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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.
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