IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Fig.4 Peak deformations of grid plate
and stiffeners, top and bottom
plates and intermediate shells
of GP and CSS are modeled
with plate / shell elements. The
finite element mesh generated
with CAST3M code, issued by
CEA France is shown in Fig.2.
Free vibration analysis
Natural frequencies and
mode shapes should be known
to simulate the deformations
correctly in the subsequent
impact analysis, which are
determined by free vibration
analysis. For this CAST3m, is
used. A few important mode
shapes are depicted in Fig.3.
Since, upon impact, the nature
of deformation is bending of
top plate, the mode depicting
the bowing of top plate that
occurs at 20 Hz, is the critical
mode as far as impact is
concerned.
Impact analysis
With the understanding of
natural vibration behaviour and
energy balance, the duration
of impact and subsequently,
peak force are defined as a
triangular pulse in a classical
way as follows:
Velocity at time of impact
vmax
= √2.g.h
= 2 m/s
Impulse at time of impact (mv)
= 1900 N-s
The duration of impulse (t)
= 0.025 s
Peak force (Fmax)
= 76 k N
Dynamic analysis of grid plate
along with core and core
support structure was carried
out using CAST3M code and
peak deformations and stresses
are extracted. Fig.4 shows the
deformation patters at a critical
instant and peak value is found
to be ~ 0.1 mm and the
associated Von Mises stress is
found to be 20 MPa, which is
local in nature. These values
are found to be insignificant
from the structural integrity
considerations.
II.5. Hardfacing of Bottom Plate of Grid Plate Assembly
Grid plate of PFBR is a
massive structure consisting of
a two plates (top and bottom of
~6.5m in diameter and large
number of sleeves in which foot
of the sub assemblies rest. This
support structure also acts as
boundary between cold and hot
sodium in the reactors. Both the
grid plate assembly and the
core support structure are
made of 316LN stainless steel
and immersed in flowing
sodium. Hence, there should
not be any self welding between
these components at their
contact location. Hardfacing
using Ni base hardfacing alloy
(AWS Ni-Cr-B alloy) is
proposed on two annular
grooves machined on the
bottom plate of the grid plate.
These grooves are located
towards the periphery of the
grid plate and hence, diameters
of these grroves are close to
that of the grid plate itself and
total length (circumference) of
single harfaced deposit was
close to 21 m. The component
is designed in such a way that
area of contact between grid
plate and the core support
structure is confined to these
hardfaced grooves in the
bottom plate.
As the Ni base hardfacing
alloys are highly prone to
18 PROTOTYPE FAST BREEDER REACTOR