IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
of core top and absorber rods
and vertical accelerations of
core subassemblies which are
of concern to reactor safety
(Table 1), (2) primary
membrane stress intensities for
the main vessel and inner
vessel (Table 2) and (3)
generation of high strain
energies at the critical portions
which can enhance the
buckling risks of main
vessel and inner vessel
(Table 3).
In view of increased
displacements, stresses and
strain energies at critical
locations when the reactor vault
is connected with the
remaining structures of NICB,
it is preferable to isolate the
reactor vault, by directly
constructing it from the base
raft without inter-connecting it
with the adjacent buildings of
NICB, from the seismic loading
considerations. This also
provides some flexibility to
design and analyse the reactor
assembly components
independently without linking to
the finalisation of design of rest
of the civil structures. This can
be adopted for future FBRs.
III.A.2. Modal Analysis of PFBR Fuel Subassembly
PFBR Fuel subassembly (FSA)
consists of 217 fuel pins held
vertically in the form of a
bundle within a hexagonal
wrapper tube. Flow induced
vibrations (FIV) in FSA is a
potential cause for fatigue,
wear and vibration induced
fretting in the subassembly
parts which can lead to
reactivity noise and premature
failure of fuel pins and other SA
parts. The coolant flow in SA is
in highly turbulent regime and
it can cause flow-induced
vibration of fuel pins and FSA
as a whole. The overall
subassembly vibration will be in
its cantilever mode. The SA
vibration will also depends
upon the sodium exit velocity,
restraints at the supports in the
grid plate and the buttons and
the interaction of the discharge
coolant jets with control plug
parts positioned above the
subassembly as well as with
surrounding subassembly jets.
As part of the FIV
experimental program on FSA,
modal measurements were
carried out on SA in water to
estimate the modal parameters.
Theoretical modelling of FSA
was also done using ANSYS
FEM software to extract the
natural frequencies and mode
Table 1 : Modal frequencies in water
Mode
Case-1
(Hz)
shapes. Beam element was
assigned to discretize the
model and the full SA is divided
in into six sections. In the
analytical modelling, two cases
were taken as support
boundary conditions for FSA.
Case 1. Hinged support at
the location where FSA rests on
the sleeve top and simply
supported at the discriminator
location.
Case-2
(Hz)
1 3.7 3.9
2 22.2 24.0
3 64.6 66.2
4 126.0 128.1
5 147.9 149.0
R&D FOR FBRs 27