Preliminary Neutronic Design of High Burnup OTTO Cycle Pebble Bed Reactor
Topan Setiadipura1*,Dwi Irwanto2, Zuhair1
1Center forNuclear Reactor Technology and Safety, National Nuclear Energy Agency(BATAN), Puspiptek Area, Serpong, 15310, Indonesia
2Nuclear Physics and BiophysicsResearch Group,Pyshics Dept.,Institut Teknologi Bandung ,JlGanesha 10, Bandung, 40132 , Indonesia
A R T I C L E I N F O A B S T R A C T AIJ use only:
Received date Revised date Accepted date Keywords:
High temperature gas-cooled reactor;
Pebble bed reactor;
Once-through-then-out cycle;
High burnup;
Pebble bed type High Temperature Gas-cooled Reactor (HTGR) is among the interesting nuclear reactor design in term of safety and flexibility for co-generation application. In addition, strong inherent safety characteristic of the pebble bed reactor (PBR) which based on natural mechanisms improve the simplicity of the PBR design, in particular the Once-Through-Then-Out (OTTO) cycle PBR design.
One of the important challenge of the OTTO cycle PBR design, and nuclear reactor design in general, is to improve the nuclear fuel utilization which shown by higher burnup value. This study performed apreliminary neutronic design study of a 200MWt OTTO cycle PBR with high burnup while keeping the safety criteria of the PBR design.Safety criteria of the design was represented by the maximum power generation per pebble fuel of 4.5 kW/pebble. The maximum burnup value also limited by the tested maximum burnup value which maintain the integrity of the pebble fuel.The optimized parameters in this study were the fuel enrichment, heavy metal (HM) loading per pebble, and average axial speed of the fuel. An optimum design with burnup value of 131.1 MWd/Kg-HM was achieved in this study which is much higher compare to the burnup of the reference design HTR-Module. This optimum design use a 17% U- 235 enrichment with 4g HM-loading per pebble fuel.
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INTRODUCTION
Pebble bed type high temperature gas-cooled reactor (HTGR) is among the interesting nuclear reactor design in term of safety and flexibility for co-generation application. In addition, strong inherent safety characteristic of the pebble bed reactor (PBR) which based on natural mechanisms improve the simplicity of the PBR core design.
Further simplification of the PBR design is offered by the once-through-then-out (OTTO) cycle PBR concept. Different with the conventional multipass cycle PBR, in OTTO cycle PBR, the pebbles only pass the core oncemaking the burnup measurement and fuel reloading devices unnecessary. This design
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also offers a better potential of high temperature heat production for co-generation [1,2]. A peu-a- peu (PAP) cycle PBR offer a more simple reactor design[3], however this simplification sacrificesan important feature of PBR, the ability to have a continual online refueling capability of the PBR as in the multipass and OTTO cycle PBR.
PBR design is very attractive to fulfill the electricity and heat demands in Indonesia, a country with very large population and natural resources. In addition, a small PBR design is highly appropriate with the small and distributed energy (electricity and/ heat) demand in Indonesia. Studies on the PBR designsand its applications in Indonesia including the development of tools for the design analysis of PBR have been performed in Indonesia [4-7].Recent calculations on the OTTO cycle PBR design show a
reviving interest of PBR application in Indonesia, in particular the OTTO cycle PBR for its simplicity and better high temperature potential. The core power of 200MWt was considered to be suitable for the Indonesian demands [8,9].
Main purpose of this research was to perform a preliminary neutronic design study of a200MWt OTTO cycle PBR with high burnup.This study can contribute to current initiative on the PBR design in Indonesia, particularly on the equilibrium core neutronic design and its optimization. Generally, increasing the burnup performance of the OTTO cycle PBR while maintaining the safety characteristic is one of important challenge for this design [10].
DESIGN PRINCIPLES
Basically, current design study used the HTR-Module [11] as the reference design.
However, different with the HTR-Module design which applied the multipass cycle, current study use the OTTO cycle design.Reactor design parameters are given in Table. 1. Including the parameters which was optimized in this study.
<Table 1>
Current design employed standard pebble fuel design based on Tristructural-Isotopic (TRISO) coated particles as illustrated in Figure 1.This fuel design assure a sound fission product retention capability of the design resulting in low release of radioactive material to the environment in any condition of the core including the most severe postulated accident. Graphite reflector which also act as the core structure, in addition to the significant content of graphite in the fuel, improve the thermal characteristic of the core due to high heat conductivity and capacity of the graphite.
Neutronically, significant graphite material compositionin the reactor improve the thermal neutron spectrum of the core due to its effective neutron thermalization capability. An inert He gas coolant avoidany chemical or physical reaction that might disrupt the neutron economy of the core.
<Figure 1>
A small diameter core conceptwas applied in this design. The diameter of the core is 3 m to allow the control rod only in the radial reflector without penetrating the core, also to keep the thermal capability to transfer the heat from the core
only by natural mechanisms giving the inherent safety of the core.
Height of the core is 4.8 m to fulfill the criteria given by Teuchert et.al [12] that the OTTO cycle PBR core should be less than 5m in height.
This condition will avoid the Xenon (Xe) oscillation by allowing sufficient transfer of neutrons from the top part of the core which contain fresh fuel pebbles to the bottom part containing older fuel pebbles.
The design also employed very low power density,as low as 5.8 W/cm3.Although this power density is higher compare to the 3 W/cm3 of HTR- Module design, but this is about 1/20 of the light water reactor’s power density.This means that the amount of energy and heat produced inside the reactor is volumetrically low so thatin an extreme accident condition with no forced colling system available, natural mechanisms such as conductive and radiative heat trasnfer is enough to remove the remaining heat so that no fuel damage and melt will occurs.This is supported by tests and analyses performed in Germany, Japan, China, South Africa and the USA[13].
In a moving core PBR (multipass dan OTTO cycle PBR), the lifetime of the core can be divided into several phases. Initially,with certain condition the core will achieve its first criticality.By adding more pebble fuel the core will have the initial core with full power, then as the fuel loading continue the core will have a start-up or running-in phase in which the neutron flux, power density profile, and the effective multiplication factor (keff) are still changing. Finally, the core will achieve the equilibrium condition that will last along the lifetime of the core. The phases prior to the equilibrium phase are sometime jointly called as pre-equilibrium phase. The core performance of the PBR design is usually represented by the performance of the core at the equilibrium, hence equilibrium calculation is important and practically the first phase in designing the PBR core[14].
A parametric survey of the uranium enrichment, heavy-metal (HM) loading per pebble, and axial fuel speed were perform in this study to achieve a higher burnup compare to the HTR- Module design which have an average burnup of 80 GWd/t-HM.The pebble fuel able to withstand a burnup up to 150 MWd/Kg-HM [1], therefore it is desirable to design a PBR with more effective fuel utilization which shown by much higher burnup value. A nuclear reactor design with effective fuel utilization is important to support the sustainability of nuclear reactor application in term of cost and fuel supply. HTR-Module design use 7.8% U-235 enrichment and 7 g HM-loading, while in current study the parametric survey of uranium enrichment
exploited the limit of 20%defined by Internatioanl Atomic Energy Agency (IAEA) for Low Enriched Uranium (LEU) [15].Analysis to quantify the effect of using higher enrichment, e.g. to the overall cost of the design, is beyond this optimization study.
Integrity of the pebble fuel is the main safety criteria of the PBR design to keep its sound inherent safety characteristic. Therefore, the safety criteria used in current study was the maximum energy generation per pebble fuel ball which will keep the integrity of the pebble fuel. Teuchert et.al reportedthat 5.7 kW/pebbleis the technical limit of the pebble fuel to keep its integrity [16]. However, for HTR-Module design in particular, Teuchert et.algive a lower limit of 4.5 kW/pebble [17]. In current design study the 4.5 kW/pebble was used as the safety criteria. This value of power generating limit per pebble fuel also used as the safety criteria in other PBR design studies [18,19]. A detail design of control rod and dynamic parameter of the core to assure the shutdown capability of the core are beyond the scope of current study. However, adding to the previous design criteria, it was desirable that the equilibrium keff to be as low as possible.
Improving the burnup value under the limit of certainmaximum power density while keeping the desired core power of 200 MWt and reducing the core height following OTTO cycle criteria was a challenge to be solved in this study.
CALCULATIONAL METHOD
As a moving core reactor, the burnup analysis of PBR should also consider the axial movement of the pebble ball. The burnup equation to be solved in this reactor is given in Eq.1 [20]
∂ Nk
∂ t +∂ Nk
∂ z υ=ϕ
∑
i=1 m
Niσfiyik+ϕ
∑
s=r q
Nsσasγsk+
∑
j=n p
Njλjαjk−λkNk−ϕNjαjk
(1)
Nk = atomic concentration of isotope k υ = axial fuel speed
Φ = flux of the core region
σfi = fission cross section of isotope i σas= absorption cross section of isotope i λj = decay constant of isotope i
yik = yield of isotope k due to fission in isotope i
γsk = probability that neutron absorption in isotope s produces isotope k
αjk = probability that decay of isotope j produce isotope k
The strategy to perform the burnup analysis of PBR used in this research was by coupling a neutron transport analysis including burnup analysis and additional simulation to move the fuel following the OTTO cycle fuel movement. The neutron transport and burnup analysis was performed by a continous energy Monte Carlo code called MVP- BURN[21]. The statistical geometry model in the MVP-BURN code is apropriate to model the double heterogeneity of PBR core and fuel design correctly in a simple way. Its monte carlo method also preferable to have a more accurate neutronic calculation compare to the standard diffusion approximation. Several studies on the PBR design also used MVP-BURN as the main calculational tools [3,10].
Calculational and coupling method used in this study is same with the method applied in MCPBR code [22]. The heavy metal burnup chain used in this calculation, as shown in Figure 2, comprise of 28 heavy metal from Th-232 up to Cm- 246 [21]. This burnup chain able to accommodate the uranium and thorium fuel cycle, although the current study was limited to uranium cycle. JENDL- 4.0 [23], the latest nuclear data library from Japanese Atomic Energy Agency (JAEA), was used in this study.
<Figure 2>
Geometrically, the cone-shape at the bottom of the PBR core was omitted in the current calculational model. The core was model as cyclindrical r-z geometry as shown in Figure 3. The geometrical model is divided in to 20 region in axial direction, and radially 5 region. This region mesh dimension considering the neededcalculation accuracy and also computational time [22]. This model is acceptable, in particular for preliminary design study, due to low neutron flux in the bottom of the core as also performed in other PBR core analysis [3,22,24].
<Figure 3>
The method applied in this calculation able to simulate the the whole lifetime phases of PBR core including the pre-equilibrium and equilibrium phase. Hence, the method can be applied to perform the optimization of the equilibrium condition and also the pre-equilibrium condition. In this research, the calculation start with the core fully loaded with specific reactor design (fuel composition and average axial fuel speed). This method was chosen due to its robustness in which the pre-burnup calculation to have pebble fuels with different
burnup values to fill the initial core is not needed.
Then as the fresh fuel continue loaded from the top core and the lowest fuel dischargefrom the core following the OTTO cycle procedure, the core will enter the running-in period shown by the changing of keff and other parameters, and finally reach the equilibrium condition.
RESULTS AND DISCUSSION
Transition of keff from the initial core up to the equilibrium core in the equilibrium analysis for different U-235 enrichment, HM-loading, and average axial fuel speed are given in Figures 4 – 7.
In those each figures, as expected, for all the design with same parameter except the average axial fuel speed initial keffare the same. Value of the initial keff
given in those figures are too high for a practical design of PBR core. These results was due to the computational method used in this study, in which initial core was simply loaded with fresh fuel.
Practically, the initial core can be loaded with many different loading strategy involvingdummy graphite pebbles which will give a different initial and running-in phase condition. The tools and method used in this study are capable to perform an analysis of any initial core loading, however the current study was focused on the performance of the equilibrium condition.
<Figure 4>
<Figure 5>
<Figure 6>
<Figure 7>
For the equilibrium condition, different average axial fuel speed affect the equilibrium keff. As can be seen from each of those figures, higher axial fuel speed will increase the equilibrium keff.This is due to the lower fuel residence time in higher axial fuel speed core making the core having, generally, a more fissile nuclides and finally resulted in higher equilibrium keff. This lower fuel residence time also will decrease the discharge burnup of the fuel. Figure 7 show that for 4 g HM- loading with 17% U-235 enrichment, an axial fuel speed of 0.8 cm/day did not give a critical equilibrium core.
Both lower HM-loading and U-235 enrichment will decrease the equilibrium keff.Change of HM-loading per pebble effect the balance between fissile inventory and moderation level.
Lower HM-loading will decrease the fissile inventory but improve the moderation level, vice versa.In the 4 g and 5 g HM-loading design, the
effect of low fissile inventory is stronger compare to the moderation level improvement, hence the equilibrium keff is decreasing with lower HM- loading. This effect is not always the case, effect of the HM-loading to the equilibrium keff depend on wether the design is over or less moderated [25].
The results show that the current design to achieve high burnup using 4 g and 5 g HM loading are an over moderated design.It can be understood that the design options to find the desired equilibrium keffare to decrease the axial fuel speed, HM-loading per pebble, and fuel enrichment.
<Figure 8>
<Figure 9>
<Figure 10>
Calculation results of the power density pro- file of the equilibrium cores are shown in Figures 8 – 10. Effect of different axial fuel speed to the maxi- mum power density is given in Figure 8. It shows that higher axial fuel speed is preferebble for the safety aspect due to its low maximum power den- sity. However, increasing the axial fuel speed is also decrease the burnup of the core.Figure 9 shows that decreasing the HM-loading per pebble which prefer- able to increase the burnup of the core will also in- crease the maximum power density which is dis- pleasing for the core design.Effect of different en- richment to the maximum power density is shown in Figure 10. Higher fuel enrichment will decrease the maximum power density, however, as discussed ear- lier it will also have a higher equilibrium keff. These results show that the axial fuel speed, HM-loading, and fuel enrichment need to be optimized to have an optimum burnup while keeping the maximum power density below the technical limit.
<Figure 11>
<Figure 12>
Results of the optimization study of the axial fuel speed, HM-loading, and fuel enrichment to the burnup value and maximum power density are given in Figure 11, while Figure 12 shows the optimiza- tion of these parameters to burnup and equilibrium keff.Both figures show the characteristic of OTTO cycle PBR optimization involving fuel enrichment, HM-loading, and axial fuel speed parameters. Based on the parametric survey results, for the 5 g HM/pebble design with fuel enrichment 20% and axial fuel speed 0.8 cm/day fulfill the maximum power density limit and have a burnup value of 131.1 MWd/Kg-HM, the equilibrium keff was 1.15.
For this design, increasing the axial fuel speed will further increase the equilibrium keff, while decreas- ing the axial fuel speed will increase the max. power density beyond the technical limit.A lower fuel en- richment of 17%decreased the equilibrium keff
to1.08, however the max. power density was in- creased to 4.3 kW/pebble. For the design with 4 g HM loading per peeble, in general, the burnup is higher but the max. power density also become in- creasing. A burnup of 145.7 MWd/Kg-HM can be achieved with 4 g HM/pebble and 20% enrichment while maintaining the maximum power density limit, however the equilibrium keffwas 1.12.Combin- ing the desired criteria of optimum burnup, the max- imum power density constrain, and the low equilib- rium keff, the optimization study show that the de- sign with 4 g HM/pebble, 17% fuel enrichment, and axial fuel speed of 1 cm/day was the optimum de- sign. It has a burnup value of 131.1 MWd/Kg-HM with maximum power density of 4.1 kW/cm3, and equilibrium keffof 1.07.Burnup of this optimized de- sign is much higher compare to the burnup value of reference HTR-Module design and also the 100MWt OTTO cycle PBR design by Teuchert et.al [12]. Although the maximum power density of this design is below the technical limit to assure the in- tegrity of the pebble, a thermofluid analysis is needed in the next phase of this design study. Basi- cally, the thermal load effect to the pebble due to this max. power density can be reduced because this maximum power density occurred at the top part of the core containing a quite fresh pebble ball and low temperature He coolant..
CONCLUSION
Preliminary neutronic design of a 200MWt OTTO Cycle PBR with high burnup of 131.1 MWd/Kg-HM was achieved with 17% U-235 enrichment and loading of 4 g-HM/pebble. The burnup improvement of this design is quite significant compare to the HTR-Module and previously proposed OTTO cycle PBR. Maximum power generation of the design was 4.1kW/pebble which fulfill the criteria of lower than technical limit 4.5 kW/pebble. This optimized OTTO cycle PBR design is an interesting nuclear reactor design to be used for electricity and co-generation application in developing country, such as Indonesia, particularly, due to its efficient nuclear fuel utilization and simplicity. Also,in general,due tostrong inherent safety of PBR design. Further study which include initial core optimization, and thermofluid aspect of
this PBR design are some of the important agenda to be studied in the near future.
ACKNOWLEDGMENT
The authors thank to the Neutronic Group at Center for Nuclear Reactor Technology and Safety of BATAN for the discussion on the design princi- ples of PBR design.
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Table 1. Reactor design parameter.
Parameter Unit Value
Core
Power MWt 200
Diameter / Heigt cm 300 / 480
Height of void (above the active core) cm 40
Max. power generation per pebble kW/pebble 4.5
U-235 enrichment % optimized in this study
HM-loading per pebble g optimized in this study
Average of axial fuel speed cm/day optimized in this study
Average burnup MWd/Kg-HM optimized in this study
Fuel pebble
Diameter cm 6
Thickness of outside graphite shell cm 0.5
TRISO coated fuel particle
UO2 Kernel radius cm 0.025
Density of UO2 Kernel Kg/m3 10.4
Coating type (inside – out) buffer/I-PyC/SiC/OPyC
Thicknes of each coating cm 0.009/0.004/0.0035/0.004
Density of each coating g/cm3 1.05/1.9/3.19/1.9
Fig.1. Illustration of the pebble fuel elemen and TRISO fuel particle used in Pebble Bed Reactor design.
Fig.2. Heavy Nuclide Chain used in the burnup calculation [21].
Fig. 3. Cylindrical r-z geometry used in the calculation to model the PBR core. In the figure, dN/dS is the change of the nuclide density along the fuel pebble path.
0 500 1000 1500 2000 2500 1
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5
v=0.8cm/day;20wt% v=0.9cm/day;20wt% v=1cm/day;20wt%
Operation Time (days)
Effective Multiplication Factor
Fig. 4. Effective multiplication factorstransition from initial core to equilibrium core for 20% U-235 enrichment and 5 g HM/pebble.
0 500 1000 1500 2000 2500 1
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5
v=0.8cm/day;17wt% v=0.9cm/day;17wt% v=1cm/day;17wt%
Operation Time (days)
Effective Multiplication Factor
Fig. 5. Effective multiplication factors transition from initial core to equilibrium core for 17% U-235 enrichment and 5 g HM/pebble.
0 500 1000 1500 2000 2500 1
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5
v=0.8cm/day;20wt% v=0.9cm/day;20wt% v=1cm/day;20wt%
Operation Time (days)
Effective Multiplication Factor
Fig. 6. Effective multiplication factors transition from initial core to equilibrium core for 20% U-235 enrichment and 4 g HM/pebble.
0 500 1000 1500 2000 2500 1
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5
v=0.8cm/day;17wt% v=0.9cm/day;17wt% v=1cm/day;17wt%
Operation Time (days)
Effective Multiplication Factor
Fig. 7. Effective multiplication factors transition from initial core to equilibrium core for for 17% U-235 enrichment and 4 g HM/pebble.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0.00
1.00 2.00 3.00 4.00 5.00 6.00
3.44 4.06 5.07
v=0.8cm/day;20wt% v=0.9cm/day;20wt% v=1cm/day;20wt%
Axial Region (top to bottom)
Power Density [kW/pebble]
Fig. 8. Effect of different average axial speed to the maximum power density. (200 MWt, 20%, 4gHM/pebble)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0.00
1.00 2.00 3.00 4.00 5.00 6.00
3.77 5.07
4gHM/pebble 5gHM/pebble Axial Region (top to bottom)
Power Density [kW/pebble]
Fig. 9. Effect of different HM-loadings per pebble to the axial power density profiles for 20% U-235 enrichment, 4 g HM/pebble, at average axial fuel speed 0.8 cm/day.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0.00
0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50
4.07
3.44
v=1cm/day;20wt% v=1cm/day;17wt%
Axial Region (top to bottom)
Power Density [kW/pebble]
Fig. 10. Effect of different U-235 enrichments to the axial power density profiles for 4 g HM/pebble, at average axial fuel speed 1 cm/day.
Fig. 11. Parametric survey results of average axial velocity, HM-loading, and U-235 enrichment to the maximum power density and burnup.
Fig. 12. Parametric survey results of average axial velocity, HM-loading, and U-235 enrichment to the equilibrium keff and burnup.
