Plutonium-238 glowing from its own heat

Plutonium-238, is a radioactive isotope of plutonium with a half-life of 87.7 years. Because it is a very powerful alpha emitter that does not emit significant amounts of other, more penetrating and thus more problematic radiation, this isotope is used for radioisotope thermoelectric generators (RTGs) and radioisotope heater units. One gram of plutonium-238 generates approximately 0.5 watts of power.

Plutonium-238 was the first isotope of plutonium to be discovered. It was synthesized by Glenn Seaborg and associates in 1941 by bombarding uranium-238 with deuterons. Neptunium-238 is made as an intermediate product, which then decays to form plutonium-238. Plutonium-238 decays to uranium-234 and then further along the radium series to lead-206.

Reactor-grade plutonium from spent nuclear fuel contains various isotopes of plutonium. Pu-238 makes up only a percent or two, but may be responsible for much of the short-term decay heat because of its short half-life. This is not useful for producing Pu-238 for RTGs because difficult isotopic separation would be needed.

Pure plutonium-238 is prepared by irradiation of neptunium-237, one of the minor actinides that can be recovered from spent nuclear fuel during reprocessing, or by the irradiation of americium^[1] in a reactor. In both cases, the targets are subjected to a chemical treatment, including dissolution in nitric acid to extract the plutonium-238. A 100 kg sample of light water reactor fuel that has been irradiated for three years contains only about 700 grams of neptunium-237, and the neptunium must be extracted selectively.

To produce plutonium-238 in a form less likely to undergo a chemical reaction and more safe for technological purposes near humans, the plutonium-238 isotope is reacted with water enriched with oxygen-16 to form ²³⁸Pu¹⁶O₂. This compound generates fewer neutrons because oxygen-16 has a much lower (α,n) transfer nuclear reaction rate than do other isotopes of oxygen. This technology was first developed by Los Alamos National Laboratory during the 1960s and 1970s to provide radioisotope thermoelectric generator power for cardiac pacemakers. Of the 250 plutonium-powered cardiac pacemakers Medtronic manufactured, twenty-two were still in service more than twenty-five years later, a feat that no battery-powered pacemaker could achieve.^[2] This same RTG power technology has been utilized in spacecraft such as Cassini–Huygens and New Horizons and other devices, such as the Mars Science Laboratory, for long-term nuclear power generation.

The United States currently has limited facilities to produce plutonium-238.^[3] Since 1993, all of the plutonium-238 the U.S. has used in space probes has been purchased from Russia. In total, 16.5 kilograms have been purchased.^[4] In 2009 the U.S. Department of Energy requested funding to restart domestic production, after production has been restarted it would take at least five years to get enough for a single spacecraft.^[5][6] As of late 2011, domestic production has yet to be restarted.^[6]

Jim Adams, deputy director of planetary science at NASA, says that there is enough of the fuel for NASA missions to around 2022. He says if NASA does not get more after that, "then we won't go beyond Mars anymore. We won't be exploring the solar system beyond Mars and the asteroid belt."^[6]

Len Dudzinski of NASA has said that American access to Plutonium-238 is the reason why it is the only country to have sent a science mission further than Mars, where solar power drops off rapidly.^[6]

Wade Caroll of DOE stated that by 2017 the first new batches of plutonium-238 might be available for NASA.^[7]

See also[link]

• Atomic battery

References[link]

External links[link]

• Story of Seaborg's discovery of Pu-238, especially page 34-35.
• NLM Hazardous Substances Databank – Plutonium, Radioactive