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
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