INTRODUCTION.  Radioisotope Thermoelectric Generator is an electrical generator.  Fuel-Radioactive material.  Uses the fact that radioactive materials generate heat as they decay into nonradioactive materials.  This released heat is converted in to electricity by using See beck effect using an array of thermocouples.

 The output obtained in a RTG is a steady output voltage and its power capacity is a few 100 W.  RTG provides an uninterrupted and reliable source of heat and electricity in remote and harsh environment such as deep space.  Considered as a type of battery and so used as power sources in satellites, space probes and unmanned remote facilities.  It provides power and heat for spacecrafts to many years.  Also known as space batteries or nuclear batteries.

PRINCIPLE: THERMOELECTRIC EFFECT  The phenomenon in which a current is produced in a circuit containing two or more different metals when the junctions between the metals are maintained at different temperatures Also called see beck effect

CONSTRUCTION  Based on the standards of nuclear technology, Main component is a sturdy container, full of radioactive material (fuel).  Walls of the container are pierced by thermocouples.  Thermocouple is made of two kinds of metal ( or semiconductors) that can both conduct electricity.  Commonly used thermoelectric materials are Germanium alloys, Lead telluride and Telluride of Antimony, Germanium and Silver.

OPERATION  Walls of the container are pierced by thermocouples.  Other end of the thermocouple is connected to a heat sink.  Passive radioactive decay in radioactive material causes it to produce heat.  Heat flow through thermocouple and out the heat sink, generating electricity in process.  The two metals of the thermocouple are connected to each other in a closed loop. If the two junctions are at different temperatures, an electric current will flow in the loop.

SELECTION OF FUELS  It should produce high energy radiation. Energy release per decay is proportional to power production per mole.mole  For spaceflight use, the fuel must produce a large amount of power per mass and volume (density). Density and weight are not as important for terrestrial use, unless there are size restrictions. The decay energy can be calculated if the energy of radioactive radiation or the mass loss before and after radioactive decay is known.massvolumedensitydecay energy

FUELS(ISOTOPES)  Plutonium-238Plutonium-238  curium-244curium-244  strontium-90 strontium-90  polonium-210 polonium-210  promethium-147,  caesium-137, caesium-137  cerium-144, ruthenium-106, cobalt-60, etc..ceriumruthenium-106cobalt-60

EFFICIENCY  Higher efficiency means less radioactive fuel is needed to produce the same amount of power and therefore a lighter overall weight for the generator.  Thermocouples used in RTGs are very reliable and long lasting, but are very inefficient.  So efficiency above 10% have never been achieved and most RTGs have efficiency between 3 to 7 %.

SAFETY  Only kind of problems RTGs are subject to use radioactive contamination, which is harmful to environment.  To minimize the risk of fuel leakage, fuel is stored in individual modular units with their own heat shielding. This modular units are surrounded by a layer of Iridium metal and encased in high strength graphite blocks.

 These two materials are corrosion and heat resistant. Surrounding the graphite blocks is an aero shell, designed to protect the entire assembly against the heat of reentering the earth’s atmosphere.  Fuel is also stored in ceramic form, that is heat resistant, minimizing the risk of vaporization and aerosolization. The ceramic is highly insoluble.

HALF LIFE PERIOD:  The half-life must be long enough that it will release energy at a relatively continuous rate for a reasonable amount of time.half-life  The amount of energy released per time (power) of a given quantity is inversely proportional to half-life. An isotope with twice the half-life and the same energy per decay will release power at half the rate, per mole.powermole  Typical half-lives for radioisotopes used in RTGs are therefore several decades, although isotopes with shorter half-lives could be used for specialized applicationsradioisotopesisotopes

APPLICATIONS  Used as power sources in satellites, space probes and unmanned remote facilities.  Used as power sources for navigation beacons, radio beacons, light houses and weather stations.  Used at places where solar cells are not viable.  Most desirable power source for unmanned and unmaintained situations needing a few 100 watts or less of power of durations too long for fuel cells, batteries and generators.

ADVANTAGES  Chain reactions do not occur in RTGs, so heat is produced at a fully predictable and steadily decreasing rate that depends only on the amount of fuel isotope and its half-life

DISADVANTAGES  Usually thermocouples are used for conversion of energy, but their efficiency is very less between 3 to 7 percentage, so it affects the efficiency of the RTG. If the radioactive material is leaked it will affect the environment harmfully.

CONCLUSION  An accidental power excursion is impossible. On the other hand, heat generation cannot be varied with demand or shut off when not needed.  The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.  One mission proposed using the electricity to power ion engines.. ion engines  Therefore RTG’s assure good alternatives to the conventional generation methods if used with great care….