United States Nuclear Regulatory Commission - Protecting People and the Environment

"Alternative Requirements for Fuel Assemblies in Design Features Section of Technical Specifications" (Generic Letter 90-02, Supplement 1)

July 31, 1992




The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement
to Generic Letter (GL) 90-02 to clarify the limitations on the
application of currently NRC-approved analytical methods and to withdraw
and replace the model technical specifications (TS) which were
recommended by GL 90-02, to be consistent with realistic reconstitution


GL 90-02 provides guidance for a line-item change for "Fuel Assemblies"
in Section 5, "Design Features," of the TS.  This change provides
flexibility in the repair of fuel assemblies containing damaged and
leaking fuel rods by reconstituting the assemblies.

Historically, licensees have concluded that the reconstitution of a fuel
assembly of an approved design with similar fuel rods does not represent
an unreviewed safety question and therefore have performed fuel
reconstitution under the provisions of 10 CFR 50.59.  In addition, the
staff has approved the use of stainless steel filler rods in fuel
assemblies at low-power core peripheral locations to replace fuel rods
damaged by baffle jetting or to reduce the neutron flux in the reactor
vessel in response to pressurized thermal shock considerations.  Previous
reconstitution practice also included a few cases involving the use of
filler rods or vacancies, which were limited to two per assembly and to
one or two assemblies in the core.

In May 1988, the staff approved a licensee's request to implement a TS
change that provided a greater flexibility in fuel reconstitution by
removing the peripheral location restriction on dummy rods and relaxing
the limitations on fuel rod substitutions in the "Design Features" of the
TS for that licensee.  The staff did not consider the limitations on the
application of approved analytical methods.  Later, the staff selected
this TS as part of a TS improvement for a model TS for the line-item
change to permit fuel reconstitution, and issued GL 90-02 on February 1,
1990.  The model TS provided with GL 90-02 permits the licensee 


Generic Letter 90-02, Supp. 1      -2-

to substitute Zircaloy-4 fuel rods with stainless steel filler rods or
vacant water spaces if the substitution is justified by cycle-specific
reload analyses using an NRC-approved methodology.  In the generic
letter, the staff defined "NRC-approved methodology" as any one of those
methods acknowledged in the final safety analysis report and applied to
support the original operating license or subsequent methodologies that
have been submitted to and accepted by the NRC staff as amendments to the
operating license.  However, the model TS were in error, since a broad
range of fuel configurations were identified that extend well beyond the
scope of applications that have been justified by the tests and analyses
for the fuel design and the design methods currently approved by the NRC.

GL 90-02

Upon receiving the industry's applications to implement GL 90-02, the
staff found that the generic letter had prompted many licensees to
incorrectly assume that their currently approved analytical methods could
be extended to proposed configurations permitted by the model TS in GL
90-02.  This resulted in the staff having to perform plant- and cycle-
specific TS changes to allow necessary fuel reconstitution, which lead to
increased staff and licensee expenditures.  The staff is issuing this
supplement to clarify the limitations on the application of currently
NRC-approved analytical methods used in the analysis of reconstituted

All analytical methods currently approved by the NRC were based upon
appropriate tests and analyses for specific fuel assembly configurations. 
Current NRC-approved methodologies apply to only a few of the
reconstituted fuel configurations allowed for by the model TS.  The
extreme range of the reconstituted fuel configurations allowed for by the
model TS is outside the scope of application of these NRC-approved
methodologies.  Applying these approved methods to configurations for
which they were not intended, such as generalized "dummy rod" and "vacant
water space" configurations, leads to safety concerns about the
conformance of the fuel assembly to specified acceptable fuel design
limits that are necessary to preclude the fuel cladding from failing. 
When responding to GL 90-02, licensees should have evaluated the
applicability of the test data used to derive the correlations and limits
for the departure from nucleate boiling ratio (DNBR) or for the critical
power ratio (CPR) for proposed configurations.  The licensees should also
have considered the effect on the mechanical design such as the effect of
differential thermal expansion on the proper seating of the fuel rod or
on the relaxation of the spacer spring which could lead to 

Generic Letter 90-02, Supp. 1      -3-

fretting wear.  In addition, the licensees should have analyzed changes
in the fuel design that affect the grid strength or the mass, stiffness,
and fundamental frequency of the fuel assembly to ensure that the seismic
and loss-of-coolant accident (LOCA) design loading conditions will not
cause any structural deformation that could prevent fuel coolable
geometry or control rod insertion.  The staff did not approve many of the
requests to implement the model TS in GL 90-02 because these requests did
not meet the analytical justification requirements discussed above.

To resolve the error that prompted many licensees to misinterpret the
applicability of existing methodologies in implementing the GL 90-02, the
staff presents the following modified guidance to provide flexibility in
reconstituting the fuel assemblies, based on the considerations mentioned


The reconstitution of a fuel assembly to replace damaged and leaking fuel
rods is not considered to be an unreviewed safety question if the
repaired fuel assembly constitutes a previously approved design.  The
licensee may perform such a reconstitution under the provisions of 10 CFR
50.59 without prior approval of the NRC staff if (1) an unreviewed safety
question does not exist, and (2) the reconstituted fuel does not require
a change to the "Design Features" section of the TS.

The staff considers an NRC-approved methodology to be any methodology
that the NRC staff has explicitly approved in a written safety
evaluation, or a plant-specific technical specification basis.  That NRC-
approved methodology must be used only for the purpose and the scope of
application specified in the reviewed document as approved or modified in
the NRC approval documentation.  In general, the scope of application for
generic methods is limited to fuel configurations that are represented by
fuel assembly test configurations used to validate an approved

In a few cases, the NRC has approved the use of a fuel assembly with one
or two filler rods at an interior core location for specific reload core
designs.  Such fuel loadings were justified on the basis of the
applicability of fuel test configurations with unheated rods (simulating
control rod guide tubes) and/or clearly conservative analytical methods
for the reconstituted fuel assembly.

Where filler rods fuel configurations are to be proposed for use, the
staff encourages licensees and fuel vendors to submit generic topical
reports that justify the specified fuel configurations 


Generic Letter 90-02, Supp. 1       -4-

with filler rods and that define and justify the analytical methods for
core analysis to support fuel reconstitution.  Applicable data from fuel
assembly tests must support the specified fuel configurations to the
extent normally required for the nuclear, thermal-hydraulic, and
mechanical design of a new fuel type.  The justification of the core
analysis methods must address the effect on core-wide analyses of
permissible core configurations with the reconstituted fuel.  The staff
has issued the drafts of the new Standard Technical Specifications (STS)
for public comment, including the following specification for fuel
assemblies under the Design Features Section:

      The reactor shall contain [  ] fuel assemblies.  Each 
      assembly shall consist of a matrix of zircaloy clad fuel rods with
      an initial composition of natural or slightly
      enriched uranium dioxide as fuel material[, and water
      rods].  Limited substitutions of zirconium alloy or
      stainless steel filler rods for fuel rods, in accordance
      with NRC-approved applications of fuel rod configura-
      tions, may be used.  Fuel assemblies shall be limited to
      those fuel designs that have been analyzed with applica-
      ble NRC staff-approved codes and methods, and shown by
      tests or analyses to comply with all fuel safety design 
      bases.  A limited number of lead test assemblies that
      have not completed representative testing may be placed
      in non-limiting core regions.

Licensees may propose this modification as a line-item improvement to
accommodate limited fuel reconstitution based on NRC-approved generic
topical reports or may develop similar plant-specific specifications, if
needed.  The NRC has not reviewed topical reports to justify a
generalized fuel configuration with filler rods.  Therefore, the staff
will accept this TS but not the model TS described in GL 90-02.  Although
the staff does not require that licensees withdraw previously approved
amendments to the TS of that form, licensees, in the future, should
interpret the phrase "NRC-approved methodology" in such TS in accordance
with the guidance of this supplement to GL 90-02.  Licensees planning to
submit future TS amendments to permit fuel reconstitution with filler
rods are encouraged to ensure that the analytical methods to support
these amendments are approved well in advance of the operational need.


The NRC staff has determined that core alterations performed in
accordance with the guidance provided in Generic Letter 90-02 do not
ensure acceptable protection against loss of integrity of the fuel
cladding.  General Design Criterion (GDC) 10 requires that 


Generic Letter 90-02, Supp. 1      -5-

the reactor core be designed in accordance with the specified acceptable
fuel design limits (SAFDL).  This Supplement 1 to GL 90-02 clarifies
that, when revising their TS to permit fuel reconstitution, licensees
need to justify the applicability of existing NRC-approved methodology or
develop a modified methodology which is applicable for the safety
evaluation, in order to ensure that proposed configurations of
reconstituted fuel assemblies conform to the SAFDL.  Although this
supplement contains a change in staff position with respect to meeting
the analytical requirements for revising TS on fuel reconstitution,
licensees are not required by this supplement, or by GL 90-02, to change
their TS.  However, the staff has concluded that this change in staff
position is a backfit which is necessary in order to ensure compliance
with GDC 10.  The basis for the determination is set forth in the
preceding discussion of this supplement.  Accordingly, pursuant to 10 CFR
50.109(a)(4)(i) a backfit analysis is not required.

This supplement does not seek to collect any information, and hence, the
Paperwork Reduction Act does not apply.

No response is required to this letter.  If you have any questions
regarding this matter, please contact the technical contact listed below. 


                                  James G. Partlow
                                  Associate Director for Projects
                                  Office of Nuclear Reactor Regulation

List of Recently Issued NRC Generic Letters

Technical Contact:  Laurence E. Phillips, NRR
                    (301) 504-3232

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