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.

Design

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Diagram of the RTG used for Cassini probe

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.

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.

The radioactive material used must have a relatively short half-life so that it decays quickly enough to generate a usable amount of heat. Typical half-lives for radioisotopes used in RTGs are a few decades. The most widely used fuel for RTGs is plutonium-238, in the form of plutonium oxide (PuO2), but RTGs in some Russian lighthouses instead used strontium-90 for reasons of economy.

Safety

It should be noted that RTGs use a different process of heat generation to 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, fission itself does not normally occur inside an RTG; 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. Diagram of a general purpose heat source module used in RTGs Enlarge Diagram of a general purpose heat source module used in RTGs

But 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.

Use

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 12–17. 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-1.

In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses and navigation beacons powered by RTGs. Powered by strontium-90, 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 plutonium-238 powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life". As of 2003 about 150 were still in use.

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 plutonium-238 which decays with a half-life of approximately 85 years. RTGs using this material will therefore lose a factor of $1 - \sqrt[{85}]Template:0.5$ or ca. 0.81% of their capacity per year. 23 years after production, such an RTG would produce only 470 W × 0.991923 ~= 390 W — or roughly 83% — of its initial output. 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, so the thermocouples work at about 80%.

This life span was a 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.

RTG Array Example

A set of radioisotope thermal generators, arranged in an array, can provide sufficient power for a manned spacecraft.

The power output of an RTG is proportional to its heat flow rate. The GPHS model illustrated above is designed for radiant cooling and increases its heat flow rate through use of cooling fins along the side of the RTG. The ends of each RTG unit have significantly less heat transfer surface than the sides, and therefore have less heat flow. Most of the GPHS model’s useful heat flow occurs through its sides, and that’s where the majority of its thermocouple devices are located to take advantage of this.

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Possible RTG Array Orientations

Radiant transfer of heat can be thought of as being from surface to surface, even if the “surface” is the ground or a control surface framing the Martian sky. Radiant transfer occurs from hot surfaces to cold surfaces. Thus, because of the large temperature difference, more net heat transfer will occur between a hot RTG and the cold Martian surface than between two RTG’s at the same temperature. Thus, when placing more than one RTG together, it is desirable to place them end-to-end in a linear configuration rather than parallel. The New Horizons space probe, the Cassini space probe, and most space probes powered by more than one RTG use this layout or some variation. If the RTG’s are laid out parallel or in a ring where their main heat transfer surfaces face each other, they will produce less power because they radiate heat back and forth to each other, reducing their net heat transfer. Each RTG in the array should present as small a cross section to the others as possible, to reduce heat transfer between the units. Likewise, to take maximum advantage of the cold Martian environment, the RTG’s should present as small a cross section to the heated habitat as possible while maximizing their exposure to open space. Thus, a set of linear arrays, each pointing straight out from the habitat, is the most efficient configuration for attaching an RTG array to a manned spacecraft.

Each GPHS RTG can be relied on for 315W of electrical power. If the power requirements of the habitat are 5000W, this can be met by 16 RTG’s. These can be arranged in 4 linear arrays of 4, each supported by its own framework and in excess of 4m long.

It is also worth noting that this array of 16 RTG’s, with an efficiency of 3% to 4%, will put out in excess of 10000W of heat for every 315W of electricity produced. If some power loss is acceptable, this >160000W of heat produced by the array can also be tapped for heating purposes.