Proceedings of GLOBAL 2005 Tsukuba, Japan, Oct 9-13, 2005

Paper No. 510

Neutronic Design Study of Small Long-live PWR with

(Th,U)O2 Fuel

Topan Setiadipura, M. Nurul S., Yuli Astuti and Zaki Su’ud

Dept. of Physics Bandung Institute of Technology

Jl. Ganesa 10 Bandung, Telp. 022-253-4094

e-mail: szaki@fi.itb.ac.id

ABSTRACT: Preliminary research on the utilization of (Th,U)O2 fuel in small long live PWR has been performed. In this paper we discuss the results of neutronic optimization of the core design with the core power level from 20 MWt up to 300 MWt. The core is designed to be able to operate (critically) 10 year without refueling. The core reactor design is a cylindrical 2 dimension R-Z (radial and axial) with tall type core. The optimization is run by varying the enrichment of the U-233 from 3w/o to 9 w/o with 0.25w/o interval, and the configuration of the core region based on this enrichment, also varying the fuel fraction from 35 % to 63.5%. The optimization also try to reduce the active core geometry as small as possible Neutronic calculation is run on SRAC code with library of nuclide data from JENDL-3.2/3.3. For 20 MWth reactor, the optimum design achieved in this research is the core with diameter 100cm and height 200cm, able to operate for 10 year with maximum excess reactivity 4.6% ∆k/k. Fuel fraction of the optimum design is 63.2%, with power density 16.5 W/cc. For 40 MWth and larger design, the excess reactivity is about 5% but the power level is higher. From this research we also able to conclude some property of the thorium based fuel to the U-233 enrichment and the moderation ratio or fuel fraction. As an example the following figure shows the multiplication factor change during burnup for 20MWt reactors. KEYWORDS : thorium, (Th,U)O2, refueling, PWR, excess reactivity, active core geometry, moderation

ratio, SRAC.

I. INTRODUCTION Small long life nuclear power plant with moderate economical aspect is an important candidate for electric power generation in remote area, such as many part outside Java-Bali area in Indonesia. Such nuclear power reactors match with the necessity and planning of many cities and provinces outside Java-Bali islands. Small long life nuclear reactors, however, usually has many problems. If we use standard power density then the size of the core will be small, need high enrichment of fissile material, and has large reactivity swing during burnup which then can not fulfill the fourth generation nuclear reactor criteria. In order that the reactors can be operated long time continuously without refueling or fuel shuffling, it is necessary to have relatively large internal conversion ratio so that we can obtain optimal design with relatively low excess reactivity during long time burnup. Designing thermal reactor with relatively high internal conversion ratio is a difficult job. Here we employ many strategy to achieve that goal including the usage of thorium cycle, employing Pa231 nuclide in the fuel, and also adopt tight lattice concept.

II. DESIGN CONCEPT In order to get good design for long life PWR which can be operated 10 years or even more without refueling or fuel shuffling, first we propose the usage of thorium cycle. Thorium cycle in the thermal environment is superior than uranium cycle in term of producing core with high internal conversion ratio. As second step we introduce tight lattice core concept t increase fuel volume fraction and together with employing higher enrichment strategy we can get small core. In order to further reduce reactivity swing during burnup we add Pa231 to reduce initial excess reactivity at the beginning of life (BOL), due to its high capture cross section so that acts such as burnable poison, while supplying U233 at the later stage of burnup by conversion process to U233 after two neutron capture processes and beta decay. By arranging this three strategy optimally the good design of long life PWR without on site refueling or fuel shuffling with relatively low excess reactivity during burnup can be achieved. III. CALCULATION METHOD

Proceedings of GLOBAL 2005 Tsukuba, Japan, Oct 9-13, 2005

Paper No. 510

The neutronic calculations in this study were run on SRAC code system. The flow of calculation used in this study is shown in Fig. 1. Cell calculations were performed using library based on public neutron data library JENDL.3.2. The result of this calculation were used in the whole core calculation using CITATION code which is embedded in the SRAC code system. In whole core calculation two dimensional R-Z geometry multigroup diffusion calculation were carried out every year.

Fig. 1 SRAC Calculation flow diagram IV CALCULATION RESULTS AND DISCUSSION In this study we consider small and very small long life PWR with the thermal power range between 20MWt and few hundred MWt. Table 1 show parameters used in cell burnup calculation based parametric survey for few hundred MWt small long life PWR with Pu231 enhanced thorium fuel cycle. The results are shown in in Figs. 1-4.

Table 1. general parameter for survey

Fig.1 shows that 0.2% Np237 can slightly reduce burnup k-inf change but the more significant reduction of k-inf swing comes from addition of 0.2% of Am231 .

fuel fraction 60% and enrich 2% U233

1

1.05

1.1

1.15

0 2 4 6 8 10burnup (y)

k-in

f oxide0.2% Np0.2% Pa

Start

SRAC PUBLIC LIBRARY

Cell CaLCULATION Fig.1 k-inf change during burnup for Th based fuel, Th-Np237

based fuel and Th-Am231 based fuel

U233-Np237

1.02

1.04

1.06

1.08

1.1

1.12

0 2 4 6 8 10 12

year

k-in

f3% U233-0.9% Np5% U233-3.3% Np7% U233-6.6% Np9% U233-10.12% Np10% U233-14.3% Np13% U233-32% Np

Homogenization

Fig. 2 The effect of Np237 content to k-inf change during burnup for Th-Np237 based fuel

Fig. 2 shows that higher amount of Np237 combined with higher U-233 enrichment results in better burnup k-inf pattern change during long life burnup without refueling or fuel shuffling. The use of 10% U-233 enrichment together with 14.3%Np237 give the burnup pattern relatively flat.

U233-Pa231

1.04

1.05

1.06

1.07

1.08

1.09

0 2 4 6 8 10 12year

k-in

f

3% U233 - Pa 0.7%6% U233 - Pa 4.0%7% U233 - Pa 5.5%8% U233 - Pa 7.2%

Fig. 3 The effect of U233 and Pa231 contents to the k-inf

change during burnup for Th-Am231 based fuel Fig. 3 shows that higher amount of Pa231 combined with higher U-233 enrichment results in better burnup k-inf pattern change during long life burnup without refueling or fuel

Enrichment 2- 13% 233U Burnable Poisons(BP) 0.2-12% 231Pa

0.2-32% 237Np Fuel-clad-mod fraction 60%-10%-30% Power Density 10-35 watt/cc

SRAC USER LIBRARY

FlCore

Calculation

Result

Finish

Proceedings of GLOBAL 2005 Tsukuba, Japan, Oct 9-13, 2005

Paper No. 510

shuffling. The use of 8% U-233 enrichment together with 7.2%Pa231 give the burnup pattern relatively flat. This value is smaller than that of U-Th-Np237 sustem. Fig. 4 shows the k-inf pattern change during long time burnup for fixed 8% volume combined with various Pa231 content change.

enrichment U 8% and Pa 7.6%- 8%

1.02

1.024

1.028

1.032

1.036

1.04

0 5 10 15 20 25 30year

k-in

f

7.6% Pa7.8% Pa8% Pa

Fig. 4 The effect of Pa231 content to k-inf change during

burnup for Th-Pa231 based fuel Fig.4 also show that if the amount of Pa231 is too low, the initial excess reactivity is high and then sharply decreases before reach relatively stable area. But if Pa231 amount is too much then initial excess reactivity will be low and after sharp decrease at the beginning of life it becomes too low compared to stable area (around 10~25 year). Therefore optimal amount of Pa231 should be chosen. Next we will discuss an example of parametric survey results for small power reactors. Table 2 shows general characteristics of 20MWt long life PWR without refueling or fuel shuffling.

Table 2 Paramters for 20 MWt samples Parameter Specification

Power (Thermal) 20 MWt

Refueling Periode 10 Year

Fuel (Th,U,Pa)O2

Cladding Zircalloy (Zr)

Coolant Light Water (H2O)

Fuel Enrichment 12w/o-16w/o U-233

Smear Density 90 %

Fuel Volume Fraction 60%

Pin Cell Type Circular Cylinder

Cladding thickness 0.07

Pin pitch 1,4

It is shown that in order to reduce the core size, higher U-233 eneriched fuel is used. Thicker cladding thickness is also chosen to anticipate long life operation burden.

0.998

1

1.002

1.004

1.006

1.008

1.01

1.012

1.014

0 2 4 6 8 10 12

Burn Up (Year)

K-ef

f

Pa 9%Pa 10%Pa 12%Pa 15%

Fig. 5 The effect of Pa231 content for 20 MWt long life PWR

burnup pattern change. In general higher Pa231 resulted in lower reactivity swing during long life burnup withour refueling or fuel shuffling. However, beyond 12%, the addition of Pa231 is not significantly further reduce reactivity swing.

Fig. 6 Core configuration for 420MWt long life PWR without refueling or fuel shuffling

After comparing the performance of U233-Th-Pa231 and U233-Th-Np237 fuels parametric survey results we decided to continue the detail design using U233-Th-Pa231 fuel due to its better performance than that of U233-Th-Np237 fuel. Fig. 6 shows optimized configuration of small/medium sized 420 MWth long life PWR without refueling or fuel shuffling during its operation. Here we devide the active part of thecore into three areas with different fuel composition. Table 3 gives parameters of the optimized 420MWth long life PWR core. It is shown that the enrichment of U-233 is

Proceedings of GLOBAL 2005 Tsukuba, Japan, Oct 9-13, 2005

Paper No. 510

ranged from 6-9%, while Pa231 content is ranged between 4.3 – 9%.

Table 3. Specification of PWR 420 MWth core

Power 420 MWth

Operation period 28 year

Shielding Zircalloy Coolant Light water (H20) Fuel Th-Pa-U oxide Enrichment U-233 6%-7%-9% Percentage Pa-231 4.3%-7%-9% Fuel, cladding&moderator volume fraction 60 %-10%-30%

Power Density 28.1 watt/cc Dia. Pin 1.224 cm Pitch 1.4 cm Core geometry Cylinder 2-D (R-Z)

Cell geometry Square Cell

Active core size 130 cm x 280 cm

Reflector width 30 cm Fig. 7 shows effective multiplication change suring long life burnup without refueling or fuel shuffling for the 420 MWth PWR. It is hsown that maximum excess reactvity during burnup is about 0.5% dk/k for about 25 years of operation.

Multiplication Factor

0.996

0.998

1

1.002

1.004

1.006

0 5 10 15 20 25 30 35

year

k-ef

f

Fig. 7 Effective multiplication change during long life burnup without refueling or fuel shuffling for 420 MWt PWR

Now we move to the optimized 20 MWt very small long life PWR core. The example of calculation results is shown in Figs. 8 and 9. While related core configurations are shown in Figs. 10 and 11. As shown in Fig. 8, 10% Pa213 in this configuration give excess reactivity during 10 years of burnup

without refueling and fuel shuffling about than 1% dk/k. While Fig. 9 shows that for core configuration as shown in Fig. 11, 12% of Pa231 give excess reactivity less than 1% during 10 years long life burnup process. V. CONCLUSION

Neutronic design study of small long-life Pressurized Water Reactor (PWR) core loaded with thorium oxide fuel and 231Pa has been performed. From the parametric survey and optimization process it is found that proper combination among tight lattice concept, the use of thorium cycle and the employ of 231Pa gives good long life PWR which can be operated for 10 years or more without refueling and fuel shuffling with reasonably low excess reactivity during burnup.

Optimization of 420 MWt long life PWR resulted in a long-life core design which operation time of 28 years, with maximum excess reactivity during burnup of about 0.5% ∆k/k and flatted power distribution during its operation. Similarly for 20 MWt very small long life PWR we found maximum excess reactivity less than 1% dk/k for 10 years of operation without refuling or fuel shuffling. ACKOWLEDGEMENT The authors would like tothank to Indonesian RUT XI Research project and Indonesian Ministry of Education B Project for their support to this research REFERENCES 1.. Yuli Astuti and Zaki S : “Preliminary design study of 40-

100MWth Small PWR Using Thorium-Uranium Based Fuel”,

Tokyo Tech COE INES – Indonesia International Symposium,

Bandung, Indonesia, March 2-4, 2005

2. Topan S. D. and Zaki S : “Neutronic Study Design of Very

Small Long Life PWR with (Th,U)U2 Fuel”, Tokyo Tech COE

INES – Indonesia International Symposium, Bandung,

Indonesia, March 2-4, 2005

3. M. Nurul S . and Zaki S: “Design Study of Small Long Life

Th-U Fueled PWR”, Tokyo Tech COE INES – Indonesia

International Symposium, Bandung, Indonesia, March 2-4,

2005

Proceedings of GLOBAL 2005 Tsukuba, Japan, Oct 9-13, 2005

Paper No. 510

13.25% U-

233

13.25% U-

233

13.5% U-

233

13.75% U-

233 14% U-233 14% U-233 Reflektor

13.25% U-

233

13.25% U-

233

13.5% U-

233

13.75% U-

233 14% U-233 14% U-233 Reflektor

13.25% U-

233

13.25% U-

233

13.5% U-

233

13.75% U-

233 14% U-233 14% U-233 Reflektor

13.75% U-

233

13.75% U-

233

13.75% U-

233

13.75% U-

233 14% U-233 14% U-233 Reflektor

14% U-233 14% U-233 14% U-233 14% U-233 14% U-233 14% U-233 Reflektor

14% U-233 14% U-233 14% U-233 14% U-233 14% U-233 14% U-233 Reflektor

Reflektor Reflektor Reflektor Reflektor Reflektor Reflektor Reflektor

Fig. 10 The core configuration of 20 MWth long life PWR with 10% Pa231 loaded in fuel.

13.75% U-

233 14% U-233

14.25% U-

233

14.25% U-

233

14.25% U-

233 14.75%U-233 Reflektor

14% U-233 14% U-233 14.25% U-

233

14.25% U-

233

14.25% U-

233 14.75%U-233 Reflektor

13.25% U-

233

13.25% U-

233

14.25% U-

233

14.25% U-

233

14.25% U-

233 14.75%U-233 Reflektor

14.25% U-

233

14.25% U-

233

14.25% U-

233

14.25% U-

233

14.25% U-

233 14.75%U-233 Reflektor

14.25% U-

233

14.25% U-

233

14.25% U-

233

14.25% U-

233

14.25% U-

233 14.75%U-233 Reflektor

14.75%U-

233

14.75%U-

233

14.75%U-

233

14.75%U-

233

14.75%U-

233 14.75%U-233 Reflektor

Reflektor Reflektor Reflektor Reflektor Reflektor Reflektor Reflektor

Fig. 11 The core configuration of 20 MWth long life PWR with 10% Pa231 loaded in fuel