IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
III.A.5. Investigation of Thermal Striping at the
Bottom of the Control Plug
When non-isothermal fluid
streams mix adjacent to a
structure, the surface of the
structure experiences
temperature fluctuations and
can develop cracks due to
thermal fatigue. This
phenomenon is called thermal
striping. In fast breeder
reactors, the coolant coming
out from the fuel subassemblies
is hotter than the coolant
coming out of the blanket and
absorber rod subassemblies.
The improper mixing of the jets
issuing out from these
subassemblies results in
temperature fluctuations in the
fluid. This may lead to the
structural damage of control
plug components like shroud
tubes, lattice plate, skirt etc.
An experiment has been
carried out in SAMRAT model to
understand the mixing
behaviour and to determine the
extent of thermal striping in the
above core area of PFBR.
SAMRAT is a 1/4 scale model
of PFBR primary circuit. Demineralized
water is used as
simulant in this model.
It has been established by the
earlier researchers that air or
water can be used to simulate
thermal striping phenomenon
in sodium if Reynolds (Re)
number and Peclet (Pe) number
are sufficiently large (Re >
20,000 and Pe > 600) both in
prototype and model. The fluid
thermal fluctuations as
measured using the simulant
fluid, can be transposed to the
prototype condition if
temperature is measured
outside the thermal boundary
layer. The attenuation of
temperature fluctuations within
the boundary layer can be
determined accurately using
sodium experiments.
In the present work mixing in
the fuel-blanket interface
region was studied. Hot water
at 343 K was sent through the
fuel zone and cold water at 308
K was sent through blanket
zone. To measure the fluid
thermal fluctuations, fast
response thermocouples (time
constant better than 15 ms)
were used.
Temperature (C)
70
60
50
40
30
20
Temperature_mixing zone
Temperature_cold water
Temperature_hot water
0 2 4 6 8 10
Time
Fig.1 Time series plot of temperature
at mixing region
Fig.1 shows a typical time series
plot of the temperature as
measured in the mixing zone.
From the statistical analysis of
data the mean temperature as
seen by each thermocouple was
estimated. These temperature
data are presented in a
normalized form to transpose
the result into reactor case by
substituting the prototype
parameters. The normalization
is done in the following
manner.
where,
T = Mean temperature
of the fluid as measured by the
thermocouple mounted in CP
T h = Mean temperature
of the hot inlet fluid
(Thermocouple mounted at the
SA exit)
T c = Mean temperature
of the cold inlet fluid
(Thermocouple mounted at the
SA exit)
∆Trms = Root mean
square of temperature
fluctuations.
32 R&D FOR FBRs