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Radioisotope thermoelectric generator

From Academic Kids

A radioisotope thermoelectric generator (RTG) is a simple electrical generator which obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity using an array of thermocouples. RTGs can be considered as a type of battery and have been used as power sources in satellites, space probes and unmanned remote facilities.

Contents

Design

The design of an RTG is simple by the standards of nuclear technology: the main component is a sturdy container of a radioactive material (the fuel). Thermocouples are placed in the walls of the container, with the outer end of each thermocouple connected to a heat sink. Radioactive decay of the fuel produces heat which flows through the thermocouples to the heat sink, generating electricity in the process.

Diagram of an RTG used on the
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Diagram of an RTG used on the Cassini probe

A thermocouple is a thermoelectric device that converts thermal energy directly into electrical energy using the Seebeck effect. It 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.

Fuels

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Inspection of Cassini spacecraft RTGs before launch

The radioactive material used in RTGs must have several characteristics:

  • The half-life must be long enough so that it will produce energy at a relatively continuous rate for a reasonable amount of time. However, 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. Typical half-lives for radioisotopes used in RTGs are therefore several decades, although isotopes with shorter half-lives could be used for specialized applications.
  • For spaceflight use the fuel must produce a large amount of energy per mass and volume (density), density and weight is not as important for terrestrial use, unless size requirements are small.
  • Should produce high energy radiation that has low penetration, mainly Alpha radiation. Beta radiation can give off considerable amounts of Gamma/X-ray radiation through bremsstrahlung secondary radiation production, thus requiring heavy shielding. Isotopes must not produce significant amounts of gamma, neutron radiation or penetrating radiation in general through other decay modes or decay chain products.

The first two criteria limit the number of possible fuels to less than 30 atomic isotopes within the entire isotope table of elements. Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242 and thulium isotopes have also been studied. Of the above, 238Pu has the lowest shielding requirements and longest half-life. Only three candidate isotopes meet the last criteria (not all are listed above) and need less than an inch of lead shielding to control unwanted radiation. 238Pu (the best of these three) needs less than 1/10 of a inch, and in many cases no shielding is needed in a 238Pu RTG, as the casing itself is adequate.

238Pu has become the most widely used fuel for RTGs, in the form of plutonium oxide (PuO2).238Pu has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels. Some Russian terrestrial RTGs have used 90Sr; this isotope has a shorter half-life, much lower energy density and produces gamma radiation, but is cheaper. Some prototype RTGs, first built in 1958 by USA Atomic Energy Commission, have used 210Po; this isotope provides phenomenally huge energy density, but has limited use because of its very short half-life and some gamma ray production. A single pound of pure 210Po in the form of a cube would be about 3 inches on a side and emit about 63,500 watts of heat (about 140W/g), easily capable of melting then vaporizing itself. 242Cm and 244Cm have also been studied well, but require heavy shielding from gamma and neutron radiation produced from spontaneous fission.

Americium-241 is a potential candidate isotope with a longer half-life than 238Pu: 241Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the energy density of 241Am is only 1/4 that of 238Pu, and 241Am produces more penetrating radiation through decay chain products than 238Pu and needs about .7 inch worth of lead sheilding. Even so, its shielding requirements in a RTG are second in thickness (greater than) only to 238Pu.

Use

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Soviet_RTG.jpg
Soviet RTGs in dilapidated and vandalized condition, powered by 90Sr

The first RTG launched in space by the United States was in 1961 aboard the Navy Transit 4A spacecraft. One of the first terrestrial uses of RTGs was by the US Navy at the uninhabited Fairway Rock Island, in Alaska where it remained in use until its removal in 1995.

A common application of RTGs is as power sources on spacecraft, especially for probes that travel far enough from the Sun that solar panels are no longer viable. As such they are carried on Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses and Cassini. As well as this, RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 1217. RTGs were also used by the Americans for Nimbus, Transit and Les satellites. Only a few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.

In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses and navigation beacons powered by RTGs (see Bellona's report (http://www.bellona.no/en/international/russia/navy/northern_fleet/incidents/31772.html)). Powered by 90Sr, these now pose environmental and security concerns, as leakage or theft of the radioactive material could pass unnoticed for years (or possibly forever; some of these lighthouses cannot be found because of poor record keeping).

In the past, small "plutonium cells" (very small 238Pu powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life". As of 2003 about 150 were still in use. They pose a hazard if the wearer dies and the generator is not removed before cremation (although they are designed to survive cremation). Designs for pacemakers and artificial hearts have been considered that use other methods to generate electricity, including Stirling engines, betavoltaic cells (generation of electricity by capturing charged beta radiation directly) and atomic batteries.

Although not strictly RTGs, small samples of radioactive material called radioisotope heater units are also used by various spacecraft for heating including the Mars Exploration Rovers, Galileo and Cassini.

Life span

Most RTGs use 238Pu which decays with a half-life of 87.7 years. RTGs using this material will therefore lose <math>1 - {0.5}^{1/87.7}<math> or 0.787% of their capacity per year. 23 years after production, such an RTG would produce at <math>0.5^{23/87.7} = 0.834<math> of its starting capacity. Thus at a starting capacity of 470 W, after 23 years it would have a capacity of 0.834 * 470 W = 392 W. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well; at the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for Voyager 2, therefore the thermocouples were at that time, working at about 80% their original level.

This life span was of particular importance during the Galileo mission. Originally intended to launch in 1986, it was delayed by the Space Shuttle Challenger accident. Due to this unforseen event the probe had to sit in storage for 4 years before launching in 1989. Subsequently, its RTGs had decayed somewhat, necessitating replanning the power budget for the mission.

Efficiency

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Radioisotope_thermoelectric_generator_plutonium_pellet.jpg
Image of (mostly) thermally isolated, RTG pellet glowing red hot due to incandescence.

RTGs use thermoelectric "couples" or "thermocouples", to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3-7%. However studies have been done on improving efficiency by using other thermoelectric generating technologies. Higher efficiency means less radioactive fuel is needed and therefore also a lighter overall weight for the generator, a critically important factor in spaceflight launch cost considerations.

Energy conversion devices which rely on the principle of thermionic emission can achieve efficiencies between 10-20%, but require higher temperatures than those at which standard RTGs run. Some prototype 210Po RTG have used thermionics and potentially other extremely radioactive isotopes could provide power by this means, but short half-lives make these infeasible. Several space bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on many space probes.

Thermophotovoltaic cells which work by the same principles as a photovoltaic cell, except that they convert infrared light (rather than visible light) emitted by a hot surface, into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric couplers but can be overlaid on thermoelectric couples, potentially doubling efficiency. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells degrade faster than thermocouples, especially in the presence of ionizing radiation. Further research is needed in this area.

Thermophotovoltaic cells and silicon thermcouples degrade in the presence if ionizing radiation. There is a patented proposal to extend cell life by moving the radioactives to fresh cells.

Dynamic generators, unlike thermoelectrics, use moving parts to mechanically convert heat into electricity. Unfortunately those moving parts can wear out and need maintenance, which may not be possible for certain applications like space probes. Dynamic power sources also cause vibration and RF noise. Even so development by NASA on a next generation RTG called a Stirling Radioisotope Generator (SRG) that uses Free-Piston Stirling engines to produce power. SRG prototypes demonstrated an average efficiency of 23%. The pistons float magnetically and there is virtually no friction between moving parts. Theoretically, an SRG could continue running for decades without maintenance. Vibration can be reduced through dampening and counter piston movement. The most likely future use for STG's may be for example, future Mars Rovers where vibration is less of a worry.

Safety

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RTGmodule.gif
Diagram of a general purpose heat source module used in RTGs

It should be noted that RTGs use a different process of heat generation from that used by nuclear power stations. Nuclear power stations generate power by a chain reaction in which the nuclear fission of an atom releases neutrons which cause other atoms to undergo fission. This allows the rapid reaction of large numbers of atoms, thereby producing large amounts of heat for electricity generation. However, if the reaction is not carefully controlled the number of atoms undergoing fission (and the heat production) can grow exponentially, very rapidly becoming hot enough to destroy the reactor.

Chain reactions do not occur inside RTGs, so that such a nuclear meltdown is impossible. In fact, some RTG are designed so that fission itself does not occur; forms of radioactive decay which cannot trigger other radioactive decays are used instead. As a result, the fuel in an RTG is consumed much more slowly and much less power is produced.

In spite of this, RTGs are still a potential source of radioactive contamination: if the container holding the fuel leaks, the radioactive material will contaminate the environment. The main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere.

There have been five known accidents involving RTG powered spacecraft. The first two were launch failures involving U.S. Transit and Nimbus satellites. Two more were failures of Soviet Cosmos missions containing RTG-powered lunar rovers. Finally, the failure of the Apollo 13 mission meant that the Lunar Module, which carried the RTG, reentered the atmosphere and burnt up over Fiji. The RTG itself survived reentry of the Earth's atmosphere intact, plunging into the Tonga trench in the Pacific Ocean. The US Department of Energy has conducted seawater tests and determined that the graphite casing, which was designed to withstand reentry, is stable and no release of plutonium will occur. Subsequent investigations have found no increase in the natural background radiation in the area.

In order to minimise the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion and heat resistant. The plutonium fuel is also stored in a ceramic form that is heat resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble.

There are no nuclear proliferation risks associated with plutonium-238 - that is to say, it is unsuitable for making nuclear weapons. The major reason is that plutonium-238 does undergo spontaneous fission and thus emits neutrons randomly, causing the chain reaction to start too early in the triggering process. This can cause the bomb to fizzle and greatly reduces its reliability and power. Moreover, plutonium-238 is hot and radioactive and this would complicate the bomb manufacturing.

See also

References

External links

fr:générateur thermoélectrique à radioisotope de:Radioisotopengenerator

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