IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
The variation in concentration
of Hydrogen in nitrogen during
steam wash is shown in Fig.2.
Before the introduction of
steam, hydrogen was observed
in the concentration range of
0.05% in the nitrogen stream
(region A of Fig.2.). This can be
attributed to the reaction
between sodium and residual
moisture in the nitrogen gas.
Abrupt increase in hydrogen
concentration was observed at
region C and steam supply was
cut off to reduce further
reaction. Nitrogen flow rate
was increased to reduce the
hydrogen concentration to safe
limit of 2%. This sharp increase
of hydrogen concentration may
be due to condensation of
steam in the chamber and may
be prevented by preheating of
the system. Once the hydrogen
concentration reached less than
1%, steam was let in (region D).
The steam supply should be
regulated with respect to the
hydrogen concentration
H 2
conc. (%)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Steam cut off
N 2
20 (l /min)
steam let in
N 2
10 (l /min)
FSA in
chamber
A
B
C
0 20 40 60 80 100 120 140 160 180 200 220 240 260
D
Time(min)
steam let in
N 2
20 (l /min)
steam cut off
N 2
20 (l /min)
Fig.2 Hydrogen released during
steam cleaning of FBTR FSA by
steam-nitrogen process
E
monitored with the sensor, and
the nitrogen flow rate has to be
adjusted to control the
concentration of steam and
hydrogen. In addition to
hydrogen concentration, the
rate of rise of hydrogen signal
has to be considered while
regulating the nitrogen flow
rate and steam supply. It is
recommended to have an
automated system which
receives the hydrogen
concentration signal from the
sensor and adjust the flow rate
of nitrogen and supply of steam
to the chamber.
Water-wash
In order to remove any unreacted
sodium and sodium
hydroxide sticking on the
subassembly, water washing
was carried out by filling and
circulating DM water from a
reservoir using a pump. Water
level in the chamber was raised
slowly by letting in water from
the bottom. Hydrogen was
continuously monitored and
whenever it exceeds 2%, water
was dumped immediately from
the cleaning chamber to
prevent hydrogen build-up. The
process was continued by
maintaining the hydrogen
concentration within safe limit
till the whole of FSA was
submerged in water. Once the
hydrogen concentration
reached near background
value, it was presumed that the
Fig.3 View of FSA
after cleaning
cleaning operation was
completed. The water in the
chamber was circulated to
ensure the complete removal of
sodium hydroxide. The wash
water was quantitatively
collected for sodium
estimation. The FSA was lifted
using EOT crane, visually
examined, dried and stored.
The FSA after the cleaning
process is shown in Fig. 3.
The entire operation has
given enough confidence over
the design and operation of the
cleaning system and the
hydrogen sensor developed at
chemical group. The campaign
also demonstrated the technical
feasibility of safe transfer of
sodium wetted FSA to the
cleaning chamber in ambient
atmosphere. The parameters
critical in the cleaning process
were identified.
40 R&D FOR FBRs