United States Nuclear Regulatory Commission - Protecting People and the Environment

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

                                                    SSINS No.:  6835
                                                        IN 87-44

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C.  20555

                               September 16, 1987


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 thinning were 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.  





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                                                            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 experienced 
the most degradation will be taken out of service by closing the corresponding 
isolation valve.  

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                                                            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
Page Last Reviewed/Updated Friday, May 22, 2015