Transcript Slide 1

PH0101 UNIT-5 LECTURE 8
Introduction
Type of Converter in Nuclear cell
Types of Nuclear cell
Betavoltaic cell
Thermocouple based nuclear cell –Principle,
construction and working
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1. Introduction
The Nuclear cell (or Nuclear battery) is a device that directly
converts the heat energy produced by a radioactive isotope into
electrical energy. The Nuclear cell (or) nuclear battery can also
be called as Atomic battery and Radioisotope battery.
• The technique used in nuclear cell is different from that used
by nuclear power stations.
• The power production in
nuclear power stations are
based on the principles of fusion and fission in which the
heat is liberated by splitting and fusing the radioactive
material such as uranium to get a huge amount of heat.
• But the nuclear cell working on the principles of
thermoelectric effect (or) Seebeck effect and produce much
smaller amount of energy.
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• In 1913, a English scientist H.G.J. Moseley demonstrated
the first nuclear battery using a radium
emitter.
• He observed that the flow of charged particles created
radium produces current.
• The nuclear battery often referred
thermoelectric generator (RTG).
as
by
radioisotope
• This system, which directly converts the heat, released
during radioactive decay into electricity.
• Such
generators
function
thermionic converters.
with
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thermoelectric
or
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A number of researches have been carried out to identify the
suitable radioactive isotope, which need lowest shielding and
long life. Among these,
1. Plutonium-238( 238Pu)
2. Americium-241(241Am)
3. polonium-210 (210Po)
4. Stronsium-90 (90Sr)
5. Cesium-144 (144Cs)
are found to be best candiated for Nuclear cell .
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The table shows the materials and their life span used as
fuel in nuclear cell.
Element
Polonium-210
Half-life
(years)
0.378
Watts/g
(thermal)
141
Plutonium-238
Cesium-144
Strontium-90
Curium-242
86.8
0.781
28.0
0.445
0.55
25
0.93
120
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2. Type of Converter in Nuclear cell
Depending upon the conversion technique, the nuclear cells
are divided into some of the major categories as
Thermal converters
Thermionic converter- based on the principles of
thermionic effect (or) thermionic emission.
Radioisotope thermoelectric generator – based on the
principles of thermoelectric effect (or) Seebeck effect.
Thermo photovoltaic cells- Based on the principles of
photovoltaic effect
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Non-thermal converters
Direct charging- based on the principles of capacitor.
Beta voltaic – the beta particles are emitted from
radioactive materials and are used as heat source for a
diode to produce electricity on the similar principles of
photovoltaic effect.
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3. Types of Nuclear cell
Basically the nuclear cell can be classified as two types
•
•
High voltage Nuclear cell
Low voltage Nuclear cell
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High-voltage Nuclear cell
In this type of nuclear cell, the beta emitting isotope is used for
the emission of charged particles.
This radioisotope is attached to one electrode and acts as an
emitter.
A collecting electrode collects the emitted beta particles on the
other side.
In between the emitter and collector, a vacuum (or) a solid
dielectric is placed.
The charged particles are travel through this electrolyte and
reach the collector.
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Concept of High voltage nuclear cell
Beta particles
Emitter
Collector
Vacuum tube
•
In this type, a huge voltage is developed
•
The sources of radioisotope used in this technique are
Strontium-90, krypton-85 and tritium.
•
This type of cell is called as Betavoltaic cell.
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4. Betavoltaic cell
A betavoltaic cell (or) betavoltaic battery is a nuclear battery
that converts energy from beta particles (electrons) released
by a beta emitting radioactive source, such as tritium, into
electrical power.
The principle is similar that of photovoltaic effect. The beta
emitting source emits the beta particles.
This energy is used for the formation of electron-hole pair that
is responsible for the production of voltage across the PNjunction.
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These cells have a specific power of 24 watts per kilogram
and can operate with full loading over 10 years of operating
cycle.
The efficiency is about 25 percentages
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A betavoltaic cell
Ejected electrons
Beta-emitting
Radioisotope
PN junction
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Low voltage Nuclear cell
A low voltage nuclear cell works on the principles of
thermocouples or Seebeck effect.
Three different concepts have been employed in the low-voltage
nuclear cell are,
Thermopile – the array of thermocouple is used to produce
current
Ionized gas –the ionized gas is passed between two dissimilar
metal to produce current
Phosphor technique- in this method, the radioactive energy is
converted into light energy. Later this light energy converted into
electrical energy using a Photocell.
A low-voltage type gives
about
1 volt with current14 in
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Lecture-8
microamperes (μA).
5. Thermocouple based nuclear cell
A nuclear cell (or) radioisotope thermoelectric generator (RTG)
is a simple device which converts the heat energy released by
the radioactive materials into electricity.
The radioisotope such as Pulutonium-238 is releasing the heat
energy by the process of decay and the released heat is then
converted into electricity by an array of thermocouples
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The nuclear cell is based on the principles of thermoelectric
effect (or) Seebeck effect.
According to this effect ‘ When a temperature difference is
maintained at the junctions of two dissimilar metal, a voltage is
developed in an external circuit.
German physicist Thomas Johann Seebeck discovered this
effect in 1821.
Metal-A
T2
T1
Metal-B
V
Principle of Nuclear cell (Seebeck effect)
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Construction
The nuclear cell consists of a safety container in which a
radioactive material is kept and used as fuel.
The thermocouples are placed on the walls of the container
and their outer ends are connected to a heat sink.
During the decay, the radioisotope emits the heat and it flows
across the thermocouple and reaches the sink.
The heat energy later converted into electrical energy based
on the thermoelectric principle
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A Nuclear cell (thermocouple based)
Safety container
238Pu
(Heat
source)
Cold
Sink
V
Thermocouples
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The thermocouple is known as thermoelectric converter
consist of two dissimilar metal (or) semiconductor and
responsible for the heat conversion into current.
The thermocouples in nuclear cell are connected to each
other in a closed loop.
Metal thermocouples have low thermal-to-electrical efficiency.
However, the carrier density and charge can be adjusted in
semiconductor materials such as bismuth telluride and silicon
germanium to achieve much higher conversion efficiencies.
When the two metals of thermocouples are at different
temperatures, an electric potential will exist between them.
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• When an electric potential occurs, electrons will start to
flow, making electric current.
• The efficiency of the battery is about 3 to 7 percentage.
• The efficiency is depends on the amount of radioactive
materials used.
• The voltage developed in nuclear cells depends on the
types and amount of radioactive materials used.
• To produce high voltage, a Polonium-210 can be used
which produce the energy of 140 watts per gram.
• To produce low voltage, Plutonium-238 can be used
which produce 0.55 watts per gram of material.
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The characteristics of the radioactive materials used in
nuclear cell are,
The half-life must be long enough so that it will produce
energy at a relatively continuous rate for a reasonable amount
of time.
For spaceflight use, the fuel must produce a large amount of
energy per mass and volume (density).
Should produce high energy radiation
penetration, mainly Alpha radiation.
that
has
low
Isotopes must not produce significant amounts of gamma,
neutron radiation or penetrating radiation in general through
other decay modes or decay chain products.
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Polutonium-210 isotope provides phenomenally huge energy
density, but has limited use because of its very short half-life
and some gamma ray production.
A kilogram of pure 210Po in the form of a cube would be
about 95 mm on a side and emit about 63.5 kilowatts of heat
(about 140 W/g),
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Applications
• They are used as power source for spacecraft
• They are used in pacemakers
• They are used in unmanned power facilities.
• They used as portable batteries in electronics applications
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