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Dry storage systems and aging management
H.Issard,
AREVA TN, France
IAEA TM 47934
LESSONS LEARNED IN SPENT FUEL MANAGEMENT
Vienna, 8-10 July 2014
AREVA TN
TN International
IAEA TM 47934 Lessons Learned in Spent Fuel Management Vienna, 8-10 July 2014 2
Summary
Dry storage systems and AREVA Experience
Aging management: Deployed systems / inspection
mitigation
New aging management program in the long term
Aging analysis for confirmation of safety of extended
interim storage
Conclusion
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AREVA Experience dry storage systems
Experience of used fuel storage designed by AREVA: Metal cask
Metal casks (TN 24):
� Dual purpose : storage and transport
� Forged carbon steel shell
� Bolted lids & metal seals
� Long Industrial Experience :first cask in 1985
� Monitoring: temperature, interlid pressure
� Inspections and Surveillance records : no
significant event
� Safety review for storage extension.
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AREVA Experience dry storage systems
Experience of used fuel storage designed by AREVA : canister system
»Canister based systems (Nuhoms®):
�Storage System components:
� Dry shielded canister
� Horizontal storage module
� Transfer cask
�Auxiliary equipment
�Long Industrial Experience : first system in 1989
�Inspections and Surveillance records : no significant event
�visual inspection
�monitoring: temperature
�Safety review for extension: lead canister inspection.
2014: AREVA has received the license for Nuhoms® MP 197HB transportation
cask, for the transport of high burnup fuels up to 62 GWd/MTU
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AREVA Experience dry storage systems
Experience of used fuel storage designed by AREVA : vaults
Vaults (Cascad type):
�Vaults: storage only
�storage pits. UNF are placed in leak-tight
welded canisters (single element or multi
element).
�Long Industrial Experience : in operation since
1990
�Storage conditions monitored
�Safety records : no significant event
�Safety review for extension
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AREVA Experience dry storage systems
►Leaking fuels
►Capsule Canister for Handling and Storage of Fuel Rod Capsules
►Dimension for handling and storage similar to a Fuel Assembly
Spacer plate
Bottom end piece
Transport
head
Fuel rod
capsules
Corner lining strip
Tie rod Transport head
Spacer plate
Corner lining strip
Tie rod
Bottom end piece
For use at a
PWR:
loading capacity
up to 108 Fuel
Rod Capsules
For use at a BWR:
loading capacity up to
33 Fuel Rod Capsules
• AREVA´s technology in
encapsulation of
defective Fuel Rods.
• Drying method : meet
the strict requirements of
maximum water content
for storage.
• Can guarantee a
reduction of
residual water
content << 1 g per
fuel rod.
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Operational Experience dry storage systems
Nuclear operators and cask vendors: broad experience in the world in used fuel storage
Aging management program and deployed systems / inspection mitigation
� Periodical inspections
• visual for containment corrosion
• Surveillance of air inlets and outlets
• dose rate
� Permanent monitoring
• Storage parameter measurements : temperature, inter-lid pressure
• gamma and neutron dose rate
� Inspections of content after cask opening
• Seal inspection, Moisture, Fuel integrity
� Time limited aging analysis : ex. creep or corrosion tests
� Analysis based on surrogates : ex. He build up
Lessons learned :
� No accident and no event,
� But very limited experience of cask opening
� Some aging issues need further justification (influence of drying)
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Dry storage feedback from inspections after cask opening
US – Post Irradiation Examination (PIE – in hot cell) of spent fuel
assemblies stored 15 years in Castor-V/21 cask
� no significant degradation to cask or FA’s
UK – PIE of PWR-UO2 fuel (58 GW d/tU) stored 20 years under air
� no change in the cladding oxide thickness
Japan – Post irradiation examination
� 2 MOX BWR rods irradiated to 20 GW d/tU were stored for 20 years in an air
capsule
• no release of fission gas/He and no change in fuel/cladding microstructure
� Integrity inspections at Tokai (with metal cask) and Fukushima-Daiichi (with TN24
cask) performed after 5 and 10 years of storage - 52 BWR assemblies (≈30GW
d/tU)
• No release of inner gas (Kr-85) and no defect observed on the spent fuel assemblies
(visual checking)
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Development of new aging management program
For years, a storage period of up to 40 years was considered as ‘long-term’ and sufficient in considering decisions and deployment of back-end fuel cycle and/or final waste management options.
Extension of dry storage : Some consider an extension of the storage duration significantly, even beyond one century.
Address the need to store used fuel with higher burn ups (62 Gwd/t for PWR or 70 Gwd/t for BWR ) and MOX fuel
Used fuel should be retrievable for further use
=> Safety of interim storage in the long term ; need of new aging management program
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Aging management to assure long term performance
Focus on ensuring primary safety functions
� Criticality control
� Confinement/containment
� Shielding
Focus on components providing safety functions
� Canister/cask and transport package= Primary for confinement/containment =
Can provide criticality control
� Concrete storage module = shielding
� Canister/cask internals and fuel = defense in depth for containment criticality
control
Focus on activities to provide assurance
� Monitoring
� Inspection
� Mitigation
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Aging management program to ensure long term performance
Aging management program (AMP) updated with results of new analysis of degradation of dry storage components:
� Cask or canister material
� concrete
� sealing system
� coatings
� neutron shielding materials
� neutron poison materials
Actions
� Tests, models
� Inspection program
� Mitigation
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Aging analysis for confirmation of safety for extended storage
Analysis of cask or canister material
� Main function : containment of used fuel.
� Submitted to ambient atmosphere and humidity.
� Most important degradation process : atmospheric Stress Corrosion Cracking
(SCC), or Chlorine induced SCC.
� Inspections performed on existing canisters have shown no signs of SCC.
� However, investigations are underway for Chlorine induced SCC evaluation for
welded canister system in marine environment: amount of Chlorine in ambient
atmosphere, assessment of deliquescence of salt, evaluation of corrosion
through inspection of existing storage systems, modelling corrosion and SCC.
� Mitigation techniques (stress reduction), coatings or material changes could
be considered for increasing weld durability.
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Aging analysis for confirmation of safety for extended storage
Inspection to support Analysis
� Inspection of cask or canister material for salt deposition
� Analysis Chlorine induced SCC
Tool carrier in use during training
SaltSmartTM device (Louisville solution, USA)SaltSmartTM delivry tool
in dry run on canister surface
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Aging analysis for confirmation of safety for extended storage
Analysis of concrete
� Main function: radiological shielding and physical protection for the
Canister against a wide range of postulated natural hazards.
� The NUHOMS® system provide an independent, passive system with
substantial structural capacity to ensure the safe dry storage of used fuel
assemblies.
� The long term concrete degradation is associated with temperatures and
radiation levels: chemical degradation, carbonation, corrosion of
embedded steel, coupled mechanisms, dry-out and thermal degradation of
mechanical properties. Results from nuclear and non nuclear industrial
experience and inspection are satisfactory.
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Aging analysis for confirmation of safety for extended storage
Analysis of sealing system
� For metal casks, the function of the metal seals is the
containment
� Long term issues are the corrosion of bolts and the
corrosion of metal seals.
� The behaviour of these components has been studied
in storage conditions with satisfactory results by CEA
(France) and CRIEPI (Japan). These studies cover long
term resistance (leak tightness tests) of metal seals
with Al or Ag outer jackets. Corrosion tests on such
seals have also shown good resistance .
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Aging analysis for confirmation of safety for extended storage
Analysis of neutron shielding materials
� Function : to protect the public and the operators form the neutrons radiations
� Industrial experience : no increase of the dose rate during storage.
� In-service neutron shielding ageing: irradiation or thermal oxidation processes.
� Accelerated aging tests: various temperatures and O2 partial pressure
� To predict the long term properties, a non-empirical model is applied, taking
into account the diffusion-limited oxidation.
• The model simulates very confidently weight losses (which are then converted into
hydrogen atoms loss) and oxidation profiles.
• good agreement between experimental results and simulated data.
0
5
10
15
20
25
30
35
90 100 110 120 130 140 150 160 170
Temperature (°C)
Hy
dro
ge
n lo
ss
(%
)
1 year
3 years
7 years
10 years
20 years
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Aging analysis for confirmation of safety for extended storage
Analysis of neutron poison materials
� Main function : to prevent criticality.
� Evaluation of creep : Industrial experience (casks in vertical position) has
shown no observation of creep of neutron absorbers due to control of
temperatures over extended storage times.
� Wet corrosion and blistering (for Boral, a porous neutron poison material)
• water entering pores in the material during loading
• Vaporization during vacuum drying
• Blistering may cause dimensional changes affecting criticality considerations
� Material used now : no corrosion/blistering
� Thermal aging effects: decrease in tensile and yield strength.
� Embrittlement due to radiation exposure: unlikely
� Nowadays neutron poison materials degradation negligible
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Aging analysis for confirmation of safety for extended storage
Used fuel behaviour in dry storage: many IAEA technical
documents
Creep of Used fuel cladding (Zr Alloys)
� At dry storage temperatures between 300 and 400 °C, the cladding
undergoes strain due to creep.
� Temperature decreases continually during dry storage, no significant creep
strain expected
� Creep strain is largely determined by fuel rod internal pressure and fuel rod
temperature time history.
� Provided that the maximum cladding temperature does not exceed 400°C,
creep under storage will not cause gross rupture of the cladding.
� Creep self limiting phenomenon: not a critical threat to used fuel integrity
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Aging analysis for confirmation of safety for extended storage
Used fuel behaviour in dry storage: many
IAEA technical documents
Hydrogen effects – embrittlement
� Storage temperature will decrease to the point
that hydrides precipitate in the Zr cladding .
� Hydrogen uptake depends on material,
irradiation history and oxide thickness.
� Hydrogen uptake and hydride precipitation
decrease the ductility of irradiated zirconium
alloys.
� The precipitation of radial hydrides can reduce
significantly the ductility.CEA –R-6084
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Aging analysis for confirmation of safety for extended storage
Pressure increase due to He-generation
� Used fuel pellets: production of helium from alpha decay during storage
� For very long storage periods (hundreds of years), the production of
helium in MOX fuel becomes comparable to the amount of fission gases
produced during reactor irradiation.
� For the considered storage duration (several decades), the pressure
increase is limited
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New AMP developments and studies
Aging/degradation Stressor and risk Solution mitigation Need further work
UNF Creep T, P, Bu : rupture Temperature limit No
UNF oxydation T, BU, gas composition :
rupture in transport
Inert gas No
UNF H2-effects Clad alloy, T, P, Bu:
Rupture in transport
Drying temperature limit
Time limit for transport
Complementary tests
UNF DBTT Clad alloy, T, P, Bu :
rupture in transport
Drying temperature limit
Time limit for transport
Complementary tests
Canister stainless steel Chlorides, resid stress:
SCC rupture
Control, Prevent salt
deposit, SCC resistant
welds
New coating, selection
treatment
Cask closure lid Chlorides, resid stress:
SCC rupture
Control, Prevent salt
deposit, SCC resistant
New coating, selection
treatment
Concrete Corrosion Temperature limit
Material selection
No
Aluminium from basket Creep Temperature limit
Material selection
Complementary tests
seals Corrosion Assessment of corrosion No
Coatings corrosion Assessment of corrosion Complementary tests
Neutron shielding Loss of properties Temperature limit Complementary tests
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Conclusion
The overall experience is a safe and reliable storage performance.
Aging management plan associated with extension of storage
New investigations and tests underway for better understanding and managing the degradation of casks or canister materials
Used fuel behaviour is a key issue for the management of the back end of the nuclear fuel cycle.
Confirmatory demonstration is needed (already started) with high burn up fuel.
Involvement of AREVA in these actions