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Progress in Nuclear Energy 50 (2008) 308e313www.elsevier.com/locate/pnucene

Power density effect on feasibility of water cooledthorium breeder reactor

Permana Sidik*, Naoyuki Takaki, Hiroshi Sekimoto

Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-17, O-okayama, Meguro-ku, Tokyo 152-8550, Japan

Abstract

Breeding is made possible by the high value of neutron regeneration ratio h for 233U in thermal energy region. The reactor is fueled by 233UeTh oxide and it has used the light water as moderator. Some characteristics such as spectrum, h value, criticality, breeding performance andnumber density are evaluated. Several power densities are evaluated in order to analyze its effect to the breeding performance. The h valueof fissile 233U obtains higher value than 2 which may satisfy the breeding capability especially for thermal reactor for all investigated MFR.The increasing enrichment and decreasing conversion ratio are more significant for MFR< 0.3. The required enrichment and conversion ratiodo not change significantly caused by power density change for very tight lattice cell (MFR< 0.3), however, its strongly depends on the powerdensity change for higher MFR (MFR� 0.3). Breeding condition of all investigated power densities can be achieved for burnup � 30 GW d/t atMFR¼ 0.3 and it requires about 3.5% of required 233U enrichment. Number density of 233Pa decreases significantly with decreasing powerdensity which leads the reactor has better breeding performance because lower capture rate of 233Pa.� 2007 Published by Elsevier Ltd.

Keywords: Breeding; Thorium; Light water coolant; Power density; Conversion ratio; Enrichment

1. Introduction

Self-sustaining system of the reactor which relates to thebreeding capability is very essential to improve the reactor ca-pability for extending the sustainability of nuclear fuel resource.Fuel breeding capability generally attributed to the fuel conver-sion capability of new fissile nuclide from fertile nuclide whichthe fuel production of fissile obtains equal or more than requiredsupply fissile fuel (fresh fuel). The commercialized reactorswhich are mainly light water and heavy water reactors, producethe energy by consuming uranium fuel with lower conversionratio capability (Duderstadt and Hamilton, 1976). Based onthe proven technology of water cooled reactors, someresearchers have investigated some new reactor designs ormodification water cooled reactor designs in order to have betterconversion ratio. Fast reactor designs which have higher breed-ing condition attract some researchers. The breeding conditionof fast reactor can be easily maintained by hard neutron

* Corresponding author. Tel./fax: þ81 3 5734 2955.

E-mail address: permana@m.titech.ac.jp (P. Sidik).

0149-1970/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.pnucene.2007.11.013

spectrum. However, fast reactor has some obstacles due to thesafety concern of void reactivity (Duderstadt and Hamilton,1976; Hibi and Sekimoto, 2005; Takaki, 2000). As the proventechnology which utilizes the capability of neutron thermalspectrum, water cooled reactor shows better safety characteris-tic which related to negative void reactivity coefficient (Duder-stadt and Hamilton, 1976), but, it is difficult to achieve breedingcondition (Sidik et al., 2006). Reducing moderator to fuel ratio(MFR) is one of the option to improve the breeding condition(Iwamura et al., 1999; Okubo et al., 2000; Hibi et al., 2000;Sidik et al., 2006). Some investigations have been done to in-crease the breeding capability by using heavy water coolantto replace the light water coolant (Hibi et al., 2000; Sidiket al., 2006, 2007). The fuel material with higher breedingcapability in thermal or epithermal spectra region is requiredto overcome the lower conversion ratio capability of watercooled reactor. Thorium fuel with fissile 233U is the fuel candi-date which shows the superior h value in thermal and epithermalregion than those of other fissile nuclides (Michael and Otto,1998), and it can provides the negative void reactivity coeffi-cient (Kim and Downar, 2002). Thorium cycle has been

309P. Sidik et al. / Progress in Nuclear Energy 50 (2008) 308e313

demonstrated its breeding capability in thermal neutron spec-trum region, and it also obtains higher performance of burnupand more proliferation resistance (Freeman et al., 1989; Nuttinet al., 2005; Michael and Otto, 1998; Turan, 2000; Akie et al.,1991).

In the present study, feasibility of water cooled thoriumbreeder reactor has been investigated. The reactor is fueledby 233UeTh oxide and it has used light water as moderatorand coolant. Some characteristics such as flux, h value, critical-ity and breeding performances are evaluated for different mod-erator to fuel ratios (MFRs) and burnups. The different fluxlevel of the reactor is important effect due to the intermediatematerial production of 233Pa. Therefore, the different powerdensities are considered for evaluation in order to analyze theeffect of different power density to the breeding capabilitydue to the different flux level of the reactor.

1

ru

nit leth

arg

y [#/c

m2s]

MFR = 1.0

MFR = 0.322.5 W/cc

2. Design parameter and calculation method

The basic reactor core design parameters of investigated sys-tems are shown in Table 1. The average power density in the fuelpellet is varied from 22.5 to 90 W/cc, and then the cell-averagedpower density becomes a function of MFR. The average linearheat rate in fuel pellet can be estimated based on the employedfuel pellet power densities which are equal to 3 to 12 kW/m. TheMFR is varied from 0.0 to 2.0 and the discharged fuel burnup isalso changed from 6 to 50 GW d/t. No moderator condition(MFR¼ 0.0) is studied as comparative study to evaluate theneutron spectrum condition which similar to the fast reactorcondition. The MFR¼ 2.0 is investigated regarding the typicalvalue of MFR for PWR standard.

The study has been performed by using the nuclear equilib-rium state model (Sekimoto and Takaki, 1991). The nuclearequilibrium state model has been used to estimate the reactorcharacteristic of several reactor types (thermal reactors andfast reactors) (Seino and Sekimoto, 1998; Mizutani andSekimoto, 1998; Waris and Sekimoto, 2001; Sidik et al.,2006, 2007). The iterative calculation system (Mizutani andSekimoto, 1997) is employed by coupling the equilibriumfuel cycle burnup calculation and cell calculation of PIJ mod-ule of SRAC (Okumura et al., 2002). The employed nucleardata library was JENDL 3.2 (Nakagawa et al., 1995). The con-version ratio here is calculated by Eq. (1). The obtained valueof the conversion ratios is evaluated by using the equilibrium

Table 1

General parameter of reactor core

Thermal power output [MWt] 3000

Fuel pellet average power density [W/cc] 22.5e90

Fuel pellet diameter (inner) [cm] 1.310

Fuel pin outer diameter [cm] 1.452

Moderator to fuel ratio (MFR) [e] 0.0e2.0

Burnup [GW d/T] 6e50

Supplied fuel type 233The233U oxide

Cladding Zircaloy-4

Coolant H2O

atom composition. This equation is employed based on thefertile and fissile nuclides with the contribution from interme-diate nuclides such as 234U and 233Pa.

CR¼Capture rateð232Thþ 234Uþ 238Uþ 240PuÞ�capture rate 233Pa

Absorption rateð233Uþ 235Uþ 239Puþ 241PuÞð1Þ

3. Spectra and h value

Moderator condition in the reactor is important to distin-guish whether the reactor becomes a fast reactor or thermal re-actor. A thermal peak occurs because the neutron spectrumbecomes softer by adjusting moderator material in the reac-tors. The significant effect of changing the neutron spectrapattern is shown by changing the MFR (Sidik et al., 2006,2007). Several power densities are evaluated for analyzingits important effect to the neutron spectrum changes. The ob-tained neutron spectra for different moderator to fuel ratio(MFR) are shown in Fig. 1. Harder neutron spectra are effec-tively achieved by decreasing MFR. Thermal peak decreaseswith decreasing MFR and its location shifts to higher energyregion. The small scale is used for evaluating the spectra pro-file of no moderated case to evaluate the burnup effect whichis shown in Fig. 2 for different power density. The thermalpeak still occurs in very small scale although it is almostnegligible and its peak increases with increasing burnup. How-ever, for moderated case as shown in Fig. 3, the thermal peak

0.110-2 10-1 100 101 102 103

Relative flu

x p

e

Energy [eV]

MFR = 0.0

45 W/cc65 W/cc90 W/cc

Fig. 1. Neutron spectra profiles for lower energy of 6 GW d/t.

10-10

10-9

10-8

10-7

10-6

10-5

0.0001

0.001

0.01

0.01 0.1 1 10 100

Relative F

lu

x p

eru

nit L

eth

arg

y [#/cm

2s

]

Energy [eV]

MFR = 0.0

50 GWd/t

36 GWd/t

18 GWd/t

6 GWd/t

22.5 W/cc45 W/cc65 W/cc90 W/cc

Fig. 2. Neutron spectra profiles for different burnup of MFR¼ 0.0.

310 P. Sidik et al. / Progress in Nuclear Energy 50 (2008) 308e313

appears much higher and the thermal peak decreases with in-creasing burnup. This phenomenon explains that in fast reactorwith no moderator gives almost negligible thermal peak, andits spectra trend is opposite to the thermal reactor. Therefore,

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10 100

Relativ

e F

lu

x p

eru

nit L

eth

arg

y [#/cm

2s]

Energy [eV]

MFR = 0.3

50 GWd/t

36 GWd/t

18 GWd/t

6 GWd/t

22.5 W/cc45 W/cc65 W/cc90 W/cc

Fig. 3. Neutron spectra profiles for different burnup of MFR¼ 0.3.

the spectra profile of fast reactor (no moderated case) becomessofter and the spectra profile of thermal reactor becomesharder for higher burnup. Lower power density shows higherthermal peak due to the softer spectrum caused by lowerflux level of the reactor. The effect of different power densityis significant for moderated case as shown in Figs. 1 and 3.However, its effect is almost negligible for no moderatedcase as shown in Fig. 2. Breeding capability can be estimatedby h value of fissile nuclide. Fissile 233U has been used in thisreactor and some additional 233U from the conversion processof 232Th after it converted into 233U by absorption neutron. Asmentioned above, the neutron spectra profiles change bychanging the MFR. Therefore, by changing the MFR, it canbe estimated the h will also changes.

The obtained results of the number of neutrons producedper neutron absorption h are shown in Fig. 4 as a functionof MFR for several fissile nuclides. The h value is calculatedby one group microscopic cross-section each fissile nuclide inequilibrium condition. It can be seen that 233U is superior tothe other fissile nuclides except for MFR< 0.3, its h valuelower than 241Pu. For no moderated case (MFR¼ 0), the h

value of fissile plutonium is superior to fissile uranium. Fissile233U may satisfy the breeding capability especially for thermalreactor because the h> 2.

4. Enrichment and breeding

To achieve the critical condition of targeted burnup, the re-quired 233U enrichment is adjusted as initial fissile nuclide. Theobtained conversion ratio is evaluated in the equilibrium state.The required enrichment and conversion ratio are also investi-gated for different power densities which related to the fluxlevel of the reactors. Fig. 5 shows the enrichment and conver-sion ratio as a function of MFR for different power densities.The enrichment and conversion ratio decrease with increasingMFR. The MFR effect is strongly effective to increase the en-richment and conversion ratio for MFR< 0.3. Fig. 6 shows theenrichment and conversion ratio decrease with increasingburnup for no moderated case. In case of moderated condition,enrichment increases and conversion ratio decreases when

U233

U235

PU239

PU241

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8Moderator to Fuel Ratio [-]

Fig. 4. h value of fissile as MFR function.

1

2

3

4

5

6

7

8

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

En

rich

men

t [%

]

Co

nversio

n R

atio

[-]

Moderator to Fuel Ratio [-]

6 GWd/t

22.5 W/cc45 W/cc65 W/cc90 W/cc

Fig. 5. Enrichment and conversion ratio as a function of MFR for 6 GW d/t.

1

2

3

4

5

6

7

8

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

0 10 20 30 40 50 60

En

rich

men

t [%

]

Co

nversio

n R

atio

[-]

Burnup [GWd/t]

MFR = 0.0

22.5 W/cc45 W/cc65 W/cc90 W/cc

Fig. 6. Enrichment and conversion ratio as a function of burnup for MFR¼ 0.0.

1

2

3

4

5

6

7

8

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

0 10 20 30 40 50 60

En

rich

men

t [%

]

Co

nversio

n R

atio

[-]

Burnup [GWd/t]

MFR = 0.3

22.5 W/cc45 W/cc65 W/cc90 W/cc

Fig. 7. Enrichment and conversion ratio as a function of burnup for MFR¼ 0.3.

311P. Sidik et al. / Progress in Nuclear Energy 50 (2008) 308e313

burnup is increased as shown in Fig. 7. Although, the thermalpeak is almost negligible, however, in the small scale evalua-tion, the thermal peak profile becomes softer when burnup isincreased as shown in Fig. 2. It means that the reactor requireslower enrichment for higher burnup. The reactor requireshigher enrichment when burnup is increased for moderatedcase because the neutron spectra become harder with increas-ing burnup as shown in Fig. 3.

The required enrichment decreases and conversion ratioincreases with decreasing power density especially for MFR>0.3 as shown in Fig. 2. In the case of MFR< 0.3, the effect ofpower density to the enrichment and conversion ratio is not sig-nificant. On the other hand, for very tight lattice cell ofMFR< 0.3, the enrichment and conversion ratio do not stronglydepends on the power density, however, for higher MFR(MFR> 0.3), its strongly depends on the power density. Fig. 7shows the breeding condition can be achieved for burnup equalor higher than 30 GW d/t for all investigated power densities atMFR¼ 0.3, and it requires of about 3.5% 233U enrichment.

5. Equilibrium nuclide compositions

The contribution of 233U is essential because of the domi-nant fissile material to the system as well as 232Th as fertilematerial. The production of 233Pa is considered to evaluatethe breeding capability which becomes the significant materialin order to improve the breeding capability. The 232Th is dom-inant for all MFR range with more than 90% in the core, and it

1018

1019

1020

1021

1022

1023

22 45 65 90

Nu

clid

e D

en

sity [#/cc]

Power Density [W/cc]

MFRincreases

Th232 U233 Pa233

Fig. 8. Nuclide densities of some nuclides for different power density.

312 P. Sidik et al. / Progress in Nuclear Energy 50 (2008) 308e313

increases with increasing MFR. 233U as well as protactinium(233Pa) decreases with increasing MFR because of the softerspectrum as shown in Fig. 8. The power density effect isstronger for 233Pa as shown in Fig. 9 that the number density

0

2 1018

4 1018

6 1018

8 1018

1 1019

1.2 1019

22.5 45 65 90

Nu

clid

e D

en

sity [#/cc]

Power Density [W/cc]

6 GWd/t18 GWd/t

36 GWd/t50 GWd/t

Fig. 9. 233Pa production at MFR¼ 0.3 with different burnup.

of 233Pa increases with increasing power density. This is thereason why when lower power density, the breeding perfor-mance becomes better because the effect of 233Pa becomessmaller.

6. Conclusion

The performances of water cooled thorium breeder reactorhave been investigated. Several power densities are evaluatedin order to analyze its effect to the breeding performance. Thespectrum becomes softer for higher MFR and lower powerdensity. Fissile 233U may satisfy the breeding capability espe-cially for thermal reactor because the h> 2 for all investigatedMFR. The MFR change is strongly effective to increase theenrichment and conversion ratio for MFR< 0.3. For very tightlattice cell (MFR< 0.3), the enrichment and conversion ratiodo not strongly depends on the power density, however, forhigher MFR (MFR� 0.3), its strongly depends on the powerdensity. For all investigated power densities, the breeding con-dition can be achieved for burnup � 30 GW d/t at MFR¼ 0.3,and the required enrichment is about 3.5% 233U. The powerdensity effect is stronger for 233Pa which shows the numberdensity of 233Pa decreases with decreasing power density.This condition leads the reactor has better breeding perfor-mance because the effect of 233Pa becomes smaller for lowerpower density.

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