Natural circulation desing of Thobatt

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Neutronic Design Study and Natural Circulation Aspect of Long Life Small PWR

with (Th,U)O2 Fuel

Topan Setiadipura1, Utaja

2

1Computational Field, Center of Nuclear Informatic Development BATAN INDONESIA

Telephone: + 62 21 756 0905

Fax : + 62 21 756 0923

Email : [email protected] ;

2PRPN BATAN

[email protected]

ABSTRACT

Development of an innovative nuclear reactor design, Long Life Small PWR with (Th,U)O2 fuel is inprogress. 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 tobe 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 280oC and 290

oC respectively.

Keywords : small long live PWR, thorium, natural circulation.

1. INTRODUCTIONSmall 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 lowtransportations of nuclear materials. In order

that the reactor can be operated long time

continuously without refuelling it is necessaryto 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 mechanismwhere the flow of the coolant in the reactor is

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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 belost. 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.

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 withoutrefueling 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 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

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.

Figure.2 Reactor core

The general parameter of the reactor design

including the fuel is shown in the table 1.

Parameter Spesifikasi

Power

(Thermal)20 MWt

Lifetime 10 Year

Fuel(Th,U)O

2 +

Pa-231

Structure Zircalloy (Zr)

Coolant H20

U-233

enrichment

7.5w/o-16w/o

U-233

Smear Density 90 %Fuel Volume

Fraction60%

Pin Cell TypeRectangular

Cell

Clad thickness 0.07 cm

Pin pitch 1,4 cm

Fuel height 195cm

Fuel pellet

radius0.612cm

Reflector H2OReflector width 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

Figure.3 Coolant channel

3. CALCULATIONAL METHODAND 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 isshown below

onacceleratiformfrictionb PPPP ++= (1)

The buoyant forces are given by

)])(()([ 00 ucrmfb LLLgP ++= (2)

where

o,m,u : 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 :

2

2

mm

e

ffriction V

D

LfP = (3)

where :

f :dimesionless friction factor(=0.02)

De : equivalent channel diameter

Vm : mean coolant flow rate

The pressure losses due to abrupt change ingeometry when the coolant is enter and exit the

channel, the relation is as follow :

2

2

m

form

VP = (4)

where :

: 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

225.0

2

m

monaccelerati

VP = (5)

The natural convection heat transfer coefficient

is calculated by calculating many parameter

such as the grashof number given as follow :

2

3

TgDGr e

= (6)

where :

: expansion coefficient of water.

T : temperature different between the

coolant and the cladding wall.

: kinematic viscousity

and the Nusselt number is calculated as follow

nGrCNu Pr)( = (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

eD

Nuh

= (8)

where 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 T. Using a computer

code, value ofT is tried to find the Gr which

resulting a natural heat flux as same as the

heat flux of the neutronic data for each axialposition. 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

Inlet Temperature (degree C)

Vm(m/s) 280 290 300

0.6 0.168079 0.012507 **0.7 0.9924 0.758192 0.514201

0.8 2.056155 1.735039 1.385538

0.9 3.374798 2.960503 2.446634

1 4.963485 4.374174 3.716405

Table.2 Chimney length and flow rate results

The results above show that with inlet

temperature 300oC the natural circulation

with Vm = 0.6m/s is not achievable.

The axial coolant temperature along thechannel for different inlet temperature and

mean flow rate is shown in the pictures at the

appendix. If the coolant flow rate is too slow

the coolant will boil as shown in the case of

inlet temperature 290oC 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, ZakiSuud, 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 foradvanced water cooled nuclear power plant

designs,Proceedings of a Technical

Committee Meeting, IAEA-TECDOC-1281.

3. L.S.Tong, J Weisman, Thermal Analysisof Presurrized Water Reactor,ANS,1979.

4. B.Nekrasov, Hydraulics, Peace Publisher,Moscow,USSR

5. M.Mikheyev, Fundamental of HeatTransfer, Peace Publisher,Moscow,USSR

6. Efrizon Umar, Prediction of Mass FlowRate and Pressure Drop in the Coolant

Channel of the TRIGA 2000 Reactor Core,

LKSTN VIII BATAN,1997.

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Natural circulation desing of Thobatt

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