IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
III.E.5. Simulated Sodium Test Results of Eddy Current
Position Sensor
Eddy Current Position Sensor
(ECPS) has been
conceptualized to detect
whether the DSR has reached
its bottom most deposited
position or not and to find out
free fall time of DSR using ECPS
signal. In the earlier version of
ECPS, primary coils housed in
DSR electromagnet side and
secondary coils housed in
sheath side were kept axially
aligned. Detailed analysis for
reactor conditions indicated
that in reactor primary coil and
secondary coils will get axially
misaligned up to a maximum of
45mm due to thermal
expansion and irradiation
induced dimensional changes.
Therefore a new configuration
of ECPS which can take care of
axial misalignment of 45mm
was conceptualized and tested
in laboratory scale. Air test was
carried out in a 1:1 model
where the presence of sodium
was simulated by an equivalent
thickness of aluminum sheet
between primary side and
secondary side coils.
In ECPS (Version-3), where
axial misalignment of 45mm is
taken care of, secondary coil
occupies whole 65mm axial
length on DSR sheath side. This
secondary coil also works as
signal transfer coil. In primary
side totally three coils (one
primary & two pick-up coils) are
housed in 105mm axial length.
Schematic of ECPS-3 is shown
in Fig.1. While conceptualizing
the ECPS-3 schematic,
constraints of space availability
of 65mm only on DSR sheath
side is taken into account and
advantage is taken of the fact
that axial misalignment of
maximum 45mm is only in one
direction i.e. DSR
electromagnet moving 45mm
below compared to secondary
sheath side coil.
The primary is excited by a
constant current source and
produces an alternating
magnetic field which induces a
voltage in the secondary coil on
the sheath side. An alternating
current starts flowing due to
induced secondary voltage
since the secondary side circuit
is closed. The magnitude of this
secondary current depends on
the impedance of the
secondary circuit. The
impedance of the sensor coil
(on secondary side) is a
function of the permeability of
the material in the sensor coil
region. Since the piston of the
DSR is made of modified 9Cr-
1Mo, which is magnetic
material, the dropping of DSR
causes a change in the
impedance of the sensor coil
resulting in a change in
secondary side current. This
change in secondary current
causes a change in the net
magnetic flux and hence in the
pick-up voltages.
Initially, the bottom pick-up
will be aligned with the
secondary coil, as the axial
misalignment takes place the
bottom pick-up will move
downwards and the top pick-up
(which is identical to the bottom
Top Pick up
Primary
Bottom Pick up
Sensor coil
Fig.1 Schematic of ECPS to
take care of axial misalignment
Secondary
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