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Information Notice No. 83-54: Common Mode Failure of Main Steam Isolation Nonreturn Check Valves
SSINS No.: 6835 IN 83-54 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 August 11, 1983 Information Notice No. 83-54: COMMON MODE FAILURE OF MAIN STEAM ISOLATION NONRETURN CHECK VALVES Addressees: All nuclear power reactor facilities holding an operation license (OL) or construction permit (CP). Purpose: This information notice is provided as a notification of a potentially significant problem pertaining to common mode failure of main steam isolation nonreturn check valves at a nuclear power facility. The apparent cause of the failure was increased friction due to over tightening of the packing gland to preclude steam leakage. The increased friction was sufficient to prevent valve closing under no flow conditions and the licensee was unable to conclude that the valves would have operated as designed in the event of a steam line break with reverse flow. 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 June 8, 1983, Portland General Electric Company reported (LER 83-06) finding all four main steam nonreturn check valves stuck open after the Trojan Nuclear Plant had shut down and steam flow had been stopped. Each of the four main steam lines is equipped with a main steam isolation valve and a main steam isolation nonreturn check valve. One of the plant's design-basis accidents is the instantaneous rupture of the main steam line in any location and the failure of one of the main steam isolation valves to close. For this postulated event, no more than one steam generator will blow down if the nonreturn check valves operate properly. Thus, this failure of the four nonreturn check valves caused a reduction in the defense-in-depth of protection which could have led to a blowdown of more than one steam generator if a break were to occur upstream of the main steam isolation nonreturn check valve in one loop and the main steam isolation valve failed in another loop. All main steam isolation valves were operable during the period in question as shown by the periodic operating test records. The apparent cause of the failure was packing-induced friction which occurred when the packing gland was tightened to preclude steam leakage past the packing and valve stem. The packing was tightened following the last refueling outage and subsequent startup in July 1982. The valves were previously repacked in the Spring of 1981. The two-year interval between repacking (Spring of 1981 8308040027 . IN 83-54 August 11, 1983 Page 2 of 2 to June 1983) seems to be too long as the packing removed from the valves was dried and brittle. This condition contributed to the frictional forces that prevented the valves from closing. As part of their evaluation of the event, the utility performed an analysis to determine if the main steam nonreturn check valves would shut during a design basis event. Based on this analysis, the utility was unable to conclude that the reverse steam flow would provide sufficient torque to close the valves. A contributing factor to the valve failure was the fact that the check valves had been inadvertently omitted from the Inservice Testing program and hence, were not routinely tested for operability. To preclude reoccurrence, the utility is planning on performing the following corrective actions prior to reaching full power following the 1983 refueling outage: 1. The check valve packing will be replaced each refueling outage. 2. Possible gland follower binding will be eliminated by increasing the inside diameter of the gland follower to allow easier movement along the shaft. 3. A testing program will be provided to adjust packing before and after heatup to ensure free movement of the disk while minimizing steam leakage through the packing. 4. An engineering evaluation will be performed to determine alternative , corrective action should this condition persist, including reevaluation of valve operation with reverse steam flow. Additionally, the main steam isolation check valves have been added to the Inservice Testing program. No written response to this notice is required. If you have any questions regarding this matter, please contact 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: R. J. Kiessel, IE (301) 492-8119 Attachment: List of Recently Issued IE Information Notices .
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