RADIOISOTOPE THERMOELECTRIC GENERATOR(RTG)
Radioisotope Thermoelectric Generator is a electrical generator.
It uses a radioactive material as the fuel and uses the fact that radioactive materials generate heat as they decay into nonradioactive materials.
This released heat is converted in to electricity by using Seebeck 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.
The first RTG launched in space by the United States was in 1961 aboard the
SNAP 3 in Navy Transit 4 A spacecraft.
One of the first terrestrial uses of RTG was in 1966 by US Navy at the uninhabitted Fairway Rock island in Alaska, where it remained in use until its removal in 1995.
Used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini, and New Horizons.
Used to power two Viking landers and for the scientific experiments left on the Moon by the Appollo 12.
In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses and navigation beacons powered by RTGs. There are approximately 1000 such RTGs in Russia. However, criticed argue that they could cause environmental and security problems, as leakage or theft of the radio active material could pass unnoticed for years (or possibly forever: some of these light houses cannot be found because of poor record keeping).
Utilized by the United States Air Force to power remote sensing stations.
In the past, small plutonium cells were used in implanted heart pacemakers to ensure a very long battery life.
Although not strictly RTGs, similar units called radioisotopes heater units are also used by various spacecraft including the Mars Exploration Rovers, Galileo and Cassini. RTGs were also used for the Nimbus, Transit and Les satellites.
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. 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. Thermocouple is made of two kinds of metal ( or semiconductors) that can both conduct electricity. They 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.
Commonly used thermoelectric materials are Germanium alloys, Lead telluride and Tellurides of Antimony, Germanium and Silver.
SELECTION OF FUELS
The half-life must be long enough that it will produce energy at a relatively continuous rate for a reasonable amount of time. And at the same time, the half-life needs to be short enough so that it decays sufficiently quickly to generate a usable amount of heat.
The fuel must produce a large amount of energy per mass and volume (density). It should produce high energy radiation that is easily absorbed and transferred into thermal radiation, preferably alpha radiation.
Most RTGs use 238 Pu which decays with a half life of 87.7 years.
RTG using Pu will diminish in power output by 1-.51/87.7 or .787% of there capacity per year. 23 years after production, such an RTG will have decreased in power by 1-.523/87.7 or 10% that is providing 83.4% of its initial output.
Thus starting capacity of 470W, after 23 years it would have a capacity of .834 x 470 = 392W.
Higher efficiency means less radioactive fuel is needed to produce the same amount of power and therefore a lighter overall weight for the generator. This is a critically important factor in space flight launch considerations. 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 â€œ 7 %.
Thermionic converter, the energy conversion device which relies on the principle of thermionic emission can achieve efficiency between 10 â€œ 20 %, but require high temperature than at which standard RTGs run.
Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and can be overlaid on top of the thermocouples, potentially doubling efficiency. Theorotical thermophotovoltaic cell designs have efficiency upto 30% but these have yet to built of conformed.
Nuclear runaway scenario is impossible.
Most RTg designs are inherently immune to nuclear meltdown or other runaway problems. 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 aeroshell, designed to protect the entire assembly against the heat of reentering the earthâ„¢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.
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.
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.
Efficiency is an important factor in spaceflight launch cost consideration.
NASA have been developing a next generation radioisotopes - fueled power source called the Stirling Radioisotope Generator (SRG) that uses free â€œ piston stirling engines coupled to linear alternators to convert heat to electricity.
SRG prototype demonstrated an average efficiency of 23%.
Use of non-contacting moving parts, non-degrading flexural bearing in test units, demonstrated no appreciable degradation over years of operations.
Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibrations can be eliminated on a concern by implementation of dynamic, balancing or use of dual â€œ opposed piston movement. Potential applications of a Stiriling radioisotope power system include exploration and science missions to deep â€œ space, Mars and the Moon.