Dry Storage and Transportation

of High Burnup Fuel

Ricardo Torres, PhD

Renewals & Materials Branch

Division of Spent Fuel Management

Office of Nuclear Material Safety and Safeguards

September 21, 2016

Webinar to State Liaison Officers & Indian Tribes
Outline

• Background

– Spent Fuel Assemblies

– Wet Storage, Dry Storage, Transportation

– Safety Requirements

• Burnup 

– What is it? Why is it important?

– What is high burnup?

– Technical Concerns - How does NRC address these?

– Licensing Considerations (continued storage, transportation)

• Conclusions

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U.S. Licensed Reactors

http://www.nrc.gov/reactors/operating/list-power-reactor-units.html

3
What are fuel rods made of?

4
How do assemblies look like?

PWR BWR

5
• After about 5-6 years in the 

reactor, fuel is no longer 

economically useful and is 

removed (~ 1/3 of fuel is 

removed every 18-24 months)

• More than 170,000 spent fuel 

assemblies (~ 2⁄3 of total) are 

stored in spent fuel pools

What happens after discharge?

Wet Storage

San Onofre Nuclear Generating Station

6
What happens after discharge?

Dry Storage

7
How do dry storage systems 

look like?

More than 87,000 

spent fuel assemblies 

are stored in over 

2300 dry storage 

systems

UxC SpentFUEL, vol.18, 

no.216, 2016

8
How do transportation 

packages look like?

30-ft drop Puncture 1,475oF fire Immersion underwater

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• Maintain fuel in a sub-critical state –

fuel remains unreactive

• Confinement / Containment – no 

release of radioactive material

• Radiation shielding – comply with onsite worker and public dose limits

• Thermal performance - heat removal, 

controlled fuel temperature

• Structural performance – maintain 

analyzed configuration

How do users assure safety 

during SNF handling?

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• Load fuel into the canister/cask 

from wet storage

• Drain

• Dry

• Backfill with helium

• Close (weld or bolt)

• Transfer to storage pad (with 

shielding overpack, as 

necessary)

• Install impact limiters for 

transport

*See Areva NUHOMS® used fuel storage video at 

us.areva.com

How is the fuel loaded?

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What is fuel burnup?

• Burnup refers to the amount of thermal energy produced over a 

given time frame divided by the initial mass of the fuel

• Units: gigawatt-days/metric ton of uranium (GWd/MTU)

www.nuclear-power.net

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Why does fuel burnup matter?

• Burnup level affects the temperature, radioactivity and physical 

makeup of the spent fuel

• Increasing burnup affects:

– Radionuclide inventory and distribution

– Heat load

– Crud thickness (deposits)

– Cladding oxide thickness and hydride content

– Fuel grain size and morphology (shape)

– Fission gas release to the plenum (end-of-life rod internal 

pressures)

13
What is high burnup?

• Generally, as the fuel reaches 

burnups of 45 GWd/MTU

– Cladding oxidation and hydrogen 

pickup accelerate

– Fuel pellet swelling increases

– Increased end-of-life rod internal 

pressures

– Changes to mechanical properties

• NRC considers:

– Low Burnup ≤ 45 GWd/MTU

– High Burnup > 45 GWd/MTU

545±80 wppm

1515 Zry-4

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Technical Issue

Cladding Creep

• What is it?

– Time-dependent deformation due to rod 

internal stresses

– Self-limiting

• Why is it an issue?

– Potential for cladding thinning

– Susceptibility to hairline cracks or ruptures

• How has NRC addressed it?

– Staff-issued guidance (ISG-11, Rev. 3)

– Separate-effects testing and modeling

ANT International, 2016

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Staff-Issued Guidance (ISG-11)

Accelerated Short-Term Testing

• Maximum calculated fuel cladding temperature should 

not exceed 400 °C (752 °F)

– Applicable to normal conditions of storage and transport, and 

short-term loading operations (e.g., drying, backfilling with inert 

gas, and transfer of the cask to the storage pad)

• Cladding stress calculations use an effective cladding 

thickness reduced by oxide later

• References cited in ISG-11, Rev. 3 provide experimental 

evidence that cladding failures are not expected for 

creep strains below 2 percent

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HBU Fuel Monitoring and 

Assessment Program (AMP)

• Surrogate surveillance program to confirm condition of 

HBU fuel in dry storage to ISG-11 expectations

• Confirmation that analyzed fuel configuration has been 

maintained, as expected, during the period of extended 

operation

• AMP expectation for HBU fuel demonstration program:

– DOE/EPRI Research Cask Program

– Or an alternative program meeting ISG-24 “Use of a 

Demonstration Program as Confirmation of Integrity for 

Continued Storage of HBU Fuel Beyond 20 Years” 

(Appendix D, NUREG-1927, Rev. 1)

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DOE/EPRI

Research Cask Program

• Intact HBU fuel stored in AREVA TN-32 bolted lid cask at 

North Anna ISFSI (Dominion VA Power)

• Nominal burnups between 53-58 GWd/MTU

• Fuel assemblies include four cladding types

– Zircaloy-4, low-tin Zircaloy-4, ZirloTM, and M5®

• Surveillance cask to be licensed to the ISG-11 

temperature limits and loaded such that the fuel cladding 

temperature is as close to the limit as practicable

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Technical Issue

Hydride Reorientation

• What is it?

– Hydrogen in the cladding dissolves during 

short-term loading operations 

– As the fuel cools down, it re-precipitates in 

a different orientation

• Why is it an issue?

– Variation of mechanical properties over 

time

– Pinch-loads during transportation

• How has NRC addressed it?

– Separate-effects testing

– Regulatory Information Summary

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Hydride Reorientation:

Ring Compression Testing

• NRC and DOE-sponsored 

testing at Argonne National 

Laboratory

– Estimate temperatures at which 

the cladding may loose ductility

– Subjected cladding specimens to 

bounding conditions for dry 

storage and transportation 

(temperature, hoop stresses)

– Provided a method for the NRC 

to conservatively review licensing 

actions until fatigue/bending data 

was obtained

Controlled displacement rates

5 mm/s typical

1.7-mm maximum displacement

Controlled temperature

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Hydride Reorientation:

Regulatory Information Summary

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Hydride Reorientation:

Regulatory Information Summary

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Hydride Reorientation:

Bending and Fatigue Testing

A transportation cask will 

experience some level of 

oscillation due to normal 

conditions of transport.

That oscillation will be transmitted 

in some way to the contents of 

the cask, the fuel elements.

The oscillation transmitted 

to the fuel elements will 

result in local stresses

The fuel cladding has the potential for fatigue failure if a large 

number of cycles are seen during transport, even if the maximum 

stresses seen by the cladding are far below the yield stress of the 

material.

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Hydride Reorientation:

Bending and Fatigue Testing

• NRC and DOE-sponsored testing at 

Oak Ridge National Laboratory

– Determine if the presence of fuel pellet 

increases the flexural rigidity (bending 

stiffness) of the fuel rod

– Determine if the presence of fuel pellet 

increases the failure strain of the cladding 

(storage and transport accident)

– Determine the number of cycles to failure 

for high burnup fuel rods at a range of 

elastic strain levels. (normal 

transportation)

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Hydride Reorientation:

Bending and Fatigue Testing

Push-pull force applied to U-Frame results in bending 

moment on the test segment

Location of test segment

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Hydride Reorientation:

Bending and Fatigue Testing

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Over-the-Road Vibration 

Studies

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Are these the only technical 

issues for HBF?

• Draft report “Managing Aging Processes in Storage” 

evaluated additional age-related phenomena for potential to 

compromise HBU fuel integrity

– Applies to periods of extended operation (20–60 yrs)

– Reviewed up-to-date technical literature and operating experience 

to define credible and non-credible degradation mechanisms

– Concludes that creep and hydride reorientation are only 

mechanisms of concern

• Report will be issued for public comment

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Conclusions

• NRC has assessed multiple technical issues related to 

potential degradation of high burnup fuel during both 

storage and transportation

• Storage

– Accelerated short-term testing results support the 

conclusion that creep and radial hydride reorientation 

will not compromise cladding integrity during dry storage 

up to 60 years

– Confirmation of these studies is provided per a surrogate 

program consistent with the guidance in ISG-24

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Conclusions

• Transportation:

– NRC and DOE-sponsored bending and fatigue testing 

results support the conclusion that cladding with 

reoriented hydrides will perform adequately

• Use of cladding-only mechanical properties is 

sufficient

– Until all data is gathered, applicants may conservatively 

assume that the cladding loses ductility at low 

temperatures, or perform consequence analyses that 

assume some degree of fuel reconfiguration

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• NRC website:

http://www.nrc.gov/waste/spent-fuel-storage.html

• NRC background on high burnup fuel:

http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bghigh-burnup-spent-fuel.html

• Information Handbook on Independent Spent Fuel Storage 

Installations 

(NUREG-1571, ML010450036 on NRC ADAMS document 

database)

• ricardo.torres@nrc.gov

More Information

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