ML082690572
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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C.
20555 March 28, 1988 NRC INFORMATION NOTICE NO. 87-44, SUPPLEMENT 1:
THIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS Addressees:
All holders of operating licenses or construction permits for nuclear power reactors that employ a Westinghouse nuclear steam supply system (NSSS)..
Purpose:
This notice is provided as a supplement to Information Noti'ce No. 87-44 to alert recipients of additional information regarding thinning of the incore neutron monitoring system thimble tubes.
It is expected that recipients will review the information for applicability to their facilities and consider actions, as 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.
Background:
During the first refueling outage at Millstone Unit 3, Northeast Nuclear Energy Company performed inspections of all incore neutron monitoring system thimble tubes.
Indications of wear were detected on 27 out of 58 thimble tubes in various locations, with the worst-indications approaching 40% throughwall loss.
Durina the recent refueling outage at Beaver Valley Unit 1, Duquesne Light Company performed inspections of all 50 thimble tubes.
Of these, 47 had indications of wear detected in various locations, with the worst indications approaching 40% throughwall loss.
Discussion:
Over the past several months, the NRC has become aware of numerous licensees in the United States that have detected thinning of the incore neutron monitoring system thimble tubes as a result of flow-induced vibration.
As discussed in NRC Information Notice No. 87-44, the failure of a thimble tube (or tubes) results in a breach of the reactor coolant system pressure boundary and creates a potentially non-isolable leak of reactor coolant.
Near of the thimble tubes has been detected at locations generally associated with geometric discontinuities or area changes along the coolant flow path.
These have included areas near the lower core plate, the core support forcing, 8803210090
IN 87-44, Supplement 1 March 28, 1988 Pace 2 of 2 the lower tie plate, and the upper tie plate.
The NRC is aware of at least 20 facilities in the United States, as well as a large number of foreign faci-lities, where thimble tube thinning has been detected.
Thus, it appears that all facilities that employ a Westinghouse-designed NSSS are susceptible to thimble tube thinning problems.
In addition, there have been several instances (both foreign and domestic) of thimble tubes experiencing leaks as a result of throughwall wear.
As previously mentioned, thimble tube wear is the result of flow-induced vibration.
It appears that the amount of vibration the thimble tubes experi-ence is determined by such plant-specific factors as the gap distance from the lower core plate to the fuel assembly instrument tube, the amount of clearance between the thimble tube and the guide or instrument tube, the axial component of the local fluid velocity, and the moment of inertia of the thimble tube.
Several licensees have voluntarily instituted inspection programs to monitor thimble tube performance.
The NRC is currently considering whether additional action in this area is warranted.
The NRC is also aware that throughwall cracking of fuel assembly instrument tubes has been detected at foreign facilities.
This appears to be the result of the vibrating thimble tube impacting the instrument tube.
The NRC is currently gathering additional information to evaluate this anomaly.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical contact listed below or the Regional Administrator of the appropriate regional office.
Sharles E. Rossi, Director Division of Operational.Events Assessment Office of Nuclear Reactor Regulation Technical
Contact:
Jack Ramsey, NRR (301) 492-1167
Attachment:
List of Recently Issued NRC Information Notices
Attachment IN 87-44, Supp. 1 March 28, 1988 Page 1 of 1 LIST OF RECENTLY ISSUED NRC INFORMATION NOTICES Information Date of Notice No.
Subject Issuance Issued to 88-09 88-08 88-07 88-06 88-05 88-0A Reduced Reliability of Steam-Driven Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Governors Chemical Reactions with Radioactive Waste Solidification Agents Inadvertent Transfer of Licensed Material to Uncontrolled Locations Foreign Objects in Steam Generators Fire in Annunciator Control Cabinets Inadequate Qualification and Documentation of Fire Barrier Penetration Seals Cracks in Shroud Support Access Hole Cover Welds Lost or Stolen Gauges Safety Injection Pipe Failure 3/18/88 3/14/88 3/7/88 2/29/88 2/11/88 2/5/88 2/2/88 2/2/88 1/27/88 All holders of OLs or CPs for nuclear power reactors.
All NRC licensees generating or pro-cessing low level radioactive waste.
All NRC broad licensees and licensees authorized to possess byproduct material as sealed sources in teletherapy units or "self-contained" irradiators.
All holders of OLs or CPs for PWRs.
All holders of OLs or CPs for nuclear power reactors.
All holders of OLs or CPs for nuclear power reactors.
All holders of OLs or CPs for BWRs.
All NRC licensees authorized to possess gauges under a specific or general license.
All holders of OLV or CPs for nuclear power reactors.