|Fig. 1: Model of a space vehicle radioisotope thermoelectric generator (RTG). (Courtesy of NASA)|
Plutonium-238, or Pu-238, has long been an integral heat source in radioisotope thermoelectric generators, or RTGs (Fig. 1), which are used to power spacecraft. This isotope of plutonium was the first discovered, synthesized by Glenn Seaborg and his associates by bombarding U-238 with deuterons to make Np-238 - which then decayed to make Pu-238. Typically, however, Pu-238 is produced by irradiating Np-237. Unlike Pu-239, Pu-238 is not fissile and cannot be used in nuclear weapons.  Its sole use is the electricity generated by converting the heat from its natural decay. In the past few decades, the United States has watched its supply of Pu-238 dwindle. The United States produced the Pu-238 it needed from 1964 to 1988 at the Savannah River Site reactors. Since then it has purchased supplies from Russia. But now that Russia's supplies are also running out, the United States must come up with a solution to ensure that the technology that depends on Pu-238, particularly spacecraft, remains feasible.
Since the early Apollo missions, Pu-238 has been a critical part of spacecraft. In particular, the RTGs that are used as batteries depend on Pu-238. In space, batteries must be durable under extremely harsh conditions, and must be safe and maintenance-free, particularly where solar energy is not feasible. Pu-238 has long been favored for this use case because of its α decay process, which does not produce much gamma radiation. The heat that results from this process can be converted to electricity with no moving parts, which makes it ideal for spacecraft. In fact, twenty-seven US spacecraft over the past 50 years have used radioisotope power systems powered by Pu-238.  Even today, RTGs are still powering Voyager 1 and Voyager 2, the furthest man-made objects from Earth. The Pu-238 and the RTGs were supplied by the Department of Energy to NASA and other federal agencies.
Since the shutdown of the Savannah River Site reactor, Pu-238 has been procured from foreign countries and processing equipment residues. These supplies are not sufficient to power future scientific endeavors. Currently, the DOE maintains 35 kilograms of Pu-238, about half of which meets the power specifications for flight.  Unfortunately, this supply could be exhausted in the next decade, via the Mars 2020 and New Horizons 4 missions. After the supply is exhausted, NASA may need to delay missions until more Pu-238 is produced or acquired.
Unfortunately, the process of producing Pu-238 is complicated and expensive. Under its Supply Project, the DOE wants potential facilities that produce Pu-238 to be able to produce at least 1.5 kilograms each year, while also handling the production process, from irradiation to separation of the Pu-238.  Its goal is to reach this production rate by 2026, and in the interim be able to produce 300 to 500 grams per year by 2019. NASA currently provides the funds for this project, funding up to $50 million per year. NASA has also given the Center for Space Nuclear Research (CSNR) a NIAC (NASA Innovative Advanced Concepts) grant to explore alternatives to the DOE process. The CSNR study identified new technologies using existing facilities that could produce the amount required by NASA, while costing significantly less than the DOE's process. 
In 2013, Pu-238 was produced in the United States for the first time in 25 years at the Oak Ridge National Laboratory in Tennessee.  Although the site is only producing small amounts of Pu-238, it was the first step towards ensuring that future discoveries, particularly in space, would not be hindered.
© Katy Shi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration (National Academies Press, 2009).
 S. S. Oakley, "Space Exploration: Improved Planning and Communication Needed for Plutonium-238 and Radioisotope Power Systems Production," U.S. Government Accountability Office, GAO-18-161T, 4 Oct 17.
 "Summary of Plutonium-238 Production Alternatives Analysis Final Report," Idaho National Laboratory, INL/EXT-13-28846, March 2013.
 S. D. Howe et al., "Economical Production of Pu-238," Idaho National Laboratory, INL/CON-11-23900, February 2013.
 S. Ferro, "NASA Resumes Production of Plutonium-238 Space Fuel After 25 Years," Popular Science, 14 Mar 13.