Natural Circulation Desing Of Thobatt

Neutronic Design Study and Natural Circulation Aspect of Long Life Small PWR with (Th,U)O2 Fuel Topan Setiadipura1, Utaja2 1 Computational Field, Center of Nuclear Informatic Development BATAN INDONESIA Telephone: + 62 21 756 0905 Fax : + 62 21 756 0923 Email : [email protected] ; 2

PRPN BATAN [email protected]

ABSTRACT Development of an innovative nuclear reactor design, 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 neutronic aspect of the reactor design already done and giving an optimum cylindrical core design with diameter 100cm and height 200cm and 0.7 dk/k excess reactivity with16.8 W(t)/cc power output and 10 years lifetime. In this paper, the thermal hydraulic with natural circulation heat removal mechanism of the reactor design will be reported. This research is to investigate the coolant flow rate and the additional chimney needed when the natural circulation heat removal mechanism is applied as the cooling mechanism of the reactor. In a natural circulation system, the flow of the coolant in the reactor only govern 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 system. To apply the natural circulation mechanism on this reactor a chimneys is added. The height of the chimney is depend on the temperature inlet on the channel, which gives its own mean coolant velocity. The lowest mean coolant velocity is 0.6m/s with 1.2cm and 16.8cm chimneys for inlet coolant temperature 280 oC and 290oC respectively. Keywords : small long live PWR, thorium, natural circulation.

1. 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 JavaBali 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. In heat removal aspect, an important feature is the natural circulation mechanism where the flow of the coolant in the reactor is 1

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 system.

be minimized by considering that the thermal limitation is still achieved. The configuration of the reactor core is as shown in the picture 1 below.

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. Thorium cycle in the thermal 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 further 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.

Figure.2 Reactor core The general parameter of the reactor design including the fuel is shown in the table 1. Parameter Power (Thermal) Lifetime Fuel Structure Coolant U-233 enrichment Smear Density Fuel Volume Fraction Pin Cell Type

Figure 1. Pa-231 conversion From the thermal hydraulic aspect, the heat removal of the reactor is using natural circulation mechanism. Additional chimneys at top of the core is needed, to have a more compact nuclear reactor this chimney should

Clad thickness Pin pitch Fuel height Fuel pellet radius Reflector Reflector width

Spesifikasi 20 MWt 10 Year (Th,U)O2 + Pa-231 Zircalloy (Zr) H20 7.5w/o-16w/o U-233 90 % 60% Rectangular Cell 0.07 cm 1,4 cm 195cm 0.612cm H2O 5cm

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(up & below) Table.1 General Parameter Illustration of the coolant channel which comprise of fuel pellet, clad and coolant flow area is given in the picture below

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 : ΔPform

Vm2 = ξρ 2

(4)

where : : pressure drop coefficient (=0.065) ξ and the coolant density is its density when at the entrance and exit position. Figure.3 Coolant channel 3. CALCULATIONAL METHOD AND RESULTS 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 shown below ΔPb = ΔPfriction + ΔPform + ΔPacceleration (1)

The buoyant forces are given by ΔPb = g[Lf (ρ0 − ρm ) + (Lr + Lc )(ρ0 − ρu )] (2)

where ρo, ρm, ρu : fluid density at entrance, mean, and exit the 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 : ΔPfriction = f

Lf De

ρm

Vm2 2

(3)

Acceleration losses is calculated as follow Vm2 ΔPacceleration = 0.25 ρ m (5) 2 The natural convection heat transfer coefficient is calculated by calculating many parameter such as the grashof number given as follow : βDe3 gΔT (6) Gr = 2

υ

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 Nu = C (Gr ⋅ Pr) n

(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 Nu ⋅ λ (8) h= De where λ is thermal conductivity of the water.

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The method used to find the natural convection heat transfer is by using the dependency of the Gr to ΔT. Using 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 research the mean coolant temperature is assumed first then the related axial temperature and density is calculated to finally calculate the chimney need for that coolant flow by comparing the equation (1) and (2). By this method, the lowest coolant flow rate that make the natural circulation is possible to applied is 0.6m/s. The additional chimney is also a function of the inlet temperature. Data of the power density of the fuel along the channel is taken from the previous neutronic calculation. Results of the coolant flow rate and the chimney is shown as follow

Vm(m/s) 0.6 0.7 0.8 0.9 1

Inlet Temperature (degree C) 280 290 300 0.168079 0.012507 ** 0.9924 0.758192 0.514201 2.056155 1.735039 1.385538 3.374798 2.960503 2.446634 4.963485 4.374174 3.716405

Table.2 Chimney’ length and flow rate results The results above show that with inlet temperature 300 oC the natural circulation with Vm = 0.6m/s is not achievable.

the coolant will boil as shown in the case of inlet temperature 290 oC and the Vm = 0.4m/s. 4. CONCLUSION Natural circulation aspect of the reactor is investigated. It is possible to applied the natural circulation as the reactor heat removal mechanism with certain flow rate and related additional chimney. To achieve more compact nuclear reactor it desirable to have a short chimney, this value is achieved when the natural circulation flow rate is 0.6m/s. 4. REFFERENCES 1. Topan S, Muh.Nurul S, Yuliastuti, Zaki Su’ud, “Neutronic Design Study of Small Long-Live PWR with (Th,U)O2 Fuel”. Proceedings of GLOBAL 2005 Tsukuba Japan, Paper No.5101. 2. “Natural Circulation data and methods for advanced water cooled nuclear power plant designs”,Proceedings of a Technical Committee Meeting, IAEA-TECDOC-1281. 3. L.S.Tong, J Weisman, “ Thermal Analysis of Presurrized Water Reactor”,ANS,1979. 4. B.Nekrasov, “Hydraulics”, Peace Publisher, Moscow,USSR 5. M.Mikheyev, “Fundamental of Heat Transfer”, Peace Publisher,Moscow,USSR 6. Efrizon Umar, “Prediction of Mass Flow Rate and Pressure Drop in the Coolant Channel of the TRIGA 2000 Reactor Core”, LKSTN VIII BATAN,1997.

The axial coolant temperature along the channel for different inlet temperature and mean flow rate is shown in the pictures at the appendix. If the coolant flow rate is too slow

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