Development of Sensor Material for Application in Temperature ...

ENGINEERING-4
Development of Sensor Material
for Application in Temperature Sensitive Magnetic Switch
EXECUTIVE SUMMARY
A temperature sensitive magnetic switch is used for enhancing the reliability of the operation of Diverse Safety Rod Drive
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Mechanism (DSRDM). The switch uses a sensor with a Curie temperature in the range 620 to 630 C. Based on the
extensive melting trials and characterisation, the Fe-Ni-Co alloy with Co-13%, Fe-36% and Ni-51% is found to possess the
Curie temperature in the required range. Differential Scanning Calorimetry was employed to evaluate the Curie
temperature of the sensor material. The problems with respect to hot working of this alloy were overcome by a careful
selection of forming temperatures and the forming process. The switch is successfully manufactured and oven tested.
OUTLINE
500 MW(e), liquid sodium cooled Prototype Fast Breeder Reactor
(PFBR) employs two diverse types of shut-down systems for controlling
and regulating its smooth functioning. These are compound
engineering mechanisms designed to operate on diverse physical
principles, but with a common goal of ensuring the prompt shut down
of the reactor upon receiving a scram signal. Of these, the Diverse
Safety Rod employs a driving mechanism, called DSRDM that
incorporates a Temperature Sensitive Magnetic Switch (TSMS), the
function of which is to cut off the energising current supplied to the
electromagnet of DSRDM, once the sodium temperature crosses the
Curie temperature. The de-energisation of the magnet results in a drop
of the Diverse Safety Rod (DSR) into the reactor core.
In the magnetic switch, the flux lines are made to pass through an
Armco cylinder, the sensor element and second piece of Armco iron
(armature and back through the central hole). This magnetic force lifts
the armature in vertical direction. During the normal operating
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temperature of sodium in the range 550 to 600 C, the sensor element is
in ferromagnetic state and has large permeability. The Curie
temperature of the sensor material is designed in such a way that it
becomes paramagnetic when the temperature of the sodium increases
beyond 625°C. This results in passage of very small amount of flux
through the armature and causes the armature to fall down and open
the circuit carrying current to the electromagnet and to drop the DSR
into the core.
The Curie point of the sensor material depends on the chemical
composition. Based on the experience gained at IIT, Madras through
arc melting route, various alloys of different compositions of cobalt in
the range 12 to 15% were prepared in Balzer's VSG30 vacuum
induction melting and casting unit. These laboratory ingots were
subjected to X-ray radiography to identify and locate the casting
defects. Gas holes were found in the ingots. Various problems
encountered during the forging process were overcome by optimising
the thermomechanical processing route. Initially, extensive cracking of
the ingot was noticed for the ingots forged using pneumatic hammer
which could not be avoided even by increasing the forging
temperatures above 1175°C. To overcome this cracking the ingots were
first homogenised at 1050°C in vacuum furnace for 10 hours and
furnace cooled. Figure 1 (a) shows the microstructure of the ingot. The
forging process was optimised to produce sound products and the
microstructure of the hydraulically forged product is given in Fig. 1 (b).
The radiography of the hot worked piece has not shown any porosity
indicating that the pores got self welded during hydraulic press forging.
Based on the Tc measurements, the alloy with (Fe36Ni51Co 13) is found to
possess the required Curie temperature. The forging was carried out as
per the optimisation sequence. The Curie temperatures measured
based on Differential Scanning Calorimetry are given in Fig. 2. The
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Curie temperature is found be about 621.5 C for both the ingot and the
forged piece. This clearly shows that the Curie temperature would not
vary significantly with thermomechanical processing and depends to a
large extent on the chemical composition. A sensor was fabricated from
this forged alloy. Radiography was performed on the forged pieces and
found that there were no defects for the sensor fabricated through
optimized forging route.
Fig. 1 :
Heat flow / mW
-2.8
-2.9
-3.0
-3.1
-3.2
-3.3
-3.4
-3.5
-3.6
The microstructure of (a) ingot and (b) Forged
piece (Magnification 200 X)
TSMS-7
Curie Temperature = 621.5
600 610 620 630 640 650
Temperature /
o C
Fig. 2: The Curie temperature of Fe Ni Co
36 51 13
o C
alloy
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ENGINEERING-4
ADDITIONAL INFORMATION
The calibration data relating the chemical composition to Curie temperature of the arc melted samples (few grams) was
generated. However, for same composition, it was found that the Curie temperature of vacuum induction melted ingots (5
kg batch size) was lower (about 25°C). Therefore the vacuum induction ingots were prepared with cobalt in the range 12 to
15% (Ni – 51% and Fe – balance) to identify the exact chemical composition.
GENERAL EXPLANATION RELATED TO THE DESCRIPTION
The Diverse Safety Rod employs a driving mechanism,
called DSRDM that incorporates a temperature sensitive
electromagnetic switch (TSMS), the function of which is
to cut off the energising current supplied to the
electromagnet of DSRDM, once the sodium temperature
crosses the Curie temperature. The de-energisation of the
magnet results in the drop of the Diverse Safety Rod
(DSR) into the reactor core. In this regard, the choice of
the candidate material for the electromagnetic switch is
rather crucial as well as limited, from the point of view of
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ensuring a Curie temperature in the range 620 to 630 C,
which is set by design considerations. The Curie point of
this sensor material depends on the chemical
composition and it is necessary to optimise the chemical
composition and thermo-mechanical processing to
achieve the required Curie temperature. Based on
extensive melting trials and characterisation, the Curie
temperature of the alloy with Ni51Fe36Co13
compositions
resulted in required Curie temperature in the range 620 –
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630 C. This alloy with the required Curie temperature is
forged based on the optimised thermomechanical
processing and the sensor components fabricated are
shown in Fig. 3. The performance of these components in
furnace is found to be satisfactory.
Fig. 3 : The sensor components of about 38 mm diameter and 40 mm
height fabricated from the forged alloy
BRIEF DESCRIPTION OF THEORETICAL BACKGROUND
Based on the requirement of the Curie temperature in the range 620-630°C, and the data available in literature, Fe-Ni-Co
alloy is decided as sensor material. The Curie temperature depends on the chemical composition of Fe-Ni-Co alloys.
Various compositions of Fe-Ni-Co were prepared using arc melting at IIT, Madras and the Curie temperatures of as melted
buttons were obtained in the temperature range 637 to 725°C.
ACHIEVEMENT
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The chemical composition of the sensor material that possesses Curie temperature in the range 620 – 630 C is identified
as Ni51Fe36Co 13. Thermo-mechanical processing of this alloy is optimized for avoiding cracking during hot forging. The
temperature sensitive magnetic switch is successfully fabricated and tested for its performance.
Further inquiries:
Dr. P.V. Sivaprasad, Materials Technology Division
Metallurgy and Materials Group, IGCAR, e-mail:prasad@igcar.gov.in
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