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IntroductionTheMcArthurRiveruraniummineislocatedin theAthabascasandstonere-gioninthenorthernpartoftheprovinceof Saskatchewan,Canada.It is theworld'slargest,high-gradeuraniumde-positwithprovenandprobablereservesofmorethan473millionpoundsU30S.It ismajorityownedandoperatedbytheCamecoCorporation.
Thispaperpresentsacasestudyoftheengineeringthatwascarriedoutinorderto artificiallyfreezetheunder-groundorebodyattheminepriortoitbeingmined.Fulldetailsof theproject
AthabascaSandstone
.~Highpressurewater
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SandandGravel
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Case Study: Thermal Analysis of ArtificialGround Freezing at the McArthur RiverUranium Mine
G.P..Newman
havebeenpreviouslydescribedbyNewmanandMaishman(2000).Theprimaryfocusof thepaperistheappli-cationofanadvancedthermalanalysistoolthatgreatlysimplifiesthefiniteele-mentmodelingrequiredduringthede-signand.monitoringstagesof projectssuchas thatcarriedoutatMcArthurRiver.
BackgroundAccordingtoNewmanandMaishman(2000),theorebodyitselfislocated550metersto 620 metersundergroundwheretheground-waterpressureisap-
proximately5500kPa.Duetothepres-enceof ahangingwallfaultstructure,theorebodyis surroundedon threesidesby fairlydry,competentground.Theotherthreesidesarecomprisedofhighlyfracturedsandstonewithsignifi-cantamountsof rubble,flowingsandandclayregions.In ordertominetheore,itwasnecessarytocreateafrozenwallbarrieraroundthethreepoorsidesoftheorebody.Thefrozenwallbarrierwasdesignedtopermitdrainageofwa-terin theoreandconsequentlyreducewaterpressurespriorto mining.Thewallwasalsorequiredtoprovidestruc-
tural supportofweak,clay/ oreground near tomining cavities.FigureI showsacross-sectionoftheorebodyandneighboringgeol-ogy.
A mechanicalfreezingsystemiscomprisedof abrinecoolinganddistributionnet-workplusaseriesof brine freezepipesinstalledinthegroundto befrozen. Typicalgroundfreezingapplicationshaveinvolveddrillingfreezeholesfromsurfaceor near
.........SouthFreezeRow
DryBasementRock
640mLevefDeveloDment
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Figure1.Cross-sectionoftheorebodyandneighboringgeology(afterNewmanandMaishman,2000)
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Low Pressure Side
Brine
72 FreezePipes
~reeze Plant-Conceptualfreezing system
High Pressure Side
Brine
530m L.evel
Figure2. Illustrationof thehighandlowpressurebrinedistributionnetworks(afterNewmanandMaishman,2000)
surfaceandtheseactivitieshavebeenwelldocumented.Inaddition,thebrinecoolinganddistributionnetworkhastypicallyincludedanammoniacom-pressorwith ammoniato brineheatexchangers.Theprocessof installinga"typical"freezingsystematMcArthurRiverwasmademoredifficultduetothelocationof thefreezepipechamberunderground.
Thefreezingchamberislocated530metersbelowground,whichmeansthatthebrinepressureswithinthefreezepipesandassociatedbrinedistributionnetworkwouldequal5000kPaif con-necteddirectlytothesurfacerefrigera-tionplant.Thisis notpracticalfromadesignoroperationsperspective.In or-derto minimizethebrinefluidpres-sures, the underground brinedistributionsystemwasisolatedfromthesurfacebrinesystemusingshellandtubebrine-brineheatexchangers.
TheillustrationinFigure2showstherelativepositionofthe800Tonrefriger-ationcapacityfreezeplantonsurface,the12"ID brinesupplyandreturnlinesinstalledin theshaft,andoneof fourshellandtubeheatexchangersonthe530m level.The low-pressurebrinenetworkon the530m leveloperateswithina 150kPato600kPapressurerangeatflowratesrangingbetween130
m3/hrand550m3/hr.Thedesignbrinetemperaturewas-40degreesCelsius.
In orderto determinetheactualgrowthof thefreezewallitwasneces-saryto installthermocouplestringsatseverallocationsaroundthefreezingre-gion.Thethermocoupleswereloweredintoacasedholecontainingafinegroutmix priorto thegroutsetting.Eachstringwascomprisedoftwelvesensorslocatedatfive-meterintervalsdownanygiventemperaturemonitoringhole.Thisenabledthetemperaturedecaytobe monitoredoffsetfromthefreezepipesin varioustypesof ground(seeFigure I for comparisonof groundtypes).Groundtemperatureswerere-cordedeverysecondday.In thiscasestudy,theactualgroundtemperaturescanbeusedtoverifythenewthermalanalysis numerical tool. This isdescribedbelow.
Thermal Modelingand AnalysisTheTEMP/w two-dimensionalfiniteelement computer program(GEO-SLOPE,2002)hasbeenusedex-tensivelyby manypracticinggroundfreezingconsultantsfordesignofartifi-cialgroundfreezingprojects.Inthepastit wascommonto applythethermalboundaryconditionforafreezepipebyassuminga fixedtemperaturedecayfunctionforearlystagesoffreezingfol-
GEOSPEC
lowedby anassumptionthatthepipesurfaceis ascold,ornearlyascoldasthebrinefortheremainderofthefreez-ingperiod.
Recently,TEMP/w hasbeenmodi-fiedsothattheuseris abletoapplyaconvectiveheattransferanalysisforthepipe-groundinterface(oranyothersur-faceconvectiveheattransferprocess).Theamountof convectiveheattrans-ferredbetweenthe groundandthechilledbrineisdependentonthegroundtemperaturerelativeto thebrinetem-peratureanditsubsequentlydetermineswhatthenewgroundtemperaturewillbe.Basedonthisapproach,thegroundwillcoolatavariablerateandtoamini-mumvaluethatis determinedby thebrineflowparametersandthediffer-ence betweenbrine and groundtemperature.
Convectiveheattransferis com-prisedof two mechanisms:energytransferduetorandommolecularmo-tion(diffusion)andenergytransferduetothemotionofafluid(advection/con-vection).Inthiscasestudyanalysis,weareconcernedwith convectionheattransferbetweenfluid in motionin apipeandthepipewallwhentheyareatdifferenttemperatures.Technically,theconvectiveheattransferisoccurringbe-tweentheinternalpipewall andthefluidbutitisacceptabletocombinetheconductiveheattransferacrossthesteelpipewallwiththeconvectivecompo-nenttoarriveatacombinedconvectiveheattransfercoefficient.
Regardlessof thenatureof thecon-vectiveheattransferprocess,theappro-priaterateequationis
q=h(Ts- Tf)whereqistheunitheatflux (W 1m2);h isthecombinedconvectiveheattransfercoefficient(W/m20C);Tsis thepipe'sexternalsurfacetemperature(0C);andTf is the fluid temperature.In theTEMP/w program,theusermustinputtheoverallheattransfercoefficientaswellasafixedtemperatureortimede-pendenttemperaturefunctionfor thefluid.Thephysicalsizeof thepipecanalsobespecifiedasanoptionif theac-tualpipegeometryisomittedfromthefiniteelementmesh.
ThedatainTableI summarizesthethermalpropertiesusedin theoriginal
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GEOSPEC
Temperature Comparison for Different Ground Types
Measured at 2m offset from freeze row In high grade ore/clay- Predicted Measured at 3.7m offset from freeze row in low grade sandy/clay- PredictedJj. Measured at 3.5m offset from freeze row in barren sandstone
4 6 8 10 12
Time (Months)
Figure 3. Comparison of computedand predicted ground temperaturesfor various rock types(measureddata after Newmanand Maishman, 2000)
thermalmodelling (NewmanandMaishman,2000)andin thecurrentanalysis.
It shouldbenotedherethatnoad-
Theseresultsclearlyshowthatthenewconvectiveheattransferboundarycon-ditionoptioncanbeappliedtoartificialgroundfreezingif oneknowsthebrine
justmentsto thefreezepipeboundaryconditiontemperaturesweremadeinthemodelwhencalibratingthethermalconductivityof theclay/ oreground.Theactualfreezepipetemperaturewascomputedasaresultof theconvectiveheatbeingremovedbythebrine.
Figure3 is a comparisonof com-putedandmeasuredgroundtempera-turesfor threeof themainrocktypesandinitialin-situgroundtemperatures.
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fluidtemperatureandfreezepipediam-eter.Theoverallconvectiveheattrans-fercoefficientcanreadilybecomputedbasedonthebrineflowrateandotherhydraulicflowproperties.
Conclusions
Artificialgroundfreezingof thehighgradeMcArthurRiveruraniumorede-positwascarriedoutunderextremeconditions.Duetoadvancesinthermal
finiteelementanalysis,it isnowpossi-bletoaccuratelydetermineheatlossesfromthesoilintothemechanicalfreez-ingsystemwithouthavingtomakeanassumptionabouttheappropriatetem-peratureboundaryconditiontouseintheanalysis.
ReferencesGeo-Slope,2002.TEMP/w Version5
UsersManual,Geo-SlopeInterna-tionalLtd.,Calgary,Canada.
Newman,GregandDerekMaishman.ArtificialGroundFreezingof theMcArthurRiverUraniumOreDe-posit.Proceedings:InternationalConferenceonGroundFreezingandFrostActioninSoils.Belgium.Sep-tember,2000.
G.P.Newman,P. Eng.,GEO-SLOPEInternationalLtd.,1400,633- 6th.AvenueS\V,Calgary,AlbertaT2P 2Y5,Tel:403-269-2002,Fax: 403-266-4851,http://www.geo-slope.com
25.0
20.0
15.0-0-; 10.0...::s1U 5.0...Q)CoE 0.0Q)I-
-5.0
-10.0
-15.0
0 2
Table1.ThermalPropertiesUsed in Analyses
Conductivity(W/ m0c) BulkVolumetricWaterContent(%)
SilicifiedSandstone 5.2 10
Clav/ Ore 1.2 50
BasementOuartzite 5.6 2
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