NuclearEngineeringandDesign236(2006)683–700 Design and development of the AHWR—the Indian thorium fuelled innovative nuclear reactor ∗ R.K. Sinha , A. Kakodkar BhabhaAtomicResearchCentre,Trombay,Mumbai400085,India Received11March2004;receivedinrevisedform26September2005;accepted28September2005 Abstract Indiahaschalkedoutanuclearpowerprogrambasedonitsdomesticresourcepositionofuraniumandthorium.Thefirststagestartedwith settingupthePressurizedHeavyWaterReactors(PHWR)basedonnaturaluraniumandpressuretubetechnology.Inthesecondphase,thefissile materialbasewillbemultipliedinFastBreederReactorsusingtheplutoniumobtainedfromthePHWRs.Consideringthelargethoriumreserves inIndia,thefuturenuclearpowerprogramwillbebasedonthorium–233Ufuelcycle.However,thereisaneedforthetimelydevelopmentof thorium-basedtechnologiesfortheentirefuelcycle.TheAdvancedHeavyWaterReactor(AHWR)hasbeendesignedtofulfillthisneed.The AHWRisa300MW ,vertical,pressuretubetype,heavywatermoderated,boilinglightwatercoolednaturalcirculationreactor.Thefuelconsists e of(Th–Pu)O and(Th–233U)O pins.Thefuelclusterisdesignedtogeneratemaximumenergyoutof233U,whichisbredinsitufromthorium 2 2 andhasaslightlynegativevoidcoefficientofreactivity.FortheAHWR,thewell-provenpressuretubetechnologyhasbeenadoptedandmany passivesafetyfeatures,consistentwiththeinternationaltrend,havebeenincorporated.Adistinguishingfeaturewhichmakesthisreactorunique, fromotherconventionalnuclearpowerreactorsisthefactthatitisdesignedtoremovecoreheatbynaturalcirculation,undernormaloperating conditions,eliminatingtheneedofpumps.Inadditiontothispassivefeature,severalinnovativepassivesafetysystemshavebeenincorporated inthedesign,fordecayheatremovalundershutdownconditionandmitigationofpostulatedaccidentconditions.Thedesignofthereactorhas progressivelyundergonemodificationsandimprovementsbasedonthefeedbacksfromtheanalyticalandtheexperimentalR&D.Thispapergives thedetailsofthecurrentdesignoftheAHWR. ©2006ElsevierB.V.Allrightsreserved. 1. Introduction • physical separation of the high-temperature high-pressure coolantfromthelow-temperaturelow-pressuremoderator; TheIndiannuclearpowerprogramhasbeenconceivedbear- • ahighconversionratiowithwellthermalizedneutronspec- ing in mind the optimum utilization of domestic uranium and trumduetocoldmoderator; thorium reserves with the objective of providing long-term • low excess reactivity in the core arising out of on-power energysecuritytothecountry.Oneoftheessentialelementsof fuelling; theIndianstrategyistoenhancethefuelutilizationusingaclosed • agreaterflexibilityinadoptingdifferentrefuellingschemes. fuelcycle.Thisentailsreprocessingofthespentfueltorecover fissile and fertile materials and its recycle back into the sys- Indiahasbeenoperatinganddevelopingimprovedversions tem. Considering this objective, the indigenous nuclear power ofitscurrentgenerationPHWRsonthebasisofoperatingexpe- program in India was initiated with Pressurized Heavy Water rience,internationaltrendsandindigenousR&Dinputsasafirst Reactors(PHWRs)usingnaturaluraniumandheavywater,and stage. basedonpressuretubetechnology.Thepressuretubeconcept, InthesecondstageoftheIndiannuclearpowerprogram,plu- usedinPHWRs,hasseveraladvantagessuchas: toniumfromthenaturaluranium-basedPHWRswillbeusedin FastBreederReactorsformultiplyingthefissilebase.Consid- eringthelargethoriumreservesinIndia,thefuturesystems,in ∗ Correspondingauthor.Tel.:+912225505303;fax:+912225505303. thethirdstageofIndiannuclearpowerprogram,willbebased E-mailaddress:[email protected](R.K.Sinha). on thorium–233U fuel cycle. While the initiation of the third 0029-5493/$–seefrontmatter©2006ElsevierB.V.Allrightsreserved. doi:10.1016/j.nucengdes.2005.09.026 684 R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 stagewilltakeplaceinthefuture,thereisaneedforthetimely Reactivity control is achieved by on-line fuelling, boron dis- development of thorium-based technologies for the entire tho- solvedinmoderatorandreactivitydevices.Boroninmoderator riumfuelcycle.TheAdvancedHeavyWaterReactor(AHWR) is used for reactivity management of equilibrium xenon load. isbeingdevelopedtofulfillthisneed. Thereare12controlrods,groupedintoregulatingrods,absorber rodsandshimrodsof4each.Thereactorhastwoindependent, 2. EvolutionoftheAHWRconcept functionally diverse, fast acting shut down systems, namely, ShutDownSystem-1(SDS-1)consistingofmechanicalshutoff Thorium is a fertile material and has to be converted into rodsandShutDownSystem-2(SDS-2)basedonliquidpoison 233U, a fissile isotope. Of the three fissile species (233U, 235U injectionintothemoderator.Thereare30interstitiallatticeloca- and 239Pu), 233U has the highest value of η (number of neu- tionshousing150in-coreself-poweredneutrondetectorsand6 tronsliberatedforeveryneutronabsorbedinthefuel)inthermal out-of-corelocationscontaining9ionchambersand3start-up spectrum. Since 233U does not occur in the nature, it is desir- detectors.Anautomaticreactorregulatingsystemisusedtocon- able that any system that uses 233U should be self-sustaining trol the reactor power, power/flux distribution, power-setback in this nuclide in the entire fuel cycle, which implies that the and xenon override. Both for the control rods and the shut off amountof233Uusedinthecycleshouldbeequaltotheamount rods,theabsorbermaterial,boroncarbide,ispackedinanannu- produced and recovered. Thorium in its natural state does not lus within 80 stainless steel tubes. The core map is given in contain any fissile isotope the way uranium does. Hence, with Fig.1. thorium-basedfuel,enrichmentwithfissilematerialisessential. The reactor core is housed in a low-pressure reactor ves- Thelargeabsorptioncross-sectionforthermalneutronsintho- selcalledcalandria.Thecalandriacontainsheavywater,which riumfacilitatestheuseoflightwaterascoolant.Onaccountof act as moderator as well as reflector. The calandria houses the itshighcostanditsassociationwithradioactivetritium,useof verticalcoolantchannels,consistingofpressuretubesrolledin heavywatercoolantrequiresimplementationofacostlyheavy topandbottomendfittings.Thepressuretubecontainsthefuel watermanagementandrecoverysystem.Theuseoflightwater cluster. A calandria tube envelops each pressure tube and the ascoolantmakesitpossibletouseboilinginthecore,thuspro- air annulus between the two tubes provides thermal insulation ducingsteamatahigherpressurethanotherwisepossiblewitha between the hot coolant channel and the cold moderator. The pressurizednon-boilingsystem.Withboilingcoolant,thereac- calandriatubesarerolled,inthetubesheetsoftopandbottom torhastobevertical,makingfullcoreheatremovalbynatural endshieldsofthecalandria. circulationfeasible.Thechoiceofheavywaterasmoderatoris Thelightwatercoolantpicksupnuclearheatinboilingmode derivedfromitsexcellentfuelutilizationcharacteristics.Consid- fromfuelassemblies.Thecoolantcirculationisdrivenbynatural eringthesecharacteristics,themainlythoriumfuelledAHWR, convection through tail pipes to steam drums, where steam is isheavywatermoderated,boilinglightwatercooled,andhasa separatedandissuppliedtotheturbine.Asimplifiedschematic verticalcore. arrangementoftheAHWRisshowninFig.2. ThefutureIndianthorium-basedreactorsystemswillbeopti- Foursteamdrums(onlyoneshowninFig.2forthesakeof mizedforthethoriumcycle.FortheAHWR,pressuretubetype clarity), each catering to one-fourth of the core, receive feed PHWRtechnologyisselectedtotakeadvantageofthevastexpe- riencegainedandinfrastructuredevelopedinthecountry.Itis desirableforthenewreactorstoincorporatepassivesafetychar- acteristicsconsistentwiththeemerginginternationaltrends.The designofthereactorhasprogressivelyundergoneseveralmodi- ficationsandimprovementsbasedonfeedbacksfromtheresults of analytical and experimental R&D. This paper describes the currentdesignoftheAHWR. 3. Overviewofthereactorconfiguration Asalreadymentioned,theAHWRisavertical,pressuretube type,heavywatermoderatedandboilinglightwatercoolednat- ural circulation reactor (Sinha and Kakodkar, 2003) designed togenerate300MW and500m3/dayofdesalinatedwater.The e AHWRisfuelledwith(Th–233U)O pinsand(Th–Pu)O pins. 2 2 The fuel is designed to maximize generation of energy from thorium, to maintain self-sufficiency in 233U and to achieve a slightly negative void coefficient of reactivity. An emergency corecoolingsysteminjectswaterdirectlyintothefuel. ThereactorcoreoftheAHWRconsistsof505latticeloca- tionsinasquarelatticepitchof245mm.Ofthese,53locations are for the reactivity control devices and shut down systems. Fig.1. CoremapoftheAHWR. R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 685 Fig.2. SimplifiedschematicarrangementoftheAHWR. water at 403K to provide optimum sub-cooling at the reac- (i.e., replacement of outer ring of irradiated (Th–Pu)O fuel 2 tor inlet. Four down-comers, from each steam drum, are con- pins with fresh ones). The vertical pressure tube configuration nected to a circular inlet header. The inlet header distributes hasguidedthestructuraldesignofthefuelassembly.Thefuel the flow to each of the 452 coolant channels through individ- assembly is 10.5m in length and is suspended from the top in ualfeeders.TheAHWRincorporatesseveralpassivesystemsto thecoolantchannel.Theassemblyconsistsofafuelclusterand fulfill several safety functions (Sinha et al., 2000). A 6000m3 twoshieldsub-assemblies.Thesesub-assembliesareconnected capacity gravity driven water pool (GDWP), located close to toeachotherthroughaquickconnecting/disconnectingjointto top of the containment serves as a heat sink for several pas- facilitatehandling. sive systems, besides acting as suppression pool and a source of water for low-pressure emergency core cooling. Achieve- Table1 mentofpassiveshutdownusingsteamoverpressuretoprovide Dimensionaldetailsofthecore the driving force and passive cooling of concrete surfaces are Totalno.oflatticelocations 505 someoftheotheruniquepassivesafetyfeaturesprovidedinthe Numberoffuelchannels 452 AHWR. Numberoflatticelocationsforcontrolrods 12 Afuellingmachineislocatedontopofthedeckplate.The Numberoflatticelocationsforshut-offrods 41 fuelling machine of the AHWR handles the fuel clusters by Latticepitch(mm) 245 means of ram drives and snout drive for coupling and making Activecoreheight(m) 3.5 aleaktightjointwiththecoolantchannel.TheAHWRhasthe Calandria flexibilitytohaveonpoweraswellasoff-powerfuelhandling. Innerdiameterofthemainshell(m) 7.4 Innerdiameterofthesub-shellateachend(m) 6.8 ThedimensionaldetailsofthecorearegiveninTable1. Length(m) 5.3 A seawater desalination plant will meet the demineralized waterrequirementsofthereactoranddrinkingwaterrequiredat Tubematerial Pressuretube Zr2.5Nb the plant, utilizing the low-pressure steam from the turbine. A Calandriatube Zircaloy-4 provisionexiststoaddtothedesalinationcapacityatthecostof Tubedimension electricalpoweroutput. Innerdiameter/WTofPressuretube(mm) 120/4 Outerdiameter/WTofCalandriatube(mm) 168/2 4. Fuelandfuelcycle Reflectorthickness(D2O)axial/radial(mm) 750/600 Moderatortemperature(K) 353 Thefuelhasbeendesignedtomeettherequirementofther- malhydraulics,reactorphysics,fuelhandlingandreconstitution Moderatorpurity(%ofD2O) 99.8 686 R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 Thefuelclusterisacylindricalassemblyof4300mmlength etal.,1999).Twoenrichmentshavebeenprovidedintheouter and118mmdiameter.Thearrangementofpinsinthefuelcluster ringtohavefavorableminimumcriticalheatfluxratios. is shown in Fig. 3(a). The cluster has 54 fuel pins arranged in ThefuelpinconsistsoffuelpelletsconfinedinaZircaloy-2 3 concentric rings around a central rod as shown in Fig. 3(b) cladtube.Thefuelpinhasapelletstacklengthof3500mmanda (AnantharamanandShivakumar,2002).The24fuelpinsinthe plenumvolumewithahelicalspringinittokeepthepelletstack outerringhave(Th–Pu)O asfuelandthe30fuelpinsintheinner pressed. The fuel pins are assembled in the form of a cluster 2 andintermediateringshave(Th–233U)O asfuel.Theinnermost with the help of the top and bottom tie-plates, with a central 2 12pinshavea233Ucontentof3.0wt.%andthemiddle18pins rodconnectingthetwotie-plates.Sixspacersalongthelength have3.75wt.%233U.Theouterringof(Th–Pu)O pinscontain oftheclusterprovidetheintermediatepinspacing.Thecentral 2 3.25wt.% of total plutonium, of which the lower half of the rod has a tubular construction with holes for direct injection activefuelhas4.0%Puandtheupperparthas2.5%Pu(Kumar of ECCS water on the fuel rods. It also contains dysprosium capsules containing dysprosium oxide in Zirconia matrix. The designdataofthefuelassemblyisgiveninTable2. TheAHWRfuelcycleisaclosedfuelcycle,envisagingrecy- cle of both fissile 233U and fertile thoria back to the reactor (Anantharamanetal.,2000).Thecurrentlyenvisagedfuelcycle timeiseightyears.Thiscomprisesfouryearsforin-reactorres- idence time, two years for cooling, one year for reprocessing andoneyearforrefabrication.Sincethe233Urequiredforthe reactor is to be bred in situ, the initial core and annual reload fortheinitialfewyearswillconsistof(Th–Pu)O clustersonly. 2 Afterreprocessing,233Uisalwaysassociatedwith232U,whose daughter products are hard gamma emitters. The radioactivity of232Uassociatedwith233Ustartsincreasingafterseparation. This poses radiation exposure problems during its transporta- tion,handlingandrefabrication.Hence,itistargetedtominimize delaybetweenseparationof233Uanditsrefabricationintofuel. Inviewofthis,aco-locationofthefuelcyclefacility,compris- ingreprocessing,wastemanagementandfuelfabricationplant, Table2 DescriptionoftheAHWRfuelassembly Parameter Value Numberoffuelpins 54 Outerdiameter(mm) 11.2 Density(g/cm3) 9.6 Fuelclad Material/thickness(mm) Zircaloy-2/0.6 Fueltype/numberofpins Innerring (Th–233U)O2/12 Middlering (Th–233U)O2/18 Outerring (Th–Pu)O2/24 Fuelenrichment(wt%) Innerring(233U) 3.0 Middlering(233U) 3.75 Outerring(Pu) 3.25(average) Upperhalf 2.5 Lowerhalf 4.0 Centralrod Tubeo.d./thickness(mm) 36/2 Numberofpins/capsule 12 Outerdiameterofpin(mm) 6 Material/o.d.(mm) ZrO2+Dy2O3 Dysprosium(wt%) 3.0 Averagedischargeburnup(MWd/t) 24,000 Averagelinearheatrating(kW/m) 10.6 Peaklinearheatrating(kW/m) 14.0 Fig.3. (a)Cross-sectionoffuelpinsintheclusterand(b)AHWRfuelcluster. R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 687 withtheAHWRhasbeenplanned.The233U-basedfuelneedsto using a slow burning absorber. In the AHWR, dysprosium is befabricatedinshieldedfacilitiesduetoactivityassociatedwith usedasaburnableabsorberwithintheclusteratalatticepitch 232U.Thisalsorequiresconsiderableenhancementofautoma- of 245mm, to make the void coefficient of reactivity negative tionandremotizationtechnologiesusedinfuelfabrication. foraveragecoreburnup. The spent fuel cluster, before reprocessing, would undergo disassembly for segregation of (Th–Pu)O pins, (Th–233U)O 5.1.2. Achievingaflatradialpowerdistribution 2 2 pins, structural materials and burnable absorbers. The Heatremovalthroughnaturalconvectionisanimportantfea- (Th–233U)O pins will require a two stream reprocessing pro- tureofthisreactor.Inordertohavegoodthermalhydraulicand 2 cess, i.e., separation of thorium and uranium whereas the neutroniccoupling,theradialpowerdistributionhastobeflat. (Th–Pu)O pins will require a three stream reprocessing pro- Thisrequirestheheightoftheactivecoretobekeptsmallwith 2 cess,i.e.,separationofthorium,uraniumandplutonium.Apart respecttothediameterofthecore.Inviewofthis,thecoreheight oftheofreprocessedthorium(45%)maybeusedimmediately hasbeenchosentobe3.5mandthecalandriavesseldiameteris in the fabrication of (Th–233U)O pins since 233U fabrication 7.4m.Thereare505latticelocationsinthecore,outofwhich 2 isrequiredtobecarriedoutinshieldedfacilities.Theremain- 452 locations are occupied by fuel and the rest by reactivity ingthoriumwillbestoredforsufficientamountoftimeforthe devices. activitytodecaytoalevelatwhich,itiseasierforhandlingwith minimalshielding.Thestoredthoriumwillbesubsequentlyused 5.1.3. Optimizingtheaxialpowerprofileforadequate forthefabricationof(Th–Pu)O fuelpins. thermalmargin 2 Inatypicalboilingwaterreactorwithbulkboiling,theaxial power profile is bottom-peaked and this increases the thermal 5. Reactorphysics margin in the top region of the fuel where the void fraction is high. In AHWR, in order to achieve a desirable axial power 5.1. Mainobjectivesofthephysicsdesign distributionforadequatethermalmargin,gradedenrichmentis usedalongthelengthofthefuelassembly.Thisisachievedby The physics design of AHWR is carried out to fulfill the altering only the plutonium content in the outer pins without followingobjectives(Srivenkatesanetal.,2000): compromising the void reactivity. The lower half of the fuel assembly is loaded with 4.0wt.% Pu and the upper half with (1) maximizetheenergyfrominsituburningof233U; 2.5wt.%Puinthoriumdioxide. (2) achieveanegativevoidcoefficientofreactivity; (3) achievegreaterthan20,000MWd/tfueldischargeburnup; 5.1.4. Achievingself-sustenancein233U (4) minimize,totheextentpossible,theinitialplutoniuminven- Theobjectiveofachievingself-sustenancein233Uhasgov- tory; ernedthereactorphysicsdesignofAHWRcore.The233Ubred (5) minimize,totheextentpossible,theconsumptionofpluto- intheclusterdecidestheself-sustainingcharacteristicofAHWR niumforgivenenergyoutput; fuel. With irradiation, the 233U content depletes in the inner (6) achieveself-sustenancein233U; (Th–233U)O pins and increases in the outer (Th–Pu)O pins 2 2 (7) deliverathermalpowerof920MWtothecoolant. duetoconversionfromthorium.Theconversionhasbeenmax- imizedbymakingthespectrumharder,i.e.,inanintermediate Toachievetheseobjectives,thephysicsdesignhasprogres- energyrangearound0.2eV. sivelyevolvedfromaseed-blanketcoredesignconcepttoacore consisting of a single type of cluster called composite cluster, 5.1.5. Minimizationofplutoniummake-uprequirement containingboth(Th–233U)O and(Th–Pu)O fuelpins(Kumar, 2 2 The plutonium pins are placed in the outermost ring of the 2000). The main considerations governing the fulfillment of clustertominimizetheplutoniumrequirement.Theplutonium theseobjectivesarediscussedinthefollowingsub-sections. used as make-up fuel comes from the discharged PHWR fuel. Thepowerfromthoriumis60%. 5.1.1. Achievingnegativevoidcoefficientofreactivityin bothoperatingandaccidentalconditions 5.2. Reactorphysicsanalyses The cluster design is mainly dictated by the objective of achievingnegativevoidcoefficientofreactivity.Thevoidcoeffi- The analyses comprise core calculations, using a 3D code cientofreactivitycanbemadenegativebymaintainingaharder forcoreoptimization,forobtainingtheoptimumfueldischarge neutron spectrum in the core. This can be achieved either by burnup, flattened channel power distribution and worth of the changingthepropertiesofthemoderatingmediumorbydecreas- reactivitydevices. ingtheinventoryofthemoderator(forexample,byincreasing the cluster size in relation to the lattice pitch). It is also possi- 5.2.1. Physicsanalysisfortheequilibriumcore ble to achieve negative void coefficient of reactivity by using Thereactorphysicsanalysispresentedheremainlypertains a burnable absorber either in the fuel or in isolated pins in an to the equilibrium core configuration, which consists of the inertmatrix.Onvoidingofthecoolant,thethermalneutronflux composite type of cluster. Detailed lattice analyses have been increasesinthecluster,andtheneutronfluxcanbereducedby performedtocalculatethevariationoflatticeparameterssuchas 688 R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 thelatticereactivity(k-infinity),themacroscopiccross-sections iscalculated.Itisseenthatthecoreburnup,powerandcoolant andtheisotopiccompositionsasafunctionofirradiation.The densitydistributionconvergeinthreetofouriterationsandthe pin-wise power distribution across the cluster, reactivity coef- optimumpowerdistributionisestimatedaccordingly.Thequar- ficients,andotherlatticecharacteristicsarealsoobtained.The ter core power distribution, calculated for the average coolant lattice evaluations have been done with WIMSD code system densityof550kg/m3 throughoutthecore,isshowninTable4. (Askewetal.,1996)andthe69energygroupsWIMSDnuclear TheburnupzonesandtheirexitburnupsarealsogiveninTable4. data library from the basic data set of ENDF/BVI.8 (IAEA, The exit burnups of the three zones are 30,000, 23,500 2002). and 20,000MWd/t. The average discharge burnup is nearly ThedesignfeaturesofAHWRforequilibriumcoreconfigu- 24,000MWd/t. The radial and axial peaking factors are cal- rationaregiveninTable3.Thecorecalculationshavebeendone culated to be 1.2 and 1.64, respectively. The limits on power using3DKINandFEMTAVG(KumarandSrivenkatesan,1984). distribution/power are derived from the minimum critical heat Thetime-averagedsimulationshavebeendonetogetoptimum fluxratio—MCHFR(CHFRistheratioofthecriticalheatflux discharge burnup and flattened channel power distribution for atanypointintheflowchanneltotheactualfluxatthatpoint), theequilibriumcoreconfiguration.Thecorepowerdistribution and it is a measure of safety margin available for the reactor hasbeenoptimizedforatotalpowerof920MW. core.TheMCHFRcalculatedat20%overpoweris1.67. t In order to achieve flux flattening, the equilibrium core has ThereactivitybalanceinAHWRisgiveninTable5.Theequi- beendividedintothreeburnupzones,whichareadjustedtoget libriumxenonloadis21.0mkandthemaximumtransientxenon the average discharge burnup of nearly 24,000MWd/t and the loadpeakingfollowingshutdownis7.0mk(US$1=3mk).This maximumchannelpowerof2.6MW.Theaveragecoolantden- isduetorelativelylowthermalfluxlevelof7.0×1013n/cm2/s. t sity in the core is 550kg/m3. The code FEMTAVG is coupled The void reactivity for equilibrium core of AHWR has been toastaticthermal–hydraulicscodeTHABNA,andthecoolant calculatedas6.0mk. density as a function of distance from inlet for every channel The major postulated initiating events, considered from the pointofreactivitychanges,arelossofregulationaccidentand cold-wateringress.Outofthese,onlylossofregulationaccident Table3 involves substantial positive reactivity addition. Both the shut PhysicsparameterofAHWRequilibriumcore downsystemsofAHWRarecapableofindependentlyshutting Parameter Value downthereactorintime. Fuellingrate,annual Numberoffuelchannels 113 5.2.2. TheinitialcoreofAHWR Pu(kg) 200 The 233U, required for the equilibrium core of AHWR will Conversionratio,233U 97% bebredinsitu.Itisenvisagedthattherewillbeagradualtran- Powerfromthorium/233U 60% sitionfromtheinitialcorethatwillnotcontainany233U,tothe Peakingfactors(maximum) equilibriumcore. Local 1.45 Radial 1.2 5.2.3. Recyclingofuranium Axial 1.64 Withseveralrecycles,the234Ucontentinuraniumincreases Total 2.85 from 6 to about 12%. It is seen that the reactivity load due to Reactivitycontrol 234UinsuccessiverecyclingofuraniumintheAHWRcausesa Boron/gadoliniuminmoderator penaltyofabout1500MWd/t.Fuelcyclecalculationshavebeen Controlrods(no.) 12(totalof18.9mk) donetooptimizecyclelengthwithrespecttotheself-sustenance Absorberrods(no.) 4(totalof7.1mk) in233Uandotherfuelperformancecharacteristics. Regulatingrods(no.) 4(totalof8.1mk) Shimrods(no.) 4(totalof3.7mk) 5.2.4. Xenonoscillations ShutDownSystem-1 41nos.(totalof80mk; ThepossibilityofxenoninstabilitiesintheAHWRisreduced 46mkwithtwo considerablyduetorelativelylowthermalfluxlevelalongwith maximumworthrods notavailable) negativevoidandpowerfeedback.Onlyfirstazimuthalmode, Absorbermaterial B4CpinsinSSshell with sub-criticality of 12mk, is close to the instability thresh- ShutDownSystem-2 Liquidpoisoninjection old in the AHWR. There are four regulating rods, one in each inmoderator quadrant,tosuppressanyfluxtiltarisingduetotheseazimuthal Safetyparameters oscillations. Delayedneutronfraction,β 0.003 Promptneutrongenerationtime,Λ(ms) 0.22 6. Descriptionofmajorreactorsystems Reactivitycoefficients,(cid:4)k/k(◦C) Fueltemperature −2.0×10−5 Coolanttemperature +3.5×10−5 6.1. Reactorblock Channeltemperature +1.0×10−5 The reactor block of AHWR consists of calandria, end Voidcoefficient,(cid:4)k/k(%void) −6.0×10−5 shields, coolant channels and associated piping, deck plate, R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 689 Table4 Optimizedcorepowerdistribution reactorcontrolandprotectionsystems,andECCSheaderwith associated piping and main heat transport (MHT) system inlet header.Thelayoutofcomponentsinreactorblockisshownin Fig. 4. The calandria is housed in a light water filled reactor vaultthatactsasaneffectiveradiationshield.Endshields,sup- Table5 ReactivitybalanceinAHWR Reactorcorestate Reactivity(mk) Reactivityswings (1)Coldtohotstandby Channeltemperature(300–558K) +2.5 Moderatortemperature(300–353K) +3.0 Total +5.5 (2a)Hotstandbytofullpower Fueltemperature(558–898K) −6.5 Coolantvoid(coolantdensityfrom0.74to −2.0 0.55g/cm3) Total −8.5 (2b)LOCAfromfullpower(coolantdensity −4.0 0.55–0.0g/cm3) (3)Xenonload Equilibriumload −21.0 Transientloadaftershutdownfromfull −7.0 power(peakatabout5h) Fig.4. Reactorblock. 690 R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 portedonconcretestructure,areprovidedatboththeendsofthe calandria. 6.1.1. Calandria The calandria is a 5.3m long cylindrical stainless steel (SS304L) vessel. It houses the reactor core, moderator, reflec- tor,andaportionofthereactorcontrolandprotectionsystems. The central portion of the calandria is called the main shell (7.4m i.d.×3.5m long). Two sub-shells of smaller diameter (6.8m i.d.) are attached to the main shell at top and bot- tom with flexible annular plates. The calandria is fully filled with heavy water and is connected to the expansion tank to accommodatevolumetricexpansionofthemoderator.Thenoz- zle penetrations, required for the moderator system, the liquid poison injection system and the expansion tank are provided in the sub-shells of the calandria vessel. The nozzle penetra- tionsforoverpressurereliefdevicesareprovidedinmainshell of the calandria vessel to protect the calandria against inter- nal pressure above the design limit occurring during acciden- tal conditions. The nominal inlet and outlet temperatures of the moderator are 328 and 353K, respectively, and the calan- dria is designed for 0.05MPa above the static head due to moderator. 6.1.2. Endshields The end shields are composite cylindrical stainless steel (SS304L)structures,filledwithamixtureofsteelballsandwater and are attached to the top and bottom ends of the calandria byinsituwelding.Theseendshieldsprovideradiationshield- ing and serve as pressure boundary to the moderator system. The end shields also support and guide the coolant channel assemblies,reactorcontrolsystemsandprotectionsystems.The Fig.5. Schematicarrangementofthecoolantchannelassembly. verticalcalandriatubesarejoinedtotheendshieldlatticetubes byrolledjoints.Lightwateriscirculatedthroughtheendshields tionbetweenthehotcoolantandthecoldmoderator.Thecoolant toremovethenuclearheatgenerated. channelassemblyislaterallysupportedwithinthelatticetubeby twobearingslocatedatthetwoendsofthetopendshieldlattice 6.1.3. Coolantchannelassembly tube.Theweightofthecoolantchannelissupportedatthetop The coolant channel houses the fuel assembly with shield- endshield.Anannulusleakmonitoringsystemisincorporated ingblocksandhassuitableinterfacesforcouplingtothemain toprovideanearlywarningofaleakageinthepressuretube,as heattransportsystem.Asuitableinterfaceisprovidedforcou- apartofthestrategytomeettheleakbeforebreakrequirement plingthefuellingmachinewiththecoolantchanneltofacilitate forthepressuretube. removalofhotradioactivefuelfromthereactorandintroduction Easyreplacementofpressuretubes,asapartofthenormal offreshfuelintothereactor.Thecoolantchannelhasfeatures maintenanceactivityisanimportantconsiderationinthedesign to accommodate thermal expansion, and irradiation creep and growth.Theschematicarrangementofthecoolantchannelwith thefuelassemblyisshowninFig.5.Theverticalcoolantchan- nel consists of pressure tube, top and bottom end fittings, and calandriatube.Thepressuretube,madeofzirconium–niobium alloy,islocatedinthecoreportion.Thecoreportionisextended with top and bottom end fittings made of stainless steel. The feederpipeisconnectedtothebottomendfittingthroughaself- energizedmetalsealcouplingandthisfacilitateseasyremoval. Thetailpipeisweldedtothetopendfitting.Thecoolantenters thecoolantchannelat533.5K,flowspastthefuelassemblyand hot coolant flows out as steam–water mixture at 558K flows out to tail pipes. The annular space between the pressure tube andcalandriatube,asshowninFig.6,providesathermalinsula- Fig.6. Fuelclusterinthepressuretube. R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 691 ofthecoolantchannelassemblies.TheAHWRcoolantchannel is designed for easy replacement of pressure tubes as a reg- ular maintenance activity without incurring a large downtime ofthereactor.Thisallowstheindividualcoolantchanneltobe replaced at the end of its design life. A longer life and easy replacement criteria have guided the selection of the pressure tube material and the design of pressure tube, end fittings and the couplings. The pressure tube is provided with an in-built reducer at the bottom end and a thicker walled top end. The pressuretubeisdetachablefromtherolledjointwiththetopend fitting.Thebottomendfittingcanbedetachedfromthefeeder byde-couplingthebottommetalsealcoupling.Thebottomend fittingissizedsuchthatitcanberemovedalongwiththepres- suretubethroughtheboreofthetopendfittingafterdetaching itfromthefeederandthetopendfitting.Topendfittingispro- videdwithtwosetsofrolledjointbores.Ashopassembledfresh pressuretubewithbottomendfittingcanbeinsertedthroughthe boreofthetopendfittingandrolledtothefreshsetofrolledjoint Fig.7. FuelhandlingsystemoftheAHWR. grooves. 6.2.2. Fuellingmachine 6.2. Fuelhandlingandstoragesystem The experience and feedback of fuelling machines of the existing PHWRs and the Dhruva research reactor have been The refuelling operation is carried out by a remotely oper- consideredforthedesignoftheAHWRfuellingmachine.The ated fuelling machine moving on rails laid on the reactor top. fuelling machine is a vertical and shielded machine designed Thefuelhandlingsystemmainlyconsistsofafuellingmachine, to handle the 10.5m long fuel assembly (Fig. 8). The fuel an inclined fuel transfer machine, a temporary fuel storage assembly of the AHWR consists of the fuel cluster, shield A blocklocatedinsidethereactorbuildingandafuelstoragebay andshieldBjoinedtogetherthroughcolletjoints.Thefuelling located outside the reactor. The temporary fuel storage block machine moves on the reactor top face to approach any indi- comprises fuel port and under water equipment. The fuel port vidual coolant channel for carrying out the refuelling opera- acts as an interface with fuelling machine for charging new tion. The function of the fuelling machine is to remove and fuel and receiving spent fuel. Underwater equipment is used insertthefuelassembly.Themajorcomponentsofthefuelling for handling the fuel within the storage block, and for trans- machineareramassembly,magazineassembly,snoutassembly, ferring the fuel clusters across the containment walls through an inclined fuel transfer machine. The temporary fuel storage blockalsocaterstobufferstorageofthefueltomeetrefuelling requirement in case of temporary outage of the inclined fuel transfer machine. The inclined fuel transfer machine transfers the fuel from temporary fuel storage block to the fuel stor- age bay located outside the reactor building. The fuel storage bay houses new fuel storage area, spent fuel storage area and the handling equipment. The design of the system has been conceptualized and following important concepts have been evolved. 6.2.1. Fueltransfersystemtotransferfuelacross containmentwalls The inclined fuel transfer machine is a tall machine con- nectingthetemporaryfuelstorageblocktothefuelstoragebay throughthecontainmentwalls.Awaterfilledpotcontainingfuel, guided in an inclined ramp, is hoisted up in the tilting leg and subsequentlyhoisteddowntounloadthefuelontheotherside. Fig.7showsthefuelhandlingsystemofAHWR.Theconcept of the inclined fuel transfer machine is most suitable because oflessrequirementofspaceinsidethereactorbuilding,online fueltransfer,smallcontainmentpenetrations,assuredcoolingof fuel throughout the transfer and passive containment isolation features. Fig.8. FuellingmachineoftheAHWR. 692 R.K.Sinha,A.Kakodkar/NuclearEngineeringandDesign236(2006)683–700 separator assembly and its trolley and carriage assembly. The theinternalstructuresofthereactorbuildingarefoundedona snoutpluglocatedinthesnoutassemblymakesaleaktightcon- commoncircularreinforcedconcretebaseraft.Thebaseraftis nectionwiththecoolantchannelendfitting.Thesnoutassembly 4m thick near the center and 5m thick near the edges, where clamps the fuelling machine to the end fitting for carrying out thewallsareconnected. the refuelling operation. The seal plug is located at the top of The AHWR reactor building has a 6000m3 capacity circu- coolant channel and acts as a pressure boundary for the MHT lar water tank in the inner containment located at an elevation water/steam. The ram assembly consists of three coaxial rams of 136m. This large water pool, called gravity driven water and the outer ram travels up to 7.6m for removal of the fuel pool, is sufficient to cool the reactor for three days follow- assembly from the coolant channel. The three coaxial rams ing any accident in the plant. The GDWP tank is made of manipulate the snout plug, seal plug, ram adaptors and shield reinforced concrete with a steel liner inside. The pool is sup- plugs for their removal, movement and installation. The mag- ported on the ring beam of the inner containment all along its azine assembly consists of eight tubes fixed on a rotor and circumference. In addition to this, two tail pipe towers sup- temporarily stores the plugs and fuel cluster. The ram adap- port it. These tail pipe towers extend right up to the base of tor is hung in the magazine tube and holds the fuel cluster the raft. Two steam drums are located within each tail pipe through a collet joint. The separator assembly is required to toweratanelevationof123m.TheGDWPisdividedintoeight senseandholdthefuelassemblyduringitsremovalandinser- compartments. tiontofacilitatejoining/disjoiningdifferentfuelassemblyparts. Thefuellingmachineheadishungtothefuellingmachinesup- 7. Passivesystemsandinherentsafetyfeaturesof port system through a X-trunion located at the ram housing AHWR of the ram assembly. The fuelling machine support system is mounted on a shielding assembly. The shielding assembly is The AHWR has several passive safety systems for reactor supportedonatrolleyandcarriageassembly.Thetrolleymoves normaloperation,decayheatremoval,emergencycorecooling, in Y direction on the rails provided on the carriage assembly. confinementofradioactivity,etc.(Bhatetal.,2004).Thesepas- The carriage assembly moves across the reactor top face in X sivesafetyfeaturesarelistedbelow: directiononfuellingmachinerails.Thedriveisprovidedbyan oilhydraulicsystem.Thefuellingmachineiscoarsealignedto • core heat removal by natural circulation of coolant during aparticularchannelthroughthetrolleyandthecarriagetravels, normaloperationandshutdownconditions; andfinealignmentisbyXfineandYfinemovementsprovided • directinjectionofECCSwaterinthefuelclusterinpassive in the fuelling machine support system. During the refuelling modeduringpostulatedaccidentconditionslikeLOCA; process the fuelling machine clamps with the channel, makes • containmentcoolingbypassivecontainmentcoolers; leaktightjoint,removesthesealplug,removesthefuelassem- • passivecontainmentisolationbywaterseal,followingalarge bly,separatestheshield‘A’,shield‘B’andfuelcluster,replaces breakLOCA; with new fuel and boxes up the channel after completing the • availability of large inventory of water in GDWP at higher reversesequencesofoperations.Theentireoperationoffuelling elevation inside the containment to facilitate sustenance of machineisdoneremotely. coredecayheatremoval,ECCSinjection,containmentcool- ing for at least 72h without invoking any active systems or 6.2.3. Fuelstoragebay operatoraction; Astoragebay,locatedinthefuelbuildingadjoiningthereac- • passiveshutdownbypoisoninjectioninthemoderator,using tor building, stores the fresh and the spent fuel under water. thesystempressure,incaseofMHTsystemhighpressuredue Thestoragepoolcapacityisdecidedbasedontherefuellingfre- to failure of wired mechanical shutdown system and liquid quencyof113fuelclusters(one-fourthofthecore)perannum, poisoninjectionsystem; twoyearscoolingspanforthefuelclusterandsixmonthsinven- • passive moderator cooling system to minimize the pressur- tory of new fuel clusters. Provision is made to monitor the izationofcalandriaandreleaseoftritiumthroughcovergas leakagefromthebay.Afail-safecraneandhandlingequipment duringshutdownandstationblackout; areprovidedinthebay. • passiveconcretecoolingsystemforprotectionoftheconcrete structureinhigh-temperaturezone. 6.3. Reactorbuilding The passive and active heat removal paths of the AHWR Theconceptofdoublecontainmenthasbeenadoptedinthe under various operational states and in LOCA are shown in designofAHWRreactorbuilding.Thecontainmentstructures Fig. 9. The design features of passive systems are described consistofaninnercontainmentwallanddome,formingthepri- inthefollowingparagraph. marycontainment.Anoutercontainmentwallanddomeform thesecondarycontainment.Theinnercontainmentwallandthe 7.1. Passivecoreheatremovalbynaturalcirculation containmentdomearemadeofprestressedconcreteandtheouter duringnormaloperation containmentwallandoutercontainmentdomearemadeofrein- forced concrete. There exists an annular space of 5.2m width During normal reactor operation, full reactor power is betweenthetwocontainments.Thecontainmentstructuresand removed by natural circulation caused by thermo siphoning