Encyclopedia
A
radioisotope thermoelectric generator 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 by the
Seebeck effect 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.
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 .
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.
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 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
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 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 fewer 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 and need less than 25 mm of
lead shielding to control unwanted radiation.
238Pu 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 .
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 , 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
The first RTG launched in space by the United States was in 1961 aboard the SNAP 3 in 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, Systems Nuclear Auxiliary Power Program units were used 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 12 through
17 . RTGs were also used 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 . 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 . There has been even an instance where the radioactive compartments were opened by a thief; it was inferred that the resulting radiation poisoning has already killed the thief.
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" were used in implanted
heart pacemakers to ensure a very long "battery life". As of 2004 about 90 were still in use. They pose a hazard if the wearer is shot in the chest with a gun. If the wearer dies and the generator is not removed before cremation the device will be subject to great heat. It is unlikely however, if the plutonium is in the form of the dioxide, that contamination will occur. Note that plutonium 238 is more able to disperse than plutonium 239, but the dioxide is an air stable solid which is normally sintered in air at a temperature much higher than that used in the cremation of human remains .
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 or 0.787% of their capacity per year. 23 years after production, such an RTG would produce at 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
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. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%, but have not been tested with actual radioisotopes. 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, NASA has worked on developing a next generation RTG called a Stirling Radioisotope Generator 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 SRG's may be future
Mars Rovers where vibration is less of a worry.
Safety
Radioactive contamination
RTGs are a potential source of
radioactive contamination: if the container holding the fuel leaks, the radioactive material may contaminate the environment.
For spacecraft, 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; and their use in spacecraft and elsewhere has attracted controversy.
However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the
Cassini-Huygens probe launched in 1997 estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its 3 RTGs during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million. If an accident which had the potential to cause contamination occurred during the launch phases , the probability of contamination actually being caused by the RTGs was estimated at about 1 in 10. In the event, the launch was successful and Cassini-Huygens reached
Saturn.
The
plutonium 238 used in these RTGs has a half-life of 87.74 years, in contrast to the 24,110 year half-life of plutonium 239 used in
nuclear weapons and
reactors. A consequence of the shorter half life is that plutonium 238 is about 275 times more radioactive than plutonium 239 . For instance, 3.6
kg of plutonium 238 undergoes the same number of radioactive decays per second as 1 tonne of plutonium 239. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same, plutonium 238 is around 275 times more toxic by weight than plutonium 239.
The alpha radiation both isotopes emit will not penetrate the skin, but can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk are the
skeleton, whose surface it is likely to be absorbed on, and the
liver, where it will collect and become concentrated.
There have been six known accidents involving RTG-powered spacecraft. The first one was a launch failure on 21 April 1964 in which the U.S. Transit-5BN-3 navigation satellite failed to achieve orbit and burnt up on re-entry north of
Madagascar. Its 17,000 Ci plutonium metal fuel was injected into the atmosphere over the Southern Hemisphere where it burnt up, and traces of plutonium 238 were detected in the area a few months later. The second was the
Nimbus B-1 weather satellite whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Launched from the
Vandenberg Air Force Base, its SNAP-19 RTG containing relatively inert
plutonium dioxide was recovered intact from the seabed in the
Santa Barbara Channel five months later and no environmental contamination was detected.
Two more were failures of Soviet
Cosmos missions containing RTG-powered lunar rovers in 1969, both of which released radioactivity as they burnt up. There were also five failures involving Soviet or Russian spacecraft which were carrying nuclear reactors rather than RTGs between 1973 and 1993.
The failure of the
Apollo 13 mission in April 1970 meant that the
Lunar Module reentered the atmosphere carrying an RTG and burnt up over
Fiji. It carried a SNAP-27 RTG containing 44,500 curies of plutonium dioxide which survived reentry into the Earth's atmosphere intact, as it was designed to do, the trajectory being arranged so that it would plunge into 6-9 kilometers of water in the Tonga trench in the
Pacific Ocean. The absence of plutonium 238 contamination in atmospheric and seawater sampling confirmed the assumption that the cask is intact on the seabed. The cask is expected to contain the fuel for at least 10 half-lives .
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.
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 most recent accident involving a spacecraft RTG was the failure of the Russian
Mars 96 probe launch on 16 November 1996. The two RTGs onboard carried in total 200 g of plutonium and are assumed to have survived reentry . They are thought to now lie somewhere in a northeast-southwest running oval 320 km long by 80 km wide which is centred 32 km east of
Iquique,
Chile.
Nuclear fission
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 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 not possible. 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.
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 at a high rate 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
See also
References
- ,Agency for Toxic substances and Disease Registry, U.S. Public Health Service, December 1990
External links
- - describes design criteria for small electric power sources, including details about thermoelectric pacemakers and space-based generators
