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Radioisotope thermoelectric generator

A radioisotope thermoelectric generator (RTG) is a very simple electrical generator which obtains its power from passive radioactive decay. Such a generator uses the fact that radioactive materials (such as plutonium-238) generate heat as they decay into non-radioactive materials. The heat used is converted into electricity by an array of thermocouples which then power a lighthouse or an interplanetary space probe.

Design

The design of an RTG is very simple (by the standards of nuclear technology). It consists of a sturdy container full of a radioactive material. The walls of this container are pierced by thermocouples; the other end of each thermocouple is connected to a heat sink. Passive radioactive decay in the radioactive material causes it to produce heat, which flows through the thermocouples and out the heat sink, generating electricity in the process.

A thermocouple is a thermoelectric device that converts thermal energy directly into electrical energy. 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 to decay quickly enough to generate enough heat. Typical half lives for radioisotopes used in RTGs are a few decades. The most popular fuel for RTGs is plutonium-238, in the form of plutonium oxide (PuO2). This isotope of plutonium cannot be used to make a nuclear weapon although it is one of many radioactive sources that could be used to make a dirty bomb.

Use

The most common use is in spacecraft, with a total of 38 RTGs launched on a total of 22 launches. They are used on probes that will travel to a distance from the Sun where solar panels are not practical sources of electricity. As such they are carried on Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses and Cassini. As well as this, the two Viking landers used RTGs for power. They were also used to power 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.

RTGs have been used primarily to power space probes, but the Soviet Union did construct lighthouses powered by RTGs (see ). These pose environmental and security concerns, as they are unattended and leakage or theft of the radioactive material would pass unnoticed for years (or possibly forever; some of these lighthouses cannot be found because of poor record keeping).

The Soviets made little or no use of RTGs for space missions, though they did launch 33 nuclear-powered RORSATs (Radar Ocean Reconnaissance SATellites), which tracked the movements of NATO and US naval warships and would have directed Soviet antiship cruise missiles against these targets. However, the RORSATs were powered by small fission reactors, not RTGs. The RORSATs needed to be at low altitude to stay in radar range, and solar panels large enough to have handled their power needs would have dragged them out of orbit too quickly. The RORSATs normally ejected their reactor cores into high orbit at the end of the mission, but there were two accidents, one in 1978 and one in 1983, that resulted in the cores falling back to Earth.

Though they are no longer used, small "plutonium cells", essentially "micro-RTGs", have, in the past been used in implanted heart pacemakers to ensure a very long "battery life". 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 radioactive half-life of approximately 85 years, so RTGs using it lose a factor of <math>1 - \sqrt[85]{0.5}<math> 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.

Safety

RTGs use a very different technique from that used by nuclear power stations. Nuclear power stations generate power by a nuclear chain reaction in which the nuclear fission of an atom releases neutrons which cause other atoms to undergo fission. This allows rapid reaction of large numbers of atoms producing large amounts of heat, but 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 runaway scenario 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. However, the reactor can be extremely simple.

Most RTG designs are inherently immune to nuclear meltdown or other runaway problems. The only kind of problem they are subject to is radioactive contamination: if the container holding the radioactive fuel leaks, the radioactive fuel will contaminate the environment.

Concerns have been raised about the use of RTGs on space probes. The main concern is that if an accident were to occur during launch or a subsequent passage close to Earth, radioactive material could be released into the atmosphere and subsequently harm flora and fauna. So far there have only been three known accidents involving RTG powered spacecraft. The first two were launch failures involving Transit and Nimbus satellites. The third was the failure of the Apollo 13 mission, which meant that the Lunar Module which carried the RTG reentered the atmosphere and burnt up over Fiji. However 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.

See also

• Nuclear reactor
• Radioactive isotope
• Cassini-Huygens