PH 0101 Unit-5 Lecture-8 1

Introduction

Type of Converter in Nuclear cell

Types of Nuclear cell

Betavoltaic cell

Thermocouple based nuclear cell –Principle, construction and working

PH0101 UNIT-5 LECTURE 8

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• 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.

1. IntroductionThe 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.

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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 by radium produces current.

• The nuclear battery often referred as radioisotope thermoelectric generator (RTG).

• This system, which directly converts the heat, released during radioactive decay into electricity.

• Such generators function with thermoelectric or thermionic converters.

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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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Element Half-life (years)

Watts/g (thermal)

Polonium-210 0.378 141Plutonium-238 86.8 0.55Cesium-144 0.781 25Strontium-90 28.0 0.93Curium-242 0.445 120

The table shows the materials and their life span used as fuel in nuclear cell.

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2. Type of Converter in Nuclear cellDepending 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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Beta particles

Vacuum tube

Emitter Collector

Concept of High voltage nuclear cell

• 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 PN-junction.

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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 cellEjected electrons

Beta-emitting Radioisotope PN junction

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Low voltage Nuclear cellA 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 current in microamperes (μA).

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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.

V

Metal-A

Metal-BT1

T2

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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238Pu(Heat

source)

ColdSink V

Thermocouples

Safety container

A Nuclear cell (thermocouple based)

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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 that has low penetration, mainly Alpha radiation.

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