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Issues Related to Long-term Dry Storage of UsedNuclear Fuel

Nuclear Waste Technical Review BoardMeetingLas Vegas NV, 11 June 2009

John KesslerManager, Used Fuel and HLWManagement Program

Outline

" Dry cask storage system functions" Dry Cask Storage Characterization Project (DCSC)

review

* Potential long-term degradation mechanisms

* EPRI plans for future work

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Functions of a Dry Cask Storage System thatMust be Maintained

* NUREG-1536 (NRC, 1997) identifies the functionsimportant to safety that the dry cask systems mustmaintain:- thermal performance

- radiological protection

- confinement- sub-criticality- retrievability

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Dry Cask Storage Characterization (DCSC)Project Review

" Four-year effort (1999-2002)

" Impetus: end of the initial 20-year licenses (2006)

" Investigation of Castor V/21 cask at INL

- In dry storage for 15 years

- Represented the lead (spent) fuel in dry storage in the U.S.

" Co-funded by NRC-Research, EPRI, DOE-RW, DOE-EM

" Documented in EPRI reports:

- Interim report 1000157 (2000)

- Interim report 1003010 (2001)

- Final report 1002882 (2002)

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DCSC Project Tasks

" Reopen Castor V/21 at INL- Cask cavity gas analysis- External dose rate measurements- Visual inspection of assemblies and cask internals

- Remove rods from assembly in Castor at INL

" Rod testing at ANL- Profilometry at ANL-West

- Rod puncturing for fission gas release at ANL-West- Destructive exams of Zircaloy at ANL-East

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Castor V/21 on TransporterHeaded to ot Shop

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DCSC Profilometry: No Evidence of CreepDuring Storage

The nominal as-fabricated cladding outer diameter is 10.72 mm.

EE.

E

CUc,)

1 0.75

1 0.70

10.65

10.60

0 500 1000 1500 2000 2500 3000 3500 4000

Location (mm, from rod bottom)

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DCSC: Morphology of Hydrides MostlyCircumferential -----------

Mid plane 0.5m above mid planeElECTRIC POWER

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Main Conclusions from the Four-Year DCSCEffort

" No cask functional degradation observed

" Assemblies still look the same- No sticking; no significant bowing upon removal

- No visual signs of degradation (color, oxide, damage)

" No leaks during storage

" No significant additional fission gas release to rodinternals

" No significant hydride reorientation

, Zero to minuscule creep during storage" "Creep life" remains

• Result: basis for license extensions out to --60 years

" Most severe conditions are during first 20 years© eELECTRIC POWER

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Examples ofAddress

Issues the DSCS Program did not

" High burnup used fuel" Long-term concrete degradatione Effect of marine environments (examined in EPRI

1011820, September 2005)report

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Review of Potential Degradation MechanismsOutline (from EPRI report 1003416, December 2002)

III II - ,. .Mechanism Claddingj~%.~.

Initial ExtendedI • Initial ExtendedNormalOxidation N Nfgr Y NCreep, Y NDCCG N NH2 reorientation Y NDHC N NSCC maybeH2 embrittlement NH2 migration Y N%4_ __

Annealing Y NCrud Spallation maybe NOff-Normal __ _

Oxidation due to air Y Yingress UCreep Y DEAnnealing Y DEAccident-impactFracture DE DEOxidation DE DEImpact Breach DE DECrud spallation DE DEAccident-fire ____

Stress rupture YAnnealing Y YH2 reorientation YDHC = Delayed Hydrogen CrackingDCCG = Diffusion Controlled Cavitation GrowthFgr = fission gas releaseSCC = Stress Corrosion Cracking

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Temperature-related Dry Storage SystemDegradation Mechanisms

'7NEI

* Fuel cladding creep caused by increased claddingductility and increased stress- Due to higher temperatures causing higher pressures inside the

cladding

" Hydride reorientation in the spent

" Corrosion* Degradation of neutron shielding

* Concrete dry-out and cracking

fuel cladding

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Changes as the System gets Cooler

" Mostly good things

-Reduced metal creep rates

- Reduced corrosion rates

-Reduced gamma and neutron radiation

" Potential negatives (mostly related to cladding)

-Additional hydride precipitation

Decreased ductility

* Potentially more susceptible to breakageduring storage and transportation

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Reduced Degradation with Time does not MeanDegradation Stops

* Corrosion (in oxidizing environments)

" Helium buildup inside fuel rods

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Aging Management Options

* "Initial" activities- Additional analyses to extend progress of degradation

mechanisms- Enhanced monitoring and

" "Eventually"

- Canning- Repackaging

- Over-packaging

• When is "eventually"?

inspection

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Potential Sequence of Events for Extended DryStorage (from EPRI report TR-1 08757, 1998, assumes 100-yr storage)

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Potential Sequence of Events for Extended DryStorage After an Accident Event

no effect

Continue DryStorage

fuel damaged cask OK

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Potential EPRI Work on Very Long-term Used[7 Fuel Storage and Transportation Issues

" Workshop (Fall 2009)* "Paper" analyses

• Experimental (with others?)* Licensing issues* Operational issues

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Together...Shaping the Future of Electricity

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