Plutonium-238 oxide pellet glowing from its own heat
|Parent isotopes||242Cm (α)
|Isotope mass||238.049553 u|
|Decay mode||Decay energy|
|Alpha decay||5.593 MeV|
Plutonium-238 is a very powerful alpha emitter and – unlike other isotopes of plutonium – it does not emit significant amounts of other, more penetrating and thus more problematic radiation. This makes the plutonium-238 isotope suitable for usage in radioisotope thermoelectric generators (RTGs) and radioisotope heater units – one gram of plutonium-238 generates approximately 0.5 watts of thermal 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, creating neptunium-238, 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 one or two percent, but it may be responsible for much of the short-term decay heat because of its short half-life relative to other plutonium isotopes. Reactor-grade plutonium 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 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. Significant amounts of pure Pu-238 could also be produced in a thorium fuel cycle.
The main application of Pu-238 is as the heat source in radioisotope thermoelectric generators (RTGs).
RTG 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 pacemakers Medtronic manufactured, twenty-two were still in service more than twenty-five years later, a feat that no battery-powered pacemaker could achieve.
This same RTG power technology has been used in spacecraft such as Voyager 1 and 2, Cassini–Huygens and New Horizons, and in other devices, such as the Mars Science Laboratory, for long-term nuclear power generation.
United States supply
The United States stopped producing bulk plutonium-238 in 1988; since 1993, all of the plutonium-238 used in American spacecraft has been purchased from Russia. In total, 16.5 kilograms have been purchased but Russia is no longer producing plutonium-238 and their own supply is reportedly running low.
In 2009, the U.S. Department of Energy (DOE) requested funding to restart American domestic production. It is estimated that to restart production will cost between $75 million and $90 million over five years. Since the DOE would be responsible for producing the plutonium-238 for NASA, the two agencies want to split the cost of restarting production. Congress has given NASA some of the money requested, $10 million in 2011 and the same in 2012. The U.S. Congress denied the DOE's funding request for three years in a row. In 2013, it was agreed that NASA would provide all funding for the production of Pu-238.
Between 3.3 pounds (1.5 kg) and 4.4 pounds (2.0 kg) would be produced per year to support NASA's robotic science missions, although if future human missions require plutonium-238 then even more would need to be produced. The Advanced Test Reactor at the Idaho National Laboratory and the High Flux Isotope Reactor at the Oak Ridge National Laboratory are both seen as potential producers. About 15 kg per GWyr could be created in liquid fluoride thorium reactors (LFTRs).
Jim Adams, deputy director of planetary science at NASA, said that there is enough of the fuel for NASA missions until 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". After production has been restarted it is predicted that it would take at least five years to get enough for a single spacecraft mission. In February 2013, it was reported that a small amount of plutonium-238 was successfully produced by Oak Ridge's High Flux Isotope Reactor – this was the first time the United States had produced 238Pu since production ended in the late 1980s. Jim Green, head of NASA's planetary science division, stated in March 2013 that NASA expects to receive reports back from DOE later in 2013 on a complete schedule that would put plutonium-238 on track to be produced at about 1.5 kg (3.3 lb) per year.
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- Alexandra Witze, Nuclear power: Desperately seeking plutonium, NASA has 35 kg of 238Pu to power its deep-space missions - but that will not get it very far., Nature, 25 Nov 2014
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- Story of Seaborg's discovery of Pu-238, especially pages 34-35.
- NLM Hazardous Substances Databank – Plutonium, Radioactive
|Plutonium-238 is an
isotope of plutonium
|Decay product of: