Neutronic And Natural Circulation Aspect Of Thobatt (thorium Battery)

Table of Contents International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

Neutronic and Natural Circulation Aspect of Thorium Battery (ThoBatt), a Long Life Small PWR with (Th,U)O2 Fuel Topan Setiadipura*1 , Zaki Su’ud*2 Computational Division PPIN-BATAN, Serpong, Indonesia 2 Reactor and Nuclear Physics Lab. ITB, Bandung, Indonesia *E-mail:[email protected]

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Abstract NEUTRONIC AND NATURAL CIRCULATION ASPECT OF THORIUM BATTERY (ThoBatt), A LONG LIFE SMALL PWR WITH (Th,U)O2 FUEL. Development of an innovative nuclear reactor design called Thorium Battery (ThoBatt), a Long Life Small PWR with (Th,U)O2 fuel is in progress. Small long-live 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. The core is design to be able to operate (critically) 10 years without refueling by introducing the thorium based fuel, tight lattice core concept, and additional burnable poisson. The core reactor design is a tall cylindrical 2 dimension R-Z (radial and axial). The optimization is run by varying the U-233 enrichment and also the configuration of the core based on this enrichment, besides the fraction of additional Pa-231 which is added as a burnable poisson. The optimization also try to reduce the active core geometry to have a more compact core. The neutronic calculation is run on SRAC code package with library of nuclide data from JENDL.3.2. The optimum design achieved is a cylindrical core with 200cm height and 100cm n diameter, 0.7% Δk/k excess reactivity with 16,8W/cc power output. Further development of the reactor design in this paper is the natural circulation aspect of the reactor. Using the precalculated power density the feasibility of applying the natural circulation as heat removal mechanism is investigated. The chimney needed to apply the natural circulation with flow rate 0.6m/s is small to make the compact core design possible in the future development, the temperature of bulk coolant and the clad is also investigated. Keywords: Mo-99, uranyl nitrate, subcritical, neutronic, monte carlo

1. Introduction Development of an innovative nuclear reactor design called Thorium Battery (ThoBatt), a Long Life Small PWR with (Th,U)O2 fuel is in progress1,2. 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 province outside Java-Bali islands. Such nuclear power reactors can increase the nuclear reactor contribution to supply the electric power demand with low transportations of nuclear materials. In order that the reactor can be operated long time continuously without refuelling it is necessary to have relatively large internal conversion ratio so that we can obtain optimal design with relatively low excess reactivity during burn up. Designing such reactor is a difficult job, here we employ several concept to achieve that goal. This reactor design also apply the natural circulation

as a heat removal mechanism to have a better safety aspect, more economically moderate design, and also more compact reactor design. Generally, in the 21st century, it is expected that the world energy demand is continue growing to meet the needs of increasing population and economic growth, especially in developing countries. One of the options to resolve the energy crisis is to introduce advanced nuclear power system to the world. However, global dispersion of nuclear power plants leads to a concern for the nuclear proliferation and an increase of uranium demand causes its earlier exhaustion3. Hence, proliferation resistance and application of other fuel such as thorium is an important alternatives of the advanced nuclear power system. The use of thorium as a fertile material in nuclear fuel has been of interest since the dawn of nuclear power technology due to its abundance and to potential neutronic advantages. And study about the thorium based fuel reactor is intensive3-8. Internally in BATAN there are also several research based on the thorium11,12.Early projects include homogeneous mixtures of thorium and uranium 155

International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

oxides in the BORAX-IV, Indian Point I, and Elk River reactors, as well as heterogeneous mixtures in Shippingport seed-blanket reactor. However these projects were developed under considerably different circumstances than prevail at present. They preceded the proscription, for non-proliferation purposes, of uranium enriched to more than 20 w/o in 235U and in fact generally involved use of uranium highly enriched in 235U. Recent work performed with the expectation of high discharge burnup, no reprocessing, the 20 w/o limit on 235U enrichment, and with non-proliferation as a primary objective include homogeneous designs, Radkowsky Thorium Fuel9, and several other seed blanket designs. All of these designs are intended for once-through operation.5 However, the discharge burnup and the cycle length of cores loaded with thorium-based fuels were shorter than corresponding uranium-based cores due to the relatively low fissile cross-section of U233 and the high thermal absorption of thorium. Therefore, to achieve a high burnup with thoriumbased fuels in LWRs required higher fuel enrichments which led to higher fuel cycle costs. Some recent studies which have been designed to achieve improved economics by increasing the burnup of thorium-based fuels still show that in conventional LWR lattices thorium-based fuel will always have lower reactivity and therefore will not be able to achieve significantly higher burnups than uranium-based fuels with a similar fissile enrichment.5

would become a primary fissile nuclide at the burnup higher than 30 MWD/kgHM in (Th,U)O2 fuel. This fuel design has become possible as the fuel assembly components have been improved enough to be irradiated up to burnup higher than 60 MWD/kgHMrod avg6. The diagram of the Pa-231 conversion shown in the picture below

Figure 1. Pa-21 Conversion The ThoBatt reactor design is an alternative for the long-life core in the above cycle concept. The configuration of the ThoBatt reactor core is shown in the picture.2., the enrichment of U-233 is lower in the deeper core to flatten the neutron flux and power distribution in the core.

2. Design Concept There are three major design concept applied in order to get good design for small long life PWR which can be operated 10 years without refueling or fuel shuffling. First, we propose the usage of thorium based fuel, specially (Th,U)O2 in homogeneous core type. Thorium cycle in the thermal and epithermal environment is superior than uranium cycle in term of producing core with high internal conversion ratio. Besides its advantage related to its abundant and the non proliferation issue. Second, we introduce tight lattice core concept by increasing the fuel volume fraction. Third, we add Pa-231 as a burnable poisson to optimally reduce the initial excess reactivity in the beginning of life (BOL), due to its high capture cross section, while supplying U-233 at the later stage of burn up by conversion process to U-233 after two neutron capture and beta decay. The basic principle of that fuel design is that during approximately first half of fuel lifetime, power comes from initial loading of U235 and during the second half, power comes mainly from U233 bred from Th232. U233

Figure2. Core configuration and enrichment region The ThoBatt reactor design also apply the natural circulation as a heat removal mechanism to have a better safety aspect, more economically moderate design, and also more compact reactor design. In heat removal aspect, an important feature is the natural circulation mechanism where the flow of the coolant in the reactor is only governed by natural fenomena, gravity, without external sources of mechanical energy. This system is an important design feature for an innovative reactor design because in many reactor shutdown or emergency condition, forced cooling is assumed or predicted to be lost. Besides, this system provide a significant cost-savings by the elimination of pumps and ancillary equipment and also can result in simplified and hence higher reliability safety system10. 156

International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

Illustration of the coolant channel which comprise of fuel pellet, clad and coolant flow area is given in the picture below

Figure3. Coolant channel of the rectangular fuel pin arrangement. General parameters of the core design are given in the table below Table 1. General parameters of the core Parameters Specification Power (thermal) 20 MWt Refueling Periode 10 year Fuel (Th,U,Pa)O2 Cladding Zircalloy (Zr) U-233 Enrichment 7.5% - 13.75 % Fuel Volume 60 % Fraction Pin cell type Rectangular cell Cladding Thickness 0.07 cm Pin pitch 1.4cm

3. Calculational Method The core reactor design is a tall type cylindrical 2 dimension R-Z (radial and axial). Optimization of the neutronic aspect is run by varying the enrichment of U-233 and the configuration of the core based on this enrichment, combine with the Pa-231 addition which act as a burnable poisons that reduce the excess reactivity in the beginning of the cycle and keep the core critically in the later cycle. The optimization also try to reduce the active core geometry as small as possible. Neutronic calculation is run on SRAC code with lbrary of nuclide data from JENDL-3.2. The cell calculation is run on the PIJ module which applying the collision probability method, parametric survey is run to find optimum cell parameter for the design. The core calculation is run on the CITATION module a multi-dimensional diffusion code, core configuration parametric survey for different enrichment U-233 and fraction of Pa-231 is run to find the optimum design. Both PIJ module and

CITATION is under the SRAC code package. The neutronic aspect of the reactor design giving an optimum cylindrical core design with diameter 100cm and height 200cm and 0.7% Δk/k excess reactivity with 16,8W/cc power output and 10 years lifetime. And for the thermal hydraulic calculation, to determine the coolant flow through the core, specially through the coolant channel, the buoyant forces were compare to the resultants losses from the friction losses, form losses, and the acceleration losses. All the calculation is using NKS units. The relation is shown14-18 below

(eq.1) The buoyant forces are given by (eq.2) where ρo, ρm, ρu : fluid density at entrance, mean, and exit channel respectively. G : gravity constant Lu,Lf,Lc Height of the upper reflector, fuel, and the chimney. The frictional pressure losses result from wall friction and turbulence in the uniform cross section channel. These losses calculated as follow :

(eq.3) where : f :dimesionless friction factor(=0.02) De : equivalent channel diameter Vm : mean coolant flow rate The pressure losses due to abrupt change in geometry when the coolant is enter and exit the channel, the relation is as follow :

(eq.4) where : ξ : pressure drop coefficient (=0.065) and the coolant density is its density when at the entrance and exit position. Acceleration losses is calculated as follow

(eq.5) 157

International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

The natural convection heat transfer coefficient is calculated by calculating many parameter such as the grashof number given as follow :

(eq.6) where : β : expansion coefficient of water. ΔT : temperature different between the coolant and the cladding wall. ν : kinematic viscousity and the Nusselt number is calculated as follow (eq.7) where Pr is Prandlt number which characterizes the physical properties of the coolant fuid, and the constants C and n is depend on the value of Gr and Pr multiplication. From Nusselt number then the natural convection coefficient is calculated as follow

(eq.8) where λ is thermal conductivity of the water. The method used to find the natural convection heat transfer is by using the dependency of the Gr to ΔT. By developing a computer code, value of ΔT is tried to find the Gr which resulting a natural heat flux as same as the heat flux of the neutronic data for each axial position. Then, the axial temperature distribution of the coolant and the clad is calculated also the density of the coolant along the channel. So, in this calculation the mean coolant temperature is assumed first then the related axial temperature and density is calculated to finally calculate the chimney needed for that coolant flow by comparing the equation (1) and (2).

4. Results And Dicussions The variations of Pa-231 addition with various U-233 enrichment is shown is the pictures below

Figure.4. K-eff for various Pa-231 addition with enrichment U-233 10% From the results above it is shown that the k-eff of the core is higher with bigger U-233 enrichment as it should be, and by increasing the Pa231 addition to the fuel the k-eff fluctuation along the burn up is flatten, and it can also increase along the burn up as shown from the picture.5. for Pa-231 15%. Then for the same Pa-231 addition, the U-233 enrichment variations effect to the k-eff are investigated. The results are shown below

Figure 5. K-eff for various U-233 enrichment with Pa-231 addition 9% From the results above it shown that the affect of Pa-231 which flatten the k-eff fluctuation along burn up is higher for higher enrichment. To achieve an optimum neutronic core parameter (limited only its reactivity) the last characteristic is constrained by the increase of the initial k-eff of the core. So, a parametric survey to find an enrichment configuration of the core region and the additional Pa-231 which make an optimal neutronik parameter is run. The results of the most optimum core configuration is shown in the picture below

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International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

Figure 6. K-eff for various U-233 enrichment for Pa-231 addition 9% Power distribution from the neutronic calculation above is used for the termal hydraulic calculation of the core. Data used for thermal hydraulic calculation is from the 10% Pa-231 addition above. To make the core more compact as desire from the design goal, we investigate the feasibility to apply the natural circulation as the core cooling mechanism and investigate the chimneys needed for given coolant flow rate and futher investigate the temperature of the material related to safety such as the coolant and the clad. From the calculation desribed in the section.3 , the results are given in the table below

Figure 7. Bulk coolant temperature for Ti=280o C From the results above the bulk coolant can be maintain not to boil except for flow velocity 0.4m/s. We can see more explicit by investigating the density of the coolant as given in the figures below

Table 2. Chimneys height (m) for various flow rate and Ti Figure 8. Coolant density along the channel for Ti= 280o C The maximum natural convection heat transfer is slightly above the half of the pin height and by decreasing the coolant flow rate the maximum point is getting higher as shown in the figure below for Ti=280o C,.

It is shown from the results above that the minimum flow rate to apply the natural circulation is 0.6m/s, and the additional chimneys needed is so small. By applying the natural circulation flow, the temperature of the bulk coolant along the channel for different initial coolant temperature (as the coolant enter channel) is given in the figures below

Figure 9. Natural Convection heat transfer for Ti=280. 159

International Conference on Advances in Nuclear Science and Engineering in Conjunction with LKSTN 2007 (155-160)

And the clad temperature of the cladding for flow rate 0.6m/s is hown below, which is still in the safety margin.

4. 5. 6. 7. 8.

Figure 10. Clad temperature for various Ti at flow rate 0.6m/s

4. Conclusion Neutronic and natural circulation aspect of ThoBatt advance reactor core design,a small long live PWR with (Th,U)O2 is already investigated. The design able to operate critically for 10 years with a small initial excess reactivity. This is achieved by combining the thorium based fuel, tight lattice concept, and additional Pa-231 as burnable poisson. The optimum core design is achieved by specific arrangement of the enrichment configuration. The design also able to apply the natural circulation as the cooling mechanism to make the core more compact and the chimneys needed is small to support an integrated primary system reactor in the future development.

9. 10.

11. 12. 13. 14.

Acknowledgment Author (TS) are very gratefull to Ir.Utaja from PRPN-BATAN for the discussion on the development of the natural circulation method applied in the thermal hydraulic calculation.

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