Plutonium-238

Plutonium-238
Plutonium-238 oxide pellet glowing from its own heat Full table
General
Name, symbol	Plutonium-238,238Pu
Neutrons	144
Protons	94
Nuclide data
Half-life	87.7 years
Parent isotopes	242Cm (α) 238Np (β−) 238Am (β+)
Decay products	234U
Isotope mass	238.049553 u
Spin	0
Decay mode	Decay energy
Alpha decay	5.593 MeV

Plutonium-238 (also known as Pu-238 or ²³⁸Pu) is a radioactive isotope of plutonium that has a half-life of 87.7 years.

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.

History

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.

Production

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^[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. Significant amounts of pure Pu-238 could also be produced in a thorium fuel cycle.^[2]

Applications

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.^[3]

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.^[4]

United States supply

The United States stopped producing bulk plutonium-238 in 1988;^[5] 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.^[6][7]

The United States Pu-238 inventory supports both NASA (civil space) and other national security applications.^[8] The Department of Energy maintains separate inventory accounts for the two categories. As of March 2015, a total of 35 kg of Pu-238 was available for civil space uses.^[8] Out of the 35 kg inventory, 17 kg remains in good enough condition to meet NASA specifications for power delivery; it is this pool of Pu-238 that will be used to fabricate 1 multi-mission radioisotope thermoelectric generator (MMRTG) for the 2020 Mars Rover mission and 2 additional MMRTGs for a notional 2024 NASA mission.^[8] 21 kg will remain after that, with approximately 4 kg just barely meeting the NASA specification.^[8] This 21 kg can be brought up to NASA specifications if it is blended with a smaller amount of newly produced Pu-238 having a higher energy density.^[8]

Sustained year-to-year funding maintains the production infrastructure and knowledge base in order to avoid significant recapture costs.^[8] Approximately $50 million per year, formerly funded by Department of Energy, was transitioned to a full cost recovery model as part of the FY 2014 federal budget.^[8] NASA has also provided additional funding to refurbish critical equipment at LANL.^[8] Department of Energy manages the operation of its nuclear facilities in order to ensure nuclear safety/security, to meet mission needs, and to obtain synergies with programs.^[8] A project to re-establish Pu-238 production capability has a total estimated cost range of $85-$125 million over 9 years, but actual project costs are likely to increase since available funding has not supported the planned pace, thus drawing out the schedule.^[8]

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.^[9] 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.^[7] 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".^[10] After production has been restarted it is predicted that it would take at least five years to get enough for a single spacecraft mission.^[11] 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.^[12] 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.^[12][4]

On February 20, 2015, a U.S. Department of Energy official reported that there is enough plutonium the U.S. stockpile (12 kg) to fuel three more MMRTGs like the one used by the Curiosity rover.^[13] One is already committed to the Mars 2020 rover,^[13] and the other two could be available by late 2021.^[13]

See also

• Atomic battery
• Plutonium-239

References

External links

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