Radioisotope thermoelectric generator

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Template:TOCright 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. RTGs are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries and generators to provide economically, and in places where solar cells are not viable.

Contents 1 Design 2 Fuels 3 Use 4 Life span 5 Efficiency 6 Safety 7 RTG Models 8 See also 9 References 10 External links

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.

Image:Rtgcutout.jpg

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

Image:Inspection of Cassini Radioisotope thermoelectric generator.jpg Image:New Horizons 1.jpg 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 are not as important for terrestrial use, unless there are size restrictions.
• 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 25 mm of lead shielding to control unwanted radiation. ²³⁸Pu (the best of these three) needs less than 2.5 mm, 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 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), 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 18 mm worth of lead shielding. Even so, its shielding requirements in a RTG are the second lowest of all possible isotopes: only ²³⁸Pu requires less.

Use

Image:Soviet RTG.jpg

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 in 1966 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 used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini and New Horizons. In addition, RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12through 17. RTGs were also used by the Americans for the Nimbus, Transit and Les satellites. By comparison, 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). Powered by ⁹⁰Sr, they are very reliable and provide a steady source of power. However, critics argue that they could cause environmental and security problems, 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).

There are approximately 1,000 such RTG's in Russia. All of them have long exhausted their 10-year engineered life spans. They are likely no longer functional, and may be in need of dismantling. Some of them have become the prey of metal hunters, who strip the RTG's metal casings, regardless of the risk of radioactive contamination.

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).

Although not strictly RTGs, similar units called radioisotope heater units are also used by various spacecraft including the Mars Exploration Rovers, Galileo and Cassini. These devices use small samples of radioactive material to produce heat directly, instead of electricity.

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}</math> or 83.4% of its starting capacity. Thus, with 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 in early 2001, the thermocouples were working at about 80% of their original capacity.

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

Image:Radioisotope thermoelectric generator plutonium pellet.jpg 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 technologies to generate electricity from heat. Achieving higher efficiency would mean 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 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 also 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 most space probes.

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

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%, and higher efficiency can be achieved with the use of greater temperature differentials between the hot and cold ends of the generator. The use of magnetically non-contacting moving parts, non-degrading flexural bearings, and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibration can be reduced through damping and counter piston movement. The most likely future use for STG's may be future Mars Rovers where vibration is less of a worry.

Safety

Image:RTGmodule.gif

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 such a nuclear meltdown is impossible. In fact, some RTGs are designed so that fission does not occur at all; rather, 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. However, this event is extremely unlikely with current RTG cask designs.

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 reentered the atmosphere carrying an RTG and burnt up over Fiji. The RTG itself survived reentry of the Earth's atmosphere intact, and plunged 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 should occur. Subsequent investigations have found no increase in the natural background radiation in the area. The Apollo 13 accident represents an extreme scenario due to the high re-entry velocities of the craft returning from cislunar space. This accident has served to validate the design of later-generation RTGs as highly safe.

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. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the earth's atmosphere. 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.

The Plutonium-238 used in RTGs has a half-life of 88 years, as opposed to the plutonium-239 used in nuclear weapons and reactors, which has a half-life of 24,100 years. Several tons of plutonium-239 have been released into the atmosphere by over 2,000 nuclear weapon tests. The plutonium-238 used by RTGs is designed to prevent release under the worst possible scenario.

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

RTG Models

Name & Model	Used On (# of RTGs per User)	Max Electical output (watts)	Max Heat output (watts)	Radioisotope	Fuel (Max kg of Radioisotope used)	Total Mass (kg)
SRG	in prototype phase, MSL	~110 (2x55)	~500	Pu238	~1	~27
MMRTG	in prototype phase, MSL	~110	~2000	Pu238	~4	26-34
GPHS-RTG	Cassini (3), Galileo (2) Ulysses (1)	300	4400	Pu238	7.8	55.5
MHW-RTG	Voyager_1 (3), Voyager_2 (3)	160	2400	Pu238	~4.5	39
SNAP-19	Viking (2), Pioneer_10 (4), Pioneer_11 (4)	35	525	Pu238	~1	???
SNAP-27	Apollo 12-17 ALSEP (1)	73	1480	Pu238	3.8	20
Beta-M	Soviet unmanned lighthouse	10	230	Sr90	.26	560

See also

• Nuclear reactor
• Radioactive isotope
• Cassini-Huygens
• Atomic battery
• Betavoltaics
• Optoelectric nuclear battery
• Radioisotope heater units

References

• Safety discussion of the RTGs used on the Cassini-Huygens mission.
• Bellona's report on RTG lighthouses.
• Nuclear Power in Space (PDF)
• Detailed report on Cassini RTG (PDF)
• Detailed lecture on RTG fuels (PDF)
• Detailed chart of all radioisotopes
• Stirling Thermoelectic Generator

External links

• Small electric power sources (PDF) - describes design criteria for small electric power sources, including details about thermoelectric pacemakers and space-based generators
• SpaceViews: The Cassini RTG Debate
• Stirling Radioisotope GeneratorTemplate:Nuclear Technology

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