IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Fig.2 Computational mesh of 30 o
sector of FSA
Fig.3 Steady state temperature of
fuel pins and nitrogen (in K) for
5 kW decay power
between the fuel centre and
clad surface is estimated as ~
1 K only due to very low decay
power of 5 kW. Hence, the fuel,
clad and the gap between them
are lumped into a single
material having equivalent
properties. The finite volume
mesh of the FSA generated
using STAR-CD code is
presented in Fig. 2. The decay
power of the FSA is given as
volumetric heat source in the
fuel pins. Appropriate values of
surface emissivity have been
taken for clad and hexcan,
considering the fact that
sodium is coated on the
surfaces.
Predicted steady state
temperatures of the fuel pins,
stagnant nitrogen within the
FSA and hexcan, for 5 kW
decay power, are presented in
Fig. 3. As the heat transfer
takes place in the radial
direction and towards the heat
sink which is outside the
hexagonal sheath, the pin
surfaces facing the sheath are
at lower temperature compared
to pins surfaces facing the
centre of the FSA. It is seen that
the temperature of central pin
clad is 1557 K. Since the clad
temperature is exceeding 923
K, forced cooling of the FSA is
essential. Similar maximum
clad temperature is estimated
for various values of decay
powers (from 1 to 5 kW). It is
Fig.4 Evolution of central pin clad temperature during loss of cooling
(5 kW decay power)
found that for a decay power of
1 kW, the clad temperature is
883 K which is within the limit
of 923 K. For 2 kW decay
power (corresponding to
internal storage for 2
campaigns), the clad
temperature is 1118 K. Hence,
FSA with decay power less than
or equal to 1 kW can be
handled in FTC without any
forced cooling. For higher
powers, forced cooling is
essential.
The transient temperature
evolution of spent FSA central
pin clad in FTC is presented in
Fig. 4, for the case of 5 kW
decay power. The normal fuel
handling time of spent FSA is
30 - 45 min for its travel from
primary sodium pool to EVTP.
When the FSA reaches EVTP
after this time, the clad
temperature is ~773 K. At
EVTP location, the FSA is taken
out of the sodium filled transfer
pot and the clad temperature
again starts to increase when
forced cooling is unavailable
for the FSA inside FTC. It takes
15 min for the central pin clad
temperature to reach the
limiting value of 923 K. After
this it takes a long time (~4 h)
to reach the steady state
temperature of 1557 K. Hence,
it is established that loss of
cooling can be sustained for
about 15 min.
FUEL CYCLE 113