Fuel Behavior in Long-term
Management of Spent
Light-Water Reactor Fuel
International Conference on
Management of Spent Fuel from
Nuclear Power Reactors
May 31 – June 4, 2010
Albert Machiels
Senior Technical Executive
Topics
Managed Storage
Structural Materials Ageing
Dry Storage & Transportability of Spent Fuel
Thermal Creep
Hydriding
Re-orientation
Delayed Hydrogen Cracking
Summary/Discussion
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Issues for Consideration (from IAEA)
• (12) Can we confidently model current fuel and material
behaviour for long-term storage? What are the
technological criteria for ensuring that long-term storage
be sustainable?
• (26) How sustainable is storage for the long term?
• (28) Is there a possibility for an international consensus
on the future strategy for fuel management?
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LWR Power Block
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LWR Power Block
Managed Storage
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LWR Power Block
Managed Storage
Geologic Repository
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LWR Power Block
Managed Storage
Geologic Repository
FR Power Block
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LWR Power Block
Managed Storage
Geologic Repository
FR Power Block
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Managed Storage (continued)
• Interim nature: functional requirements such as retrievability
• Protection of public health and environment
– Safety (shielding, subcriticality, confinement/containment)
• Potentially long storage period: century scale
• Systems: passive or low-complexity active
•  Emphasis on materials degradation phenomena
– Security
• Material accountability (theft)
• Protection against malicious acts
– Public acceptance
• Public dislike for radioactivity, and especially wastes
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Structural Material Ageing
• Ageing Management
– Evolved in the context of “License Renewal
Application” for operating nuclear power plants beyond
their original licenses
• US Code of Federal Regulations, Title 10, Part 54
– Provides well-tested approach for successful
interactions between licensees and regulators
• Integrated plant assessment
• Time-limited aging analyses (TLAA)
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Dry Storage & Transportability of Spent Fuel
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Dry Storage & Transportability of Spent Fuel
Over 1250 Casks Loaded in the US
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Dry Storage & Transportability of Spent Fuel
• Scenario: Spent UOx Fuel in Dry Storage for 50-150
years followed by transportation
• Material issues:
– Normal Conditions
• Maintaining high level of cladding integrity
• No substantial alteration of normal assembly
geometry
– Accident Conditions
• Maintaining subcriticality
Emphasis on fuel rod cladding
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Material Degradation Issues
• Mechanisms
– Air Oxidation
– [Cladding Stress
Corrosion Cracking]
– Thermal Creep and Creep
Rupture
– Hydride Re-orientation
[and Migration] and
Impact on Cladding
Mechanical Properties
– [Delayed Hydrogen
Cracking (DHC)]
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Material Degradation Issues
• Mechanisms
– Air Oxidation
– [Cladding Stress
Corrosion Cracking]
– Thermal Creep and Creep
Rupture
– Hydride Re-orientation
[and Migration] and
Impact on Cladding
Mechanical Properties
– [Delayed Hydrogen
Cracking (DHC)]
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• Driving Forces
– (Peak) Temperature
– Cladding stresses
Thermal Creep
• Prevention of creep rupture: limit diametric creep strain to < 1%
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Thermal Creep
• However, any strain value is acceptable as long as local
cladding stress remains below yield strength
– Fuel rods are closed systems: Creep deformations tend to
be self-limiting
– Limiting peak temperatures are likely to be relatively high,
even taking annealing effects into account
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Initial Hydride Morphology
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Hydride Morphology (after heating to ~400°C
and cooling under “low” stresses)
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Hydride Morphology (after heating to ~400°C
and cooling under “moderate” stresses)
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Hydride Morphology (after heating to ~400°C
and cooling under “high” stresses)
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Impact on Cladding Mechanical Properties
Impact of Re-orientation
on Room Temperature
Mechanical Properties
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Hydride Re-orientation – Impact on Cladding
Mechanical Properties
Mechanical Properties
vs. Testing Temperature
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Delayed Hydrogen Cracking
Recent controversy about rate controlling
process for DHC propagation (cfr. Journal of
Nuclear Materials)
– Y.S. Kim et al.: “Precipitation First Model”
– Hydride precipitates at a crack upon
the imposition of a tensile stress
– Concentration gradient results in
diffusion of hydrogen to the crack tip
– M. P. Puls et al.: “Diffusion First Model”
– Stress gradient results in diffusion of
hydrogen to the crack tip
– Crack grows if hydrogen concentration
> solubility limit for hydride precipitation
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When Does It Matter?
200
200 MPa Without Creep
150 MPa Without Creep
100 MPa Without Creep
200 MPa With Creep
150 MPa With Creep
100 MPa With Creep
• Normal Conditions
Hoop Stress (MPa)
– Maintaining high
level of cladding
integrity
– No substantial
alteration of
normal assembly
geometry
150
100
50
0
10
20
Time (yrs)
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30
40
When Does It Matter … Not?
• Accident Conditions
– Maintaining subcriticality
• Transportation
applications
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When Does It Matter … Not?
• Accident Conditions
– Maintaining subcriticality
• Transportation
applications
• Risk Information
• Criticality Safety Information
•Impact of misloading
•Impact of fuel reconfiguration
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Summary
• Managed Storage
• Thermal Creep and Hydride Re-orientation in the context
of dry storage and transportability
– IAEA Coordinated Research Program (CRP) on Spent
Fuel Performance Assessment and Research (SPAR)
• TECDOC-1343 (SPAR-I)
• TECDOC-wxyz (SPAR-II): to be published
– EPRI 1015048 “Spent Fuel Transportation Applications
– Assessment of Cladding Performance: A Synthesis
Report” (2007)
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Discussion
• Can we confidently model current fuel and material
behavior for long-term storage?
– Qualified yes!
– Modeling of entire system is important
– Confirmatory surveillance/demonstration program
• What are the technological criteria for ensuring that longterm (dry) storage be sustainable?
– Ageing management for structural materials
– Transportability and transportation safety
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