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ORNL-TM-3964.txt
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ORNL-TM-3964.txt
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ORNL-~TM- 3064
Contract No. W-Ti05-eng-26.
CHEMICAL TrCHMCLOGY DIVISION
NEUTRON-INDUCED TRANSMUTATION OF HIGH-LEVEL RADIQACTIVE WASTE
H. C, Claiborne
DECEMBER 1972
NOTICE
Thlt report was prepared az sn account of work
tha linbed Secrex Gummnlnt__ Nekhwr
CAX RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
aperated by
UNION CARBIDE CCRPORATICON
for tae
U.S. ATOMIC ENERCY COMMISSION
iii
CONTENTS
Abs tract - - - " - - » - ” » - » + - - + - - . - - - - - -
AN
Cn
3
9.
10.
Appendix I: # Comparison of RCGs Calcuiated ty LaVerme (Ref
ersl
IneroduClion & o o o v o & o o o o o « = =« o = o s o .
SUHMEYY o s - s s o s = s+ e s s e s a s 4 o s e . . o
Method for Determining the Hazard of Radicactive Waste
Nuclesr Calculational Methed . . . . . . . . . . . . .
Reactor Type and Standard Conditians . . . . . . . . .
Contribution of Eack Component to the hHazardi of the
Waste from a PWR Spent-Fuel Processing Plant ., . . . .
Transmitaticn of Fission Preduct Waste . . . . . . . .
7.1 Maximm Buraout-to-Troduction Ratios for Fission
Pz‘wucts - a & & & & @ » L - - - - - - > - - -» -
7.2 Reactor Residence Times Reauired for Fission
PI‘OdnCt mo“t * - . + -, 4 - » - - . - - - - - -
7.3 Application of Transmuetation Schemes . . . . . .
Actinide Recycling Ina PWR . . . . « ¢+ o ¢ o - . .
801 HMheet . ’ L J » . - - - A - - - - - - - - » - -
8.2 Chemical Processing for Waste Marsgement
Simplification . & v ¢ ¢ 4 4 b e e e b e s . e
8.3 Effect of Recycle on Resctivity and Flux . . . .
8.4 Effect of -Recy<cling on Fazard Messure . . . . . .
8.5 Effect of Fecyeling on the Hazards of Chemical
Processing angd ruel rFabrication . . . . . . . . .
Conclusions and Recoammendations . . + « + o » o « » «
Reference s - - * - - L - - - - * - - - - & . - - L - -
Regulations (Ref. &) . .
- - - -
- - - .
)
iv
Aopendix II: Radioactivity and Hazard Messure of Each Actinide
Nuclide a2s 2 Funetion of Time After Discharge for the Standerd
Case and After the 50th Recycle .
- - - . - . . - . - . -
Appendix III: ZXHazard Reduction Achieveble by Erhanced Removal
of Actinide Zlements (Ref. 19) !
s v = * . - » T s e *r - v .
NEUTROR~INDUCED TRARSMUTATION OF HIGH-LEVEL RADIOACTIVE WASTE
H, C. Claiborne
ABSTRACT
The possibility of reducing the potential hazard of high-
level radiocactive waste by neutron-induced transmutation has
received little study. In this report the mvailagble information
on fission product transmatation is reviewed and discussed, the
contribution of individusl actinides Tto the potential hazard of
the waste is calculated, and expected hazard reduction factors
that would result from recycle through a PWR are calculated for
the actinide waste from chemical processing of spent fuel.
It is not practical to burn fission product wastes in power
reactors because the neuiron fluxes are tco low, Developing
special turner reactors with the reguired neutrca flux cf the
order of 1017 n/ eml-sec or burning in the blankets of thermo-
nuclear reactors is beyond the limits of current technology.
It seems that ultimate storage in deep geclogical formations,
such as bedded salt, remains the best method for {ission product
disposel.
When plutonium and uranium extraction efficiencies exceed
0%, a significant reduction in the long-term hazard potential
of the waste can be obtained by similar removal of neptuniwm,
americium, and curium (the other ectinides beirg very small
contributors). Consegquently, it seems reasonable to concentrate
on developing economical chemicel processes to extrect these
three actinides for seperste storage or for recycling through
the reactors that produce them.
The results of cuch recyceiing calculaztions show that the
long-term hazard potential. of the waste from light water reactors
may te reduced by factors up to 200 if no more than 0.1% of the
actinides are discarded to the waste in ealh pass through the
reprocessing plent. lLarger reductions of the hazard potential
of' the waste will become practical if methods are developed to
Froduce sharper separations between tne actinides and fission
products as the spent fuel is processed.
1. INTRODUCTION
The management of high-level, long-lived radicactive wastes associ-
ated with a2 highly developed nuclear nower economy based on fission
regctors will present & formidable problem to present and Twiture gener-
ations, Schemes for management of these wastes that Lave been under
serious consideration involve conversion of the squeous wastes to solid
forms with subsequent storsge in man-made vaults or in deep geological
formations such as bedded salf.
The possibility of ultimate disposal into deep space or the sun
(the only method for complete and permanent removal from the earth) has
begun tc raceive more considerafion because of the recent and prejected
advances in space technology. The only otker known method of ultimate
disposal (in contrast to permanent storage) is to transmute or burn out
(fission in the case of some of the actinides) long-lived radiocactive
nuclides to stabie or short-lived nuclides by expesure to & neutron
flux.
Studies have bheen madel’z cn the posaipility of using special high-
flux "burner reactors” to reduce the stockpile of the "problem fission
products" 85Kr, =0 L3es,
Sr, and The excess neutrons from controllsed thermo-
3,k
these fission products and the waste actinides,
nuclear reactors have also been sugsested for use in transmutation cf
Aside from the problems associated with burning fission preducts
(which are discussed later in this report), 908r and ~31cs decay to com-
pletely innccucus ievels in less than 1000 years, a time for which ceon-
tairment in appropriate geclosicel formetions can be provided with good
assurance, The nuclides 85Kr and 3H with shorter half-lives are even
more suitable for long-term siorage in geologicel formations. The
igotope 1297 (half-life,16 million years) is one of the sxcepticnal
fission products that has an extremely long life but is produced in
such lew o-nZsuatraticns that its hazard may possibly be reduced tc
wopropristely low levels by isotopic dilution (i.e., by wmixing with
stable isotoves of the same chemical element).
In contrast, many of the actinides that are produced by transmutation
of uranium and thorium in reactors have half-lives in the thousands of
years, occur in large quantities, and are ncot suitable for isctopic dilu-
tion becauce stable forms of these slements do not exist. Consequently,
an aven stronger motive exists for completely destroying or restricting
the accumlation of these slpha-emitters since predictions of the tectonics
of geologizal formations for 105 to 10" years have a lower confidence
level compared to those for the order of 1000 years. In present concepts
of power reactors, it is planned that only 99.5 to 99.9% of the uranium,
plutoniuvm, and thorium will be recycled., Consequently, it is customarily
assumed that all other heavy elements (Cf, Bs, Cm, Am, Np, Pa, Ac, Ra,
ete. ) will be rejected as waste along with the 0.1 to 0.5% of the U, Pu,
and Th that goes to the waste in the present generation of spent fuel
reprocessing plants,
The hazard potential of this actinide waste can be reduced by recy-
cling the actinides through the power reactors prcducing them; eliminaticn
oceyrs by Tfission at points in the reaction path. The primary objective
of this work was to determine the extent of the reduction of the radio-
logical hazard of the waste streams from chemical processing plants and
the effect on the neutror eccnomy of a pressurized water reactor (PWR)
caused by recycling of the actinides (except for the smell smounts lost
in the waste streams) back through the reactors producing them. In
addition. the individusl contribution of each actinide %o tThe waste
hazard was determined as a funciion of decay time and compared with the
hazerd from all the wacste, which inciudesthe fission products, nuclides
produced from structural materials, actinides, and all decay products.
In the following sections the bases for calculations are given and
pertinent results are gresented and discussed. A modified version of
ORIGEN,5 an isotope generation and depletion code, and its associated
nuclear library was used in all the calculations,
The author wishes to agknowledge the many helpful suggestions and
criticisms by J. P. Nichols and the careful review of this work by him;
J. 0. Blomeke, and M. J. Beli.
2. CSUMMARY
It is generally impractical to appreciably change the haéard potential
of fission product wastes by transmuting these wastes with neutrons in
nuclear reactors. Developing special burner reactors with the reguired
neutron fiux of the order of 10t7 n/cm?'sec or burning in the;hlankets of
thernconuclear reactors is beyond the limits of current techno%ogy. It
appears thet ultimale storage in deep geological formations is the best
method for fission product disposal since less thar 1000 years are re-
guired to reduce their radiocactivity to an innocuous level, aitime span
for which tectonic stability can be essentially assured in fo%mations
such as bedded salt.
In contrast tc the fission products, meny of the actinid%s in the
waste from spent-fuel processing have half-lives of thousandsiof years
and are not suitable for isotopic dilution. Consequently, afstronger
motive exists to find an alternative method of restricting the accumi-
lation of these alpha emitters since the tectonics of geolog;cal forma-
tions cannot be predicted with as high a confidence level for the longer
periods that are required for their decay to innocuous levels.
The determinetion of the extent of the reduction of the radiclogical
hazard of the waste streams from chemical processing plants and the effect
on the neutron economy of a PWR caused by recycling of the qbtinides was
I
the primary cbjective of this study.
The relative importance of the contribution that the various com-
ponents make to the hazard messure (the total water required to dilute
each nuclide of a mixture to its RCG') of the waste from a2 PWR spenti-fuel
processing plant is sanown in Table 1. Beyond about 4 years, the acti-
nides ard their daughters dominzte from & hazard viewpoint. When RCGs
5,9
are used that are liess conservative than the recommended default
values of the Code of Federal Fegulations, the importan:% of the acti-
nides diminish scmewhat for decay times greater than 107 years.
The actinide waste hazard is controlled by the americium and curium
vp to lOL years, At longer decay times the long-lived ?37Np and its
+* .
Radiation Concentration Guide value, which was formerly called MFC,
\A
Table 1. Relative Contribution of Actinides anéd Their laughters to the
Hazard Measure of the Weste and of Each Actinide and Its Daughters
to Actinide Waste with 99.5% of U + Pu Extracted
Weter Required for Dilution to the RCGE (% of total
Muelides to waber required for the mixture) for Decay Times (yr) of:
Waste 10° 5 x 10° 10+ 10° 1?
411 Components of Waste:®
Actinides 0.3 ol ok 98 99
Fission Products C 99+ 5 6 2 1
Structuial Q.04 i 0.2 0.03 b ox 10'LF
Actinide Waste:b
Americium 51 56 2h 8 8
Curium 41 37 5S e
Neptunium 0.2 0.3 12 80 89
0.5% U + 0.5% Pu 8 7. 5 3 1
Other 5x 102 1x 100 5x10°2 6 x 1075 ail
8
a'Using CFR RCGs and recommended default velues for the unlisted muelides.
bRound-off may cause ccivrmn not to total 100,
daughters 2egin 1o dominate. Another important point is that the
remaining actinides, namely, Ac, Th, Za, Bk, Cf, and Es, mzke 2 negli-
givle contribution tc the hazard of the waste, The import of these
results is that in any wasie management system in which at least 99.5%
oL the uranium and plutonium is extracted, a significant further reduc-
tion in the actinide waste hazard can be obtained by removal of most
of the smericium,. curium, and neptuniuvm from the weste. If 99.5%
removal of these three actinides is 2lso effected, the uranium and
plutonium become controlling and it would then be profitable (from a
wzste hazard viewnoint) to increase the extraction efficiency of these
latter elements, particularly the plutonium.
Tne effect of recycling of 99. 5% 2md 99.9% of the actinides other
than U or 2u on the bazard measure is shown in Table 2 in terms of a
hazzrd reducticn factor as a funchion of postirradiation decay time.
The hazard reduction factor used here is defined as the ratio of the
water required for diiution of the waste to the RCE for the standard
case (nc removal of the actinides other than Pu + U at the indicated
extracticn efficiency) to that required to dilute the waste afier each
successive reactor irradiation cycle.
These resuits show that wren recycling is practiced, the hazard
. measure of the waste is apprdximately proportional to the neptunium,
americium, =nd curium sent to the waste sinces the hazard reduction
factor is about five times greater when 0.1% of the actinides is sent
to the waste alter each c¢ycle than that for tne 0.5% case. This obtzins
logically because the reactor discharge composition is little affected
Ty a change of only 0.4% of recycled actinides in the feed stream. In
addition the standard case iz alss little affected by whether 0.1% or
0.5% of U + Pu is present since the americium and curium predominate at
snaller decay times and neptunium after 105 years. It follows that if
99.99% remcval of all actinides is effected, the hazard reduction
factior for the actinide waste will increase by about a factor of
10 up to around 2000 at 106 vears, The table also shows that
the hazard reduction factors decrezse asymptotically with the
number of reeycles, which is a result of the bulidup
Table 2.
Effect of Recycle of Actinides Other Than U and Pu on the
Hazardé Measure of Waste from PWR Spent Fuel Processing
Recycle
Water Recuired for Dilution to RCG,a Ratio of Standard
to Recycleb Case (Hazard Reduction Factor) for
Decay Times (yr) of:
No. 10° 10° 0¥ 107 10°
Actinide Extrection Efficiency, 9%.5%:
0 12 15 18 28 52
1 9.3 1z 13 20 L6
2 8.2 10 11 18 Lk
3 7.6 8.h S.2 17 L3
L 7.2 7.4 8.3 17 L2
5 6.8 6.5 7.5 17 L2
10 5.8 h.7 5.8 17 42
20 5.1 3.8 4.9 17 Le
30 5.0 3.6 k.6 17 42
Actinide Extraction Efficiency, §9.9%
o 58 73 8o 137 256
1 Lk 59 6k 96 22k
z 38 L8 52 7 213
3 36 Lo Lh 8k 210
L 33 35 39 83 209
5 32 31 16/ 83 208
10 27 22 27 83 207
20 - 18 22 82 206
30 - 17 21 82 206
EI'Using CTR RCGs and recommended defszult values for the uniisted nuciides.
°Chemical processing assumed al 150 days alter reactor discharge: one
cy¢le represents 3 years of reactor operation,
8
&
of the higher transuranics, and that effective equilibrium is attained
in 20 cycles mcre or less,dspending con the decay time.
when the RCGs tvaed dy Bell6 ard those calcvlated by LaVerne9 are
used in place of the recommended defanlt valves for the unlisted ruclides
in the Code cf Federsl Regulatiorns, tha hazard reduction fa¢tors become
5.5 and 10 rfspectively. The corrasponding values ©ar 99 9% extraction
of the actinmides ere 25 and 49. Although the RCGs calculated byhlaflerneg
are more realistic than the more conservative recommended default values,
the Code of Federal Reguleticns must be follovad in nuclear reactor
design and operation.
Reeyeling of the actinides and achicvirg a 99.9% extraction effi-
ciency reduce the hazard measure of the actinides at eguilibrium to the
same order as that of the long-lived fission products (1291, 932:, 93mNb,
99Tc, angd 35"5) for the longer decay times. the hazard measure of the
actinides being abcutl twice fhat of the long-liived fissicn precducts at
1000 years and dropping to aboul one-nalf of the fission produet veiue
at 106 years. However, if 1291 is eliminated as a hazzrl} by isctopic
dilution {or separate storage), the actinides would continue to control
the total waste hazard potential. An actinide extraction efficiency of
9G,99%-% along with the recycling is recuired before the hazaré measure
of the total waste hazard potential is controlled by the long-lived
. : iz
fission preoducts cther than 91.
At some point, however, Turther
extracticon of actinides from the waste will become senseless because
- e d T Y e 1 - P - 3 - 2 - - + -
tve Wwill then have a Long-torm nazard noitentizl that 1c less than
that of naturally occurring fcrmations of uranium ard thorium. (See,
\
O\
Tor example, the arguments presented in ref,
The decrease in the average materizl nsutron moltiplicaticon for a
typical DPWR containing recycled actinidec was only 0.8%. This loss of
regeTivity can be cormpensated by increasing the fissile enrichment of
the ra2acter by only about 2% {e.g., from 3.3 to 3.! L% erricament in 2
typical DWR).
L
Recycling of reacter actinide waste will increase the radiation
oroblem zssociated with chemical procassing and fuel fabrication because
of the increased radiocaltivity of tae reactor feed and discharge
sireams. After a few recycles,zfigcf builds ur to be the grestest
source of neutrons and reaches 1042 reutrons/sec per metric “on of
spent fuel at 150 days ai'ter discharge. A reduction ol 3 FJactor ol
300 is possible if the eplifernium is removed. This caz be accomplished
by not recycling Cf even though thares is an increase in the Cf proguc-
tion with curium buildup. Significant °7°CT buildup occurs from
' 240 2 \
successive neutron captures starting with © “Cr and SOCf, whoze
Precursors are Ehgfik engd 2503&.
Recycling of ectinides through a resctor adds to the inventory
of hazardous materiels but will probably have nc messursbie affect on
the potential severity of design pasis asccidents, Tfie hzzard measure
of the actinide wastc based on irgestion was incressed by only 1294
after 60 recycles. The total is sbout cme-ienth of that for the
fission products. If the hazerd measure is based on inhalsaticn,
recycling increases the potential hezaxd by 2 factor of 2 ot dis-
charge with the everage in the reasctor being significantly higher.
The actinides have an irhalaticn nazard measure of & facior of 7
higher ti:an the fission prodecus at discharge. The above stalemenis
assume that the reccncentration factors in the envirconment are sdproxi-
nately the same for actinides and fission producis. ?Present informsiiom,
however, indicates that certain tission products (e.z., S azd I) gre
reconcentrated %o a greater extent in the epvironment. This hHxx the
effect of causing the fission products to be the dexinant source of
both ingestion and inhalation hkazard during reactor operation. The
actinid= ccncentratioz in & rescicr, however, is not significant in
aralyzirg the "maximum credible sccident” {MCA) since the sctinide
compounds cannot be sigrnificently dispersed into the stnmeiphere by
any credidble resctor accident. Transmutation of lission products
in burner reactors would, of course, edd tc the notentisl hazard
of the MCA because the welatiie fission products are controlling in
an accident analysis.
Recycle of setinider in the LiIF3Rs should preduce even higher
nazard veductinn fattors since the sverage fission-to-capture ratio
af She artinides should be nigher in 2 Tast remcior than in 52 thermal
one. The author her foumd it difficplt o suantify this effect bhocmuse
of the current paucity of neutron cross-reciion date for the higher
actintdes in Tail soectre. FTast oross-section data for the higher
actinide; should de developed 32 that recycling siudies can 5e made
it aizs appears that recycling of the actinides is particulerly
suited Tor & Fluid Suel reactor such &z thz HSIR. & processing schexne
Za3 beern visuvalirzed thal recycler efsentinliy all the urasnium, neptunium.
thoariun, and most of the other actinides. Considersbly less americium
and curiyx sre produced cospared to & PR, Wwnich consideradly zisgplifie:z
the wesle cisposal prchien. In addition, being a fluid fuel resctor,
the prodlens srising Iroo Fabrication snd handiing of kesvy neutron.
exitting fuel elemernir are eliminated,
3. METHCD FOR DETERMINING THE HAZART OF RADIOACTIVE WASTE
In cgqparifig the potential hezerd from different mixtures of radic-
sctive materials, a stendard method is required for determining a specific
value for each mixture that is s reflection of its biological hazerd. The
specific eetivity alone is insufficient since bicleogicszl factors are not
included.
The controlling consideration of hazard from the viewpoint of lonz-
term storage or disposal of radioactive materials is the danger of their
dissolution or dispersal irn unéerground waiter wilh subsequent ingestion
by humen beings. Cansecuently, a good measure of the ingesticn hazazd
associated with a xmixture of radionuclides of widely varying sctivities
is the quantity of water required to Gilute the radioactive mixture to a
concentration low encugh to permit unresicicied use of the water: the
larger the amount of water requirad, the greatsr the potentisl hazard.
The hazard measurs for the mixture is determimed by summing the amount
ol wnter recuired to dilute each individual muelide to its Radiation
Concentration Guide velue {or RCG, which was formerly calleq MPC) for
wrstrictes use of water., This methnd, which was used in & previous
unrks on the hezards of long-term storage of radicactive wastes, was
selected for use in the study. The method has the virtue ¢of =implicity
in soplicaticor and relates to the maximum veluve oo the hazerd since no
consideration i given to fractionation and paths of travel to human
beings. e most rxecert digiussion ofF cther methods of evaluafing the
hatard poteniinl of radicactive weste is given by Gera and Jacobs,7
who also propose & new hazard mesasure that invoives both the ingestion
hazaré uzod in this study, the inhsjation hazard, andg the prohability of
beling taxen up by humans. Determination of these probabilities is very
Sifficuit, however, 3ince statistical data regerding the prcbability of
gezidents ané other radicactivity releases, Inrluding their conseguences
in 21} phases of radiocactive waste manasgement, are not readily available
or orgily estimated,
The RUGz vsed In this study were taken frem the Code of Federal
Regalntian:,s which iy currently the guide for unrestricted use of
12
water in which these ruclides mey be dissclved. For nuclides with
unliéteg RCGs, the recommended defeult values were used, nemeiy,
3 x 107" uCi/ml for beta-decay ruclides with half-lives grester than
2 ar and 3 x 107 Cifm> for miclides that Gecay by elpha emission or
sponteneous fission, These default values represent a conservative
estimate of the RCGs, Some of the results in this report are alse
compared on %the basis of the KCGs used by Bell and Bfi.lcn:a6 and those
recently calculated by IaVernme’ feor unlisted muclides. Bell and Dillen
used € x 1077 and 2 x 10'6 Ci/m3 for 2°7Re and 229m,reSpectively,an&
unity for all other unlisted nuclides. LaVerne calculated RCGs for all
the uniisted nuclides and 5 x 1077 and b x 10™! Ci/m> for >°?Ra gnd
2291"‘:3, respectively, the two muclider thet contributed to most of the
differences that occurred dque 4o the particuliar RCGs that were uced.
13
i
b,
TUCLEAR CALCULATIONAL METHOD
The puclear calculations dwring reactor irradiation and after
discharge were made with 2 modified version of the nuclide generation
and devleticon code SRIGZN.’ The calculation daring irradiation is based
on three neutron energy groudps, namely, thermal, a 1/t energy distribu-
tior in the resconance region, and a fast group. The crozs sections in
the libzrary had been predetermined from basic data vy weighiing with a
typical P¥R neutron energy spectrum. Xore details of the original code
and cross-section library, wiich included data for actinides only up to
254
Cni, are given in refs, 10 mnd 11.
For calenlaticns involviag recyeling of the actinides, it was
necessary to erxpsnd the iibrary te include some higher transuranics and
inerease the calcnlaticual sccpe of the ORIGEN code. Cross-section and
decay data for the following muciides were added to the PWR sctinide
Library: ahou’ 2hamflp,‘ahofip: ahk?u’ 2hs?u: 2h5am, ah5Cm, 2h6cE’ 2&7Cm¢
2hecfl’ ehgcm, 35°cm, 2‘;Bk, 2503&; 2&9Cfr esonJ ESICf: 252cr, 253cr,
25h6£, 35333. actinides higher then einsteinium were not expected to
heve a significant effect because they all decay (X-deczy, along with 2
little spontaneous fission) with short nelf-lives, thus preventing buildup
of the nuclides beyond 2”7Zs. The calculations confirmed this expectation.
The decay michod and neutor irterastion provabilities are such that no
significant amounts af the actinides can be remcived from the reaction-
fiécay chain except by fission. Cross sections end decsy constants for
the transuranic elements thzt vere adled to the library were taken fronm
ref, 12, '
.
A calculfi?icn af the meteriel maltiplicaticn constant or k_ was
added to the code since it was necessary to know the effect of actinide
recycle on the veactiviiy. Althouvgh the kfl aalculation igrores core
i=akege and control rods or other control poisons, the results, which
wouid not be adeguate for ithe core physics, seer adeguate for reiative
comparisons. Neutron yields per {ission as a function ol energy wers
saken from the SNDF/B-IT dets file’> for most of the Tissile muclides.
for those not incliuded ir that fle, the neutron yielids were taken or
infervred Irom tne pudblications by Gordeeve and :‘:‘fiirenkin_.l4 Hopikins
14
‘ me 1 . 16 Lo s _ . o o
and' Diver, 2 and Clark. The effective neutron yield from fission of
each ?uclide hy resonance energy neultrons was obiained by weighting the
energy dependent yieids with a 1/E neutron flux., For fast fissions,
the figsion spectrum was used &8s the weighting ITunrction.
Otker code chenges include a recycle option for any number of
actinides, en ebility to specify removal of any number of actinides
afier an arbitrary decay time subseguent to reactor discharge for
recycling or further decay of the remaining materials, ard ar ability
to account for the fissions of all the fissionable materials.
15
5. REACTOR TYPE AND STANDARD CCNDITIONS
The reactor selected for this study was the Diablo Canyon. which is
typical of & PWR design. When operating at equilibrium, the fuel is 3.3%
enriched uranium with a burnup of 33,000 MWd/metric ton of uranium. It
was assumed that this burnup was obtained by continuous coperation at e
specific power of 30 MW/metric ton cver a three-year pericd. Tor the
usually assumed plent factor of 0.8, intermittent operation at a specific
power of 38 MW/metric ton for 80% of the time would produce the same
burrup. Sinee {for the time periods involved), the waste hazard measure
resulting from a particular burnmup is not a sensitive funetion of any
-reascnable cperaticn schedule, it was deemed unnecessary to complicate
the cealculeations and analysis by considering a perticular operation
zchedule.
The fuel regicn is divided intc thres zones with each ons conteining
about an equal weight of fuel (approximately 28.3 metric tons of uranium).
The central zone is discherged yesrly ané the remaining fuel shuifled
inward with the cuter zone being recharged with fresh fuel.
In the caleulations it was necessary to ignore control rods and to
assume that the neutron flux wes woifcrm throughout a region, ana that
the regions were neutronically unccupled. A calculstion cycie comprised
three years of irradiation time between charge and discharge of a zone.
This procedure gives the correct values (within the accuracy of the
assumptions) for the discharge compositicn after the irradiation cycle.
The average coxposition, neuntron fiux, and km for the entire reactor
loading =Ty over one-vear cycles hecause of the yesriy charge and
éischarge and are noct expiicitly given in the output of the CRIGEN code.
However, these sveresge values cer be constructed easily from the ouiput
of & calculaticn cycie.
The “standard” Tor compsring the effect of actinide recycie on the
actinide weste hazard measure was the waste obitined by removing a stiru-
lated percentege of uranium amd plutonium at 150 days after discharge
i sending the remaining quantities to waste along with sl11 the other
ectinides, and all actinicde daughters generated since discharge from
the reactor.
16
6. CONTRIBUTION OF EACH COMPONENT TO THE HAZARD OF THE
WASTE FROM A PWR SPENTFUEL PROCESSING PLANT
The results of the calculations presented in this section show the
relative importance of the contribution that the varicus corponents make
to the hazard of the waste from z PWR spent-fuel processing plant for
the previcusly described standard conditions and 99.5% recovery of
uranium apd plutonium; i.e., 0.5% of U and Pu and 100% of ali other
components are discharged ac waste and stored some place after suitable
processing.
Table 3 shows the percentage contribution of the actinides (inc¢lnding
their decay pro&acts) to the total hazard measure (water regquired for
dilution of the content of one metric ton to the RCC for the mixture) of
the waste as determined with three different sets of RCGs and the effect
of removing 1291
from the waste. Beyond sbout 00 years, the actinides
and their deughters dominate from a hazard measure viewpoint and show no
significant effect up to about 1()ll years due tc the different sets of
RCGs. At greater times, the relative importence of the actinides dimin-
ishes somewhat when the RCGs of rcf. 6 or ref. 9 (see Appendix I) are
used for the unlisted nuclides in place of the recommended derault values
in the Federal Code of Regulstions. Most of this difference can be
atfributed to the difference in RCGs for the nuclides of the 233U decay
chain (4n+l series), particularly those for 2297 and “%pa.
Toe remaining contribution to the hazard measure is almost all
(strustural clements are not important) from fission products with 1221
supplying 88% of this total at 103 years and rising to 98.4% at 106 vears.
Essentialiy all of the remaining fission product hazard for the longer
By 9 Py s 135
17 2 < WS e
times is contributed by the ~No and -
The yelative contributions of each actinide and itz daughter o
the total hazard memsure resulting from the mixture of the actinides
and their davghters are given in Table L, which shows that wp to J.O1¥
years the actinide waste hazard is mostly controlled by the americium
and curiuvm with no significant differences resulting from the different
RCGs. At mmch greater decasy times,the long-lived 2371\?;3 (2.1 x 106 year
17
Tzhle 3. Relative Contribution of Actinides apd Thelr Daughters
to the Total Waste from PWR Spent-Fuel Processing
Contribution of Actinides and Their Davghters (%)
at Decay Times (years) of:
iy =
102 & x10% 103 10" 10° 5 x 10° 10°
Using CFR RCGs and Recommended Default Values for Unlisted Nueclides:
1297 tpesent 0.3 9.3 97.5 93.8 97.8 90.2 99.1
129
I removed 0. 34 96.7 99.6 99.1 99.8 99. 9+ 99. G+
Using CFR RCGs and Values from Ref. & for Unlisted Nuclides:
129, present 0.3k gk, 3 97.5 92.5 59.3 70.8 61.6
1290 removed 0.3 96.7 . 99.6 58.8 55.7 9B.4 99.0
Using CFR RCGs and Vaelues from Ref. § for Unlisted Nuclides:
129I
present C.34 ok, 3 97.5 92.6 72.9 78.L 73.8
129
T removed 0.3k 96.7 99.6 g8.9 96.4 98.9 09, 4
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half-life) and its daughters begin to dominate. Another important point
is that the remaining actinides along with their deughters, namely, Ac,
Th, Pa, Bk, Cf, and Bs, meke a negligible coniribution to the waste
hazard. The contribution of uranium to the hazard of the U + Fu mixture
alone varied from negligible to a maximum of 25% at 106 years. The import
of these results is that in any waste menagement system in which at least
939.5% of the uranium and plutonium is extracted, a significant reduction
ir the actinide waste hazard can only be obtained by removel of most of
the americium, curium, and neptunium from the waste. If 99.5% removal of
these three actinides is a2lso effected, the uranium and plutcnium become
controlling and it would then pay (from 2 waste hazard viewpoint) to
increase the extraction efficiency of these latter elements, particularly
the plutonium.
The absolute values of the contribution of each cocmponent to the
nazard measuvre in cubic meters of water per metric ton of fuel are shown
in Tabie 5. To put these values in perspective, consider the required
2.3 x 1039 m3/metric ton for dilution of all the muclides to the RCG
after decaying 100 years. This volume of water is approximately equal
to the yearly flow of the Mississippi River into the Guif of Mexico.
Note that the last two rows in Table 5 are based cn the RCGs given in
refs. 6 and 9, respectively, for nuclides unlisted in the Code of Federal
Regulations, which (for beyond 10k years) results in an increasingly
smaller hazard measure that is about a factor of 67 and 37 lower, respec-
tively, at 106 years.
The apparent large quantity of water regquired for dilution to the
RCS for just one ton of fuel tends to megnify the potential hazard. Wo
reasconable scengrio can be constructed that visuelizes repid mixing or
dissolution of waste that has been processed into a very slightly soluble
form. The ingestifin hazard measure refers to potential long-term solution-
ing. However, consideration of such quantities of water dces present ore
argument for decreasing thé”quantity of actinides for ultimate disposal
by recycling the actinides back %hrough the power reactcrs prodncing'
ther. On the other hand, Bell and Dillon6 (using their RCGs) point out
that, after aging 1000 years, the actinide hazard measure of waste stored
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TR
for the RCGs that are not listed, particularly those in the ~-“U decay
c¢hain (bn+1 series), so that 3 better evalustion of the hazards of very
long-term storsge cen be made.
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wacte siream for each actinide and fts daughters.
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2hka on the hazard measure of the waste. When the higher actiniiles are
ircluded, the hnzard measure of the waste increases slowly up o a mexi-
mim factor of near R at a little over loh y=ars compared to that osteinec
vhen they are .eglected. Note, aowever, there is very little difference
in the values for the activity messured in curies.
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As previousiy mentioned, the concept of burning the “problem fizsien
. 85 : I37~ . N : -
sroduets” JKr, ©~Sr, and ~2!Cs in nuciefr reactors has been studied by
1,2
Steirherg and co-workers.” ’” In this section, their work is discussesd
3,4
briefly aleng with the gasted use of controlled thermonuclesr
Y ‘g SUEK
reacLors.
™he prodiem fissien products cannot be elimineted by any system cf
Tission power resctors operating in either a stagnaut oy expanding nficlear
pawer eccnhomy since the production rate exceeds the elimination rate by
burnout and decay. Only at eguilibrium will the production and removal
retes be equal, # condition that is never attained irn power reactors.
Equilibrium can be obtained, however, for a system that includes the
stockpile of fission nroducts as part of the system inventory since the
stockpile will grow until its decay rate ecuals the net production rate
of the system. Far the projected muclesr power eccnomy, however, this
will reaquire a very large stockpile with its associafed potential for
release of large quantities of hezardous radioisotopes to the environ-
ment. It is this stockpile that must be greatly reduced or eliminated
from the biosphere. A method suggesfed by Steinberg et g}_ is transmi-
tation in "burner reactors,” which are designed to maximize neutron