1 reported
on November 8, 1978, a field modification pertaining to the lower vessel 
head insulation support system which was not seismically analyzed at the 
time of modification. A subsequent stress analysis indicated that the ASME 
Code allowable stress  and the yield stress for the operating base 
earthquake (OBE) on some of the instrument guide tubes were exceeded. 

For the North Anna No. 1 field modification, the lower vessel head 
insulation was supported by angle brackets that are clamped onto the 
gridwork to about 20 of the 53 instrument guide tubes.  This additional load 
on the guide tubes was not considered in the original stress analysis. 

The maximum stress occurs in the socket welds between the Thimble guide 
tubes and the Instrumentation nozzles that penetrate the vessel.  The 
maximum stress was about 70 percent of the ultimate and, therefore, 
violation of the pressure boundary would not be expected. 

VEPCO plans to modify the supporting arrangement by building a framework 
underneath the insulation, which will take the loading off the guide tubes. 

All holders of operating licenses and/or construction permits should be 
aware of the potential problem of exceeding the allowable stress and the 
yield stress for OBE. 

This Information Notice is provided as an early notification of a possibly 
significant matter.  It is expected that recipients will review the 
information for possible applicability to their facilities.  No specific 
action or response is requested at this time.  If further NRC evaluations so
indicate, an IE Circular or Bulletin will be issued to recommend or request 
specific licensee actions.  If you have questions regarding the matter, 
please contact the Director of the appropriate NRC Regional Office. 

No written response to this information notice is required.
```

*Page Last Reviewed/Updated Friday, May 22, 2015*

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