United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-64: Reactivity Insertion Transient and Accident Limits for High Burnup Fuel


August 31, 1994

                               LIMITS FOR HIGH BURNUP FUEL


All holders of operating licenses or construction permits for nuclear power
reactors and all fuel fabrication licensees.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to recent information on high burnup fuel
performance that could affect previously approved fuel burnup limits and
enthalpy limits of highly exposed fuel.  It is expected that recipients will
review the information for applicability to their facilities and consider
actions, as appropriate, to minimize adverse effect on design and operation of
reload cores and to properly evaluate their 10 CFR Part 21 reporting
responsibilities if continuing evaluation indicates limits should be reduced.
However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required.

Description of Circumstances

Recent experimental data have suggested that high burnup fuel may be more
prone to failure during design-basis transients and reactivity insertion
accidents (RIAs) than previously thought.  Recent data on the relationship
between fuel failure enthalpy and burnup for pressurized water reactor fuel
rods tested in foreign experimental facilities indicate lower failure
initiation enthalpy thresholds (measured in differential cal/gm) than was
considered in the evaluation of currently approved fuel burnup limits.  The
NRC is closely following work to confirm the validity of the preliminary
results by verifying the conditions of the experimental tests.  The NRC is
also reviewing data reduction, data interpretation, and the implications of
the observed failure results at low enthalpy rise levels on the safety
evaluation of LWR design-basis transients and accidents.  The NRC is scheduled
to complete its review of the high burnup fuel data by the summer of 1995.
The results of this study may invalidate the basis for the previous staff
approvals of fuel design topical reports and fuel failure and fuel burnup

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August 31, 1994
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Recent information suggests that fuel rod damage during reactivity transients
can increase with high burnup.  Various investigators have reported a "rim"
effect, consisting of a high burnup peripheral region of the fuel pellet, in
which the solid fuel matrix has a small grain size and is surrounded by
fission gas bubbles (Ref. 1).  This porous microstructure could enable fuel to
disperse in powder form during transients that heat and expand the fission gas

A recent study for the Electric Power Research Institute (EPRI) was conducted
on fuel irradiated up to burnups of 60 GWD/MTU (Ref. 2).  The objective was to
evaluate the degradation of mechanical properties in cladding and assembly
structural components caused by oxidation, hydriding, and radiation
embrittlement.  While strength increases, ductility was found to be very low
and dependent on hydrogen content and fluence, both of which increase with
burnup.  This appears to contradict a previous belief that metal annealing at
operating temperatures would prevent radiation damage from accumulating after
some initial period.

Recent integral tests with high burnup fuel under reactivity transient
conditions have indicated that the bounding peak fuel enthalpy that may cause
fuel cladding failure decreases significantly with increasing fuel exposure.
Of the fuel rods tested, the one with the highest burnup (approximately
65 GWD/MTU) failed at an enthalpy level of only 30 cal/gm with a reactivity
insertion corresponding to less than 15 cal/gm; some fuel dispersal may be
associated with this failure.  An earlier integral test at the intermediate
burnup level of 32 GWD/MTU showed cladding failure at about 85 cal/gm
(Ref. 3).  If the scatter of data points for high burnup failure is similar to
that observed in lower burnup failure data, the recent cited data are not
conclusive regarding the enthalpy failure limit for a specific fuel rod.
However, the transient data, the declining cladding ductility with increasing
burnup, and the fuel pellet "rim" effect, when considered together, suggest
that the trend of these results may be real.

LWR fuel suppliers and reactor operators (licensees) may wish to consider this
preliminary information in planning for fuel cycle designs, particularly those
fuel design changes and longer core operating cycles which may approach or
exceed approved fuel burnup limits.  A higher burnup fuel may be required to
meet more restrictive enthalpy limits for fuel failure, fragmentation,
and dispersal than those permitted by the acceptance criteria of the standard
review plan (Ref. 4).  The NRC review of pending and future requests to extend
burnup beyond the limits previously approved for existing fuel designs (up to
approximately 60 GWD/MTU, fuel rod average) will more carefully consider the
adequacy of the reactivity insertion experimental data included or referenced
by such requests, until the implications of these high burnup experimental
data on licensing safety evaluations are resolved.

.IN 94-64
August 31, 1994
Page 3 of 3

On October 26, 1994, the NRC will present additional technical information on
this subject at the NRC 22nd Water Reactor Safety Information Meeting.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate NRC project

/s/'d by Robert F. Burnett/s/'d by AEChaffee/for

Robert F. Burnett, DirectorBrian K. Grimes, Director
Division of Fuel Cycle SafetyDivision of Operating Reactor Support
  and SafeguardsOffice of Nuclear Reactor Regulation
Office of Nuclear Material Safety
  and Safeguards

Technical contacts:  L. E. Phillips, NRR
   (301) 504-3232

   E. D. Kendrick, NRR
   (301) 504-2891

   J. Roth, NMSS
   (301) 415-7257

1.  References
2.  List of Recently Issued NRC Information Notices
3.  List of Recently Issued NMSS Information Notices
.Attachment 1
IN 94-64
August 31, 1994
Page 1 of 1


1.M. E. Cunningham, et al., "Development and Characteristics of the Rim
Region in High-Burnup UO2 Fuel Pellets," Journal of Nuclear Materials,
Vol. 188, page 19, June 1992.

2.G. P. Smith, et al., "Hot Cell Examination of Extended Burnup Fuel
from Calvert Cliffs-1,' EPRI TR-103302, November 1993.

3.P. E. MacDonald, et al., "Assessment of Light-Water-Reactor Fuel
Damage During a Reactivity-Initiated Accident," Nuclear Safety, Vol.
21 No. 5, September-October 1980.

4."NRC Standard Review Plan," NUREG-0800, Rev. 2, July 1981.

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