7th US/German Workshop on Salt Repository Research, Design, and Operation

Holger VölzkeBundesanstalt für Materialforschung und –

prüfung (BAM)Ken B. Sorenson

Sandia National Laboratories (SNL)

Washington, DCSeptember 7-9, 2016

Issues on Aging of Spent Fuel Storage Systems

Sandia National Laboratories is a multi mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration

under contract DE-AC04-94AL85000. SAND2016-8277 C

Contents

Background of SNL-BAM collaboration

BAM research activities associated with extended interim storage issues and aging management

SNL activities associated with extended interim storage issues and aging management

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Background of SNL-BAM collaboration

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SNL and BAM collaborate in the areas associated with the back-end of the commercial nuclear fuel cycle; specifically on packaging, transportation, and storage of spent nuclear fuel.

A Memorandum of Understanding (MOU) between SNL and BAM was established by 2012. Bilateral meetings/workshops take place twice a year.

Extended interim storage of spent fuel and HLW needs to be addressed as a major issue in both countries due to delayed disposal siting procedures.

Various technical issues concerning degradation effects of casks and inventories during extended periods of interim storage have been identified and specific investigations are being performed.

BAM Research Activities

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BAM Research Activities related to Extended Interim Storage Issues

BAM Seal Investigations Metal Seal Systems Elastomer Seal Systems

Polymer Degradation Effects investigated by BAM

BAM research activities related to extended interim storage: General remarks

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The need for the extended interim storage of SNF and HLW casks towards disposal implicates additional challenges for the nuclear waste management strategy in Germany.

Improved knowledge and data bases about the long term performance of casks and their inventories is essential for future extended storage licenses.

Recently, governmental research programs have been adjusted in Germany to address technical and scientific issues in this area.

BAM has identified the need for demonstrating the extended long term performance of seals and polymers used in dual purpose casks and has already initiated specific R&D programs. Additional or extended R&D programs are scheduled.

BAM research activities related to extended interim storage (1)

Long term performance of metal seals, elastomeric seals, and polymers for neutron shielding purposes

Preliminary investigations and laboratory tests performed by BAM for many years

Project “LaMEP” (Long term investigations on metal seals, elastomer seals, and polymers) funded by BMWi/GRS for 3 years (2015-2018).

Major Goals are: - Extension and continuation of initial test series to gain further

systematic material data, - Development and validation of analytical approaches for the

extrapolation of selected material or component properties to longer periods of time.

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BAM research activities related to extended interim storage (2)

Project „New Developments in the Long term Dry Interim Storage of Spent Nuclear Fuel and Vitrified High-Level Waste“ Funded by BMUB for 3 years (2015-2018)

Project Lead: GRSSub-contractors: BAM and Öko-Institut Main topics:

1. State of the art and review of national and international developments in interim storage of spent nuclear fuel and HLW

2. Aging management3. International activities4. Non-accessible areas (cask inventories)5. Organizational and ethical aspects

BAM key topics:- National and international regulations and guidance- Aging Management: technical aspects, integrated approach, gap analyses- Evaluation of international activities in the area of long term dry interim

storage7

Test ParametersTemperatures: +20°C +75°C +100°C +125°C +150°CHolding times since: 02/2009 01/2014 11/2010 01/2014 02/2009Seal type Al + Ag Al + Ag Al + Ag Al + Ag Al + Ag

BAM laboratory tests with continuous leakage rate measurement during seal loading and unloading

Long term performance of Helicoflex® metal seals concerning seal force, useable resilience and leak tightness

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BAM metal seal investigations (1)

Restoring seal force (Load Fr) reduction depending on holding time and temperature

0

200

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600

0,005 0,05 0,5 5 50 500 5000

Load

[N/

mm

]

Holding Time [weeks]

150� C

20 � C

100 � C

Ag-Seals

40 Years 100 Years

75� C

125� C

58%

85%

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0,005 0,05 0,5 5 50 500 5000

Load

[N

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Holding Time [weeks]

20 � C

100� C

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40 years75� C

125� C

50%

79%

100 years

Al - Seals

Computer tomography scan of an Al-seal after 3 months at 150°C

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BAM metal seal investigations (2)

Major outcome so far:Seal function and leak tightness are not affected by seal force reductions

Relevance for: Auxiliary seals in spent fuel and HLW casks Primary seals in LLW/ILW casks

Major topics:1. Investigation of the low temperature behavior down to -40°CResults published by Matthias Jaunich, Wolfgang Stark, and Dietmar Wolff: Low Temperature Properties of Rubber Seals Kgk-Kautschuk Gummi Kunststoffe, 2011. 64(3): p. 52-55. A new method to evaluate the low temperature function of rubber sealing

materials Polymer Testing, 2010. 29(7): p. 815-823. Comparison of low temperature properties of different elastomer

materials investigated by a new method for compression set measurement Polymer Testing, 2012. 31(8): p. 987-992.

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BAM elastomer seal investigations (1)

2. Aging of elastomer O-rings under thermo-mechanical loadsInvestigation program with selected rubbers (HNBR, EPDM and FKM) tested as O-rings with an inner diameter of 190 mm and a cross sectional diameter of 10 mm since May 2014. The O-rings are oven-aged at four different temperatures (75 , 100 , 125 , 150 °C). They are examined after various times between one day and 5 years. In order to be able to compare between compressed and relaxed rubber, the

samples are aged in their initial O-ring state (Fig. 1) As well as compressed between plates (Fig. 2) with a deformation of 25 %

corresponding to the actual compression during service Furthermore, we are aging samples in flanges that allow leakage rate measurements

(Fig. 3).

Fig. 1 Undeformed O-rings

Fig. 2 Half O-rings compressed between plates Fig. 3 O-ring in flange for leakage measurements 11

BAM elastomer seal investigations (2)

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BAM elastomer seal investigations (3)

The test set-up can lead to heterogeneous aging caused by diffusion-limited oxidation (DLO) effects. These effects depend on material, dimensions, time and temperature. Heterogeneous aging results in distorted bulk properties such as compression stress relaxation and compression set (CS) suggesting that HNBR has better performance than EPDM at 150 °C but which is not the case at 100 °C.

CS represents the resulting permanent deformation after the load is removed (CS=100% means no elastic recovery anymore).

The presence of heterogeneous material properties due to thermo-mechanical aging was shown by hardness measurements across the seal cross-section.

If DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts and Arrhenius graphs.

According to the extrapolations to 23 °C, exemplary CS values of 50 % and 80 % would be reached after approx. 10 years and 29 years, respectively for HNBR and after approx. 400 years and 1100 years, respectively for EPDM.

Selected results from aging investigations of elastomer O-rings under thermo-mechanical loadsKömmling A, Jaunich M, Wolff D, Effects of heterogeneous aging in compressed HNBR and EPDM O-ring seals, Polymer Degradation and Stability (2016), doi: 10.1016/j.polymdegradstab.2016.01.012.

Ultra-High Molecular Weight

Polyethylene (U)HMW-PE for neutron

radiation shielding

CASTOR® HAW28M cask design by GNS

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Polymer degradation effects investigated by BAM (1)

Basic requirement: Sufficient long term neutron radiation shielding without safety relevant degradationDegradation effects to be considered: Temperatures (max. 160 °C and decreasing during

storage) Thermal expansion Structural changes from semi-crystalline to

amorphous Gamma radiation (decreasing during storage)

Structural damages and/or crosslinking Hydrogen separation

Mechanical assembling stresses Stress relaxation

Gamma irradiation tests by BAM (at room temperature) Low dose irradiation (60Co source): 0.5 – 60 kGy High dose irradiation (conservative max. storage dose): 600 kGy

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Further gamma irradiation tests with material blocks of 10*10*50 cm³

0 kGy

50 kGy

100 kGy

200 kGy

400 kGy

600 kGy

UHMW-PEHMW-PE

Exemplary outcomes for UHMW-PE show an increase of the insoluble, crosslinked fraction after high dose gamma irradiation

Further investigations planned: Thermal aging of(U)HMW-PE at elevated

temperatures Combination of radiation and thermal aging Development of adequate prognostic

methods to allow extrapolation of long term material performance

Polymer degradation effects investigated by BAM (2)

DOE Program Objectives: Storage and Transportation R&D Closing the High Burnup Spent Fuel Cladding Technical Gap

These activities are funded through the U.S. Department of Energy, Office of Nuclear Energy. BAM and SNL meet on a regular basis to collaborate on technical issues, and development and analysis of data

BAM and SNL are also collaborators on a DOE/Euratom sponsored International Nuclear Energy Research Initiative (I-NERI) entitled:

Assessing the Integrity of High Burnup Spent Nuclear Fuel in Long Term Storage and Transportation

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SNL activities associated with the BAM/SNL collaboration

Preparing for extended storage and eventual large-scale transport of spent nuclear fuel (SNF) and high-level waste (HLW)

Developing the technical basis for: Extended storage of used nuclear fuel Fuel retrievability and transportation after extended storage Transportation of high burnup used nuclear fuel

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DOE Program Objectives:Storage and Transportation R&D

What is the fuel cladding issue? Reactor operations

• Hydrogen is absorbed into the cladding• Fission gas build-up increases rod internal

pressures cladding hoop stress• Longer operating times and high burnup fuel

(> 45 GWd/MTU) increase these effects Dry storage

• The drying cycle creates a thermal spike that brings H back into solution

• H precipitates preferentially with radial orientation under subsequent cooling and cladding hoop stresses

• Degradation of cladding mechanical properties due to re-oriented hydrides

Transportation loadings• Normal conditions of transport induce

cladding stresses due to over-the-road (rail) loadings

• Cladding integrity during transport needs to be demonstrated

Billone, “Update on Testing to Evaluate Radial Hydrides”, U.S. NWTRB Meeting Presentation, Feb 17, 2016, ANL

Hanson, “DOE R&D in Support of High Burnup Spent Fuel Dry Storage and Transportation”, EPRI ESCP Int’l Subcommittee Meeting, Apr 14, 2016, PNNL

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Closing the high burnup used fuel technical gap associated with fuel cladding

Mechanical testing on high burnup fuel cladding:

Ring compression testing at Argonne National LaboratoryDetermines Ductile-to-Brittle Transition Temperature(DBTT) of cladding, an indicator of material brittleness• Highly dependent on internal rod pressure• Dependent on maximum temperatures seen during drying

Billone, “Update on Testing to Evaluate Radial Hydrides”, U.S. NWTRB Meeting, Presentation, Feb 17, 2016, ANL

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Closing the high burnup used fuel technical gap associated with fuel cladding

Mechanical testing on high burnup spent fuel:

Cyclic Integrated Reversible-Bending Fatigue Tester (CIRFT) at Oak Ridge National Laboratory• Determines the flexural stiffness of the cladding/fuel “system”

• Stiffness provides a measure of fuel/cladding strength under mechanical loads• CIRFT testing provides valuable insights regarding

• Pellet-Clad interactions• Pellet-Pellet interactions

• CIRFT testing provides a good measure of fatigue strength

All figures and picture: Wang, et al., “CIRFT Testing of High-Burnup Used Nuclear Fuel from PWRs and BWRs”, U.S. NWTRB Meeting, Presentation, Feb 17, 2016, Oak Ridge National Laboratory

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Closing the high burnup used fuel technical gap associated with fuel cladding

Thermal analyses on high burnup spent fuel in storage:

Thermal analyses on the DOE/EPRI Confirmatory Demonstration Project cask loading with high burnup fuel:

• Best estimate given fuel vendor data and current drying processes• Regulatory thermal analyses penalty factors removed• Best estimate maximum cladding surface temperatures are much less than the regulatory limit of 400 °C • Indicating:

• Less Hydrogen going into solution that can re-precipitate in a radial orientation• Lower internal rod pressures that effect H radial hydride reorientation (PV=nRT)

Hanson, “DOE R&D in Support of High Burnup Spent Fuel Dry Storage and Transportation”, EPRI ESCP International Subcommittee Meeting, Apr 14, 2016, PNNL.

Maximum cladding surface temperature for each assembly in the demo cask 20

Closing the high burnup used fuel technical gap associated with fuel cladding

Normal Conditions of Transport (NCT) Loadings: Shaker table and Over-the-Road Truck and Rail (shaker table only) tests were conducted on a surrogate PWR assembly to estimate realistic loadings that a spent fuel assembly may see during transport.

• Accelerations and strains were obtained on both the surrogate assembly and the conveyance• Placement of instrumentation was informed by analyses

McConnell, “Sandia Shaker Table and Over-the-Road Vibrations Studies”, US NWTRB Meeting, Feb 17, 2016, SNL.

Large margin of safety relative to either elastic or fatigue failure criteria

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Closing the high burnup used fuel technical gap associated with fuel cladding

FY16 Activities: Ring Compression Tests: Conduct additional RCTs

to better understand DBTT Zirlo results at 350 °C:• Higher H content?• Lack of radiation-hardening annealing at 350 °C?

CIRFT Tests:• Tests on TMI (low Sn Zirc) and Surry (Zirc) samples• Tests on lower sections of fuel rod

Thermal Analyses:• Conduct best estimate analyses of other licensed storage designs

Thermal Tests:• Conduct thermal benchmark test to obtain maximum cladding surface temperatures using higher design temperatures/pressures

Transportation NCT:• Plans are underway to conduct a full-scale ENSA ENUN-32P rail cask with surrogate assemblies to obtain over-the-rail loading data

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Closing the high burnup used fuel technical gap associated with fuel cladding

SNL and BAM collaboration addresses various technical issues regarding aging effects on spent fuel storage systems and regular workshops are held to share information.

BAM investigations on aging effects of metal seals, elastomer seals and polymers don’t indicate a major safety issue regarding the long term performance of such materials and components so far. Investigations are going to be continued and expanded.

SNL focusses on the technical gap associated with fuel cladding:- Further testing will focus on cladding response and performance under

realistic temperatures, hoop stresses, and external stresses,- Indications are that cladding, including for high burnup fuel, will continue to

perform its safety functions during extended storage and normal conditions of transport.

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Summary