United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 88-09: Thimble Tube Thinning in Westinghouse Reactors

                                                            OMB No.: 3150-0011
                                                            NRCB 88-09


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

                                  July 26, 1988


NRC BULLETIN NO. 88-09:  THIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS


Addressees:

All holders of operating licenses or construction permits for Westinghouse 
(W)-designed nuclear power reactors that utilize bottom mounted instrumenta-
tion.

Purpose:

The purpose of this bulletin is to request that addressees establish and 
implement an inspection program to periodically confirm incore neutron 
monitoring system thimble tube integrity. 

Description of Circumstances:

The incore neutron monitoring system thimble tubes extend 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 sealed at 
the leading (reactor) end, but are open at the 10-path transfer device to 
allow insertion of an incore neutron detector during flux mapping.  By design, 
the thimble tubes, over most of their length, serve as a portion of the 
reactor coolant system (RCS) pressure boundary.  (A description of a typical 
incore neutron monitoring system is contained in NRC Information Notice No. 
87-44, "Thimble Tube Thinning in Westinghouse Reactors," dated September 16, 
1987.)  Thus, wear of the thimble tubes results in degradation of the RCS 
pressure boundary and can also create a potentially non-isolable leak of 
reactor coolant.  Furthermore, thimble tube thinning could result in multiple 
thimble tube failures beyond a facility's design basis during flux mapping 
operations or a transient event.

As discussed in NRC Information Notice No. 87-44, Supplement 1, "Thimble Tube 
Thinning in Westinghouse Reactors," dated March 28, 1988, thimble tubes are 
experiencing thinning as a result of flow-induced vibration.  Thimble tube 
wear has generally been detected at locations associated with geometric 
discontinuities or area changes along the flow path (such as areas near the 
lower core plate, the core support forging, the lower tie plate, the upper tie 
plate, and the vessel penetration).  Many licensees have detected thimble 
wear.  





8807200246
.                                                                 NRCB 88-09
                                                                 July 26, 1988
                                                                 Page 2 of 4


There have also been several instances (both foreign and domestic) of thimble 
tubes experiencing leaks.  In addition, a review of the available data 
indicates that most leaks have occurred during flux mapping (while either 
inserting or retracting the probe).  

Discussion: 

The amount of vibration the thimble tubes experience 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, the thickness of the thimble tube, and the moment of inertia of the 
thimble tube.  A review of the available data indicates that it is not 
possible to accurately predict thimble wear rates.  Thus, it appears that the 
only effective method for determining thimble tube integrity is through 
plant-specific inspections and periodic monitoring. 

There are currently no inservice inspection or testing requirements for 
thimble tubes.  The NRC staff believes that this may have resulted in 
significant thimble tube degradation having gone undetected, creating a 
condition that may be adverse to safety.  To ensure that addressees are in 
compliance with General Design Criteria 14 "Reactor Coolant Pressure Boundary" 
of 10 CFR 50, Appendix A and to minimize (through early detection of thimble 
tube thinning) the likelihood of a potentially non-isolable leak of reactor 
coolant, the NRC staff requests that addressees perform the actions described 
below. 

Actions Requested:

1.   Each addressee is requested to establish an inspection program to monitor 
     thimble tube performance.  This inspection program should include:

     ø    the establishment, with technical justification, of an appropriate 
          thimble tube wear acceptance criterion (for example, percent 
          through-wall loss).  This acceptance criterion should include 
          allowances for such items as inspection methodology and wear scar 
          geometry uncertainties.

     ø    the establishment, with technical justification, of an appropriate 
          inspection frequency (for example, every refueling outage).

     ø    the establishment of an inspection methodology that is capable of 
          adequately detecting wear of the thimble tubes (such as eddy current 
          testing).

2.   Addressees are requested to implement the inspection program in 
     accordance with the following schedule, as applicable.  In addition, 
     addressees are requested to take appropriate corrective actions (such as 
     isolation or replacement) should a thimble tube fail to meet the 
     acceptance criterion established in Item 1 above.
.                                                                 NRCB 88-09
                                                                 July 26, 1988
                                                                 Page 3 of 4


     a.   All holders of operating licenses which are in their first fuel 
          cycle and all holders of construction permits - at their first 
          refueling outage.

     b.   All holders of operating licenses that (prior to receipt of this 
          bulletin) had already established an inspection program to monitor 
          thimble tube integrity consistent with that requested in Item 1 
          above and, based upon the results of the last inspection, took ap-
          propriate corrective actions for thimble tubes that failed to 
          satisfy the established acceptance criterion - in accordance with 
          their established inspection frequency. 
          
     c.   All holders of operating licenses that (upon receipt of this 
          bulletin) are in a cold shutdown that is expected to be of 
          sufficient duration to allow inspections of the thimble tubes to be 
          performed - prior to restart.

     d.   All others - prior to restart from the next cold shutdown that is of
          sufficient duration to allow inspections of the thimble tubes to be 
          performed (but no later than restart from the next refueling outage 
          scheduled to commence after receipt of this bulletin).

Reporting Requirements: 

1.   Records generated during the development of the inspection program re-
     quested by Item 1 of Actions Requested, as well as the results of the 
     inspections requested in Item 2 of Actions Requested, shall be documented 
     and maintained in accordance with plant procedures.

2.   Addressees who cannot meet the schedule described in Item 2 of Actions 
     Requested are required to submit to the NRC an alternative schedule with 
     justification for the requested deviation.

3.   Addressees are required to, in accordance with the following schedule, as
     applicable, submit a written response that a) confirms that an inspection
     program consistent with that requested in Item 1 of Actions Requested has
     been established and b) confirms that inspections of the thimble tubes 
     have been performed and that appropriate corrective actions were taken. 
     
     a.   Addressees implementing the inspection program in accordance with 
          Parts a, c, or d of Item 2 of Actions Requested - within 30 days of 
          completion of the thimble tube inspections.

     b.   Addressees implementing the inspection program in accordance with 
          Part b of Item 2 of Actions Requested - within 90 days of the 
          receipt of this bulletin.

.                                                                 NRCB 88-09
                                                                 July 26, 1988
                                                                 Page 4 of 4


4.   Regarding thimble tube degradation that is detected, addressees are re-
     minded of the reporting requirements of 10 CFR 50.72 and 10 CFR 50.73. 
     For example, 10 CFR 50.72 requires a report for "... any event, found 
     while the reactor is shut down, that, had it been found while the reactor
     was in operation, would have resulted in the nuclear power plant, 
     including its principal safety barriers, being seriously degraded or 
     being in an unanalyzed condition that significantly compromises plant 
     safety" and 10 CFR 50.73 requires a report for "... any event or 
     condition that resulted in the condition of the nuclear power plant, 
     including its principal safety barriers, being seriously degraded..."

Although not required by this bulletin, addressees are encouraged to work col-
lectively to address technical concerns associated with this issue (such as 
development of an appropriate thimble tube acceptance criterion and an appro-
priate inspection frequency), as well as to share thimble tube operational 
experience and inspection results. 

The written reports required above shall be addressed to the U.S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C.  20555, 
under oath or affirmation under the provisions of Section 182a, Atomic Energy 
Act of 1954, as amended.  In addition, a copy shall be submitted to the ap--
propriate Regional Administrator.  

This request is covered by Office of Management and Budget Clearance Number 
3150-0011 which expires December 31, 1989.  The estimated average burden as 
defined in 5 CFR 1320.7 is approximately 3000 man-hours per licensee response.  
Comments on the accuracy of this estimate and suggestions to reduce the burden 
may be directed to the Office of Management and Budget, Room 3208, New 
Executive Office Building, Washington, D.C.  20503, and to the U.S. Nuclear 
Regulatory Commission, Records and Reports Management Branch, Office of 
Administration and Resources Management, Washington, D.C.  20555.

If you have any questions about this matter, please contact one of the 
technical contacts listed below or the Regional Administrator of the 
appropriate regional office.




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  Jack Ramsey, NRR
                     (301) 492-1167

                     Shou-Nien Hou, NRR
                     (301) 492-0904 

Attachment:  List of Recently Issued NRC Bulletins


Page Last Reviewed/Updated Tuesday, July 23, 2013