MINOR ACTINIDE TRANSMUTATION IN GFR600 KEYWORDS

15 Pages·2011·0.92 MB·English

MINOR ACTINIDE TRANSMUTATION
FUELCYCLEAND
MANAGEMENT
IN GFR600
KEYWORDS:transmutation,gas-
cooled fast reactor, nonuniform
ZOLTÁN PERKÓ,a* JAN LEEN KLOOSTERMAN,a and minoractinidecontent
SÁNDOR FEHÉRb
aDelftUniversityofTechnology,DepartmentofRadiation,RadionuclidesandReactors
Mekelweg15,Delft,Netherlands
bBudapestUniversityofTechnology,InstituteofNuclearTechniques
Mu˝egyetemrakpart3,Budapest,Hungary
ReceivedSeptember24,2010
AcceptedforPublicationMarch10,2011
Within the Generation IV initiative, the gas-cooled than 3%. By feeding MAs as well (constant MA content
fast reactor (GFR) is one of the reactors dedicated to strategy),thereactivityhasasteadyincreasefromcycle
minor actinide (MA) transmutation. This paper summa- tocycle,predominantlydueto238Pubreedingfrom237Np.
rizestheresearchperformedwiththeGFR600reference The effects of the isotopic composition of the pluto-
designinordertoassessitsMAburningcapabilities.For nium and MAs were also examined by performing cal-
the study, modules of the SCALE program system were culationswithdataspecifictothespentfueloftraditional
used. western pressure water reactors and Russian type
Single-cycleparametricstudieswereperformedwith VVER440 reactors. Despite the considerably different
cores having different MA content and spatial distribu- MA vectors, no significant deviation was found in their
tion. It was shown that the addition of MAs to the fuel overalltransmutation.However,thePucompositionhad
greatlyreducedthereactivitylossduringburnup.More- astrongeffectonthereactivityandthedelayedneutron
over,thehighertheMAcontentofthecore,thehigherthe fraction in the first cycles.
fraction of it that was fissioned; however, the more the Finally, cores having nonuniform MA content were
delayed neutron fraction and the fuel temperature coef- investigated.ItwasfoundthatthoughtheMAdestruction
ficient degraded. Significant reduction can be achieved efficiency was significantly higher in the middle of the
in the amounts of neptunium and americium, while cu- corethanattheedge,movingsomeoftheMAsfromthe
rium isotopes accumulate. outer regions to the center resulted in only minor im-
Thestudyofmultipleconsecutivecyclesshowedthat provement in their destruction. However, the spectral
by adding only depleted uranium (DU) to the repro- changes caused by the rearrangement increased the
cessedactinidesinfuelfabrication(pureDUfeedstrat- k-effective,whichallowedhigherburnupsandincreased
egy),upto70%oftheinitiallyloadedMAscanbefissioned MA destruction. Unfortunately, some of the safety pa-
inthefirstfivecycles.Moreover,thereactorcanbemade rameters of the reactor degraded.
criticalduringthattimeiftheinitialMAcontentishigher
I. INTRODUCTION prominentinsustainabilitybyhavingaclosedfuelcycle
and a self-breeder core.1 In such a system all actinides
In2002theGenerationIVinitiativedraftedthemost would be recovered from the spent nuclear fuel of the
importantrequirementsthatfuturenuclearreactorsshould reactor during reprocessing, and they would be used in
meet and embraced six reactor concepts that have the fuel fabrication for the same reactor; thus, only fission
highestpotentialtodoso.Oneofthedesignsisthegas- products ~FPs! and actinides due to reprocessing losses
cooled fast reactor ~GFR!, which is anticipated to be wouldbesenttothegeologicalrepositories.Atthesame
time,onlyfertileisotopeswouldhavetobeaddedtothe
*E-mail:[email protected] recovered actinides during fuel manufacturing because
NUCLEARTECHNOLOGY VOL.177 JAN.2012 83
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
oftheefficientbreeding.Thesewouldobviouslyresultin II. MODELING GFR600
a significantly reduced spent fuel output and in a better
use of fissile materials compared to today’s reactors. As a reference GFR, the CEA-designed “effective”
TheGFRisalsoenvisionedtoplayanimportantrole GFR600conceptwaschosen,featuring600-MW~thermal!
innuclearwastemanagement.Presentreactorsaremostly power, plate-type fuel, and high-pressure helium cool-
operatedinanopenfuelcycleinwhichthespentfuelis ant. The exact details of the reactor are easy to find in
notreprocessedandallactinidesendupaswastetogether mostrelevantarticles~seeRefs.3,7,and8forexample!;
with the fission products. This is not only disadvanta- here, only the most important parameters are repeated.
geousfromthewastemanagementbutalsofromthefuel The core consists of 112 fuel assemblies ~FAs! ar-
economicspointofview,sinceononehandtheactinides rangedinsixringsaroundacentralpiece;theseannular
increase the volume, heat load, and radioactivity of the regions are referred to as the six regions of the core
nuclearwasteandrepresentitslong-termdanger;onthe @region 1 being the outermost, region 6 the innermost
other hand, many fissile isotopes are discharged instead ring,plusthecentralpiece~coloredlightgreenandred!
ofbeingreusedinthermalreactors.Hence,itisdesirable inFig.1,respectively#.Eachassemblyishexagonal,has
toshifttoanuclearsystemwherethespentfuelisalways a120-degrotationalsymmetry,andcontains21fuelplates
reprocessed, all actinides are recycled, and only fission in3compartments,eachhaving7parallelplatesfixedby
productsaredisposedof~togetherwiththereprocessing theassemblywrapperandtheY-shapedcentralmechan-
losses!. The reuse of uranium and plutonium is already ical restraint ~see Fig. 2!. The fuel concept is based on
possibleasmixed-oxide~MOX!fuelinappropriatether- dispersed fuel—the ceramic actinide compound ~acti-
mal reactors, but minor actinides ~MAs! are not usable nidecarbide,70V0V%a!isembeddedinaninertceramic
today since they are not fissile in present commercial matrix~SiC,30V0V%!formingtheCERCERfuel.The
reactors ~nor is it daily practice to recover and reuse structuralmaterialisalsoSiC~cladding,assemblywrap-
plutonium from spent MOX fuel!. The situation is dif- per, etc.!, while the reflector is Zr Si . In the reference
3 2
ferent in fast reactors, where because of the increased fuel,theactinidecompoundismadeupby84n0n%bUC
energyofneutronsallactinidesaremorefissionableand ~natural uranium! and 16 n0n% PuC ~legacy plutonium
the buildup of heavier isotopes via capture is less prob- fromspentfuel!.Thesemainparametersaresummarized
able. This leads to the idea of using them as actinide inTable I.
burners in order to effectively destroy actinides gained It was investigated how MAs can be destroyed by
byreprocessingthespentfuelofotherreactors.Themain adding them to the fuel. In these burnup calculations it
focus is on the elimination of MAs ~neptunium, ameri- wassupposedthattheysubstitutesomeoftheuraniumin
cium,andcurium!astheyrepresentsignificantheatload thefuel;hence,theactinidecompoundwaschosentobe
andneutronsourceinthespentfuelandarenotthermally made up by 16 n0n% PuC, X n0n% MA carbide, and
fissile. ~84-X! n0n% UC ~the exact composition in the nonuni-
In the European GCFR-Specific Targeted Research formcaseisdetailedinSec.IV.B!.Toexaminetheeffects
Program ~STREP!, an extensive study was done on the of the isotopic composition of the recycled plutonium
MA transmutational capabilities of GFRs. The calcula- andMAs,twosetsofdatawereusedinourstudycorre-
tionswerebasedonaone-dimensionalmodeloftheCEA- sponding to the spent fuel of western pressurized water
designed “effective” GFR600 reactor concept. Among reactors ~PWRs! and Russian VVER440 reactors, re-
otherissuesanalyzedwasthepotentialofburningMAs— spectively. The former composition is based on the
stemmingfromlegacyspentfuel—bymixingthemuni- “Pu-2016”scenariostudyofCEAandrepresentstwice-
formlytothefuel~forashortreview,seeRefs.2and3; recycled MOX fuel expected to be accessible in 2016
fordetails,seeRefs.4and5!.Tofinalizethiswork,afull ~Ref. 2!, while the latter is characteristic of 45 GWd0
three-dimensional ~3-D! model of the reactor was built tonneUburnupand5yearsofcooling~Ref.9!~theexact
usingtheSCALEprogramsystem,6andadditionalburnup isotopiccompositionscanbeseeninTableII!.Intherest
calculationswereperformedtoconfirmearlierresults,to ofthispaper,thesetwocompositionswillbereferredto
analyze the effects of the isotopic composition of MAs as the PWR and theVVER cases, respectively.
and plutonium, and to study the performance of cores
having spatially nonuniform MAcontent.
II.A. The KENO-VI Model of Reactor
Thestructureofthispaperisasfollows.InSec.IIa
shortintroductionisgiventotheGFR600design,andthe To study the actinide transmutational capabilities,
modelsusedinthecalculationsarepresented.SectionIII theTRITON6 module10 of the SCALE program system
details the effects of adding MAs uniformly to the fuel. was used that couples traditional SCALE modules to
Section IV describes the transmutational capabilities of performautomaticburnupcalculation.Ateverytimestep,
GFR600—in Sec. IV.A the results of single and multi- first resonance self-shielding was performed with the
cycle uniform are presented, while in Sec IV.B those of
nonuniformMAburningareinterpreted.SectionVcon- aVolumepercentage.
tains a short summary of the results. bMolpercentage.
84 NUCLEARTECHNOLOGY VOL.177 JAN.2012
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
Fig.1. TheKENO-VImodelofGFR600~thevisualizationwasdonewithKENO3D!.
the fuel inside the plates. Next, a transport calculation
wasdonebytheKENO-VIMonteCarlocriticalitymod-
ule, after which the KMART6 module collapsed fluxes
and cross sections to the three-group format that the
ORIGEN-Sburnupmodulerequirestodeterminethenew
compositionofthefuel,subsequentlyfinishingthetime
step. The computations were based on the 238-group
ENDF0B-Vcross-section data library.
TheKENO-VImodelfollowstheexactgeometryof
thereactor~seeFig.1!.However,asnodetailedreflector
concept had been designed for GFR600, both the axial
and the radial reflectors were modeled as homogeneous
mixtures of reflector, coolant, and structural material in
correspondencewithearlierstudies.5Thecompositionof
the reflectors can be seen inTable III.
The temperatures of the fuel, the cladding, and the
reflector are 9908C, 6658C, and 5658C, respectively.
Burnup periods of 1300 effective full-power days were
chosen corresponding to the planned 5% FIMAburnup
Fig.2. TheNEWTmodeloftheFAsofGFR600. of the fuel. When studying consecutive cycles, cooling
periodsof5yearswereinsertedbetweenthecampaigns
~i.e., burnup periods!.
BONAMIandNITAWLmodules~intheunresolvedand Based on the results of the transport calculations,
the resolved resonance regions, respectively!, applying certainquantitiesweredeterminedononehandtobetter
the SYMMSLABCELLoption ~corresponding to an in- understandthechangeofreactivityduringburnupandon
finite array of parallel fuel, cladding, and coolant slabs! the other hand to study the safety of the reactor. For the
withfuel0cladding0coolantratiosof35%010%055%~in formerpurpose,theworthoftheisotopeswasintroduced
accordancewiththeiractualvolumefractionsintheFA! ~see Sec. II.C!; for the latter, the fundamental delayed
andsettingthewidthofthefuelslabequaltothewidthof neutron fraction was determined ~see Sec. II.D!. As a
NUCLEARTECHNOLOGY VOL.177 JAN.2012 85
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
TABLE I
MainCharacteristicsofGFR600
CoreParameters AssemblyParameters
Thermalpower 600MW NumberofFAsincore 112
Powerdensity 103MW0m3 NumberofplatesperFA 21
Coreheight0diameter 1.95m01.95m Fuelcomposition~UPuC0SiC! 70030V0V%
Coolantinlet0outlettemperature 4908C08508C Fueltemperature 9908C
Coolantmaterial Helium Claddingmaterial SiC
FuelParameters NaturalUComposition
SiCdensity 3.21g0cm3 235Ufraction 0.7n0n%
UPuCdensity 13.62g0cm3 238Ufraction 99.3n0n%
Porosity 15%
TABLE II TABLE III
IsotopicCompositionofPWRandVVERSpentFuel* CompositionoftheReflectors
PlutoniumVector MAVector AxialTop AxialBottom0
Reflector RadialReflector
Fraction~n0n%! Fraction~n0n%! Material ~%! ~%!
Isotope PWR VVER Isotope PWR VVER
Hecoolant 40 20
238Pu 2.7 2.71 237Np 16.86 48.89 SiCstructure 10 10
239Pu 56 54.86 241Am 60.64 31.56 Zr3Si2reflector 50 70
240Pu 25.9 23.38 242mAm 0.23 0.11
241Pu 7.4 12.27 243Am 15.69 14.65 *Involumefractions.
242Pu 7.3 6.78 242Cm 0.02 0.001
241Am 0.7 0 243Cm 0.07 0.049
244Cm 5.14 4.43
245Cm 1.25 0.26
Therefore,asecondmodelofthereactorwasassem-
246Cm 0.1 0.05
bled for the FTC calculation. While building this new
*Notethatthemostsignificantdifferencesareduetothehighercool- model, it was sought to remain as close to the actual
ingtimeofthePWRspentfuel. design as possible; that is why the NEWT module11 of
SCALE was chosen. In this two-dimensional discrete
ordinatesmethodcode,itwaspossibletoapplytheexact
geometry of the FA~except for the third dimension, the
safetyfeaturethefueltemperaturecoefficient~FTC!was
axialleakagewasonlytakenintoaccountbyabuckling
alsocalculatedasafunctionofburnupandMAcontent;
factor, specifying the 195-cm height of the assemblies!.
however, it required a new model of the reactor to be
However, because of the unusual shape of the GFR600
built ~see Sec. II.B!.
FA ~containing thin fuel plates aligned in three direc-
tions!,averyfinegridstructurewasneededforNEWTto
II.B. The NEWT Model of the Reactor
work properly ~see Fig. 2!.
One of the most important advantages of using the It was concluded that the calculations for the full
KENO-VI module was that a precise description of the coreapplyingtheexactgeometryofeveryfuelassembly
complicatedgeometryoftheGFR600wasfeasible.How- wouldhavetakentoomuchtime.Hence,firstaunit-cell
ever,whendeterminingtheFTC,thedirectcalculations calculation was made for each region with the actual
carriedoutwiththeMonteCarlotransportcodeapplying geometry and a white boundary condition ~NEWTdoes
the 3-D model of the reactor proved to be inconclusive, not support periodic boundary condition on edges that
asthedifferencebetweenthek-effectiveforthereference are not parallel with the x- or the y-axis!. Then, the ho-
andanelevatedfueltemperaturewassmallandhadhigh mogenized macroscopic cross sections were filled into
variance.Consequently,thecoefficientsobtainedthisway the full core NEWTmodel so that it could be used with
didnotshowcleardependenceontheburnuportheminor amuchroughergrid~seeFig.3!.Foreverytimestep,two
actinidecontent;thedeviationswereburiedinthevariance. criticality calculations were performed—one with the
86 NUCLEARTECHNOLOGY VOL.177 JAN.2012
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
( ~s ! ~t!F ~t!
a i,r,g r,g
~s ! ~t!(cid:2) g(cid:2)~t,r,f! ~2!
a i,r
( F ~t!
r,g
g(cid:2)~t,r,f!
and
( ~ns ! ~t!F ~t!
f i,r,g r,g
~ns ! ~t!(cid:2) g(cid:2)~t,r,f! . ~3!
f i,r
( F ~t!
r,g
g(cid:2)~t,r,f!
InEqs.~2!and~3!thesummationsareoverthethree
energygroups~thermal,resonance,andfast!requiredby
ORIGEN-S.
To take into account the quantity of the isotopes as
well, their macroscopic worth can be defined as
W ~t!(cid:2)N ~t!w ~t! , ~4!
i,r i,r i,r
Fig.3. TheNEWTmodelofthewholecoreofGFR600with
homogenizedFAs. whereN ~t! isthenumberdensityofisotopeiinregion
i,r
ratthegiventime.Finally,thetotalworthoftheisotopes
canbeintroducedastheiraveragemacroscopicworth~in
nominalfueltemperatureandonewitha1008Cincreased the rest of this paper, worth will always refer to this
temperature—then the k-effective values were used to value!:
gaintheFTCvalues@seeEq.~8!later#.Withthismethod
theFTCshowedanexplicitdependenceonboththeMA 6 V
W~t!(cid:2) (W ~t! r , ~5!
cfroonmtetnhteoMftohnetecoCraeralondcatlhceulbautironnusp,,wuhnilcikhewtheoresescdaetrtievreedd i r(cid:2)1 i,r Vcore
aroundtheNEWTvalues.Cross-sectionprocessingwas where
done the same way as in the KENO-VI model.
V (cid:2)volume of the fuel in region r
r
II.C. Worth Calculations V (cid:2)volume of the core.
core
To measure the influence of the individual isotopes
Addinguptheworthofallisotopesinthefuel,weobtain
on the reactivity, traditionally the reactivity weights are thefissionabilityofthefuel~Fiss ~t!(cid:2)( W~t!!.By
introduced as w (cid:2) ~ns ! (cid:3) ~s ! ~Refs. 12 and 13!, fuel i i
i f i a i addingtheworthofcladdingandcoolantisotopestothis
where the brackets indicate spectrum-averaged values.
value, the fissionability of the core is determined:
Using this definition, a space- and time-dependent mi-
croscopicworthcanbeintroducedforeachisotope“i”in
each region “r” of the reactor as Fiss~t!(cid:2) ( Fiss ~t! . ~6!
l
l(cid:2)~fuel,cladding,coolant!
w ~t!(cid:2)~ns ! ~t!(cid:3)~s ! ~t! . ~1!
i,r f i,r a i,r The changes of the fissionability—especially in the
The meaning of Eq. ~1! is quite straightforward; it uniform cores—closely follow the changes of the reac-
showswhetheranisotopeatthegivenregionofthecore tivity.4,14 This provides a powerful tool to examine the
contributespositivelyornegativelytotheneutronecon- behavior of the reactor; by analyzing the worth of the
omy at the given time. The nuclei with positive micro- individual isotopes, a good understanding of the basic
scopic worth ~w ~t! (cid:4) 0! can be regarded more as processes taking place in the reactor can be gained.
i,r
fissionable,whilethosewithnegativemicroscopicworth Three types of isotopes can be distinguished in the
~w ~t!(cid:5)0!canberegardedmoreasabsorberisotopes, fuel as follows:
i,r
since the former produce more and the latter produce
1. Fissionable isotopes: heavy metal isotopes hav-
fewer neutrons than they absorb.
ingpositiveworth~theirtotalworthisreferredto
The spectrum-averaged values necessary to deter-
as HMp worth!
mine the microscopic worths can easily be calculated
usingthethreegroupfluxesandcrosssectionscomputed 2. Absorber isotopes: heavy metal isotopes having
by the KMART6 module via collapsing the 238 group negativeworth~theirtotalworthisreferredtoas
values estimated by KENO-VI: HMn worth!
NUCLEARTECHNOLOGY VOL.177 JAN.2012 87
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
3. Fission products: isotopes produced by fission, Using the TSUNAMI-3D module15 of SCALE ~apply-
plus the Si and C isotopes of the fuel ~their total ing the 3-D KENO-VI model of the reactor! k-effective
worth is referred to as FPworth!. sensitivity coefficients @S ~t!# were obtained for all
k,Sxi,,rg
reactions~x! ofthefuelisotopes~i! inallenergygroups
Naturally,thetotalfuelworthisgivenbythesumof
~g! and regions ~r! of the core, then self-shielding cal-
the HMp, HMn, and FPworth at any time.
culationswerecarriedoutforthereference~T (cid:2)9908C!
ref
andtheincreasedfueltemperature~T (cid:2)10908C!using
II.D. Safety Assessment per
the CSAS-MG sequence. The resulting cross sections
of all relevant reactionsc of the fuel isotopes were read
Two safety parameters of the reactor were calcu-
from the AMPX working libraries @si,r,~990!~t! and
lated, the fundamental delayed neutron fraction ~DNF! x,g
si,r,~1090!~t! microscopic cross sections#, then the sen-
and the FTC. The former substantially influences the x,g
sitivitycoefficientswereusedtocalculatethek-effective
dynamicsofthereactorandourabilitytooperateitwhile
change due to isotope i ~Dk ~t!! and the isotopic contri-
the latter is a measure of the most important prompt i
butions to the FTC ~FTC ~t!!:
feedback effect. i
ThefundamentalDNFwasestimatedbysimplytak-
ing the delayed to total neutron production ratio ~as no Dki~t!(cid:2)ke~9ff90!~t!(((Sk,Sxi,,rg~t!
adjoint calculations were performed, the more usual ef- r x g
fective DNF was not determined!:
(cid:2)(cid:2) si,r,~1090!~t!(cid:3)si,r,~990!~t!
(cid:6) x,g x,g ~9!
nD~E!Sf~rs,E,t!F~rs,E,t!dr3dE sxi,,gr,~~990!!~t!
b~t!(cid:2) (cid:2)(cid:2)
and
n~E!S ~r,E,t!F~r,E,t!dr3dE
f s s
(Dk ~t!
1 i
( (6 (~nD!i,gNi,r~t!~sf!i,r,g~t!Fr,g~t!Vr FTC~t!(cid:2) 100 ki~990!~t! (cid:2)(FTCi~t! . ~10!
(cid:2) i r(cid:2)1 g eff i
6 Withthistechniquetheimportanceoftheindividual
( ( (~n! N ~t!~s ! ~t!F ~t!V
i,g i,r f i,r,g r,g r isotopeswasrevealed,andthedependenceoftheFTCon
i r(cid:2)1 g
theburnupandtheMAcontentofthecorebecameclear.
(cid:2)(b ~t! . ~7!
i
i
III. THE EFFECTS OF ADDING MAs TO THE FUEL
In Eq. ~7!, ~n ! is the average number of delayed,
D i,g
while~n! istheaveragetotalnumberofneutronspro-
i,g
duced by the fission of isotope i, induced by a neutron First,theeffectsofsubstitutingsomeoftheuranium
withanenergyingroupg.Furthermoreb ~t! isthecon- in the fuel with MAs were investigated. Using the
i
tribution of isotope i to the delayed neutron fraction. KENO-VImodelofthereactorburnup,calculationswere
TheFTCwasestimateddirectlybyusingk-effective performedwithcoreshavinguniforminitialMAcontent
values from criticality calculations with the reference of different amount and origin. The results discussed in
and a 1008C increased fuel temperature: this section correspond to the first cycle, and they show
(cid:3) (cid:4)
thattheMAshaveadecisiveinfluenceonthereactivity,
1 k~1090!~t!(cid:3)k~990!~t! 1
FTC~t!(cid:2) eff eff . ~8! as well as on the safety parameters and the MA trans-
100 k~990!~t! K mutational capabilities of the reactor.
eff
AsdiscussedinSec.II.B,whendeterminingtheFTC
III.A. Effects on Reactivity
thek-effectivevaluesfromtheNEWTmodelwereused
in Eq. ~8! since the coefficients calculated with the
Thechangeofreactivityduringburnupincoreshav-
KENO-VI model were too scattered, showing no clear
ing different initial VVER MA content can be seen in
trends.
Fig. 4. Two important effects should be observed: the
At last, to better understand the dependence of the
FTC on the burnup and the MA content of the core,
cThereactionstakenintoaccountareelasticandinelasticscat-
FTCvalueswerealsocalculatedwithperturbationtech-
tering,fission,radiativecapture,andn,2n;moreover,varia-
niquesforfourspecificcases@atthebeginningofburnup tionduetothechangeinnwasaccountedfor,asthesensitivities
~BOB! and at the end of burnup ~EOB! in the core to all these are automatically calculated by TSUNAMI-3D
having 0% and 10% uniform initial PWR MAcontent#. ~Ref.15!.
88 NUCLEARTECHNOLOGY VOL.177 JAN.2012
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
higheramountof237Nppresent.Theincreaseintheworth
ofthefissionableminoractinideisotopesduringthecam-
paign also gets more significant with the MA content,
mainlyduetotheincreasedproductionof 242mAmfrom
241Am. Obviously, both these effects decrease the reac-
tivity loss.
In Ref. 14 it is shown that two major differences
arise between the PWR and the VVER cases: Both the
initialreactivityandthereactivitylossduringburnupare
higher in the VVER case. The former is caused by the
higherinitial241PucontentoftheVVERplutonium,while
the latter is explained by the more rapidly decreasing
241Pu and the slowly increasing 242mAm worth ~due to
theloweramountof 241Ampresentintheshortercooled
VVER spent fuel!.
Fig.4. Thereactivityduringburnupincoreshavingdifferent III.B. Effects on Safety Parameters
initialVVERMAcontent~X!.e
In the previous section it was shown that the addi-
tion of MAs to the fuel has beneficial effects on the
reactivityandmakesitpossibletoreachhigherburnups
additionofMAsdecreasestheinitialreactivityd andthe
and consequently higher MA destruction. The safety
reactivitylossalike.Thesearethesameeffectsthatwere
of the reactor is, however, negatively affected by the
shown with PWR MAs in earlier studies.2 Obviously,
presence of MAs. As can be seen in Fig. 6, the DNF
both are advantageous from the reactor operation point
decreases with the increasing MA content, though its
ofviewsinceasmallerreactivityhastobesuppressedin
decline during burnup is almost entirely independent of
thebeginningandintroducedduringburnup~athighini-
that. For understanding, see Fig. 7, where the plotted
tial PWR MA contents the reactivity swing even turns
positive!. Moreover, the decreased reactivity loss en- values are 100{~biX%~0!0(ibi0%~0!!; i.e., the contribu-
tionofisotopegroupitothedelayedneutronfractionat
ables longer campaigns and higher burnup values, con-
BOB is compared to the DNF at BOB in the core hav-
sequently increasing the MA destruction ~seen later in
ing 0% initial VVER MA content. The majority of de-
Figs. 13 and 14!.
layed neutrons is produced by plutonium ~239Pu and
The decreased initial reactivity can be explained by
241Pu primarily! and uranium ~238U mainly!; moreover,
theworthofabsorberisotopes.TheMAmixhasamore
the decreased number of delayed neutrons produced by
negative microscopic worth than the uranium it substi-
the decreasing amount of uranium present in the fuel is
tutes ~though this negative microscopic worth becomes
basically compensated by the MA isotopes ~mostly by
smaller as the MAcontent of the fuel increases!; hence,
the reactivity decreases with its addition.14f 237Np!. The true cause of the decrease in the DNF is
that the contribution of the unchanged amount of pluto-
Thedecreasedreactivitylossduringburnup,aswell
nium in the fuel decreases since the MAs make the
asthedifferencesbetweenthePWRandtheVVERcase
spectrum harder, increasing the prompt, but leaving the
canbeexplainedbytheworthofthefissionableisotopes.
delayed neutron yield unaffected.
AscanbeseeninFig.5,theplutoniumworthdominates
In Ref. 14 it is shown that the DNF is higher in the
thetotalHMpworthineverycase,anditsdecreasedur-
VVER case ~mainly due to the higher amount of 241Pu
ingburnupgetssmallerwiththeincreasingMAcontent;
and 237Np! but also has a more rapid decrease during
in the PWR case it even stays constant. This is mostly
burnup~as241Puisbeingconsumedmorerapidly!.gThis
causedbythemorerobustproductionof 238Pufromthe
difference, however, disappears in later cycles.
The FTC during burnup calculated with the NEWT
dThisistrueonlyforsmallMAcontents.Substitutingahigher
model of the reactor can be seen in Fig. 8 for cores
amountoftheuraniumwiththeMAmixstartstoincreasethe
having different initial PWR MA content.h Note that
initialreactivity.
eThevaluesgiveninthelegendsaretheXvaluesinSec.II. justliketheDNF,theDopplercoefficientalsodecreases
fTheturn-backoftheinitialreactivityathigherMAcontentsis
causedbythefactthatthemicroscopicworthoftheMAmix gTheDNFatBOBis'0.360%and'0.325%incoreshaving
and the uranium becomes less negative with the increasing 0%and10%uniforminitialPWRMAcontents,respectively,
MAcontentduetothecausedspectralhardening;hence,after whilethedecreaseduringthefirstirradiationis0.015%with
a point the substitution of some uranium with stronger ab- both cores compared to the more than 0.02% decrease ob-
sorberMAsiscounterbalancedbytheincreaseintheworthof servedintheVVERcase.
therestoftheabsorbersinthecore. hTheVVERcaseisbasicallyidenticaltothePWR.
NUCLEARTECHNOLOGY VOL.177 JAN.2012 89
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
Fig.5. BreakdownofHMpworthincoreshavingdifferentuniforminitialPWRandVVERMAcontent.ThecomponentsofHMp
worthareasfollows:total~blue(cid:2)!,Pu~blue(cid:3)!, 235U~red▫!,Cm~red(cid:7)!, 242mAm~red(cid:5)!.Axesontheleftbelongto
thebluegraphs,axesontherighttotheredgraphs.AllworthiscomparedtothetotalHMpworthatBOBincoreshaving
3%uniforminitialMAcontent;i.e.,theplottedvaluesare100{WX%~t!0~( W3%~0!!.
i i:Wi~0!(cid:4)0 i
Fig.7. TheDNFatBOBcomparedtoreferencecaseincores
Fig.6. TheDNFduringburnupincoreshavingdifferentini- havingdifferentinitialVVERMAcontent.Therepre-
tialVVERMAcontent.NotethattheadditionofMAs sented DNF producing isotope groups are as follows:
decreases the DNF, but the change during burnup is total~blueline!,Uisotopes~blue(cid:2)!,Puisotopes~blue
unaffected. x!,andMAisotopes~red▫!.
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Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
238UdecreaseandthespectralhardeningduetotheMA
accumulationbothdecreasetheFTC,whileathighMA
contentsthespectralsofteningduetotheMAconsump-
tion counterbalances the effect of the decreasing 238U.
III.C. Effects on Spent Fuel Properties
AstheGFRisplannedforoperationinaclosedfuel
cycle, its spent fuel will have to be reprocessed after
every irradiation, and the recovered actinides will have
to be incorporated in the new fuel of the reactor. It is
important to know what conditions are to be expected
duringtheseprocedures;hence,theeffectsthattheaddi-
tionofMAstothefuelwouldhaveontheoverallradio-
activity, the thermal heat, and the neutron source of the
spentfuelwereanalyzed.Thesequantitiescaneasilybe
Fig.8. TheFTCduringburnupincoreshavingdifferentuni- computedwiththeORIGEN-SmoduleofSCALE,andin
forminitialPWRMAcontent. SCALE 6 they are automatically included in the TRI-
TON calculations.All results in this section correspond
to the spent fuel from the whole core ~containing 14.8
in absolute value with the addition of MAs. However, tonnesofheavymetalsand800kgofFPs!afterthefirst
unlike the DNF, the change of the FTC during burnup cycle ~i.e., time 0 is the discharge moment!.
depends on the initial MA content of the core—at low The MAs have the most significant effect on the
MAcontentstheFTCdecreasesinabsolutevalue,while neutron source of the spent fuel. Immediately after ir-
at high contents it stays almost constant. As discussed radiation for 1300 days the total neutron source of the
in Sec. II.D, the isotopic contributions to the FTC were spentfuelfromthefullcorehaving3%initialVVERMA
calculatedforfourspecificcaseswithperturbationmeth- content is ;5(cid:6)1011 n0s, which is roughly 8 times the
ods; the results are presented in Fig. 9. Uranium-238 is valuecalculatedfortheMAfreecore~6.5(cid:6)1010 n0s!i;
clearlydominant,andspectrumeffectscanalsobeseen. moreover,thisdifferenceonlyincreasesduringthecool-
In the MA free core the ;785-kg decrease in the 238U ing period to over a factor of 13 @neutrons from sponta-
contenttogetherwitha70-kgincreaseintheMAamount neousfissionand~a,n! reactionsareaccountedfor,the
correspond to a decrease of (cid:3)0.21 pcm0K in the FTC delayedneutronspresentinthefirstfewminutesafterthe
during burnup, while in the core having 10% initial reactor is shut down are neglected#. With the further
PWR MA content the 610-kg decrease of 238U along addition of MAs, the neutron source basically rises lin-
with the 314-kg decrease of MAs basically leave the early to ;1.4 (cid:6) 1012 n0s at 10% initial VVER MA
FTC unchanged. Consequently, at low MAcontents the content.j
Thereasonforthisincreasecaneasilybeunderstood
by taking a look at Fig. 10. Notice that the curium iso-
topes clearly dominate the neutron source of the spent
fuel during the whole cooling period.As is discussed in
Sec. IV.A, the more curium that is present at the begin-
ningofburnup,themoreisleftattheend,consequently
resulting in stronger neutron emission.
Anotherimportantaspectduringreprocessingisthe
heat load of the spent fuel. Just like the neutron source,
the thermal power also increases with the MA content
of the fuel, however to a lesser extent. The heat pro-
duced by the spent fuel from the full core having 10%
and no initial VVER MA content at discharge ~i.e.,
immediately after the first irradiation! is 4.7 MW
and 2.7 MW, respectively ~6 MW and 2.7 MW in the
PWRcase!.Thismodestincreaseisbecauseasignificant
iThesituationissimilarinthePWRcase,thevaluesareslightly
higher~6.7(cid:6)1011n0sand7.1(cid:6)1010n0s,respectively!,just
Fig.9. The isotopic contributions to the FTC coefficient in likethedifference~approximately9times!.
coreshavingdifferentuniforminitialPWRMAcontent. j1.9(cid:6)1012 n0sinthePWRcase.
NUCLEARTECHNOLOGY VOL.177 JAN.2012 91
Perkóetal. MINORACTINIDETRANSMUTATIONINTHEGFR600
Fig.10. Thetotalneutronsourceofnuclidesinthespentfuel Fig.12. Theactivityoftheheavymetalelementsinthespent
ofthefullcorehaving5%initialVVERMAcontent fuel of the full core having 5% initial VVER MA
during the cooling period following the first irradia- contentduringthecoolingperiodof5yearsfollowing
tion.Delayedneutronsareneglected. thefirstirradiation.
portion of the thermal power can be attributed to the ofneptunium,americium,andplutoniumisbasicallysin-
FPs,whichbuildupverysimilarlyinthedifferentcores gularly due to 238Np, 242mAm, and 238Pu.The expected
if the burnup is the same. heat load during fuel manufacturing from the actinide
Actinides from the spent fuel will have to be incor- mix recovered from the spent fuel of cores having 10%
poratedintothenewfuelofthereactor;hence,itisworth and0%initialVVERMAcontentafter5yearsofcooling
takingalookattheirheatloadseparately.Duringmostof is27.9W0kgand3.3W0kg,respectively~31.6W0kgand
the cooling period, curium isotopes are responsible for 3.3W0kg in the PWR case!.
the bulk of the heat ~see Fig. 11!, while at the end plu- The addition of MAs also results in a more radio-
tonium isotopes become increasingly dominant ~as cu- active spent fuel; however, this increase is very modest
rium isotopes have shorter half-lives!. The curium heat as the short-term radioactivity is dominated by the FPs
productionismainlydueto242Cmand244Cm,whilethat ~just like the thermal power!. The total radioactivity of
the spent fuel immediately after irradiation from cores
having 10% and no initial VVER MA content is 2.7(cid:6)
1019 Bqand2.6(cid:6)1019 Bq,respectively~2.8(cid:6)1019 Bq
and2.6(cid:6)1019 BqinthePWRcase!,whiletheexpected
valuesfromthereprocessedactinidesduringfuelmanu-
facturingare72GBq0gand43GBq0g~64GBq0gand33
GBq0g in the PWR case!. As can be seen in Fig. 12,
mostly plutonium is responsible for the activity.
IV. TRANSMUTATIONAL CAPABILITIES
Themaingoalofourinvestigationwastoassessthe
MAtransmutational capabilities of the GFR.As the re-
actor is planned for operation in a closed fuel cycle in
which the spent fuel is always reprocessed and all the
recovered actinides ~including the MAs! are incorpo-
ratedintonewFAs,itwasenvisionedthattheextraMAs
thataresoughttobedestroyedwouldsimplybeaddedto
the recovered ~or the initial! actinides and no specific
Fig.11. Thethermalpoweroftheheavymetalelementsinthe
spent fuel of the full core having 5% initial VVER target assemblies would be embedded into the design.
MAcontentduringthecoolingperiodof5yearsfol- First, spatially uniform transmutation was consid-
lowingthefirstirradiation. ered where the MA content of the fuel had no spatial
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