Materials Performance and Aging Considerations
for Power Reactor (PR) and Research Reactor (RR)
Spent Nuclear Fuel in Storage
Robert L. Sindelar
June 2, 2010
IAEA International Conference for Management of Spent Fuel from
Nuclear Power Reactors
VIC - Austria, AS
Outline
Spent Nuclear Fuel
Research Reactor (RR) –
Aluminum Alloys
Power Reactors (PR) –
Zirconium Alloys
Al-Clad RR Plate Fuel
Zircaloy-Clad PR Fuel
RR Fuel in Wet Storage
PR Fuel in Wet Storage
RR Fuel Dry Storage Facility
PR Fuel in Storage Casks
Wet Storage Considerations
Water Quality
Dry Storage Considerations
Drying
Environmental Controls
Objective: Limit DDegradation
Cladding During Storage
Establish environmental conditions/controls so as to limit materials
degradation during interim storage
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Functions of a Storage System for Spent Nuclear Fuel
Thermal Performance
Radiological Protection
Confinement
Sub-Criticality
Retrievability
The fuel cladding is a critical confinement barrier and structural
material for fuel integrity for the storage period and also for safe fuel
retrieval and handling pending final disposition
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PR Fuel Cladding – Pre-Storage Condition
Hydride
Microstructure
Hydrogen in
Cladding
UO2 Fuel
Zircaloy
Cladding
Radiation
Damage
Microstructure
Oxide Scale
[Zirconia:
ZrO2]
PR fuel clad cross section showing hydride microstructure
in cladding [micrograph courtesy of EPRI]
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RR Fuel Cladding – Pre-Storage Condition
Maintains
Metallurgical
Fuel/Clad Bond
Up to 50 mm of
Oxide Scale
[Boehmite:
γ-AlO(OH) or
Al2O3•H2O]
MTR Design (Plate) Fuel
Radiation
Damage
Microstructure
Flux Zone Lower Boundary
with General Corrosion
MTR post-irradiation Irradiated fuel plate RR fuel clad cross section
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[ANL micrograph]
Zirconium Alloys – Wet Storage
Formation of passive layer of ZrO2 leads
to very slow corrosion, especially at
pool temperatures
US experience: 200,000 assemblies in
water pool storage with no failures
Water controls are specified for water
activity control
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Aluminum Alloys – Corrosion with Poor Water Quality
Corrosion
Nodules
In
Fuel
Region
Edge
Deterioration
Through-Clad
Pit
Side Plate
Corrosion
Fuel Plate
Side Plate
Side Plate
Fuel
Plate
Crevice
Nodules
Side Plate
Blisters
Corrosion in
screw locations
Hole left by
missing screw
Various Corrosion Modes On Aluminum-Clad Plate Fuel
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Wet Storage – Water Quality
Water quality is defined by a set of parameters that are
used to characterize the water physical and chemical
conditions. It includes:
pH;
conductivity;
dissolved impurity species;
undissolved solids;
colloids;
organic substances;
biological organisms; and
temperature
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Aluminum Alloy Water Quality Parameter Limits
Recommended physical-chemical parameters, limits, and monitoring frequencies for water in fuel
decay and storage basins [from IAEA Water Quality Management document]
PARAMETER
VALUE (LIMIT)
MONITORING
FREQUENCY
4.5 to 7
weekly
< 10 μS/cm
weekly
< 5 mg/l
Every 6 months
Cu Concentration
< 0.1 mg/l
Every 6 months
Cl Concentration
< 0.1 mg/l
Every 6 months
Nitrate (NO3-), mg/l
< 10 mg/l
Every 6 months
Sulphate (SO42-), mg/l
< 10 mg/l
Every 6 months
Fe
< 1.0 mg/l
Every 6 months
Al
< 1.0 mg/l
Every 6 months
Temperature
< 45°C
monthly
Radioactivity level (*)
see note 1
weekly
pH
Conductivity
Solids
(*) Water Radioactivity level and the presence of radioisotope species should be measured each time a water sample is drawn or one time per week. A gamma scan is
recommended to measure the presence of radioisotopes that would have come from failed fuel (e.g. Cs-137). No specific limits are set. The presence of radioisotope
species should be evaluated on case-by-case basis. Measurement of the activity from filters and resin columns should be performed to detect the presence of leaking
fuel.
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Corrosion Surveillance for RR Fuel
Corrosion Surveillance Program
Periodic Removal of Fuel Clad Material Specimens
Evaluate Corrosion Modes and Rates of Corrosion
Compare to Basin Chemistry Records for Immersion Period
Program in INTERNATIONAL ATOMIC ENERGY AGENCY,
“Recommended practices for water quality management in
research reactors & spent fuel storage facilities,” IAEA
Nuclear Energy Series, to be published 2010.
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Fuel Dry Storage – Drying Considerations
PR Fuel
No zirconium oxyhydroxides
Drying: remove free water, care to cool slowly to avoid
formation of radial-oriented hydrides (embrittled
microstructure)
RR Fuel
Aluminum oxyhydroxides from reactor operation and
wet storage
Need high temperatures to remove Boehmite (~ 500°C)
ASTM Drying Guide: C1553-08
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Dry Storage – Example of a Dryness Specification
Limit Water in Sealed Canister for Hydrogen Deflagration or Pressure
Concern
Corrosion reaction producing Hydrogen:
2 Al  4 H 2O  Al 2O3  H 2O  3H 2
Avoid Deflagration Hazard of H2 in Air
Limit of 4% H2: Wwater (ml) = 38.7 Volume of canister (m3)
Avoid High Pressure
FW = 292505
V
PH
2
273.15 + T
FW is free water in ml
V is canister volume in m3
PH2 is H2 pressure in atmospheres
T is temperature in °C
Limit Water in Sealed Canister for Materials Degradation Concern
Avoid Corrosion of Fuel and Internals
Aluminum 6061 Exposed to
250 °C Water Vapor for Six
Months (Initial Relative
Humidity 100%)
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PR Fuel Dry Storage Degradation Phenomena
Normal Operation Degradation Phenomena:
Creep: Limit temperature to avoid 1% creep strain to avoid creep rupture
Hydride Embrittlement: Avoid formation of brittle hydride microstructure
Delayed Hydride Cracking: Avoid high H content and stress
Off-Normal Degradation Phenomena:
Ingress of air and/or water detrimental to cladding:
Cladding corrosion – additional wastage; additional hydrogen build-in
Fuel swelling - oxidation to U3O8 can split cladding
Key Reference: R.E. EINZIGER, et. al.,“Data Needs for Storage and
Transportation of High-Burnup Fuel,” Radwaste Solutions, March/April
2005
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RR Fuel Dry Storage Degradation Phenomena
Normal Operation Degradation Phenomena:
Creep: Limit temperature to avoid plate deformation
Diffusion Through Cladding: Limit temperature to avoid breakthough
Off-Normal Degradation Phenomena:
Ingress of air and/or water detrimental to cladding:
Vapor corrosion of aluminum alloys is significant at high temperatures
Vapor corrosion of aluminum-based fuel is similar to aluminum alloys
Reference: R.L. SINDELAR, et. al., “Acceptance Criteria for Interim Dry
Storage of Aluminum-Alloy Clad Spent Nuclear Fuels,” March 1996;
IAEA Wet and Dry Storage Guide for RR Fuel: in preparation
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Dry Storage – Creep Analysis of RR Fuel
Parametric Analysis
Finite Element Model (ABAQUS)
of MTR Plates
Coble Creep Deformation
Mechanism
Grain Size/Temperature (100350°C)/Time (to 10,000+ years)
Deformation Targets
Plate Slump of 0.1 inches
Plate Slippage of 0.1 inches
from Slot
Ref. WSRC-TR-95-0121, SRNL, 1995
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Dry Storage – Vapor Corrosion Tests for RR Fuel
Vapor Corrosion Testing of Al-Clad and U-Al Materials: Al1100, Al5052, Al6061, U-Al Alloys
 Water Vapor/Air/Temperature (to 250°C) with & without Gamma Radiation
Capsule Test Configuration
Surface Oxide Morphology
Al1100, 1 Week Exposure
Corrosion function of (t, T, RH, alloy) + radiation
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Summary
Imperative for Interim Storage: Limit DDegradation to Fuel to
Enable its Safe Retrieval
Initial Condition of Fuel from Reactor Operation and Post-Operation
History
Wet Storage Controls
PR Fuel Storage in Water is Not Limited
RR Fuel Storage in Water Needs Water Quality Controls
Dry Storage Controls
Limit Moisture in a Dry Storage System to Limit Corrosion and
Pressurization in Sealed Systems; Drying Practices for Specific Fuel Type,
Fuel Condition, Canister System to Limit Remaining Water
Environmental Parameters to Limit Degradation Phenomena
With Appropriate Controls, Either Wet or Dry Storage Systems
Can be Successful for Extended Interim Storage for PR and RR
Spent Nuclear Fuel
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