Technical Influence of High Burnup UOX
and MOX Water Reactor Fuel on Spent
Fuel Management
(IAEA Consultancy)
R.E. Einziger (USNRC) and Z Lovasic (IAEA)
Presented at
International Conference on Management of Spent Fuel from Nuclear Power
Reactors
May 31 - June 4, 2010
Vienna, Austria
Main Contributors
• W. Goll (AREVA NP GmbH, Germany) - Chair;
• A. Kumar (Bhabha Atomic Research Centre,
India);
• S. Kusuno (Institute of Applied Energy, Japan);
• N. Tikhonov (Federal State Unitary Enterprise
Leading Institute, Russian Federation);
• P. Cook (British Nuclear Group, United Kingdom);
• J. Kessler (Electric Power Research Institute, United States of
America);
• R. Einziger (NRC, United States of America).
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Purpose of the Work
• Compile data on high burnup UOX and
MOX fuels
• Evaluate Potential Influence of high
burnup UOX and MOX on spent fuel
management
• Make countries aware of the technical
ramifications of the fuel changes for the
back-end of the fuel cycle.
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Assumptions in the
Study
• Only MOX and UOX were evaluated.
• Only zirconium alloy clad oxide fuels
have been considered.
• Issues will be identified in general.
Solutions will not be pursued.
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UOX Fuel
• The uranium is enriched up to 5% in
PWR and BWR fuel and natural to
slightly enriched in HWR fuel.
• Most fuel rods are filled with helium
gas
• The PWR Zircaloy-4 was
metallurgically treated in such a way
as to form circumferential hydrides
during irradiation, while the BWR
Zircaloy-2 had a random grain
texture.
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The major differences
between the high and
low burnup LWR fuel
•
•
•
•
Fissile content is higher
Fission product content is higher,
Fission gas pressure is higher,
Cladding mechanical properties are
different,
• Fuel rim effect is greater.
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MOX Fuel and UOX Fuel
Comparison
• Pellet and rod design of MOX fuels is generally
similar to UO2 fuels
• Increased MOX internal rod pressure
• The mechanical design of MOX fuel assemblies
is similar
• Some MOX assembly designs include
additional water rods to improve moderation
• Rod placement in assembly
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Fuel Characteristics not
Considered in Evaluation
• Unimportant
– Cladding oxidation
– CRUD
– Pellet- cladding gap
– Fuel rod bowing
• Minor influence
– Pellet fracturing
– Fuel oxidation
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What was considered
in the investigation
Neutronics
Fission gas release
Mechanical Properties of the Cladding
Pellet Rim
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Where high burnup affects the
backend fuel cycle?
Wet
Storage
Criticality
Dry
Storage
Transportation Disposal Reprocessing
L
L,W
L, W
L,W
L, W
L, W
L, W
Heat removal
L, W
L
Shielding
L,H, W
L, H, W L,H, W
L,H, W
L,H, W
Containment/Confinement
L?,
H?,W
L,H, W
L,H, W
L,H, W
L,H, W
Retrievability
L, H,W
L,H, W
L,H, W
L,H, W
L,H, W
L,H, W
L,H, W
Operations/construction
L,H, W
L = LWR fuel, H = HWR fuel, W = WWER fuel.
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Wet Storage;
Possible impacts of HBU UOX and
MOX fuels
• Upgrade of the pool facility with respect to heat
removal, pool cleanup systems
• Regulatory issues.
• Criticality - additional neutron poison material in
the pool water or in storage racks
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Dry Storage and
Transportation;
Possible impacts of HBU UOX and MOX
fuels
• Redesign of the cask heat removal and shielding systems,
• Decrease in the number of spent fuel assemblies that can
be placed into a single transport /storage cask,
• Increased decay time in the pool prior to placement in dry
storage,
• Redesign of the structural support for the spent fuel
assemblies.
• Lower maximum fuel temperature for MOX fuel
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Reprocessing;
Possible impacts of HBU UOX and
MOX fuels
• Redesign of some systems may be
required.
• Recalibration of radiometric
instrumentation
• Higher discharges into environment
and HLW stream
• Reprocessed MOX – additional
challenges due to lower Pu solubility
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Repository
Possible impacts of HBU UOX and MOX
fuels
• Higher source terms of the radionuclides
– potentially higher release to the groundwater,
– additional shielding during spent fuel transfer from
the transportation cask.
• Higher temperatures
– Smaller waste containers,
– longer decay times at the surface prior to loading
• Repository space may have to be increased or
decreased depending if combined with
reprocessing
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REPU and MOX
Fabrication
Possible impacts of HBU UOX and MOX
fuels
• Different isotopic concentration in recycled fuel
(less Pu-239, more Pu-238 and 240)
• Increased enrichment of REPU or an increased
amount of plutonium in MOX fuel is required to
meet the same burnup target,
• Increases in shielding may be required for
REPU and MOX fuel fabrication operations.
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Non-proliferation;
Possible impacts of HBU UOX and MOX fuel
• High burnup UOX, REPU, and MOX fuels
tend to be more proliferation-resistant,
because of the higher specific activity of
each of these fuel types and because of
less favourable fuel isotopics for
proliferation (less Pu-239 and more Pu240 and Pu-238).
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Conclusions
• Experience has indicated the safe
feasibility of using high burnup UOX and
MOX fuels in the reactor,
• It still appears that longer cycles, higher
radiation and heat load may enhance
some characteristics of SNF that could
require additional attention and specific
investigation in SNF management.
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