Fuel Behavior in Long-term Management of Spent Light-Water Reactor Fuel International Conference on...

Fuel Behavior in Long-term Management of Spent Light-Water Reactor Fuel

International Conference on Management of Spent Fuel from Nuclear Power ReactorsMay 31 – June 4, 2010

Albert MachielsSenior Technical Executive

2© 2010 Electric Power Research Institute, Inc. All rights reserved.

Topics

Managed Storage

Structural Materials Ageing

Dry Storage & Transportability of Spent FuelThermal CreepHydriding

Re-orientationDelayed Hydrogen Cracking

Summary/Discussion


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?


LWR Power Block


LWR Power Block

Managed Storage


LWR Power Block

Managed Storage

Geologic Repository


LWR Power Block

Managed Storage

Geologic Repository

FR Power Block


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


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)


Dry Storage & Transportability of Spent Fuel



Over 1250 Casks Loaded in the US



• 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


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)]


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)]

• Driving Forces

– (Peak) Temperature

– Cladding stresses


Thermal Creep

• Prevention of creep rupture: limit diametric creep strain to < 1%


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


Initial Hydride Morphology


Hydride Morphology (after heating to ~400°C and cooling under “low” stresses)


Hydride Morphology (after heating to ~400°C and cooling under “moderate” stresses)


Hydride Morphology (after heating to ~400°C and cooling under “high” stresses)


Impact on Cladding Mechanical Properties

Impact of Re-orientation

on Room Temperature

Mechanical Properties


Hydride Re-orientation – Impact on Cladding Mechanical Properties

Mechanical Properties

vs. Testing Temperature


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


When Does It Matter?

• Normal Conditions– Maintaining high

level of cladding integrity

– No substantial alteration of normal assembly geometry

0 1 0 2 0 3 0 4 0 5 0

1 0 0

1 5 0

2 0 0

Time (yrs)

Hoo

p S

tres

s (M

Pa)

200 MPa Without Creep



200 MPa With Creep

150 MPa With Creep

100 MPa With Creep


When Does It Matter … Not?

• Accident Conditions

– Maintaining subcriticality

• Transportation applications


When Does It Matter … Not?

• Accident Conditions

– Maintaining subcriticality

• Transportation applications

• Risk Information

• Criticality Safety Information

•Impact of misloading

•Impact of fuel reconfiguration


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)


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 long-term (dry) storage be sustainable?

– Ageing management for structural materials

– Transportability and transportation safety

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