ANL/NE‐12/18 

NRC Job Code V6060: Extended In‐Situ and Real Time Monitoring  

Task 3: Long‐Term Dry Cask Storage of Spent Nuclear Fuel  

____________________________________________________ 

Argonne National Laboratory 

   About Argonne National Laboratory Argonne is a U.S. Department of Energy laboratory managed by UChicago Argonne, LLC  under contract DE‐AC02‐06CH11357. The Laboratory’s main facility is outside Chicago,  at 9700 South Cass Avenue, Argonne, Illinois 60439. For information about Argonne,  see http://www.anl.gov. 

                                Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither  the United States Government nor any agency thereof, nor UChicago Argonne, LLC, nor any of their employees or officers, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of document authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, Argonne National Laboratory, or UChicago Argonne, LLC  

 

ANL/NE‐12/18

 

 

 

NRC Job Code V6060: Extended In‐Situ and Real Time Monitoring  

Task 3: Long‐Term Dry Cask Storage of Spent Nuclear Fuel  

____________________________________________________ 

 

prepared by: 

J.D. Lambert, S. Bakhtiari, I. Bodnar, C. Kot, J. Pence Nuclear Engineering Division Argonne National Laboratory  for:  

I.G. Prokofiev Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission March 2012 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

v  

Fig.S-1. Forty-three NAC-MPS dry casks containing the total spent fuel inventory of the decommissioned Connecticut Yankee reactor (Courtesy: NAC International, Inc.)

EXECUTIVESUMMARY

Over the last25yearsover1500drycaskstoragesystemshavebeen loadedwithspent nuclear fuel at commercial light‐water reactor sites around the U.S. (e.g., Fig.S‐1).Thesedrycaskstoragesystemscontainapproximatelyonequarterof the65,200MTUofspent fuel accumulatedbyDecember2010; this proportion increaseswith time as spentfuel pools fill up and long‐cooled spent fuel is moved to dry storage to make room forfreshlydischargedfuel.Althoughexaminationof fuelat36.5GWd/MTUafterdrystoragefor 14.2 years indicated no adverse effects of storage on the fuel or its dry cask, theconsiderably longer storage timesnowbeingconsidered,higher fueldischargeburn‐ups,andexternalenvironmentalfactors,raiseconcernsaboutthelong‐termperformanceofdrycaskstoragesystemsandthespentfueltheycontain.

TohelpaddresstheseconcernsthepresentreportfulfilstherequirementforTask3of the U.S. Nuclear Regulatory Commission’s Office of Nuclear Regulatory Research JobCode V6060 “to informNRCregulatorsofthemonitoringand inspectionrequirementsandavailablecapabilitiestomonitorperformanceoflong‐termdrycaskstoragesystems.”

First,tosetthescene,thehistoryandthetypesofdrycaskstoragesystemthathavebeendeveloped and used in the U.S. are brieflydescribed,asarepresentregulationsregardingmonitoring.

Secondly, the prior performance testingof dry cask storage systems in the U.S. andJapan, including the examination of spent fueltodeterminetheeffectsofstorage,andcurrentareas of investigation into cask stability arereviewed. Such review clarifies what needs tobemonitored.

Thirdly, potential methods for moni‐toring dry cask storage systems are discussedat length. Themethods are categorizedby theparameters (orphenomena)beingmonitored,which include: fuel failure, fuel relocation,system cooling, canister temperature, canister corrosion, canister leakage, bolted caskleakage,structuralstabilityofconcreteover‐packs,andsurfaceγdoserate.

Themethodsarerankedaccordingtotheirstateofdevelopment,i.e.,whetherthey:(i)arecurrentlyused in the field, (ii) are likely fielddeployable in thenear term(in1‐3years),or(iii)needlonger‐termdevelopment(4ormoreyears)andthenprobableuseinadry cask demonstration program. Table S‐1 summarizes the potential methods ofmonitoring dry cask storage systems; the table identifies the relevant sections in thedocumentwherethemethodsarediscussed.

vi  

Table S-1

Potential Methods of Monitoring Dry Cask Performance during Normal Operation

Parameter/ Phenomenon

Monitoring Method/Stage of Development Current Field

Practice For Near-Term

Field Deployment For Longer-Term

Development

Fuel failure

None

Pitch-catch speed of sound (SoS) across canister gas space1

[§5.6]

Current method sufficient

Fuel relocation

None Changes in DCSS

thermal and γ images [§5.3.5; §5.7.1]

γ tomography of DCSS [§5.7.3]

Canister temperature

Inferred from T/C reading at surface and COBRA

calculations

Thermal imaging using borescope [§5.3.5; §5.4.1]

U/S or fiber optic temperature sensors on canister surface

[§5.3.3; §5.3.4]

Change in DCSS cooling

Inspection of cooling vents for debris

Temperature-indicating RFIDs and/or SoS devices

in vents [§5.8.2]

Current methods sufficient

Canister corrosion

None

Borescope inspections; SaltSmart™ coupons

for brine deposits [§5.4.1]

Miniaturized EMAT monitoring of welds and surface layers of canister [§5.4.2]

Canister leakage

None2

Outlet air monitored for increase in SoS due to helium leakage

[§5.5]

ΔTBT using fiber optic temperature sensors

on canister [§4.3.2.2] to annunciate large leak (vertical DCSSs only)

Bolted cask

leakage

Alarmed on ΔP in space between

inner and outer lids

Current method sufficient

Overpack structural stability

Visual inspection of concrete surfaces

Programmed visual inspection; Schmidt Hammer; γ imaging

[§5.7.1; §5.8.1]

U/S pulse velocity; Smart Pebbles™ [§5.8.1; §5.8.2]

Surface γ dose rate

Routine radiological surveillance of DCSSs

Integrate with visual inspection [§5.4.1] and γ imaging [§5.7.1]

Integrate with DCSS γ tomography

1Xenon and krypton from failure(s) will reduce SoS in helium fill gas,

method works only if unobstructed path for sound exists across canister gas space. 2Welded inner and outer lids assumed to preclude canister leakage.

 

vii  

Weconcludethatavarietyofnon‐obtrusivemethodsarefeasibleforthelong‐termmonitoringofdrycaskstoragesystems.Methodsrangefrommeasuringthespeedofsoundin air as ameans of detecting helium leaks from canisters to traditional non‐destructiveexaminationmethodsfordeterminingstructuralintegrityofconcrete.

Somemethods—suchasmonitoringoverheatingbymeasuringthespeedofsound

in the cooling air that exits a dry cask storage system—couldbedeveloped andusefullydeployed rather quickly at independent spent fuel storage installations. Other methods,suchasSaltSmart™ coupons tomeasure saltdepositiononcanisters (tobeused in fieldtrialslaterthisyear),mayneedrefinementbeforewidespreaduse.Yetothermethods,liketheuseofγ‐raytomographyfordetectingfuelrelocation,requirenotonlyfurtherR&Dbutalso expensive equipment. Theymay bemore applicable to the dry cask demonstrationprogrambeingconsideredbytheNuclearRegulatoryCommissionthantouseinthefield.

Specificrecommendationsregardingmonitoringofdrycaskstoragesystemsare:

Electromagnetic acoustic transducers should be actively pursued for detectingcanistercorrosionbecausetheycanmonitortheconditionofweldsontheinnersideofacanisterwallandbecausetheydonotrequireacouplingmedium.

Thecurrentmethodofmonitoringforaleakinbolteddrycasksbyachangeinpressureinthespacebetweeninnerandouterlidshasworkedwellandneedsnoimprovement.

Measuring the speedof sound todeterminehelium leakage fromcanisters, airtemperature,andpossibly fuel failureshouldbedevelopedasausablemethodbyminiaturizingcurrent“pitch‐catch”ultrasoniccavitiesforuseinthefield.

Detecting fuel failurebyKr‐85 activitywarrantsminimal further developmentbecause the isotope’s 10.7‐year half‐life restricts its use to cooling times lessthanaboutfiftyyears.

Visualinspectionandthermal/γimagingofdrycaskstoragesystemsneedtobeautomatedtoallowreliabledetectionofchangeinfuelconfiguration.

Methods now used tomonitor the integrity of concrete buildings and bridgesshould be further evaluated for application to dry cask storage systems. TheSchmidtHammertest,theindirectmeasurementofultrasonicpulsevelocity,andSmartPebbles™ that can detectchlorideintrusionarepromisingtechniques.

Anoverallrecommendationisthat: Monitoringmethodsmust be carefully evaluated for application to the unique

geometriesandlimitedcomponentclearancesindrycaskstoragesystems.

viii  

ix  

AcronymsandAbbreviations

ACI AmericanConcreteInstituteALARA AslowasreasonablyachievableANL ArgonneNationalLaboratoryAREVA Frenchnuclearcompany(mergingofCOGEMA,FRAMATOME,CEAIndustrie)ASTM AmericanSocietyforTestingandMaterialsBWR BoilingwaterreactorCastor CaskforstorageandtransportofradioactivematerialCCD ChargecoupleddeviceCFS Concretefilledsteel(cask)CRIEPI CentralResearchInstituteofElectricPowerIndustry(Japan)DCDP DrycaskdemonstrationprogramDCSS DrycaskstoragesystemDOE (UnitedStates)DepartmentofEnergyDSC DryshieldedcanisterEMAT ElectromagneticacoustictransducerEPRI ElectricPowerResearchInstituteESCP ExtendedStorageCollaborationProgram(DOE/EPRI/NRC)GWd Gigawatt‐dayHSM HorizontalstoragemoduleINEEL IdahoNationalEngineering&EnvironmentalLaboratoryINL IdahoNationalLaboratory(formerlyINEEL)ISFSI IndependentspentfuelstorageinstallationISG Interimstaffguidance(NRC)MASB Multi‐assemblysealedbasketMTU MetrictonnesuraniumNAC NuclearAssuranceCorporationNEI NuclearEnergyInstituteNDE Non‐destructiveexaminationNRC (UnitedStates)NuclearRegulatoryCommissionNTS NevadaTestSiteNUHOMS NutechhorizontalmodularstoragePWR PressurizedwaterreactorR&D ResearchanddevelopmentRC Reinforcedconcrete(cask)RH RelativehumiditySCC StresscorrosioncrackingSNF Spent(used)nuclearfuel

x  

SoS SpeedofsoundSRNL SavannahRiverNationalLaboratorySSC System,structureorcomponentTAN TestAreaNorthTEPCO TokyoElectricPowerCompanyTN Transnuclear,Inc.UPV UltrasonicpulsevelocityU/S UltrasonicU.S. UnitedStatesUT Ultrasonictesting  

   

xi  

 

TableofContents

ExecutiveSummary………………………………………………………………………………………………..v

AcronymsandAbbreviations………………………………………………………………………………….ix

1. Background........................................................................................................................................12. ScopeofStudy...................................................................................................................................23. DryCaskStorageSystemDesigns............................................................................................33.1 TransnuclearNUHOMS‐24PDCSS........................................................................................43.2 TransnuclearTN‐68DCSS........................................................................................................53.3 FieldMonitoringofDCSSsandSNFHandling.................................................................64. DCSSPerformanceTestingandDemonstrationProjects..............................................84.1EarlyU.S.PerformanceTesting.............................................................................................84.1.1TestsatNevadaTestSite.......................................................................................................84.1.2TestsatH.B.RobinsonNGS..................................................................................................84.2LaterU.S.PerformanceTesting.............................................................................................94.2.1ThermalTestsoftheVSC‐17DCSS.....................................................................................94.2.2ThermalTestingoftheCastorV/21DCSS...................................................................104.2.3Follow‐onWorkwithCastorV/21DCSS......................................................................114.2.3.1TheCastorV/21DCSS........................................................................................................124.2.3.2DCSSInspectionsandTests.............................................................................................124.2.3.3CaskTemperatures..............................................................................................................134.2.3.4RadiationSurveys................................................................................................................144.2.3.5SurveysofCaskatmosphere............................................................................................154.2.3.6FuelRodExaminations......................................................................................................154.2.3.7Summary..................................................................................................................................164.3JapaneseWork.............................................................................................................................174.3.1DCSSMonitoringandInspectionatFukushima‐Daiichi.........................................174.3.2CRIEPIWorkonDCSSs..........................................................................................................184.3.2.1BlockageSimulation..........................................................................................................194.3.2.2DetectionofCanisterLeakage......................................................................................204.3.2.3DetectionofFuelFailure.................................................................................................205.ImplementationofDCSSMonitoring....................................................................................215.1DryCaskDemonstrationProgram.....................................................................................215.1.1ChoiceofDCSSDesign.......................................................................................................225.2NeedforRuggedInstrumentation.....................................................................................225.3TemperatureMeasurements................................................................................................225.3.1TypeNThermocouples.....................................................................................................235.3.2JohnsonNoiseThermometers........................................................................................23

xii  

5.3.3UltrasonicTemperatureSensors..................................................................................245.3.4Fabry‐PerotFiberOpticTemperatureSensors......................................................255.3.5ThermalImaging...................................................................................................................265.4SurfaceInspectionofComponents....................................................................................275.4.1BorescopeInspection……………………………………………………………………………275.4.2ElectromagneticAcousticTransducerInspection…………………………………..285.5MonitoringCask/CanisterLeakTightness.....................................................................305.6CanisterInternalConditions.................................................................................................325.6.1GasComposition…………………………………………………………………………………..325.6.2PressureandHumidity…………………………………………………………………………325.7Gamma‐rayScanning...............................................................................................................335.7.1GammaImagingSystems.................................................................................................335.7.2‘GammaFingerprinting’DCSSs.......................................................................................345.7.3Gamma‐rayTomography.................................................................................................375.8InspectionMethodsforConcrete.......................................................................................385.8.1TraditionalNDEMethods.................................................................................................385.8.2EmbeddedSensors..............................................................................................................426.SummaryandRecommendations.........................................................................................437. Acknowledgements……………………………………………………………………………………468. References…………………………………………………………………………………………………47

Appendix

A. DryCaskStorageintheU.S………………………………………………………………………….51

Tables

1. DryCaskStorageSystemsinUseintheU.S.2009‐2011...............................................32. PeakTemperaturesfortheVSC‐17DCSSLoadedwithConsolidatedFuel.........103. Time‐TemperatureHistoryofAssemblyT11inThermalPerformanceTests

ofCastorV/21DCSS...................................................................................................................104. Temperatures(oC)inTEPCOHeatRemovalTestsofConcreteCasks..................195. SpeedofSound(SoS)inGasesat0oC…………………………………………………………...306. NDEMethodsforConcreteStructures................................................................................377. AcceptedConcreteQualityVersusMeasuredPulseVelocity....................................408. PotentialMethodsofMonitoringDCSSConditionduringNormalOperation…42

Figures

1. State‐by‐StateInventoryofSpent(Used)NuclearFuelinMTU

asofDecember2010...................................................................................................................1

xiii  

2. TheNUHOMS‐24PDCSSA.Thedouble‐liddeddryshieldedcanister(DSC)for24PWRassemblies...........4B.Afour‐unithorizontalstoragemodule(HSM)containingDSCs............................5

3. TheTransnuclearTN‐68DCSS..................................................................................................64. DryStorageCasksattheINEELin2003...............................................................................95. SnugfitofAssemblyT11inthestoragebasketintheCastorV/21DCSS...........136. TheaveragediameterprofilesofrodsfromtheT11assembly,showing

creepdownof0.6%anddepressionsatgridspacers.............................................157.ThecladdingonRodH9fromassemblyT11at1100mmabovemidplane,

exhibitingcircumferentialdistributionofhydrideprecipitates............................168. SchematicofDCSSmonitoringatFukushima‐Daiichi................................................189. SchematicoftheConcreteCasksTestedbyCRIEPI....................................................1910. ChangesofTBTandpressureinaCFScask...................................................................2011. MethodofdetectingfuelfailurebymeasuringKr‐85inDCSSatmosphere.....2112. TheSchmidtHammerusedfortestingtherelativecompressivestrength

ofconcrete....................................................................................................................................2213. BlockdiagramoftheJohnsonnoisethermometermeasurementprocess.......2414. Ultrasonicthermometrysystemwithanotchedwaveguide..................................2515. Thermalimagingwithaninfraredcamera

Left:ImageofafuseboxwithoverheatedcentralfuseRight:TheIR‐CAM59XIRcamerausedforitsimaging............................................26

16. BorescopesusedtoinspectthecoolingannulusoftheVSC‐17DCSSLeft:Toshibamodel1K‐M44H;Right:Everest/VITXLPro.......................................27

17. ImagesofcomponentsintheverticalcoolingchannelintheVSC‐17DCSS,obtainedbyCRIEPIworkersincollaborationwithINLinvestigators...........….28

18.AnEMATUTcomparedwithaconventionalpiezoelectricUT……………………...2919.EMATinspectionresultsofsamplewithsixthermalfatiguecracks(A‐E)….…2920.Speedofsoundinairat20oCasafunctionofheliumconcentration……………..3121.Hand‐heldprototypeoftheSoSLeakDetectordevelopedatANL…………………3122.Pitch‐catchU/Smeasurementofspeedofsoundtoannunciatefuelfailure

inanSNFcanister………………………………………………………………………………………3223. TheRadScan8004gammaimagingsystem;Left:theassembledsystem

Right:Schematicofdetectionhead……………………………………………………………..3324. Gamma‐rayimagesoftheMC‐10cask.Thewide‐angleview(left)and

zoomed‐image(middle)showlittlestructuraldetailbutareclearlyofdifferentshapethanthetopview(right).........................................................................34

25. Gamma‐rayimagerusedtofingerprinttheMC‐10cask.Left:Schematicoftheimager.Right:Theimagerrecordingthetopviewofthecask.................34

26. Gamma‐rayspectrometer.Upper:Thespectrometermountedonstepladder.Lower:Gamma‐rayspectrumfromtheMC‐10cask...................................................35

xiv  

27. Sketchofthemethodforγ‐raytomographyofaDCSS…………………………………3628.ReinforcedconcreteWW‐II"pillbox"onsouthcoastofEngland,

illustratingeffectsof70yearsofmarineclimateonstructuralstability...........3729. RelationshipbetweenreboundvalueQandcompressivestrengthof

concretedeterminedbytheSilverSchmidtHammer................................................3930. UPVtomographyofaconcretehighwaysignwithhoneycombingofthe

structurerevealedbyreducedvelocityofpulse..........................................................4031. Threewaystoattachatransducerandadetectoronaconcretestructure

forUPVmeasurements:direct,semi‐direct,andindirect........................................4032. ThePunditLabUPVtestermanufacturedbyGENEQInc.,Switzerland;

atransducerheadisshownatbottomleft.....................................................................4133. RH/temperaturesensorpackingprocedure……………………………………………….42

1  

1. Background

Aconsequenceofthegrowthinnuclearpower intheU.S.—combinedwitha1976policydecisionnot topursuereprocessingof commercial spentnuclear fuel (SNF)1—hasbeentheaccumulation from1960tothepresent timeof largeSNF inventoriesatreactorsites. Because spent fuel pools were never intended for permanent storage, they aregraduallyreachingtheirlimitoncapacity,despitere‐rackingofassemblies.

Tocombatthelackofwetstorageutilitiesbeganfrom1985todrylong‐cooledSNFandstoreitinaninertatmosphereinvariousdesignsofmetalandconcrete“drycasks”atfacilities known as independent spent fuel storage installations (ISFSIs), co‐locatedwithreactors.Bytheendof2010, thetotalU.S. inventoryofSNFwas65,200metric tonnesofuranium (MTU) [1], distributed as shown in Fig.1. About one quarter of this SNF is drystoredatISFSIs.2ThefractionofSNFindrystoragewillonlyincreasewithtime.

Fig.1. State-by-State Inventory of Spent (Used) Nuclear Fuel in MTU as of December 2010;

courtesy: Nuclear Energy Institute (NEI).

Inanticipationoftheopeningofanationalrepositorybythelate1990’s,theNuclearRegulatoryCommission(NRC)licensedtheuseofdrycasksforaninitialperiodoftwentyyears, with the possibility of license extension. In 1999 an extensive examination wasperformed on low‐burn‐up fuel from the Surry pressurizedwater reactor (PWR),whichhadbeenstored inahelium‐filledCastorV/21all‐metalboltedcask for~14years [2‐4].Theexaminationisdescribedin§4.2.3;briefly,itshowedthat:

                                                            1 Adecisionstillinforcetoday.2 AsofDecember2010,therewere48ISFSIswithgenerallicenses,15withspecificlicenses,andtenreactorsitespursuinggenerallicenses.FifteenISFSIswereassociatedwithpermanentlyshutdownreactors;atsixoftheseISFSIsthereactornolongerexisted,aso‐called‘ISFSIonly’or‘orphan’site.

2  

Thecaskdisplayednosignsofphysicaldeteriorationorgasleakage; Theconcretepadunderthecaskhadexperiencednodegradationorsagging; Twelve fuel rods that were examined exhibited (i) no cladding creep or

additional fissiongasreleaseattributable to thestorage, (ii)noevidenceofhydrogen pickup or radial hydride reorientation in the cladding due tostorage,and(iii)little,ifany,annealingofthecladding.

When the national repository at Yucca Mountain was delayed these encouragingresultsallowedtheNRCtograntlicenseextensionfordrycaskstoragesystems(DCSSs)onacase‐by‐casebasis,beginningwiththeCastorV/21DCSS.Today,DCSSsattheSurryISFSIhavestoredSNFfortimesapproaching30years,whiletheaveragetimeforalldry‐storedSNF is in the regionof15years.During thisbroadexperience therehavebeennosafetyincidentswithDCSS’satanyofthesixty‐threecurrentISFSIs.

Withcancellationof theYuccaMountain repositoryandnoclearpath forward forSNFdispositionemerging from the reportof thePresidentialBlueRibbonCommitteeonAmerica’sNuclearFuture[5],dry‐caskstorageofhigher‐burnupfuelfortimesgreaterthanorginallyenvisionedforSNFbeforefinaldisposition.MechanismsfordegradationofDCSSsinoperativeintheshorttermmaybecomeimportanttothesafetyfunctionsofthesystem,structureorcomponents(SSCs)oversuchaprolongedperiod.Forexample,excessivedry‐outofconcreteinanaridclimate,corrosionofconcreterebarinamarineclimate,orstresscorrosion cracking (SCC) of welds on storage canisters need to be considered, and, ifpossible,monitored.MonitoringDCSSsisthesubjectofthisreport.

2. ScopeofStudy

Overthelast212years,long‐termdrystoragehasbeenthesubjectofseveralexpertevaluations;forexample,ElectricPowerResearchInstitute(EPRI)workshopsinNovember2009andDecember2011[6,7];areportbytheU.S.NuclearWasteTechnicalReviewBoardin December 2010 [8]; and an NRC‐sponsored review by Savannah River National Lab‐oratory (SRNL)ofmaterials aging issues and agingmanagement for extended storageofSNF inMay 2011 [9]. In addition, programs dealingwith long‐term storage of SNF arebeingpursuedbothbyEPRI[10]andtheU.S.DepartmentofEnergy(DOE)[11].

ThepresentdocumentrequestedofArgonneNationalLaboratory(ANL)bytheNRCdiscusseswaystomonitortheperformanceofDCSSsoverextendedperiodsoftime.ItcanthusbeconsideredacompaniondocumenttotheSNRLreviewonmaterialsagingissues.

TheANLdocumentbeginswithabriefsurveyinSection3ofthemajordesignsofDCSSs for SNF that have been developed and used by the industry in response to thegrowing lackof storagespace inspent fuelpools.ThediversityofdesignswasdescribedfullyinarecentEPRIpublication[12].ThissectionalsodescribescurrentfieldmonitoringofDCSSsandSNFhandling.

Section 4 summarizes previous dry cask performance testing and demonstrationprojects in the U.S. and in Japan. Although techniques are available formonitoring theconditionofDCSSs, therewill be aneed forperiodic removal and inspectionof a typical

3  

assembly and a sample of its fuel pins from a demonstrationDCSS in order to supply abaselineagainstwhichmonitoredparameterscanbecompared. TheresultsobtainedontheSurryCastorV/21DCSSanditspayloadofPWRassembliesafter14.2yearsdrystoragearedescribedtoillustratewhatcanbeaccomplishedbyperiodicinspection.

Section5discussesthegeneralrequirementsforaDryCaskDemonstrationProject(DCDP),andcontinueswithadiscussionofpossiblemonitoringtechniques.Section6givesasummaryandrecommendations.

3. DryCaskStorageSystemDesignsAvarietyofDCSSdesignshavebeendevelopedbytheindustryandlicensedbyNRC

sincethemid1980’s.ThehistoryofthisdevelopmentisdescribedinRef.12.Designsaredivided into twomajor types: thosewithweldedmetal canisters of SNF, and thosewithboltedmetalcasksofSNF.Table1summarizesthetypesandnumbersofDCSSsinuseintheU.S.over2009‐2012[13‐15].AppendixAitemizesthisU.S.experiencebyDCSSvendorandbyutility,asofFebruary2012[15].

Table 1. Dry Cask Storage Systems in Use in the U.S. 2009-2012

Type Cask Configuration/

Nomenclature

Number of Casks Cask Vendor March

2009 April 2010

February 2012

Welded Canister

Reinforced Concrete Overpack VSC, W150 NAC UMS and MPC TranStor Bolted Metal Overpack HI-STAR 100 Metal/Concrete Overpack HI-STORM Horizontal Concrete Module NUHOMS Subtotal

66 211 34

12

225

412

960

66 232 34

12

280

463

1087

66 266 34

12

394

603

1375

BNG Fuel Solutions

NAC

Holtec

Holtec

Transnuclear

Bolted Cask

NAC-128 TN CASTOR MC-10

Subtotal

2

128 26 1

157

2

133 26 1

162

2

145 26 1

174

NAC

Transnuclear GNS

Westinghouse

Grand Total

1117

1249 1549

4  

A noticeable feature in the use of DCSSs has been the gradual switch from boltedDCSSs tooneswithdouble‐lidded,double‐weldedcanisters forSNF inorder tominimizecost. Today, welded canisters represent 89% of all deployed DCSSs. Of course, boltedDCSSswill remain in service until final disposition of their SNF andmust bemonitoreddespitetheirdwindlingproportioninthetotalpopulation. 

The most frequently used welded‐canister type (NUHOMS) and bolted‐cask type(TN)ofDCSSarebrieflydescribedinordertocontrasttheirdifferinggeometriesandthelikelydifferingrequirementsformonitoringthem.

3.1 TransnuclearNUHOMS‐24PDCSS

In the most popular welded‐canister DCSS—the NUTECH horizontal modularstorage (NUHOMS) system—SNF assemblies are contained in a steel “dry shieldedcanister” (DSC)with twowelded lids (or coverplates), an innerandanouter.Figure2AshowstheDSCforstoringtwenty‐fourPWRassemblies,theNUHOMS‐24Pdesign.3Usingatransfer cask a loadedDSC is inserted horizontallywith a hydraulic ram into a concrete“horizontalstoragemodule”(HSM),locatedonareinforcedconcretepad(Fig.2B).

TheHSMisassembledon‐site fromprefabricatedreinforcedconcreteslabs, three‐feet thick.TheDSC isstoredhorizontallyonasteel support frame inside theHSMand iscooled by a flow of air entering the bottom and exiting the top of theHSM via shieldedvents. The vents have wire‐mesh screens to inhibit intrusion of debris and possibleblockage of air flow. A heat shield is positioned over the horizontal DSC to minimizeheating of the concrete roof slab of theHSM.An ISFSI normally containsHSM’s that areassembled in multiples of two back‐to‐back units that share common, two‐feet thickinteriorwalls.Moredetailscanbefoundinvendorpresentations[16,17].

Fig.2. The NUHOMS-24P DCSS (Courtesy: Transnuclear Inc.) A: The double-lidded dry shielded canister (DSC) for 24 PWR assemblies;

                                                            3 VariantsincludeDCSSsfor32PWRassemblies,andfor52and61BWRassemblies(seeAppendixA). 

5  

Fig.2. The NUHOMS-24P DCSS (Courtesy: Transnuclear Inc.) B: A four-unit horizontal storage module (HSM) containing DSC’s.

3.2TransnuclearTN‐68DCSS

TheTN‐68dry storage cask is typical of themost frequentlyusedbolted‐canisterdesignof verticalDCSS.ThisDCSS is sized to accommodate sixty‐eightBWRassemblies4andislicensedtostorefuelat60GWd/MTU,afteraminimumcoolingtimeofsevenyears[18].ThecaskisshownschematicallyinFig.3. Asshown,SNFassembliesarecontainedinaboratedsteelbasketwithintheTN‐68caskbody.Thisbody—theprimaryconfinementofSNF—isaweldedcarbonsteelcylinder,with awelded carbon steel bottom and a forged andwelded flange top. A flanged lid isboltedtothecaskbodywithametallicgasketastheprimaryclosure.Anoutersecondaryclosure is a lid that is gasketed and bolted over the top of the primary lid; the spacebetweenthelidsispressurizedtodetectheliumleakagefromthecaskinterior.

InOctober 2010helium leaksweredetected from twoTN‐68 casks (with bolted

lids) at Exelon’s Peach Bottom BWR [19]. One of the leaks from a main outer lid sealappearedtorequireunloadingthecask’sSNF.Theotherleakwasassociatedwithadrainportcoverweld,whichwillsimplyberepaired.Neitherleakreleasedradioactivityandbothleakswere successfullydetectedby themonitoring system for leak tightness.Both leakswereclassifiedas‘verysmall’.

                                                            4 VariantsincludeDCSSsfor32and40PWRassemblies(seeAppendixA).

6  

Fig.3. The Transnuclear TN-68 DCSS (Courtesy: AREVA).

3.3 FieldMonitoringofDCSSsandSNFHandling

CurrentNRCregulationsrequirethatDCSSsmusthavethecapabilityforcontinuousmonitoring inorder todeterminewhencorrectiveactionmustbe taken tomaintainsafestorageconditions[20],aswellastobeabletotestandmonitorcomponentsimportanttosafety. Inpractice,however,manycomponents important tosafestorageofSNFarenotmonitored or tested. These include the fuel, cladding, neutron poisons, fuel baskets, andfuel assembly hardware. It is assumed that the continued effectiveness of these com‐ponentsduringdrystorageforthecurrentlicensingextensionperiods(~40years)canbedemonstrated through analysis and testing before loading casks and by integrity of thecanisters/casks.The technicalbasis forprojected longer termstorage isbeingdevelopedbytheNRC.

Thefunctionalityandeffectivenessofcomponentsthatprovideradiationshielding,

as well as those related to overall structural integrity (neutron shielding, overpacks/storagemodules, and concrete pads), are ensured by periodic visual inspections and/ormeasurements taken with portable equipment/instrumentation. If necessary, remedial

7  

actioncanthenbetaken.Continuousmonitoringis,ingeneral,onlyappliedtocomponentsandstructuresthataredirectlyrelatedtoconfinement.Monitoringthepressureofthedrystoragecanisters/casksisoneapproachtodetectingthelossofconfinement.However,thiscanonlybeimplementedforcertaincaskswithboltedlids.

According to current NRC guidance the continuous monitoring requirements forDCSSscanbesatisfiedbyroutinesurveillanceprogramsandactive instrumentation[21].TheNRCstaffalsoaccepts thatdouble‐liddedcanistersclosedentirelybyweldingdonotrequiremonitoringoftheseals,because“apotentialleakpathmustbreachtwoindependentwelds,sequentially,beforetheconfinementsystemwouldbecompromised”[22]. For boltedcaskstheintegrityoftheconfinementismonitoredbymeasuringthepressureinthespacebetweentheinnerandoutertopclosurelidsthroughamonitoringportintheouterclosurelid.Hermeticallysealedpenetrationsareprovidedinthesupportandcoverplatestobringouttheleadsforthepressuremonitoringsystem.

Forweldedcanisters,thetestingportsandotherpenetrationsofthelidsaresealedafterdrying, inertingand testinghavebeen completed, andduringactualdry storagenomonitoringinstrumentationisretainedtocheckforleaktightness[23].

In addition to visual inspections, radiation surveys and confinement/pressuremonitoring, temperaturesaremeasuredby theoperatorsofsomeDCSSshavingconcreteover‐packsorstoragemodules[23].Thesesystemshaveairinletportsatthebottomofthemoduleandexhaustportsatthetop.Thispermitsnaturalconvectioncoolingofthecaskorcanister. At a minimum the exhaust air temperature at the top of the cask/module ismonitored. Significant increase in this temperature indicates overheating of the storagecask/canister. The air inlet and outlet ports are also inspected daily for blockages andremedialactiontakenwhennecessary.InDSCCsusingconcretemodulestohousecanistersof SNF (i.e.,NUHOMS) the temperaturesof themetalheat shieldsprotecting the interiorconcrete surfaces may also be monitored. Again this can indirectly indicate canisteroverheating.

Themajorhandlingof fuelassemblies,casksandcanisters forDCSSsoccursduringcaskloadingandunloading,andtransporttoandfromthestoragepad/module.TheactualprocedurestoaccomplishthesetasksarecomplexanddependonthespecificDCSSdesign.Typicallythefollowingmajorstepsareinvolvedinafuelloadingoperation:

Astorage (or transport) cask togetherwith its fuel canister isplaced in theSNFpool.

SelectedSNFassembliesareplacedinthesubmergedcask/canister. The cask/canister is lifted from the pool and drained ofwater. Draining or

pumpdownofwatermaybeassistedbypressurizingwithnitrogenorheliumandmayproceedinmultiplesteps.

The cask/canister is closed bywelding or bolting the inner lid to the cask/canisterbody.Ifnecessary,remainingwateristhendrained.

Moisture from the interior is removed by drying. Drying includes bothvacuum drying and purging with inert gas and may be repeated until thepressureintheisolatedsystemremainsbelowaspecifiedlimit.

8  

Thecanisterisbackfilledwithhelium,leaktestedandtheouterlidwelded(orbolted)tothecasks.Forboltedcasksthefreespacebetweeninnerandouterlid is evacuated, backfilled with helium and instrumentation installed formonitoringpressure.

The cask/canister is transported to the storage facility and either placeddirectlyontheconcretepadvertically,orinsertedhorizontallyintoaconcretemodule.

During these operations radiation will be encountered and it is constantlymonitored to ensure that ‘as low as reasonably achievable’ (ALARA) conditions aremaintained.Obviouslyduringallpurging,pump‐down,backfilling,vacuumdryingandleaktesting operations the appropriate pressures must be monitored. Temperatures aremonitoredduringmanystagesoftheseoperations,butmostimportantlyduringthedryingoperation so as not to exceed critical limits and cause permanent degradation of thecladding. If specifiedsystemtemperaturesareexceeded,remedialaction is taken.Duringwelding of the inner lid/shield‐plug continuous hydrogenmonitoring is required. If thehydrogen concentrationexceeds2.4%, allwelding is stoppedand the canister interior ispurgedwithinertgas.

4. DCSSPerformanceTestingandDemonstrationProjects

4.1 EarlyUSPerformanceTesting

4.1.1 TestsatNevadaTestSite

TheearliestDCSSperformancetestingwascarriedoutattheNevadaTestSite(NTS)in1978aspartof theSpentFuelHandlingandPackagingProgramDemonstration. ThisprogramwasaimedatdevelopingcapabilitytoencapsulatecommercialSNFassembliesincanisters and to establish by testing the suitability of concepts for interim dry surfacestorageofSNF.Todevelopthermalmodels,testswereperformedwithasealedSNFcaskintheEngineMaintenance,AssemblyandDisassemblyfacilityatNTS[24].

APWRspentfuelassembly,threeyearsoutofthereactor(25GWd/MTU,1.25kWpower),wassealedinastainlesssteelcanister.Thecanisterwasplacedinacarbonsteelliner and then in a reinforced concrete cask. The sealed storage cask was then placedverticallyonaconcretepad.Thermocoupleswereplacedontheoutsideofthecanister,theoutsideofthesteellinerandwereembeddedintheconcreteofthecask.Athermalmodelof the sealed storage cask was developed and its predictions were found to be in goodagreementwithtransientandsteady‐statetestdata.

4.1.2 TestsatH.B.RobinsonNGS

To confirm thermal and shielding models, another DCSS demonstration programwasperformed(1988‐1989)attheH.B.RobinsonNuclearGenerationStation(NGS)oftheCarolinaPowerandLightCompanyusingahorizontalNUHOMSDCSS thatoriginallyhadthreemoduleswith21SNFassemblies [25].Beforeusingactual SNFassemblies, electricheaters were used to perform experiments with both normal and blocked flow of air.

9  

Seventy‐six thermocouples were used in both the electric heater and the SNF assemblytests.Thethermocouplesmeasuredambient,outletair,canister,heatshieldandconcretetemperatures.Twocanisterswereinstrumentedwiththermocouplesontheoutershellofthe canister, end caps, and in the center of five guide tubes of the fuel assemblies. Tomaintain canister leak tightness, the thermocouple leadswere routedoutof the canisterthroughaspecialfitting.

4.2 LaterU.S.PerformanceTesting

Later DCSS demonstrations in the U.S. were performed at the Idaho NationalEngineeringandEnvironmentalLaboratory(INEEL)5startinginthemid‐1980s.SixDCSSswere shipped to INEEL in the 1980’s and stored there on a concrete pad (Fig.4). TwoDCSSs—a VSC‐17 cask containing a welded canister of consolidated (disassembled) fuelfromtheSurryandTurkeypointPWRs[26],andaCastorV/21boltedcaskcontaining21wholePWRfuelassembliesfromtheSurryreactor—wereloadedin1985andsubjectedtothermalteststovalidatethermalperformancecodessuchasCOBRA‐SFS[27].TheVSC‐17cask was heavily instrumented with 95 thermocouples, and was used to check outtemperatures under normal operation and for various configurations of blockage of thevertical air flow around the fueled canister. The Castor V/21DCSS had a thermocouple‘lance’thatallowedtemperaturemeasurementsinacentralfuelassemblyonly[28].

Fig.4. DCSSs at the INEEL in 2003.

4.2.1 ThermalTestsoftheVSC‐17DCSS The tests included assessment of thermal performance with a full load ofconsolidated fuel in seventeen channels in the so‐called multi‐assembly sealed basket(MASB) for four ventilation blockage conditions, and for vacuum, N2 and He backfillenvironments.Measured temperatures are given inTable2. Reasonable agreementwasobtained betweenmeasured temperatures and those predicted by the COBRA‐SFS code.Fuelburnupswere27‐35GWD/MTMandcoolingtimeswere8‐15years.

                                                            5KnownastheIdahoNationalLaboratory(INL)since2005

10  

Table 2. Peak Temperatures for the VSC-17 DCSS Loaded with Consolidated Fuel*

Run Number

Backfill/ Vent Blockage

Measured Temperature (oC) DCSS

SurfaceEst. PeakConcrete

Liner Surface

Canister Surface

Peak Fuel

1 He/None 37 69 82 136 316 2 He/ Half inlets 41 76 90 145 329 3 He/All inlets 56 132 152 202 373 4 He/ In&Outlet 56 141 161 212 376 5 N2/None 40 72 85 145 366 6 Vacuum/None 41 72 86 146 384

*DCSS heat load of 14.9 kW and ambient temperature of 21-24oC

4.2.2 ThermalTestingoftheCastorV/21DCSS

Temperaturesweremeasuredinanassembly(T11)nearthecenterofthiscaskwithathermocouplelancefortheverticalandhorizontalconfigurationandforavarietyoffillgases,includingN2,He,airandvacuum(seeTable3);theresultswereusedtosupportheattransfer codedevelopment.Thehighest temperatureachieved in thecladdingwas415oCduringa3‐daytestundervacuumintheverticalconfiguration.Themeasuredtemperaturefor a He fill was 357oC in the horizontal configuration versus 344oC in the verticalconfiguration.During14.2yearsofactualsealeddry‐storageconditions,thepeakcladdingtemperatureinanatmosphereofHewith<1%airwasbelievedtohavedeclinedfrom344to155oC.

Table 3. Time-Temperature History of Assembly T11 in Thermal Performance Tests of Castor V/21 DCSS

Cask

Configuration Cask

Atmosphere Peak Cladding

Temperature (oC)Duration

(hr) Vertical Vertical Vertical

Horizontal Horizontal

Vertical Vertical

He N2

Vacuum

He N2

70%He/30% air He/<1% air

344 359 415

357 398

348

344 to 155

119 43 72

93 72

2880 1.3 x 105

TheratherminoreffectsofcaskatmosphereandcaskorientationonpeakcladdingtemperaturesuggestthatthemajormodeofheattransferfromtheSNFinthisDCSSwasbymetal‐to‐metalconduction fromtheSNFassemblies to thebasket, fromthebasket to thecanisterwall,andfromtheretothecaskexterior.Basedontheseresults,thein‐leakageofairinsmallamountsseemslikelytohaveaminoreffectoncladdingtemperaturesduring

11  

drystorage.Butthisconclusionmaybeuniquetotheall‐metalCastorV/21DCSS,inwhichcomponentclearancesaresmalltobeginwithandbecomesmallerasthermalequilibriumisachieved,allowingheattransfertooccuroverwhelminglybyconduction.

Inconcretecasks,liketheverticalHI‐STORM100,oreventhehorizontalNUHOMSconfiguration, heat transfer is more a mixed‐mode process: metal‐to‐metal thermalconduction through to the canister exterior and then by transfer of heat via a ‘chimney’effectintheHI‐STORM100orbyconvectiveheatflowintheNUHOMSconfiguration.

4.2.3 Follow‐onWorkwithCastorV/21DCSS

Becausethesetestswerenotintendedtostudyfundamentalfuelbehavior,thefuelunderwentonlyminimalcharacterizationpriortothetests,suchasvisualexaminationofthe assemblies and ultrasonic examination to ensure that no breached fuel rods wereincluded. The temperature at various locations wasmonitored during the tests and thecovergaswasperiodicallyanalyzedtocheckifanyrodswereleaking;nonewasfound.

Afterdrystoragefor14.2yearsafollow‐onprojectwasinitiatedin1999underthejointsponsorshipoftheNRC,EPRIandDOE,toexaminetheSurrySNFstoredatINEEL.Theprojectwastoprovideconfirmatorydataforlicenseestocontinuedrystoragebeyond20years,aswellasforNRCstafftouseintheirtechnicalreviews.Theobjectiveswereto:

Obtaindatatoconfirmthepredictedlong‐termintegrityofdrycaskstoragesystemsandtheirSNFunderdrystorageconditions.

ProvidedatatosupportthetechnicalbasesandcriteriaforevaluatingthesafetyofDCSSandtransportationsystems,andforextendingDCSSlicenses.

The first phase of the program involved moving the Castor V/21 DCSS from theconcrete pad to the INEEL Test Area North (TAN) Hot Shop Facility. Helium cover gassampleswereextractedandanalyzed.Detailedtemperaturereadingsof thecaskexteriorandlocationsontheinteriorwereobtained.Radiationsurveyswereperformed.Videoandphotographic inspectionsof thecaskexteriorand interiorsurfaceswerecarriedout.Thelidseals,thefuelassemblyexteriorsandselectedfuelrodswereexaminedindetail.

The DCSS was then resealed, filled with helium and returned to its pad. In thesecondphaseoftheprogramtwelvefuelrodsthatwerepreviouslyremovedfromtheT11assemblyintheDCSSweregivendetailednondestructiveexaminationatANL.Fourofthetwelve fuel rods then underwent destructive examination to determine fuelmorphologyandcladdingmicrostructure,andformechanicalpropertytestingofthecladding.

4.2.3.1TheCastorV/21DCSS

The body of the Castor V/21DCSSwas a one‐piece cylindrical structuremade ofnodularcastiron.Thematerialhadgoodstrength/ductilityandprovidedeffectivegammashielding.Thecaskwas16fthighand8ftindiameterandweighedapproximately112tonswhenloadedwithPWRfuelassemblies.Twoconcentricrowsofpolyethylenerods inthe15‐inchthickcaskwallprovidedneutronshielding.Circumferentialheattransfer finsranaround the exterior of thewallwhichwas coatedwith epoxy paint. The cylindrical fuel

12  

basket fitted tightly in the5‐ftdiametercaskcavityandwasconfiguredtohold21spentPWRassemblies.Thebasketwasconstructedofstainlesssteelandboratedstainlesssteelfor criticality control. The basket layout provided criticality control and promoted heattransferbyconvection.

Thecaskwasclosedwithaprimaryboltedstainlesssteellid(6ftdiameter,1ftthick)which had bothmetallic (inner) and elastomer (outer) O‐rings to seal the cask interiorfromtheenvironment.Asecondarylidusedincommercialapplicationswasnotusedinthetestingprogramorthe long‐termstoragemonitoringprogram(1994‐1999).Theprimarylidhadthreepenetrationsforvariousoperationssuchaswaterfillinganddrainingaswellas vacuum drying and back‐filling with gas. The primary lid used in testing was notstandard and had ten additional penetrations for fuel guide tube instrumentation(thermocouplelances)andgassampling.

4.2.3.2DCSSInspectionsandTests

AllcomponentsoftheCastorV/21DCSSweresubjecttolongtermsurveillanceanddetailedinspectionstoidentifypossibledegradations.

Concrete Pad: The concrete pad on which the Castor V/21 DCSS was storedconsisted of 2 feet of concrete on top of 12 inches of a compacted gravel sub‐base. Theconcretewas reinforcedwith twomats of reinforcementbars embedded4 inchesbelowandabovethetopandthebottomsurfacesoftheconcretepadrespectively.A20ftx20ftsection of the pad centered on the location of the cask was inspected and tested forstructuralsoundnessbythe“Schmidthammertest”[29].

Schmidttestswereperformedatninelocationsandindicatedthattheconcretemetorexceededthedesignstrengthof4050psi(28MPa).Visualinspectionofthepadshowednoevidenceofstructuralfailureordegradation(cracks,displacement,spallation,aggregatepop‐out);thesurfacewassolidandshowedonlyminorwearandweatheringeffects.Testswithastraight/tautlineoverthe20‐ftx20‐ftgridshowednosagorverticaldisplacementunderthecaskposition.

CaskExterior,Lids,BoltsandSeals:Thecaskexteriorand lidswereexposed totheweatherfor14.2years.Theentirecaskexteriorwasvideoexamined,supplementedbydirect visual inspection, for evidence of degradation. In general only minor corrosion,scratchesfromhandling,andrustwerefound.Theprimarylidwastight,buttorquevaluesof the bolts were not checked during removal. All bolts were visually examined fordeterioration—cracks,pitting, corrosion,damage to thread;nonewasevident.Themetaland elastomer O‐rings were examined remotely by video camera immediately afteropeningthecaskandagainvisuallyaftersixmonths;finallytheO‐ringswereremovedforfurther examination. TheO‐ringswere in excellent condition, aswere the seal seats andcaskflanges.

Cask Interior:The accessible part (upper portion above fuel basket) of the caskinteriorwasinspectedremotelywithvideocamerasforevidenceofdegradation.Thelowerportionofthe interiorcouldonlybe inspectedthroughthefuel tubesorsomeoftheflux

13  

traps.Aminiature(pencil)camerawasusedforthis inspection.Nosignificantdamageordeteriorationwas foundon the interior sidewalls.Only scratcheswereseen,most likelyfrom the fuel basket insertion or from its expansion during the 1985 thermal tests.Similarlynodamagewasobservedatthecaskbottomandontheadjacentsidewalls.

Somedebriswasfoundonthecaskfloorandonthehorizontalbarsofthefueltubes.Samples of the debriswere retrievedwith tape swabs and examined visually aswell asusingelectronmicroscopyanddispersivespectrometry.Theresultssuggestthatthegrainswere mainly steel slivers or steel oxide from the fuel basket. Chemical analysis wasinconclusive. All samples showed low radioactivity, primarily due to Co‐60. Crackswerefound in a number of stitch welds in the fuel basket. Thorough examination in 1999concluded that all the cracks were caused by the thermal tests that were performed in1985andthatnonewcracksdevelopedduringthestoragephase.

FuelAssembliesandRods:Thefuelassembliesunderwentvisualexaminationandwere foundtobe ingoodcondition.Basedonmeasuredlift forcestheassemblieshadnotendencytostickduringextraction,althoughtheywere‘snug’ fits intothestoragebasket(Fig.5).

At the topof theassemblies the fuel rodsvaried in length by as much as 0.5 inch, whichwas ascribed to uneven irradiation. Except forslight localized bowing there was no otherdetectable deformation of the fuel rods. To lookfor cladding failure, the cask fill gas wasperiodically monitored over the 14.2‐yearstorage period for Kr‐85 gas release; none wasdetected.

4.2.3.3CaskTemperatures

As previously described, temperatures inside the cask were extensivelymeasured during the thermal performance tests performed in 1985 prior to thestart of long‐termstorage.These testswere conductedwith the cask for varioustimes, different fill gases (helium, nitrogen, air and vacuum), and in the vertical andhorizontalconfiguration.TemperaturesinsidethecaskwereagainmeasuredinSeptember1999justpriortoitsdetailedexamination.Sincethethermocouplelanceusedin1985wasnolongeravailable,thetemperaturesin1999hadtobemeasuredwiththeprimarylidoff.After 10 minutes equilibration, when the temperature rise was about 0.1oC/min., thetemperature inthehottestzoneofthehottestassemblywas154oC.Extrapolatingtofinalequilibration,itwaslikelythatthemaximumtemperaturewouldreach155to160oC.Theresultspertainonlytotheconditionsofthecaskatthetimeofmeasurement.Withthelidinplacethefinalequilibriumtemperaturewouldbehigher.

Using portable pyrometers extensive temperature measurements of the exteriorsurfaceofthecaskweretakeninAugust1999.Asexpectedthesetemperatureswerewellbelow the values measured in 1985 (on average nearly 30oC lower). The temperature

Fig.5. Snug fit of Assembly T11 in the storage basket in the Castor V/21 DCSS.

14  

measurementsweremadeatmanylocationsalongtheheightofthecask,atfourdifferentazimuthal orientations, and also at the top and the bottom of the cask. The highesttemperatureswere recorded at the center of the top, about 105oC in 1985 and 61oC in1999.Themaximumsidetemperaturesof thecaskwereapproximately82oCand49oCin1985and1999.Thetemperatureprofilesin1999wereverysimilartothosein1985.

4.2.3.4RadiationSurveys

Gammaandneutrondoseratesweremeasuredin1985andagainin2001afterthedetailedexaminationand return to storageof the cask.Measurementsweremadeat thesame locationsalong thesideof thecask (at severalorientations)aswell as the topandbottomof thecask. Inaddition in2001completecircumferentialdoseratesurveysweremade at three locations along the length of the cask. The measuring instruments whilesimilar were not identical in 1985 and 2001. Considerable scatter was found in themeasurements,particularly forneutrondoserates,with fieldmeasurementuncertaintiesrangingfrom10%to30%.

Inboth1985and2001thepeakdoserateswerefoundnearthetopandthebottomof the cask where the flow nozzles of the PWR assemblies were located. As expected,becauseof fissionproductdecay, thepeakgammadose rates in2001decreasedby90%from those in 1985 at all side locations, from over 100mrem/hr to about 10mrem/hr.Somenon‐uniformityingammareadingsaroundthecircumferencewasidentified.ThisisduetothehigherCo‐60levelsaroundtheupperflownozzles.Atthetopandbottomofthecask the 2001 gammadose rateswere found to be about 15‐20%of the 1985 readings,consistent with the decay of Co‐60. In both cases the readings were made without thesecondarylid.Thedoseratesfor1985werewellwithinthedesigngoalof200mrem/hr.

Duetotheextremely longhalf livesof fissileradionuclides,neutrondoserates forthe1985and2001readingsshouldbethesamewithinthemeasuringuncertainties.Thisheldforthemaximumreadingsalongthesideofthecask(about16mrem/hr).However,intheactivefuelzonethe2001readingswereabouthalfthereadingsobtainedin1985.Thiswasnotduetoshieldingchangesbutrathertothedifferentinstrumentsusedin2001thatexhibitmuch less sensitivity to thermalized neutrons. The dose profiles did not indicateanydeteriorationinthepolyethyleneshielding.

Atthetopofthecaskthepeakneutrondoseratesfor1985and2001wereidentical(45mrem/hr).At thebottomof the cask the2001neutrondose rateswere consistentlyhigher than those of 1985. There were also differences in the readings at differentazimuths.Thisdisparitywasanartifactduetothedifferentlocationandorientationofthecaskinthetests.In1985thecaskwasinahorizontalpositionintheTANfacility locatedwellabovethefloor.In2001thecaskwasorientedverticallyonlyabout18inchesaboveitsconcretestoragepad.Thedetectorswereclosetotheconcretesurface,whichthermalizedtheneutronsandcausedconsiderableback‐scatterthatdistortedthereadings.

15  

4.2.3.5SurveysofCaskAtmosphere

Gas samples from inside the caskwere taken starting in 1985, following the caskperformancetest, through1999whenthecaskwas filledwithhelium.Thesampleswereanalyzed using mass spectrometry. The readings in 1985 indicated, that considerableamountsofairwaspresentinthe16samples.Theaircontaminationwastracedbacktothesamplingtechniqueandwasnotrepresentativeofthecaskfillgas.Gassamplesweretakenagainin1986,usingabettersamplingtechnique,andthereadingsindicatedthatthefillgaswasprimarilyHe(betterthan99%)withonlytracesofairconstituents.ThesampleswerealsoexaminedwithgammaspectrometryforKr‐85.Thereadingswereverylowindicatingtherewerenofuelleaks.

The fill gasof theCastorV/21caskwasagain sampled,at leastonceayear, from1994 through1999.Theanalysisof the cover gas in1994 indicatedabout2%air in thesamplewiththeremainderbeinghelium.Thiswasduetotheinadvertentingressofairintothe cask during the sampling operation. After refilling the cask with fresh heliummeasurementindicatedabout0.5%airinthecovergas.Latersamplingsindicatedariseinnitrogencontentbutthatvaluestabilizedin1996.Atthesametimeoxygenconcentrationsremained constant and the helium concentrations up to the final readings in 1999remainedcloseto99%.Thegasanalysesdidnotpointtoanyingressofairintothecask.

Radiochemicalanalysisof thegassamplesobtainedduringthestorageperiodwasperformedtocheckforthepresenceofC‐14andKr‐85.ThereweresomeanomaliesintheC‐14 data, indicating increases over certain time periods. Similarly inconsistencieswerefoundintheKr‐85data.However,thereadingswereallclosetothedetectionlimitsoftheinstruments.Basedontheoverallanalysisofthesedataitwasconcludedthattherewasnoindicationoffuelrodcladdingdamageduringdrystorage.

4.2.3.6FuelRodExaminations

Twelve fuel rods extracted from theT11assemblyweremeasuredforchangeindiameter by averaging the profilometrymeasurements made at four orientations.Theaveragecreepdownofthecladdingwasfoundtobe~0.6%.Thisvaluewastypicaloftheas‐irradiatedvaluesforPWRfuelrodsofthe Surry fuel‐cladding gap size and burn‐up, so that creep at temperature during drystorage had been small. The average profilesexhibited slight depressions at the location ofthegridspacers,asshowninFig.6.

Fourfuelrodswithmarginallyhigherdiametersthantheotherswerepuncturedforgasanalysis.Thegascontentwas96‐98%HewithtracesofN2(<0.03%)andO2(<0.01%),with the remainder fission gas. Values for fission‐gas release ranged from 0.4 to 1.1%,againvaluesthatweretypicalforPWRfuelatthisburn‐uplevel.

Fig.6. The average diameter profiles of rods from the T11 assembly, showing creep down of ~0.6%, and depressions at grid spacers.

16  

Two of the rods were sectioned andmetallographic samples were prepared andexamined. There were no unusual featuresobserved in the UO2 microstructures, exceptperhaps for the appearance of fission‐gasporosityinthecenterofpelletsabovethecoremidplane, where temperatures were highest.Againsuchbehavioristobeexpected.

The cladding had oxide layers thatvaried in thickness in the range of 20‐40μmfromthefuelcenterlinetothetopofthecore,expectedvaluesforthisburn‐uplevel. Figure7showsthetypicaldistributionofhydridesinthe claddingwall. In all cases examined hydrideprecipitates were oriented circum‐ferentially.Thus, the heat transfer test at 415oC in vacuumfor72hourshadnotcausedradial reorientationof hydrides in the cladding of this low‐burnup fuel—an important finding. There was a≤25%reductionincladdingmicrohardnessmeasuredonthecladding;thisreductionwasprobablycausedbythethermaltestinvacuuminwhichpeakcladdingtemperatureswere415oC.

4.2.3.7Summary

InspectionoftheCastorV/21DCSSanditscontentsyieldedreassuringlypositiveresults on the ability of this type of bolted DSCC to safely contain low‐burnup SNF.Specifically, the metal cask itself showed no evidence of material degradation in thecomparativelyaridclimateofIdaho.Hammertestsoftheconcreteinthevicinityofthecaskindicatedtheconcreteretaineditsdesignstrength;theconcretealsoexhibitednocracking,spallingorsaggingunderthe112‐tonweightofthecask.Thesealgasketsappearedtobeingoodconditionandevidence fromperiodicgassampling indicatedno inward leaksofairhadoccurred.TherewasnoindicationofKr‐85activityinthecaskinteriorsothatnofuelfailureshadoccurredasaresultofthedrystorage.

Assemblies were easily removed from the inner storage basket for inspection,despite their snug fit. The assemblies were in good conditionwith no unusual features.Oxidebuildupon the claddingof the limitednumberof rods (12)whichwere examinedwas similar in depth and appearance to that observed on wet‐stored fuel at the sameburnup.Therodsexhibitedtypicalcreepdownofthecladdingandevidenceofsomerod‐spacermechanicalinteraction.Thefission‐gasreleasesof0.4‐1.1%infourrodsthatwereanalyzedwerewellwithintherangemeasuredonas‐irradiatedrodsatthisburnup.Thatis,therewasnoincreaseinfission‐gasreleaseduetostorage.

No microhardness measurements were made on the as‐irradiated cladding butbasedonmeasuredvaluesonslightlylowerburnupfuelfromtheTurkeyPointPWR,someannealing of radiation damage had occurred, quite probably due to the thermal

Fig.7. The cladding on Rod H9 from Assembly T11 at 1000 mm above midplane, exhibiting circumferential distribution of hydride precipitates

17  

performancetest invacuuminwhichmeasuredpeakcladdingtemperatureswere415oC.Finally, microscopy revealed that there had been no significant radial reorientation ofhydrideprecipitatesintheZy‐4claddingdespitethishigh‐temperatureoperation.

4.3 JapaneseWork

WorkinJapanondrycaskstoragehasbeenmainlyperformedbytwoorganizations:actualmonitoringofall‐metaldrycasksbyTokyoElectricPowerCompany(TEPCO)inunit5ontheFukushima‐Daiichisite,andasetofmoregeneralizedtestswithconcretecasksbytheCentralResearchInstituteofElectricPowerIndustry(CRIEPI).

Asof June2010, theTEPCOsitehad408BWRassembliesstored innineall‐metalcasks (five large casks containing 52 assemblies each, and four smaller casks with 37assemblieseach).TheTEPCOworkwaspresentedatanIAEAworkshopin2010[30],whiletheCRIEPIworkwasdescribed inaspecialeditionofNuclearEngineeringandDesign in2008[31].

4.3.1 DCSSMonitoringandInspectionattheFukushima‐DaiichiSite

The TEPCODCSSs are double‐lidded, bolted and gasketed casks of forged carbonsteelwithboratedresinforneutronshielding.Eachcaskisstoredhorizontallyonaframe.AsinU.S.boltedcasks,thespacebetweentheinnerandouterlidsispressurizedwithHeandmonitoredforchangeinpressure(Fig.8a).Othermonitoredparametersarethesurfacetemperatureof the cask, thedifferencebetween the inlet andoutlet temperaturesof thecaskmodule,andarearadiationinthebuilding.Figure8BshowsoneoftheDCSSsin2010.

Dry cask storage operations began in 1995. First inspection of a DCSS wasperformedin2000after5yearsstorage,andasecondinspectionwasmadein2010after10 years storage. The cask with maximum heat load was inspected on each occasion.Inspectionincluded:gassamplinginordertodetectKr‐85activity;visualinspectionofthegaskets;leaktestsoftheprimarylid;andvisualexaminationofthefuelcladdingintwofuelbundles.NoKr‐85wasdetectedoneitheroccasion,measuredleakratesoftheprimarylidwasafactoroftenlowerthantherequiredleakagecriterionof<1x10‐6Pam3/s,andthefuelcladdingexhibitednodefects.

Theonlynotedabnormalitywasawhitediscolorationofgasketsurfacescausedbyresidualwater;procedureswerechangedtominimizecaskimmersioninthepool.BeforetheMarch2011tsunami,thenextplannedinspectionwastobein2015.

Remarksbya JapaneserepresentativeatanEPRI/DOE/NRCmeeting inDecember2011[7]indicatedthatthebuildinghousingtheDCSSscontainedalargequantityofsanddepositedbythetsunami.AlthoughnofurthercommentsweremadeabouttheconditionoftheDCSSs,onecanassumethat instrumentationwas lostduring theaccident. Therearenowlikelymoreurgenttasksthantheinspectionofthesedrycasks.

18  

Fig.8a Schematic of DCSS monitoring at Fukushima-Daiichi (Courtesy TEPCO)

Fig.8b Photo of all-metal DSCC at Fukushima-Daiichi before March 2011 (Courtesy TEPCO)

4.3.2 CRIEPIWorkonDCSSs

Thehighcostofall‐metalDCSSsgaverisetoconsiderableresearchbyCRIEPI intolessexpensiveconcretesystems[31].ThepreferredJapanesedesignisnowsimilartotheHoltec HI‐STORM DCSS; i.e., a double‐welded, double‐lidded SNF canister in a vertical,naturallyair‐cooledconcreteoverpack.CRIEPIresearchhasincludedworkon:

stabilityduringearthquakes,andotheraccidents[32];

usingheaterassembliestosimulateSNF,temperatureincreasesduetoblockageofthecaskairintakes[33];

usingheaterassemblies,methods tomonitorHe leakage fromanSNFcanister[34],andin‐situdetectionofcladdingfailurebymeasuringKr‐85release[35];

stresscorrosioncrackinginamarineenvironmentofboththecanister[36]andtheconcrete(anditsrebar).

19  

4.3.2.1BlockageSimulation

Theblockagesimulationswereperformedontwotypesofcask:reinforcedconcrete(RC)andconcrete‐filledsteel(CFS),showninFig.9.Theheaterinventorygaveatotalheatload of 22.6 kW; calculations suggested 80% of the heat (18.1 kW) was removed byconvectiveair flow,12%(2.76kW)was lost fromthecasksides,and8%(1.84kW)waslostfromthecasktop.MeasuredcomponenttemperaturesaregiveninTable4.

Fig. 9. Schematics of the concrete casks used in CRIEPI tests (Courtesy CRIEPI)

Table 4. Temperatures (oC) in TEPCO Heat Removal Tests of Concrete Casks

Cask Type RC CFS Blockage None 50% None 50%Inlet Air 33 33 33 33

Outlet Air 98 103 85 99 Air ∆T 65 70 52 66

Canister Surface

209 214 192 200

Guide Tube

301 306 228 235

Storage Container

91 96 83 93

The tests suggested that even with 50% blockage of the cask air intakes,temperatures inthefuelassemblieswouldbebelowvaluesthatwouldcauseconcernforfuelelementstability.However,thecontainertemperaturesforbothcasktypeswerecloseto (or above) the recommended maximum operating temperature for concrete (90oC).Testswerealsorun for full (100%)blockageof thecaskair intakes,but thetemperaturevalueswerenotgivenintheCRIEPIpresentation.

20  

Fig.10.Changesof∆TBTandpressureinaCFScask(CourtesyCRIEPI,Ref.34).

4.3.2.2DetectionofCanisterLeakage

Another cask mockup usingheaterswasused toexplorepossibledetection of leakage of He from anSNF canister. The axial variation inSNF canister temperature wasmeasured by an array of 11 thermo‐couplesonthebottom,uptheverticalside, and on the top of the canister.TheheliumatmosphereinaCFScaskwas bled down from an internalpressureof56kPAtoapressureof5kPaovera4‐dayperiod.

The difference in temperaturebetween the bottom and top of thecanister, ∆TBT,was found to increasenoticeablywithdecrease inpressure.After 4 days, ∆TBThad increased byapproximately 8oC (Fig.10). Theresearchersconcluded thatwhenHepressure was high, the temperatureintheupperpartofthecanisterwaselevatedduetothermalconvection.Asheliumleakedandpressuredecreased, therewasanincreasing lackofconvectiveheattransfer,causingthetemperatureatthecenterofthecanisterbottomtoriseandtemperatureatthecenterofthecanistertoptofall.

Leakageof the cask atmosphere could thus, in theory, bemonitoredbymeasuring∆TBT.Ifheliumleakagewasveryslow,however,itmightbedifficulttodifferentiatebetweenagenuinetemperaturedifferenceduetoleakageandsimpledriftofthethermocouplereadings.It shouldalsobenoted, that this technique isonlyapplicable toverticallyorientedcaskswith concrete overpacks or concrete shielding, where convective heat transfer plays animportantrole.

4.3.2.3DetectionofFuelFailure

Themethodofmonitoringfuelfailureduringstoragebydetectingthe514‐keV‐rayofKr‐85intheDSCCatmospherewasinvestigatedbyCRIEPIworkers[35].TheprincipleofthemethodisshowninFig.11.Ashieldedhigh‐resolutionGedetectorinthemodifiedlidofaDCSSinterrogatesthegasspaceatthelocationofnoSNF,i.e.,inaverticallyemptyslotinthe SNF basket. Trials were performed with a mockup of the detector and a kryptonsource.Theinvestigatorsconcludedthemethodwasfeasible.

21  

Fig.11. Method of detecting fuel failure by measuring Kr-85 activity in the DCSS atmosphere (Courtesy CRIEPI, Ref.35)

Therearetwodrawbacks: first, therequiredgeometry(Fig.11)meansthemethodcanonlybeusedwithspeciallydesignedDCSSs;second,Kr‐85hasahalf‐lifeof10.7years,meaningthemethodwillbeunabletodetectfuelfailureoccurringmuchbeyond40‐50yearsafterreactordischarge.TheonlyotherradioactivefissiongasisotopeisKr‐81,withahalf‐lifeof210,000years,andwithanactivityandyield far too lowfordetection. ApossibleNDEmethodofdetecting fuel failure(s) in aweldedcanister—involvingmeasurementofthespeedofsoundinthecanisterplenum—isdescribedin§5.6.

5. ImplementationofDCSSMonitoring

5.1 DryCaskDemonstrationProgram

A long‐term dry cask demonstration program (DCDP) is being considered by theNRCtoevaluateifextrapolationsmadefromshort‐termdataarevalid.Suchaprogramwillallowunforeseendetrimentaleffects to longtermDCSSperformancetobe identifiedandaccommodated.PeriodicinspectionandmonitoringofDCSSsusedintheDCDPisessentialto identifying aging mechanisms and possible issues with welded canisters and boltedcasks.Potentialdegradationof theexternalstructuresofDCSSsoververy longperiods isequally important,because thesestructuresprovide radiationprotection.Thus, effectsoftheexternalenvironmentonmetalor concreteoverpacksneed tobeevaluated, togetherwiththeimpactofheatfromtheSNFitself;periodicinspection(orwherepossible,directmonitoring)ofoverpacksisasimportantasforthecanistersorcaskstheycontain.

TheCastorV/21DCSSexaminationdemonstratedthatwithsufficientresourcesandproperfacilitiesitispossibletoconductacomprehensiveDCDP.Althoughthisearliereffortwasafterstorageofonly14years,thereappearstobenofundamentalobstacletocarryingoutaDCDP fora significantly longerperiodof time.Obviously, organizational continuityand long‐term financial support are essential for success. There are several otherrequirementsandinspectionmethodsforasuccessfulDCDPthatareoutlinedbelow.

22  

5.1.1 ChoiceofDCSSDesign

Themajority(89%)ofDCSSstodaycontainweldedcanistersofSNF.Topermittheperiodic examinationof SNF indry storage, a bolted closure, as used in theCastorV/21DCSS,seemsmorepractical;however,bothtypesofclosuremustbeincludedinaDCDPasbothareusedcommercially.Formonitoringtheinterior,closureswillneedtohavespecialfittingstorouteinstrumentationleadsandtopermitsamplingoftheinterioratmosphere.Suchroutingisproblematicfordouble‐liddedSNFcanisters.

Theinternalconditionsofacanisterorcaskatthestartofdrystorageshouldbeasprototypic aspossible. Inparticular, the initial loadingof fuel assemblies shouldbe fullyrepresentativeofindustrialpractice.Thisneedstoincludetheloadingoffuelassembliesinaspentfuelpoolenvironment,followedbytypicaldraining,dryingandinertingoperations.Forexample, in themostextensivepastdemonstrationproject—theCastorV/21DCSS—fuel assemblies were loaded into a fresh,completelydrycask, different from the cask inwhichtheyweretransported.Thisprecludedinvestigatingtheeffectsofresidualmoisture.

5.2 NeedforRuggedInstrumentation

Measurementsinthepastwerecarriedout with mainly standard equipment. Whilethe techniques, sensors, and instrumentationusedinpastSNFprojectsliketheCastorV/21test could, in principle, be adopted here, thechallenge of the DCDP is to extendobservations andmonitoring over potentiallymuch longer periods of time. This in turnimplies that sensors, cabling and electronicsmustbeofextremedurabilityandstability,orare readily replaced. Redundancy in sensors/

instrumentation could also help to overcome thelongdurationrequirement.

On the other hand, since dry storage processes progress slowly andconditions inside storage canisters are not extreme, instrumentation doesnot require fast response timesorbe capable of operating in extremeconditions, as is the case inmonitoring someaspectsofnuclearpowerplantoperations. But for dry storage of this long duration accurate measurement will be challenging,requiring instrumentation that will not degrade; or is self‐calibrating, like the Johnsonnoise thermometer [37,38]; or can be readily replaced. The instrumentation must alsoremainfunctionalinsevereenvironmentalconditionsand/oraccidentconditions.Tomeettherequirementfor longevity,simpleruggedequipmentthatisreadilycalibratedmaybetheanswer.TheSchmidtHammer,usedtoteststrengthofconcrete[28],isagoodexample(Fig.12);itcomeswithitsowncalibrationanvilsofknownstrength.

5.3 TemperatureMeasurements

Fig.12.TheSchmidtHammerusedfortestingtherelativecompressivestrengthofconcrete(advertisementphoto).

23  

To avoid degradation of high‐burnup cladding by radial reorientation of hydrideprecipitates, ANL work has shown that the temperature inside canisters during dryingmust bemaintained below 400oC [39,40]. Cladding temperatures are presently inferredfrommeasurementsonthecask/canisterexteriorandfromcalibratedheattransfermodelsoftheDCSS.

In all past dry storage demonstration projects, as well as in commercial storage,temperatures are primarily measured using standard Type J or Type K thermocouples.Thermocouples are rugged sensors with successful long‐term application in the nuclearindustry. However, they suffer from the inherent problems of drift, and thus requireoversightandperiodicrecalibration.Thisinturnmakestheirusesomewhatproblematicinaverylongtermandpassiveapplicationlikedrystorage.Recentevaluationsofemerginginstrumentation technology for nuclear power plants [37,38] suggest several alternativetemperaturesensorsthatmayalsobeapplicabletomonitoringSNFdrystoragefacilities.Thesealternativesarebrieflydiscussedbelow.

5.3.1 TypeNThermocouplesTypeN(Nicrosil‐Nisil)thermocouplesweredevelopedinthe1970sand1980sasa

lowerdriftalternativetootherbasemetalthermocouples.Sincetheyweredesignatedasastandard thermocouple type by the Instrument Society of America in 1983, Type‐Nthermocoupleshavebeenwidelyused innon‐nuclear applications formore than twentyyears.TheNicrosilandNisilalloysinTypeNthermocouplesweredevelopedspecificallytoovercome the instabilities of other base metal thermocouples. Nicrosil and Nisil alloycompositions feature increasedcomponentsoluteconcentrations(chromiumandsilicon)in the nickel base to transition from internal to surfacemodes of oxidation and includesolutes (silicon and magnesium) which preferentially oxidize to form oxygen diffusionbarriers. Moreover,TypeN thermocoupleswerespecificallydesigned for improvedhighfluence neutron performance by eliminating all elements with high neutron absorptioncross sections from the compositions of the thermoelements. TypeN thermocouples arenowwidelyavailablecommerciallyatsimilarcosttootherbasemetalthermocouplesandwithsimilarvaluesofthermoelectricvoltageoutput.

5.3.2 JohnsonNoiseThermometers

Johnson noise is a first‐principles representation of temperature. Fundamentally,

temperatureismerelyaconvenientrepresentationofthemeankineticenergyofanatomicensemble.Because Johnsonnoise isa fundamental representationof temperature, ratherthan a response to temperature such as electrical resistance or thermoelectric potential,Johnsonnoiseisimmunefromchemicalandmechanicalchangesinthematerialpropertiesof the sensor. The Nyquist equation gives the relationship between temperature,resistance,andvoltagegeneratedasfollows:

V2=4kBTR∆f,

24  

whereV2is themeansquaredvalueof thevoltage—also calledpower spectraldensity—acrossaresistorofresistanceR,kBisBoltzmann’sconstant,Tistheabsolutetemperatureof the resistor, and ∆f is the measurement bandwidth. To measure temperature usingJohnsonnoise, the frequency responseof the total systemmustbeknownaswell as theresistance. Temperature is then computed by dividing the power spectral density of thenoise voltage by 4kBR. Because of the statistical nature of the voltagemeasurement, themeasuredvaluecanbedistortedbyhighnoisecontent.Thenoiselevelcanbereducedbylongerintegrationtimeofthemeasurement.

One way of employing JNT is as a continuous, first‐principles recalibrationmethodology for a conventional resistance‐based temperature measurement technique.The direct method of measuring temperature from a resistance temperature detector(RTD)hassignificant,unavoidabledrift.TheJNTmeasurementisappliedinparalleltotheRTD leadwiresof the resistancemeasurement circuitwithoutaltering that circuit. SinceJohnson noise is a first‐principlesmeasurement, it does not require periodic calibration.The combinedRTDand Johnson‐noise temperaturemeasurement approachprovides thespeedandaccuracyofresistancethermometryaswellasautomaticcalibration.

Figure13illustratesthecombinedmeasurementprocess.TheRTDisexposedtothetemperature, and exhibits both a resistance value and Johnson noise. Since these twosignals are separable, they can be processed independently. The RTD’s resistancetemperaturevalueiscomparedwiththeJohnsonnoisetemperature,andacorrectionmadetothetransferfunction.ThiscorrectionismadecontinuouslyorperiodicallydependingontheRTDdriftandtheacceptablemeasurementuncertainties.ThecombinedRTD/Johnson‐noisesystemprovidesprompttemperaturemeasurementswithhighaccuracy.

Fig.13 Block diagram of the Johnson noise temperature measurement process (Courtesy ORNL; Ref.38).

5.3.3 UltrasonicTemperatureSensors

Although the fieldofU/S temperaturemeasurementhasmanyvariants, thewire‐

line,pulse‐echoU/SsensormaybeespeciallyadaptabletotemperaturemeasurementsinDCSSsbecauseofitsruggednature.Experimentalstudiesinreactorsafetyusingultrasonic

25  

wire‐line thermometry were performed more than forty years ago. Continuousdevelopment has advanced the technology such that ultrasonic wire‐line thermometrysystems are currently available commercially. However, the technology has not beenwidelyusedinnuclearapplications.

Ultrasonicwire‐linethermometryisbaseduponthechangeinthevelocityofsoundinawirewithtemperature.Thespeedofsoundinawirevarieswiththeelasticmodulusanddensity,asgivenintheequationbelow.Bothparametersaretemperaturedependent,butthetemperatureeffectonelasticmodulusdominatesbyaboutanorder‐of‐magnitudeover that of density. This causes the velocity of sound c to decrease with increasingtemperatureaccordingto:

c=[E(T)/ρ(T)]1/2,

whereEisYoung’smodulusandρisdensity,bothfunctionsoftemperatureT.

In U/S wire‐line temperature measurement an extensional wave is sent down awaveguide.Thereturn timeof reflectionsof the launchedwavepulseare thenrecorded.Thewire‐linecontainsaseriesofnotches,andthetimedifferencebetweenreflectionsfromthenotchesisindicativeofthetemperaturedifferencesbetweenthenotches(Fig.14).

Fig.14 U/S thermometry system with a notched waveguide (Courtesy ORNL; Ref.38).

5.3.4 Fiber‐opticTemperatureSensors

Thesesensorsarelikelytobedeployedinnuclearpowerplantsinthefuture.Fiber‐opticsensorshaveadvantageswhichmakethemattractiveforuseinharshenvironments,e.g.,resistancetocorrosion,highreliability,andrelativelyhighaccuracy.However,theyareaffected by radiation. In particular in mixed high‐neutron/gamma fields these sensorsexhibitsometemperatureoffset.Iftheseshiftsincalibrationcanbecompensatedforwithanon‐linesensingorcalibrationmodel, theFabry‐Perot temperaturesensor,orasimilarfiber‐optictechnology,couldbesuitableforDCSSs,whereradiationlevelsarelesssevere.

Distributedfiber‐opticBraggthermometryisbaseduponaseriesofBragggratingsarrangedalongthecoreofasingle‐modeopticalfiber.ThetemperaturedependenceoftheBragg wavelength of a fiber‐optic Bragg grating (FBG) originates from the thermal

26  

expansionofthefiber,whichresultsindetectablevariationintheopticalindexofthecore.FBGs are known to respond to variations in multiple parameters such as load, strain,vibration,andtemperature,andcanmeasuresomeoftheseparameterssimultaneously.

The two advantages of distributed fiber‐optic Bragg thermometry are that thesensorisnonconductive,allowingdeploymentinharshelectromagneticenvironments,andthat many sensors can be configured along a single path enabling acquisition of adistributedtemperaturemapwithasinglereadoutsystem.ThiswouldmakeitpossibletoobtaindirectlythetemperatureprofilesalongtheheightorcircumferenceofDCSSsinsteadofrelyingonmultiplesinglepointmeasurements.

Temperature measurements, at least on the cask exterior, should be performed

periodicallyoverthefullheightoftheDCSSandoveranumberoforientations.Significantchangesinthetemperatureprofiles(notmagnitudes)couldindicategrossdegradationinthefuelassemblies,e.g.,fuelrelocation.Todate,measurementoftheexternaltemperature(e.g.,on theVSC‐17DCSS)using thermocoupleshasbeenanunwieldyprocess,bedeviledbythedifficultyofmakinggoodthermalcontactwiththeconcretesurface.

5.3.5 ThermalImaging

External temperaturemeasurements could be alternatively performedby thermalimaging using infrared cameras, which are readily available commercially. For example,Figure15showstheimage(left)ofafuseboxwithacentral,overheatedfuse,whichwasobtainedwith the IR‐CAM59X IR camera (right)marketed by Sierra Pacific Innovations[41]. Such an IR camera could be set up to systematically scan the exterior of a DCSS;comparisonof scansperformedatdifferent timescould then indicate relativechanges inSNFdistributioninagraphicfashion.

Fig.15 Thermal Imaging with an Infrared Camera Left: Image of a fuse box with overheated central fuse

Right: The IR-CAM 59X IR camera used for its imaging (Courtesy: Sierra Pacific Innovations)

5.4 SurfaceInspectionofComponents

27  

5.4.1 BorescopeInspection

CorrosionisthemajorconcernforallstructuresandcomponentsofDCSSssothatperiodic inspection of the cask or canister exterior as well the interior of the concreteoverpack must be a part of a DCDP. Inside the canister SCC may be dominant, whileexterior structures may be additionally susceptible to atmospheric corrosion, includingthat from marine environments, and aqueous corrosion. Inspection can be carried outusinghigh‐definitionvideothatproducesdigitalimages.

Becauseofhighradiationlevels,inspectionoftheinteriorofconcreteoverpacksandthe surface (and possibly theweld region) of canisters needs to be performed remotelyusingradiationresistantvideocameras.Successfulsurveyshavealreadybeenperformed.ComponentsintheverticalcoolingannulusoftheVSC‐17DCSSwereinspectedin2004and2007[33,42]usingseveralminiatureborescopesavailablecommercially;twoofthemareshowninFig.16.

The borescopeswere inserted through the vent holes at the top of the DCSS andgently loweredbytheirelectrical leadsdowntheverticalcoolingannulus,whichwas76‐mm(3‐in)inwidth.Inthiswayaxialscansofthesteelcanisterandtheinnersteellineroftheconcreteoverpackwereperformedoverasignificantareaoftheannulus.SomeimagesobtainedthiswayareshowninFig.17.Measurementsofradiationinthecoolingannuluswere performedwith a Merlin‐Guerin Products Geiger‐Mueller ‐ray detector AMP‐200;radiationlevelsupto2,500R/hrweredetected.

Fig.16 Borescopes used to inspect the cooling annulus of the VSC-17 DCSS Left: Toshiba Model 1K-M44H; Right: Everest/VIT XLPro (Courtesy Winston; Ref.42)

28  

Fig.17 Images of components in the vertical cooling channel in the VSC-17 DCSS, obtained by CRIEPI workers in collaboration with INL investigators.

(Courtesy: K. Shirai et al., Ref.33)

A similar inspectionwas performed of the SNF canister in aNUHOMSHSM at anOconeePWRafter5yearsdrystorage[43];nothingadversewasobserved.InspectionwasfacilitatedbythelargecomponentclearancesintheNUHOMSDCSS.

SuchvisualinspectionistheonlycurrentlyfeasiblewaytointerrogatetheconditionofweldsonSNFcanisters.ForNUHOMSDCSSslocatedontheseacoast,visualinspectionofouterweldsforevidenceofbrinedepositscouldyieldindicationofimpendingproblems.AfieldinspectionplannedfortheCalvertCliffsISFSIlaterin2012[44]mayallayfearsofSCCofcanisterweldsandwallsinamarineclimate.NDTexpertsatTNarealsolookingattheuseofSaltSmart[44,45]couponsformonitoringsaltdepositiononSNFcanisters.

5.4.2 ElectromagneticAcousticTransducer(EMAT)Inspection ConventionalU/Sinspectionofsurfacefeaturessuchasweldsrequirestheuseofacouplingmedium(orcouplant),typicallywater,betweenthesurfacefeatureandtheprobe.Sucharequirementwouldmake itverydifficult toultrasonicallyscanDCSScomponents.Analternativeistouseanelectromagneticacoustictransducer(EMAT),whichrequiresnocouplingmedium[46]. Theprincipleofthismethodistouseahigh‐frequencycurrentinacoilheldclosetothetestsurface.ALorentzforceisthenproducedinasurfacelayerofthemetaltestarticle,settinguplatticevibrationsofthesamefrequencyandproducingU/Swaves.Theelectro‐magnetic interaction between the probe and material allows measurement to be madewithout mechanical contact and without a coupling medium. Figure 18 contrasts theoperationofastandardpiezoelectricU/StransducerwiththatofanEMATtransducer.

29  

Fig.18 An EMAT UT compared with a conventional piezoelectric UT

EMATisbeginningtobeusedasanalternativetoconventionalU/Stestingforweldinspectioninheavy‐gageausteniticsteels.Forexample,Fig.19showstheresultsobtainedbyGaoetal.[47]usinganEMATprobeintheshearhorizontal(SH)wavemodeona2‐inchthick,2‐inchwideausteniticweldzone.

(b) Inspection from upper side (c) Inspection from lower side

Fig.19. EMAT Inspection results of sample with six thermal fatigue cracks (A-E). (Courtesy; Gao et el. Ref.47)

30  

Notethatuseful informationcanbeobtainedbyscanningfrombothsidesofthetestpiece—thisabilitycouldbeextremelyuseful in interrogatingtheconditionofthecircum‐ferential closureweldson the lids of SNF canisters,where the lids are recessed into theboreofthecanister.EMATseemstobetheonlyNDEmethodforinvestigatingtheconditionof the welded inner lid of SNF canisters. Cited disadvantages of EMAT probes are theirlargersizeandlowersignalscomparedwithconventionalU/Sprobes.FurtherR&Dseemslikelytominimizethesedisadvantages.5.5 MonitoringCask/CanisterLeakTightness

TheoverarchingrequirementofdrystorageisforcompleteconfinementofSNFtoprevent the release of radionuclides to the environment. Leak tightness ensures nocorrosion insideandthat theheliumfillgasmaximizesheat transfer fromtheSNF.Thus,leak tightness must be monitored by whatever means possible. For casks with boltedclosures,confinement(leaktightness)isdirectlyconfirmedbycontinuouslymonitoringthepressureinthegasspacebetweentheinnerandouterlids;itisthemethodusednowforboltedDCSSs.Apressurechangeactivatesanalarm.

NoanalogousdirectmethodexistsfortestingtheleaktightnessofanSNFcanisterwith welded primary and secondary lids. It is assumed that the two welded lids will,together,preclude lossofconfinement.This isaveryreasonableassumption intheshortterm,butpossiblynotinthelongterm,whenweldtemperaturesapproachambientvalues,moisturecancondense,andsaltspraymaydeliquescetopromoteSSCofwelds[48].

Apossibleindirectmethodofleakdetection—forverticallystoredcanistersonly—mightbetopressurizeSNFcanistersafter loadingandsubsequentlyusethe∆TBTmethoddevelopedbyCRIEPI[31]toannunciateleaks,seeFig.10in§4.3.2.2.Suchamethodrelieseither on the placement of a number of thermocouples or other temperaturemeasuringdeviceson thecanister,or theuseof thermal imagingdescribed in§5.3.5 to indicate the∆TBTduetoleakage.

Anotherindirectmethodtocheckforheliumleakagefromacanisteristomeasurethe speedof sound (SoS) in samplesof air from theoutletventof aDCSSusing the leakdetectorfortracegasesdevelopedbyANL[49].ThemethodreliesonthefactthattheSoSisdifferentfordifferentgases,seeTable4.AsmallamountofheliummixedwithairmayincreasethemeasuredSoSsufficientlytomakealeakdetectable,asshowninFig.20.

Table 4 Speed of Sound (SoS) in Gases at 0oC

Gas SoS m/s

Gas SoSm/s

Hydrogen 1284 Air 331 Helium 965 Argon 319 Neon 435 Krypton 211

Water Vapor 405 Xenon 169

31  

Fig.20.Speed of sound in air at 20oC as a function of helium concentration

Thedetectorconsistsofanacousticcavityintowhichispumpedthesamplegasandapairofpiezoelectrictransducersattachedtothecavity,whichareoperatedinthepitch‐catch mode. Higher‐order echo can be used to enhance sensitivity. An early hand‐heldprototypeoftheSoSleakdetector,aboutfiveinchesinlength,isshowninFig.21.Smallerversionsofthedetectorcanbedeveloped,withasmalleracousticcavity,whichisoperatedatahigherfrequency;theywillbemoresensitive.

Fig.21. Hand-held prototype of the Speed of Sound (SoS) Leak Detector developed at ANL (Courtesy S-H Sheen)

ItisenvisionedthatboththeinletandoutletportsofDCSSswouldbeequippedwithminiaturized SoS detectors that send signals wirelessly to a data acquisition system foroperatoraction.Withnoheliumleakagethedetectorswouldbesensitiveindicatorsofairtemperature(theSoSincreasesslightlywithtemperature6).ButasuddenincreaseinSoSatanoutletdetectormightthenindicatealeakageofheliumfromtheSNFcanister.

WhatisnotknownisthedegreeofdilutionwiththeoutsideairoftheheliumthatleaksfromanSNFcanister,andwhetherchangescanbereadilydetected.Nevertheless,themethodisnon‐obtrusive,canbeeasilyautomatedandremotelyinterrogated,isapplicabletobothverticalandhorizontalDCSSs,andprobablycanbedeployedatlowcostatISFSIs.TheseadvantagessuggestfurtherR&Dofthemethodiswarranted.

                                                            6 By~0.6km/sec/oCovertherange10‐60oC.

32  

5.6 CanisterInternalConditions

5.6.1 GasComposition

Thegascompositionintheinteriorofacanisterisofgreatinterest.Changesinthegascompositionmayberelatedtocanisterleakageortofailureoffuelcladding(orboth).FuelfailurewoulddefinitivelybeindicatedbythepresenceKr‐85activityinthefillgas.Butasnotedbefore, theKr‐85methodrequiresamodifiedcanister lidandwillnotwork forcoolingtimesgreater than40‐50years.Ofcourse, inaDCDPit ispossible toperiodicallytakedirectsamplesoffillgasandanalyzebymassspectrometry,butnotinthefield.

ApossibleNDEmethodofdetectingfuelfailureinanSNFcanisterinthefieldusesavariant of the SoS technique described previously. Here, U/S piezoelectric probes arepositionedonoppositesidesofacanisteratalocationwherethereisfreespacebetweentheSNFbasketandinnerlid,givinganunimpededpathforsoundtotravelfromonesideofthecanistertotheother(Fig.22).ThepresenceoffissiongasinthisspacewillreducetheSoS that is measured. Although free space may exist in only certain DCSSs the methodwarrants furtherevaluation.7Piezoelectricdevicesare tolerant tohigh levelsof radiationandwouldneedtobereplacedonlyoccasionally.

Fig.22. Pitch-catch U/S measurement of speed of sound

to detect fuel failure in an SNF canister 5.6.2 PressureandHumidity

Tsaietal. (50)havepointedout thatmonitoringsuchparametersaspressureandhumidity inside an SNF canister without recourse to electrical cables threaded throughpenetrationsinthecanisterboundaryrequiresseveralenablingtechnologies,suchas:

                                                            7 ExistingdesignsmaybechallengingbutifthemethodisprovedfeasiblebyR&D,designscouldbemodifiedslightlytoprovideapath. 

33  

sensors that can operate at elevated temperatures (200oC) and high radiationlevels(1,000Rad/hr)forextendedduration,

wirelesstransmissionofsensorinformationoutofthecanister,and asensorpowersupplylongerthanthepresenttypicalbatterylifeof10years.

The biggest technical challengewill be transmissionof sensor information out oftheweldedcanister.Ultrasonictransmissionandmagneticinductivecouplingthroughthecanisterwallappeartobepossiblemethodsfordoingso,and‘proof‐of‐principle’workhasbeen performed on eachmethod. For example in 2006,Murphy and co‐workers [51,52]successfully collectedU/S signals througha6‐inch (152‐mm) thick steelwall. In2008, apatent applicationwas filed byWilhelm etal. [53] formagnetic communication throughmultiplemetalbarriers.However,themethodsrequireeitheratransduceroracoil,andapowersupply,onthe insideof thecanister. AlthoughpowercouldperhapsbeharvestedfromtheSNF activity,themethodsarefarfrompracticalusewithoutconsiderableR&D.

5.7 ‐rayScanning

Monitoring the variation in low‐level ‐ray activity detected on the exterior of aDCSSmaybeusefulinseveralways:todeterminedegradationintheshieldingpropertiesof theoverpack; to indicatespatialchange in thedistributionofSNFwithintheDCSS;or,morelikely,toindicatesomecombinationofthetwoeffects.

There are three levels of ‐raymonitoring—in ascending order of sophistication,theyare: imaging, ‘finger‐printing’ ofDCSSs for safeguardspurposes, and finally, full‐blown‐scanningtoproduce3‐DmapsofactivitywithinDCSSs.Eachisdescribed.

5.7.1‐ImagingSystems

At the simplest level, commercial scanning units, such as the RadScan 800 4 ImagingSystem(Fig.23)developedbyPajaritoScientificCorporation,whichsuperimposeactivitymeasurementsonvisualimages,havebeenusedtomapcontaminationhotspotsatseveralnuclearpowerplants[54].

Fig.23. The RadScan 800 4 Imaging System Left: The assembled system; Right: Schematic of detection head

(Courtesy: Pajarito Scientific Corporation; Ref.50)

34  

QuotingfromRef.50:“TheRadScan800unit,Fig.23(left),isaremotelyoperated,

highlycollimated, scanning lowresolution‐spectroscopydetector that is combinedwithvideocaptureanddatahandlingandanalysissoftware.Thedetectionsystemiscomprisedofadetectionheadlocatedonapan/tiltunit.Anelectronicscontrolboxisattachedtothepan/tilt unit and provides initial processing of the radiation detection signal as well aspositioningelectronics.”

“Thedetectionhead,Fig.23(right),housesaNa(I)scintillationdetectorencasedina

tungstencollimatorwitha4‐degreeaperture.Spatialresolutioncanbeincreasedusinga3‐or2‐degreeinsert.Locatednexttoandin‐linewiththecollimatorarealaserrangefinderand a conventional CCD camera with automatic zoom and aperture. To construct aradiometric overlay image the RadScan™ acquires both radiometric and conventionalimagedatafortheentireareatobesurveyed.Thesedataareobtainedinaseriesof‘singleshot’ acquisitions. Once started the acquisition process is fully automated, requiring nomanualinterventionormonitoring.”

Such a unit could be used during visual inspection of the exterior of DCSSs to

correlateareasofoverpackdegradationwithincreasedγactivity.5.7.2 ‘‐Fingerprinting’DCSSs

Atthenextlevelofsophisticationisthe‘‐fingerprinting’workthatwasperformed

atINLin2004tomonitorthecontentsofDCSSsforsafeguardspurposes[55,56].Figure24showsimagesthatwereobtainedforoneofthesixDCSSsstoredatINL—theWestinghouseMC‐10cask,whichcontainedPWRassembliesin18of24possiblelocations,foratotalof15metrictonnesofheavymetal.The‐rayimagesaresuperimposedonopticalimagesoftheMC‐10cask(acleareropticalimageofthisDCSSappearsontheextremeleftofFig.4).

Fig.24 -ray images of the MC-10 cask. The wide-angle image (left) and zoomed-image (middle) show little structural detail but are clearly of different shape than the top view (right). (Courtesy: Ziock et al, Ref.56)

35  

The ‐ray imager is shown schematically in Fig.25 (left); it consisted of a coded‐aperture mask [57] projected onto a CsI(Na)‐based, position‐sensitive ‐ray detector. Avisible‐lightvideowasco‐alignedwiththeimagingaxisandusedtorecordanopticalimagebeforeeachrun.Figure25(right)showsthe‐rayimagerlookingdownontheMC‐10caskforproductionoftheright‐handimageinFig.24.

Fig.25 -ray imager used to fingerprint the MC-10 cask Left: Schematic of the imager; Right: The imager recording the top view of the cask

(Courtesy: Ziock et al., Ref.55)

In addition to the ‐ray imager, a ‐ray spectrometer, Fig.26 (upper),was used torecordthe‐rayspectraoftheSNFwithintheDCSSs.ThespectrometerconsistedofanN‐type coaxial high‐purity germanium detector, collimated with a bismuth annulus thatrestricted the field of view to 10 degrees; Fig. 26 (lower) shows the spectrum obtainedfromtheMC‐10cask. Themajorphotopeaks fromtheSNFweredue todecayofCs‐137,Eu‐154andCo‐60.

Ziock and co‐workers concluded that the ‐ray images were dominated by the

radiation component that was scattered by the shielding of the casks. Although the sixcasks produced strikingly different images (a sort of fingerprinting), thiswaswhy theimagesthemselvesexhibitedlittlestructuraldetail—adisappointingresult.

The investigators emphasized, however, that prominent photopeaks in the ‐ray

spectramustoriginatefromunscatteredradiation,sinceComptonscattered‐raysshowedupbelow thephotopeakenergies. Hence‐rays in thephotopeakswerenotscattered byshielding.Thisconclusionhasimportantconsequences,asdescribedinthenextsection.

36  

Fig.26 -ray spectrometer (Courtesy: Ziock et al., Ref.56) Upper: The spectrometer mounted on a step ladder Lower: Gamma-ray spectrum from the MC-10 cask

5.7.3 ‐RayTomography

TheconclusionbyZiockthatγ‐raysinthephotopeaksofthecaskspectrawerenot

scatteredbyshieldingmeantthatstandardγ‐rayscanning,withmulti‐channelanalysis—asused in normal examination of fuel rods and assemblies—may be used to produce (,z)scans of fission‐product activities in vertical cylindricalDCSSs, as shown in Fig.27. Fromthesescans3‐Dplotsofactivitycouldbeconstructed.Inotherwords,γ‐raytomographyshouldbefeasible.

37  

Such detailed scanning will require ability to move the γ‐ray detector in areproducible, prescribed fashion over the cylindrical surface of DCSSs. This ability willallow slight changes to be discerned in the spatial distribution of γ activity from oneexaminationtothenext,whichmayindicatechangesinSNFconditionduringstorage.Thescanningwillbelengthy,takingdaystocomplete,althoughthishardlyseemsanissueforSNFthatwillbestoredfordecades.

Fig.27. Sketch of the method for γ-ray tomography of a DCSS

Obtaining γ activity in three dimensionsmay be the NDEmethod of choice for ademonstration program, giving the clearest picture of the situation inside sealed SNFcanisters or bolted casks. Software to construct 3‐Dmaps of γ activity is available as aresult of, for example, the need to assay drums of radioactive waste [58]. Being animmobilefissionproductEu‐154willprovidethemostaccuratepictureoffueldistribution.However,its8.6‐yearhalf‐lifewillrenderitusefulforonly40yearsafterreactordischarge.In the long term, Cs‐137 with its half‐life of 30.2 years may be the only practicalradioisotope.

Avideocameraandthermalimagingdeviceshouldbemountedwiththeshieldedγ

detector in order to allowvisual inspection and temperaturemeasurement concurrentlywithobtainingγactivitydata.Obviously,systemswillberequiredfordataacquisitionandprecisioncontrolofdetectormovementbycomputer.

38  

Fig.28. Reinforced concrete WW-II “pill box” on south coast of England, illustrating the effects of 70 years of marine climate on structural stability of concrete.

5.8 InspectionMethodsforConcrete

Reinforcedconcreteformsanintegralpartof the majority of modern designs of DCSS,primarily on the grounds of its attractive costcompared with that of all‐metal systems.Althoughconcreteplaysonlyapassiveroleintheshielding of SNF canisters or casks, nonethelessitslong‐termstabilityisessentialtoisolatingSNFfrom operators and the general public (seeFig.28).Theperiodicandsystematicexaminationofitsconditionisthereforeofhighimportance.

Fortunately, because concrete is widelyused in buildings, bridges and roads,many non‐destructive examination (NDE) methods havebeendevelopedfortestingconcrete.Themethodsare well documented; for example, by theAmericanConcreteInstitute(ACI).

5.8.1 TraditionalNDEMethods

ArecentACIsubcommitteereport [59]reviewedthe fieldofNDE testing insevenmethod areas—visual inspection, stress‐wave, nuclear, magnetic and electrical, pene‐trability, infrared thermography, and radar. An eighth area of stress‐wave methods fordeepfoundationsdoesnotapplytoDCSSsexceptinrelationtotheconcretepadsonwhichtheyareplaced.

FourofthetwentyNDEmethodsidentifiedbytheACICommitteeseemappropriatefor further consideration formonitoring changes in DCSSs: visual inspection, sonic echo(SchmidtHammer),backscatterradiometry,andhalf‐cellpotential.Afifthmethod—ultra‐sonic (U/S) pulse velocity—maypossibly be applicable tomonitoring the concretewallsandroofsofNUHOMSDCSSs.ThemethodsaresummarizedinaselfexplanatoryfashioninTable6.Briefcommentsonseveralofthemethodsfollow.

Visual Inspection: Inspection of air vents is required daily for most DCSSs. Routineweeklyormonthly inspectionof theDCSSexteriorshouldalsobecarriedout,preferablywithanautomatedvideocamerasystemsimilartotheRadScan™800.All‐weatherCCTVsystems that could be used for such surveillance abound. The ability to alarm on thedetectionofanewsurfaceartifact,andtheabilitytozoominandrecordthesame,wouldbedesirablesystemfeatures.

39  

Table 6 NDE Methods for Concrete Structures

(Courtesy: A. G. Davies, Ref.59)

Method and Principle Applications Advantages

Limitations

Visual Inspection—observe, classify and document appearance of degradation on exposed surfaces of structure

Maps degradation such as cracking, spalling, scaling or erosion, or construction defects

Straightforward method, easily recorded data; may be automated. Inner faces can be examined using a borescope

Qualitative; may be difficult to discern trends in data

Sonic Echo—Hammer impact on surface and a receiver monitors reflected stress wave. Time-domain analysis used to determine travel time

Determines location of cracks, delaminations, voids, etc.

Access to only one face needed. Uses available equipment. Can locate variety of defects

Requires experienced operator. Limited to testing members of <2 m thickness

Backscatter Radiometry—measure intensity of high-energy EM radiation reflected from the near surface region of concrete member

Determines in situ density of fresh or hardened concrete

Access to only one face needed, and suitable for fresh or hardened concrete. Rapid, portable equipment.

Requires licensed operator. Precision affected by surface material, sensitive to chemical composition

Half-cell Potential—Measure difference in voltage between rebar and reference electrode. Measured voltage indicates likelihood of rebar corrosion

Identifies regions of reinforced concrete where there is high probability of corrosion at time of measurement

Lightweight portable equipment provides indication of likeli- hood of corrosion at the time of testing

Requires electrical connection to rebar; not applicable to resin-coated rebar. No indication of corrosion rate. Experience needed to interpret.

U/S Pulse Velocity—measure the travel time of a U/S pulse over known path length

Determine the relative condition of concrete based on measured pulse velocity

Portable equipment that is relatively easy to use

Generally requires access to two sides; provides no data on depth of defect

SonicEcho: Thesonicecho,orSchmidthammer,methodiswidelyusedtodeterminethecompressive strength of concrete [29]. Figure 29 shows the relationship between thereboundparameterQdeterminedby theSilverSchmidtHammer [60],which isavailablecommercially from Switzerland, and the compressive strength of concrete. Periodic,systematicsurveillance,forexample,ofthewallsofhorizontalstoragemodules(HSMs)oftheNUHOMSDCSSsor thecylindricalwallsof theHi‐StormDCSSsby thismethodwouldhelptodeterminetheonsetofanydegradationintheconcrete.

40  

Fig.29. Relationship between rebound value Q and compressive strength of concrete determined by the Silver Schmidt Hammer (Ref.56).

BackscatterRadiometry:Thismethodmayhaveuniqueapplication—usingtheactivityoftheSNFitselftointerrogatetheconditionoftheconcreteoverpack.TheRadScan™800system, described in §5.7.1, would be the type of instrument needed to perform suchinspection. Degradation of the concrete would be indicated by an increase in gammaactivity detected on the exterior of the DSCC between one inspection and the next. Themethodwouldrequireanaccuratescanningprotocolinordertoavoidspuriousindicationsofchangeingammaactivity.

Half‐cell Potential: This method to detect corrosion of concrete rebar requires thatelectrical connectionbemaintainedwith the rebar. Themethod is therefore likely to beusefulonlyfortheDCSSsusedinadrycaskdemonstrationprogram,notforunitscurrentlyinusecommercially.

U/SPulseVelocity: TheUPVmethod ideally requiresaccess toboth sidesof a concretestructuretoallowatransducertoemitpulsesinthe20‐100kHzrangeandforadetectoronthefarsidetomeasuretime‐of‐flightthroughthestructure.Thepulsevelocityisthenarelative indicator of the concrete quality through the structure. Figure 30 shows thehoneycombingthatwasdetectedinaconcretehighwaysigncolumninthisway[61].

41  

Fig.30. UPV tomography of a concrete highway sign with honey-combing of the structure revealed by reduced pulse velocity (Ref.61).

Although local scanning of

concrete walls in the NUHOMSDCSSs might just be feasible withdetectors inserted via the airvents,UPVmeasurementscanbestbe performed having the trans‐ducer and the detector on thesame face, the 'indirect' config‐uration[62],Fig.31.Suchscanningrevealsdefectsinthesurfacelayerof the concrete. A commerciallyavailableUPVunit [63]byGENEQInc., Switzerland, is shown inFig.32. The relationship betweenthe quality of concrete and thepulsevelocityisgiveninTable7.

Fig.31. Three ways to attach a transducer and detector on a concrete structure

for UPV measurements: direct, semi-direct, and indirect (Ref.62).

42  

Fig.32. The PunditLab UPV tester manufactured by GENEQ Inc.,Switzerland; a transducer head is visible at bottom left (Ref.63).

Table 7

Concrete Quality versus Pulse Velocity

Pulse

Velocity (km/s)

Concrete Quality (Grading)

Above 4.5 Excellent 3.5 to 4.5 Good 3.0 to 3.5 Medium Below 3.0 Doubtful

5.8.2 EmbeddedSensors

Remedyingtheeffectsofcorrosion,stressfracturesandothermodesofdamageinconcrete canbebothexpensive and time‐consuming.Thus, therehasbeenan increasingemphasis recently on monitoring the ‘health’ of concrete in buildings and bridges on asemi‐continuous basis using various types of embedded sensor [64]. The sensors canprovideanearly‐warningsystemforassessingdamagebeforesafetyissuesarise.

Lange et al. [65] describe a field‐ready system of inexpensive relative‐humidity(RH)/temperaturesensorspermanentlyembeddedinconcretewhichwillcommunicatetoa small, battery‐powered data acquisition system. Figure 32 illustrates the packingprocedureused to encase theRH/temperature sensor SHT7xmanufacturedbySensirionInc., Switzerland [66]. The system was used on concrete slabs in the field; the authorscommented: “making it possible to learn about the impact of daily temperature andhumiditycycles,andtheimpactofrainevents.”

43  

Fig.32 RH/temperature sensor packing procedure (Courtesy: Lange et al., Ref.65).

SRIInternationalisintheprocessofdevelopingSmartPebbles™fortheCaliforniaDepartmentofTransportation[67].SmartPebblesarewirelesschloride‐thresholdsensors,literally the size of a pebble,which can be embedded into existing concrete or includedwhen pouring new concrete and used to detect chloride ingress. An external readerprovidespowertothewirelesssensortoenableittotransponditsIDnumberandstatus.SRI isalsodevelopingtemperaturesensorsactivated inthesameway.BothsensortypesareofgreatinterestforpotentialuseinaDCDPasaprecursortouseinthefield.

6. SummaryandRecommendations

TheprecedingsectionsdescribedthehistoryandtypesofdrycaskstoragesystemsthathavebeendevelopedintheU.S.todealwithcommercialSNF.PriorandcurrentareasofresearchonDCSSsintheU.S.andJapanwerealsoreviewed.Finally,potentialmethodsformonitoringtheconditionofDCSSswerediscussed.Table8summarizesthesemethodsbytheparameterorphenomenonbeingmonitored,andbythestageofdevelopmentofthemethod; i.e.,whetherit is:(i)currentlyusedinthefield,(ii) likelydeployableinthenearterm(1‐3years),or(iii)deployableinthelongerterm(fourormoreyears).

44  

Table 8 Potential Methods of Monitoring DCSS

Condition during Normal Operation

Parameter/ Phenomenon

Monitoring Method/Stage of Development Current Field

Practice For Near-Term

Field Deployment For Longer-Term

Development

Fuel failure

None

Pitch-catch speed of sound (SoS) across canister gas space1

[§5.6]

Current method sufficient

Fuel relocation

None Changes in DCSS

thermal and γ images [§5.3.5; §5.7.1]

γ tomography of DCSS [§5.7.3]

Canister temperature

Inferred from T/C reading at surface and COBRA

calculations

Thermal imaging using borescope [§5.3.5; §5.4.1]

U/S or fiber optic temperature sensors on canister surface

[§5.3.3; §5.3.4]

Change in DCSS cooling

Inspection of cooling vents for debris

Temperature-indicating RFIDs and/or SoS devices

in vents [§5.8.2]

Current methods sufficient

Canister corrosion

None

Borescope inspections; SaltSmart™ coupons

for brine deposits [§5.4.1]

Miniaturized EMAT monitoring of welds and surface layers of canister [§5.4.2]

Canister leakage

None2

Outlet air monitored for increase in SoS due to helium leakage

[§5.5]

ΔTBT using fiber optic temperature sensors

on canister [§4.3.2.2] to annunciate large leak (in vertical DCSSs only)

Bolted cask

leakage

Alarmed on ΔP in space between

inner and outer lids

Current method sufficient

Overpack structural stability

Visual inspection of concrete surfaces

Programmed visual inspection; Schmidt Hammer; γ imaging

[§5.7.1; §5.8.1]

U/S pulse velocity; Smart Pebbles™ [§5.8.1; §5.8.2]

Surface γ dose rate

Routine radiological surveillance of DCSSs

Integrate with visual inspection [§5.4.1] and γ imaging [§5.7.1]

Integrate with DCSS γ tomography

1Weldedinnerandouterlidsassumedtoprecludecanisterleakage.2Xenonandkryptonfromfailure(s)willreduceSoSinheliumfillgas,

superiortoKr‐85methodwhichneedsspeciallidandislimitedbyhalf‐life.

45  

Weconcludethatavarietyofnon‐obtrusivemethodsarefeasibleforthelong‐termmonitoringofdrycaskstoragesystems.Methodsrangefrommeasuringthespeedofsoundin air as ameans of detecting helium leaks from canisters to traditional non‐destructiveexaminationmethodsfordeterminingstructuralintegrityofconcrete.

Somemethods—suchasmonitoringoverheatingbymeasuringthespeedofsound

in the cooling air that exits a dry cask storage system—couldbedeveloped andusefullydeployed rather quickly at independent spent fuel storage installations. Other methods,suchasSaltSmart™ coupons tomeasure saltdepositiononcanisters (tobeused in fieldtrialslaterthisyear),mayneedrefinementbeforewidespreaduse.Yetothermethods,liketheuseofγ‐raytomographyfordetectingfuelrelocation,requirenotonlyfurtherR&Dbutalso expensive equipment. Theymay bemore applicable to the dry cask demonstrationprogrambeingconsideredbytheNuclearRegulatoryCommissionthantouseinthefield.

Specificrecommendationsregardingmonitoringofdrycaskstoragesystemsare:

Electromagnetic acoustic transducers should be actively pursued for detectingcanistercorrosionbecausetheycanmonitortheconditionofweldsontheinnersideofacanisterwallandbecausetheydonotrequireacouplingmedium.

Thecurrentmethodofmonitoringforaleakinbolteddrycasksbyachangeinpressureinthespacebetweeninnerandouterlidshasworkedwellandneedsnoimprovement.

Measuring the speedof sound todeterminehelium leakage fromcanisters, airtemperature,andpossibly fuel failureshouldbedevelopedasausablemethodbyminiaturizingcurrent“pitch‐catch”ultrasoniccavitiesforuseinthefield.

Detecting fuel failurebyKr‐85 activitywarrantsminimal further developmentbecause the isotope’s 10.7‐year half‐life restricts its use to cooling times lessthanaboutfiftyyears.

Visualinspectionandthermal/γimagingofdrycaskstoragesystemsneedtobeautomatedtoallowreliabledetectionofchangeinfuelconfiguration.

Methods now used tomonitor the integrity of concrete buildings and bridgesshould be further evaluated for application to dry cask storage systems. TheSchmidtHammertest,theindirectmeasurementofultrasonicpulsevelocity,andSmartPebbles™ that can detectchlorideintrusionarepromisingtechniques.

Anoverallrecommendationisthat: Monitoringmethodsmust be carefully evaluated for application to the unique

geometriesandlimitedcomponentclearancesindrycaskstoragesystems.

46  

7.Acknowledgements

This work was supported by the U.S. Nuclear Regulatory Commission’s Office ofNuclearRegulatoryResearchJobCodeV6060.TheauthorswouldliketoacknowledgethesupportandencouragementthroughoutthisworkofIouriProkofievoftheNRC.

The authors alsowish to thank J. C. Voglewede and S. Lindo‐Talin of theNRC fortheirvaluablecommentsonthedraftreport.andS.‐H.Sheen,H.‐T.ChienandA.C.RaptisofANLforinformativediscussionsonNDEmethods.

ThisreportwaspreparedasanaccountofworksponsoredbyanagencyoftheU.S.Government.NeithertheU.S.Governmentnoranyagencythereof,norUChicagoArgonne,LLC,noranyoftheiremployeesorofficers,makesanywarranty,expressedorimplied,orassumesanylegalliabilityorresponsibilityfortheaccuracy,completeness,orusefulnessofanyinformation,apparatus,product,orprocessdisclosed,orrepresentsthatitsusewouldnot infringeprivatelyownedrights.Referencehereintoanyspecificcommercialproduct,processorservicebytradename,trademark,manufacturerorotherwise,doesnotnecess‐arilyconstituteorimplyitsendorsement,recommendation,orfavoringbytheU.S.Govern‐mentoranyagency thereof.Theviewsandopinionsof thedocumentauthorsexpressedherein do not necessarily state or reflect those of the U.S. Government or any agencythereof,ArgonneNationalLaboratory,orUChicagoArgonne,LLC.

47  

8. References

1. Used Nuclear Fuel Storage Map, Nuclear Energy Institute (2011),http://www.nei.org/resourcesandstats/documentlibrary/nuclearwastedisposal/graphicsandcharts/used‐nuclear‐fuel‐storage‐map.

2. R.E.Einziger,H.C.TsaiandM.C.Billone,ExaminationofSpentPressurizedWaterReactorFuelsRodsafter15YearsinDryStorage,Nuc.Tech.,144,186(2003).

3. J. Kessler, Dry Cask Storage Characterization Project, EPRI Report 1002882(September2002).

4. R.E.Einziger,H.C.Tsai,M.C.BilloneandB.A.Hilton,ExaminationofSpentPWRFuelRodsafter15YearsinDryStorage,ICONE10‐22456(April2002).

5. ReactorandFuelCycleTechnologySubcommitteeReporttotheFullCommission,Blue Ribbon Committee on America’s Nuclear Future, Washington DC (June2011).

6. J.Kessler,UsedFuelandHigh‐LevelRadioactiveWasteExtendedStorageCollabor‐ation Program,November 2009Workshop Proceedings, EPRI Report 1020780(March2010).

7. EPRI/DOE/NRCExtendedStorageCollaborationProgram(ESCP)Meeting,EPRI,Charlotte,N.C.,December6‐8,2011.

8. D. B. Rigby, Ed.,EvaluationoftheTechnicalBasis forExtendedDryStorageandTransportationofUsedNuclearFuel,U.S.NuclearWasteTechnicalReviewBoard(December2010).

9. R.L.Sindelar,MaterialsAgingIssuesandAgingManagementforExtendedStorageandTransportationofSpentNuclearFuel,SNRL‐STI‐2011‐00005(May2011).

10. EPRI Research Portfolio, Used Fuel and High‐Level Waste Management,http://portfolio.epri.com/Research.aspx?sId=NUC&rId=214

11. C.Stockman,B.Hanson,H.Alsaed,C.O.Farnum,J.L.Buelt,K.B.Sorenson,S.Birk,Used Fuel Storage—Research Development OpportunitiesWork Package—YearEndReport,FCRD‐USED‐2010‐000228(September2010).

12. J. Kessler, Industry Spent Fuel StorageHandbook, EPRI Report 1021048 (July2010).

13. DryCaskStorageintheU.S.byVendor,StoreFUEL,11‐127(March3,2009).14. DryCaskStorageintheU.S.byVendor,StoreFUEL,12‐140(April6,2010).15. DryCaskStorageintheU.S.byVendor,StoreFUEL,13‐162(February7,2012).16. J.C.McLean,NUHOMSModularSpent‐FuelStorageSystem:Design,Licensingand

Construction,EPRI‐NP‐6940(August1990).17. http://us.areva.com/EN/home‐380/transnuclear‐inc.html.18. Transnuclear, Inc., TN‐68Dry Storage Cask System Preliminary SafetyAnalysis

Report,NRCAdamsAccessNumberML003684893.19. MaureenConley,TwocasksleakheliumatPeachBottom,NuclearFuel,Vol.35,22,

12‐13(November1,2010).

48  

20. StandardReview Plan for Spent FuelDry Storage Systems at aGeneral LicenseFacility, NUREG‐1536Revision 1, as based on 10CFR 72.122(h)(4) and 10CFR72.128(a)(1)(July2010).

21. ConfinementEvaluation,NRCInterimStaffGuidance–5,Revision1.22. The Design/Qualification of Final Closure Welds on Austenitic Stainless Steel

Canisters as Confinement Boundary for Spent Fuel Storage and ContainmentBoundary for Spent Fuel Transportation, NRC Interim Staff Guidance – 18(May2,2003).

23. Transnuclear, Inc., NUHOMS HD System Final Safety Analysis Report, Rev. 0,(January2007).

24. Westinghouse, Spent Fuel Dry Storage Technology Development: ThermalEvaluation of Sealed Storage Cask containing Spent Fuel, AESD‐TME‐3050(August1980).

25. EPRI, NUHOMSModular SpentFuel Storage System:PerformanceTesting, EPRINP‐6941(September1990).

26. M.A.McKinnonandR.C.Schmitt,TestingandCOBRA‐SFSAnalysisoftheVSC‐17VentilatedConcrete,SpentFuelStorageCask,PNL‐SA‐19941(April1992).

27. D.R.Rector,C.L.WheelerandN.J.Lombardo,COBRA‐SFS:AThermal‐HydraulicAnalysisComputerCode:Volume1—MathematicalModelsandSolutionMethods,PNL‐6049,Vol.1(1986).

28. W.C.BareandL.D.Torgerson,DryCaskStorageCharacterizationProject—PhaseI:CASTORV/21CaskOpeningandExamination, INEEL/EXT‐01‐00183 (August2001).

29. American Society for Testing andMaterials, StandardTestMethodforReboundNumberofHardenedConcrete,ASTMC805‐94,ASTM(1994).

30. T. Aida, T. Hara, Y. Kumano, OperatingExperience inSpentFuelStorageCasks,IAEA‐CN‐178/KN27(3June2010).

31. ConcreteCaskStorageofSpentFuel—ChallengeofEconomicStorageTechnology,Nuc.Eng.&Des.,238,1167‐1263(2008).

32. T. Saegusa, G. Yagawa and M. Aritomi, TopicsofResearchandDevelopmentonConcreteCaskStorageofSpentNuclearFuel, Nuc. Eng. & Des.,238, 1168‐1174(2008).

33. K.Shirai,H.Takeda,M.Wataru,T.Arai,J.Tani,T.Saegusa,TestingofMetalCaskandConcreteCask,IAEA‐CN178/KN24(June2,2010).

34. H.Takeda,M.Wataru,K.Shirai,T.Saegusa,DevelopmentoftheDetectingMethodofHeliumGasLeakfromCanister,Nuc.Eng.&Des.,238,1220‐1226(2008).

35. T. Matsumara, A. Sasahara, Y. Nauchi, T. Saegusa, Development ofMonitoringTechniquefortheConfirmationofSpentFuelIntegrityduringStorage,Nuc.Eng.&Des.,238,1260‐1263(2008).

36. M.Mayuzumi, J. Tani andT.Arai,Chloride‐InducedStressCorrosionCrackingofCandidateCanisterMaterialsforDryStorageofSpentFuel,Nuc.Eng.&Des.,238,1227‐1232(2008).

37. K. Korsah, et al., Emerging Technologies in Instrumentation and Controls: AnUpdate,NUREG/CR‐6888(January2006).

38. K. Korsah, et al., Instrumentatioon and Controls in Nuclear Power Plants: AnEmergingTechnologiesUpdate,NUREG/CR‐6992(October2009).

49  

39. CladdingConsiderations for theTransportationand Storage of SpentFuel, NRCInterimStaffGuidance–11,Revision3(November17,2003).

40. T.A.Burtseva,Y.YanandM.C.Billone,Radial‐Hydride‐InducedEmbrittlementofHigh‐BurnupZIRLOCladdingExposedtoSimulatedDryingConditions,NRCAdamsAccessNumberML101620301(July2010).

41. http://www.imaging1.com/thermal/IR_Cam.html#http://www.nationalinfrared.com/x20/shop/3D/574‐3/index.html.

42. P.L.Winston,InspectionandGamma‐RayDoseRateMeasurementsoftheAnnulusof the VSC‐17 Concrete Spent Nuclear Fuel Storage Cask, INL/EXT‐07‐13129(September2007).

43. T. Neider, Research and Data Needs for Very Long‐Term Dry Storage: AREVAPerspective,NuclearWasteTechnicalReviewBoard,SummerMeeting(June11,2009).

44. ExtendedStorageCollaborationProgram,NDESubcommitteeMeetingMinutes(17February2012).

45. http://www.louisvillesolutions.com/meter_description.html?.46. R.B. Thompson, “PhysicalPrinciples ofMeasurementswithEMATTransducers,”

Ultrasonic Measurement Methods, Physical Acoustics, Vol. XIX, R.N. ThurstonandA.D.Pierce(Eds.),AcademicPress(1990).

47. H.Gao,S.M.AliandB.Lopez, InspectionofAusteniticWeldswithEMATs,Reviewof Quantitative Nondestructive Evaluation, 1175‐1181, Vol.29, D.O. ThompsonandD.E.Chimenti(Eds.),AmericanInstituteofPhysics(2010).

48. L.CaseresandT.S.Mintz,AtmosphericStressCorrosionCrackingSusceptibilityofWeldedandUnwelded304,304Land316LAusteniticStainlessSteelsCommonlyUsedforDryCaskStorageCanistersExposedtoMarineEnvironments,NUREG/CR‐7030(October2010).

49. S‐H. Sheen, H‐T. Chien and A.C. Raptis,UltrasonicTechniques forDetectingHe‐liumLeaks,SensorsandActuators,B71,197‐202(2000).

50. H.Tsai,Y.Y.Liu,M.Nutt, J. Shuler, “AdvancedSurveillanceTechnologiesforUsedFuelLong‐TermStorageandTransportation,”ICEM2011‐59032(2011).

51. T.L.Murphy,“UltrasonicDigitalCommunicationSystemforaSteelWallMultuipathChannel:MethodsandResults,”LM‐06k004,February16,2006.

52. G.J. Saulnier, H.A. Scarton, A,J. Gavens, D.A. Shoudy, T,L. Murphy, M.Wetzel, S.Bard, S. Roa‐Prada, P. Das, “Through‐WallCommunication of Low‐RateDigitalDataUsingUltrasound,”IEEEUltrasonicSymposium,pp.1385‐1389(2006).

53. G.R.Wilhelm, C. Jaskolski andE. Berkenpas, “MagneticCommunicationthroughMetalBarriers,”U.S.PatentApplicationPublicationUS2008/0070499A1(March20,2008).

54. J. T. Santo,M.Maul, R. Lucerno,M. Clapham,B./Battle,D. Sluska, B. Carberry,ApplicationsofRemoteGammaImagingSurveysattheTurkeyPointPWRReactorFacility,http://www.pajaritoscientific.com/pdf/TECH_APP_REMOTE_GAMMA_IMAGING_TURKEY_POINT_FACILITY.pdf.

55. K. P. Ziock, G. Caffrey, A. Lebrun, L. Forman, P. Vanier, J. Wharton, RadiationImagingofDry‐StorageCasksforSpentNuclearFuel,2005 IEEENuclearScienceSymposium,1163‐1167(2005).

50  

56. K.P.Ziock,P.Vanier,L.Forman,G.Caffrey,J.Wharton,A.Lebrun,TheFeasibilityofCask“Fingerprinting”asaSpent‐Fuel,Dry‐StorageCaskSafeguardsTechnique,UCRL‐TR‐215943(October2005).

57. E.E.Fenimore,T.M.Cannon,CodedApertureImagingwithUniformlyRedundantArrays,App.Opt.,17,3,337‐347(February1978).

58. N.Menaa,D.Nakazawa,H.Yang,S.K.Smith,D.Petroka,M.Villani,AnIntegratedWaste Assay System using Tomographic and Segmented Gamma Scanning forNuclearPowerPlantApplications,WasteManagement’10(March2010).

59. A. G. Davies, Chairman ACI Committee 228, Nondestructive TestMethods forEvaluationofConcreteinStructures,ACI228.2R‐98(reapproved2004).

60. http://www.silverschmidt.com.61. http://www.ltrc.lsu.edu/tec_07/presentations/nondestructive‐web.pdf.62. http://www.olsoninstruments.com/pdf_downloads/catOI_upv.pdf63. http://www.proceq.com/site/downloads/pundit‐lab.html.64. E.vanEvery,A.Deyhin,EmbeddedSensorsforLife‐TimeMonitoringofConcrete,

4th.Int.Conf.,StructuralHealthMonitoringonIntelligentInfrastructure,SHMII‐4(July2009).

65. D.A. Lange, Z. C. Grasley, R. Rodden,EmbeddedSensorsforMeasuringEnviron‐mental Parameters in Concrete, Health Monitoring Systems and Sensors forAssessingConcrete,p.41‐51,AmericanConcreteInstitute(March2008).

66. http://www.sensirion.com/en/01_humidity_sensors/05_humidity_sensor_sht71.htmyyyy.

67. http://www.sri.com/rd/microsensors.pdf.

   

51  

AppendixA:

DryCaskStorageSystemsintheU.S.byVendorandUtilityasof7February2012(CourtesyUxC,Ref.15)

SUMMARY

52  

DCSSsintheU.S.byVendor

53  

DCSSsintheU.S.byUtility

54  

55  

56