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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
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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