IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
I.2. Metallographic Examination of the Mixed Carbide
Fuel Pin which has seen 154 GWd/t burn up in FBTR
Remote metallographic
examination was carried out
on a fuel pin which is
proximate to the centre of the
core. The fuel pin was cut
using diamond wafer wheel
to extract specimens from the
centre and the end of fuel
column. Sequential grinding
and polishing of the molded
specimens were carried out.
Replicas of the polished
surface of the specimens were
taken and transferred out of
the hot cell. They were
observed under microscope
after decontamination and
gold sputter coating.
Photo micrograph of fuel
cross section at the centre of
the fuel column (see Fig.1)
revealed circumferential
cracking of fuel and a distinct
zone devoid of fuel porosities
near the outer diameter.
Favorable stress-temperature
regime near the outer
diameter of the pellet has led
to hot compaction of the
pellet thereby creating a
dense zone. The porosity
exhaustion is attributed to the
creep of the fuel due to Fuel
Clad Mechanical Interaction
(FCMI). The micrographs
revealed complete closure of
the fuel-clad gap and
circumferential cracking both
at the centre and end of the
fuel column indicating that
the entire fuel column
has entered the
restrained swelling regime.
Measurements on the
micrographs further revealed
that the outer and inner
diameter of the clad has
increased due to void
swelling. The measured
values were consistent with
the diamteral changes
measured using fuel pin
profilometer.
Fig.1 Photomicrograph of the fuel-clad cross section at the centre of the fuel column.
4 FAST BREEDER TEST REACTOR