ML082690571

NRC Information Notice No. 87-44: Thimble Tube Thinning in Westinghouse Reactors

ML082690571
Person / Time
Issue date:	09/16/1987
From:	Rossi C Office of Nuclear Reactor Regulation
To:
C HAWES
References
IN-87-044, NUDOCS 8709100056
Download: ML082690571 (5)

Text

SSINS No.: 6835 IN 87-44 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 16, 1987 NRC INFORMATION NOTICE NO. 87-44: THIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS Addressees:

All pressurized water reactor facilities employing a Westinghouse nuclear steam supply system (NSSS) holding an operating license or a construction permit.Purpose: This information notice is being provided to alert addressees to potential problems resulting from thimble tube thinning in Westinghouse reactors.

It is expected that recipients will review the information for applicability to their facilities and consider actions,-if appropriate, to avoid similar problems.

However, suggestions contained in this information notice do not constitute NRC requirements; therefore no specific action or written response is required.Description of Circumstances:

During the recent refueling outage at North Anna Unit 1,.eddy.current (EC)testing identified wall thinning on approximately 23 out of 50 thimble tubes.The wall degradation occurred on the.thimble tubes just above the lower core plate, between the lower core plate. and the fuel assembly guide tubes. Several thimble tubes with greater than 35% wall thinningiwere identified, with one thimble tube thinned as much as 49%.Discussion:

The movable incore neutron detectors travel within retractable thimble tubes.The thimble tubes normally extend (as indicated in Attachment 1)..from a 10-path transfer device, through the seal table, through the bottom of the reactor vessel, and into selected fuel assemblies., The thimble tubes are supported by guide tubes within the lower vessel region and the ;fuel.assemblies, and by high-pressure conduits between the reactor vessel and the seal table.The thimble tubes are sealed at the leading (reactor) end, but are open at.the 10-path transfer device to allow insertion of an incore neutron detector.8709100056 IN 87-44 September 16, 1987 Page 2 of 3 Mechanical high-pressure seals, located at the seal table, are used to seal the area between the thimble tube and the high-pressure conduit. This seal serves as a reactor coolant system (RCS) pressure boundary since the area between the thimble tube and the high pressure conduit is at RCS pressure.Consequently, a leak in a thimble tube results in degradation of the RCS pressure boundary by creating a path for reactor coolant to bypass the mechanical seal. In order to halt the flow of leaking reactor coolant, the manual isolation valve must be closed.As indicated, the thimble tubes are supported over most of their length.However, a small portion of the thimble tube is directly exposed to RCS flow. This exposed portion is between the top of the lower core plate and the bottom of the fuel assembly.

This region is approximately 18.4 to 34.8 mm in length, depending on the reactor type., It is believed that flow-induced vibration on this exposed portion causes fretting at the adjacent guide tubes.Undetected thinning of a thimble tube could lead to the development of a non-isolable leak and a corresponding loss of reactor coolant. As discussed previously, the manual isolation valve would-have to be closed to halt the flow of leaking reactor coolant. The. leaking coolant may create an environ-ment in the vicinity of the isolation valves too hazardous for personnel to enter.Leaking thimble tubes could result in degradation of the incore neutron moni-toring system. If not isolated, reactor coolant from leaking thimble tubes can flow into the 10-path transfer device, allowing coolant to flood the other thimble tubes originating from that device. This could result in rendering inoperable more than just the leaking tube.In addition to North Anna Unit 1, incore thimble'tube thinning and leakage has been detected at facilities in France and Belgium. In this country, leaks in thimble tubes are known to have occurred at Salem Unit 1. In Licensee Event Report (LER)81-028, Public Service Electric &,Gas Co.'(PSE&G) reported that three incore thimble tubes were known to have developed leaks because of fretting.

One of these leaks resulted in the flooding of all six 10-path transfer devices, partially or completely flooding all the thimble tubes in the reactor. In addition, thinning has been detected on the Farley thimble tubes.At North Anna Unit 1, the proposed corrective action was to retract selected thimble tubes approximately 2 inches. This would move the thinned area out of the region of high turbulence.

In addition, the thimble tube that experi-enced the most degradation will be taken out of service by closing the corre-sponding isolation valve.

IN 87-44 September 16, 1987 Page 3 of 3 No specific action or written response is required by this information notice.If you have any questions about this matter, please contact the Regional Administrator of the appropriate regional office or this office.Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contact: Jack Ramsey, NRR (301) 492-9081 Attachments:

1. Typical Westinghouse Incore Neutron Monitoring System 2. List of Recently Issued NRC Information Notices 0A Attachment 1 TYPICAL WESTINGHOUSE INCORE NEUTRON MONITORING SYSTEM DRIVE UNITS CONDUITS Attachment 2 IN 87-44 September 16, 1987 LIST OF RECENTLY ISSUED INFORMATION NOTICES 1987 Information Date of Notice No. Subject Issuance Issued to 87-43 87-42 87-41 87-40 87-39 Gaps in Neutron-Absorbing Material in High-Density Spent Fuel Storage Racks Diesel Generator Fuse Contacts Failures of Certain Brown Boveri Electric Circuit Breakers Backseating Valves Routinely to Prevent Packing Leakage Control of Hot Particle Contamination at Nuclear Power Plants Inadequate or Inadvertent Blocking of Valve Movement Compliance with the General License Provisions of 10 CFR Part 31 Significant Unexpected Erosion of Feedwater Lines 9/8/87 9/4/87 8/31/87 8/31/87 8/21/87 8/17/87 8/10/87 8/4/87 All nuclear power reactor facilities holding an OL or CP.All nuclear power reactor facilities holding an OL or CP.All nuclear power reactor facilities holding an OL or CP.All nuclear power reactor facilities holding an OL or CP.All nuclear power reactor facilities and spent fuel storage facilities holding an NRC license or CP.All nuclear power reactor facilities holding an OL or CP.All persons specifi-cally licensed to manufacture or to initially transfer devices, containing radioactive material to general licensees, as defined in 10 CFR.Part 31.All nuclear power reactor facilities holding an OL or CP.87-38 87-37 87-36 OL = Operating License CP = Construction Permit