https://lynceans.org/all-posts/radioisotope-thermoelectric-generators-rtg-for-spacecraft-history-and-current-u-s-pu-238-production-status/Updated 5 March 2021
Peter Lobner
Radioisotope Thermoelectric Generators (RTG), also called Radioisotope Power Systems (RTS), commonly use non-weapons grade Plutonium 238 (Pu-238) to generate electric power and heat for National Aeronautics and Space Administration (NASA) spacecraft when solar energy and batteries are not adequate for the intended mission. In comparison to other RTG heat sources (Strontium-90, Cesium-137), Pu-238 has a relatively long half-life of 87.75 years, which is a desirable property for a long-life RTG.
Approximately 300 kg (661 lb) of Pu-238 was produced by the Department of Energy (DOE) at the Savannah River Site between 1959 – 1988. After U.S production stopped, the U.S. purchased Pu-238 from Russia until that source of supply ended in 2010.
Limited production of new Pu-238 in the U.S re-started in 2013 using the process shown below. This effort is partially funded by NASA. Eventually, production capacity will be about 1.5 kg (3.3 lb) Pu-238 per year. The roles of the DOE national laboratories involved in this production process are as follows:
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In 2015, the U.S. had an existing inventory of about 35 kg (77 lb) of Pu-238 of various ages. About half was young enough to meet the power specifications of planned NASA spacecraft. The remaining stock was more than 20 years old, has decayed significantly since it was produced, and did not meet specifications. The existing inventory will be blended with newly produced Pu-238 to extend the usable inventory. To get the energy density needed for space missions while extending the supply of Pu-238, DOE and NASA plan to blend “old” Pu-238 with newly produced Pu-238 in 2:1 proportions.
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In a 20 July 2020 news release, ORNL provided more information on the U.S. production process for Pu-238 and reported that, “the lab has been consistently increasing its Pu-238 production capabilities, aiming to produce 1.5 kilograms per year by 2026.” You can read this ORNL press release here:
https://www.ornl.gov/news/ornl-produced-plutonium-238-help-power-perseverance-marsAt the planned U.S. production rate for Pu-238, NASA should be able to conduct an MMRTG mission at about four-year intervals. If NASA MMRTG missions will be more frequent than this, the U.S. will need to purchase additional Pu-238 from another source, perhaps Canada.
5 March 2021 Update:
The Perseverance rover landed on Mars on 18 February 2021, in the planned target area in Jezero Crater. Power from the MMRTG was nominal after landing. Perseverance will spend at least one Mars year (two Earth years) exploring the landing site region.
The next NASA mission with an MMRTG-powered spacecraft is the Dragonfly mission to Saturn’s moon Titan, which will launch in 2026 and arrive on Titan in 2034.
The Voyager 1 and 2 spacecraft were launched in 1977, each with three RTGs delivering a maximum of 470 watts of electrical power at the beginning of the mission. Both spacecraft have left the solar system (Voyager 1 in 2013 and Voyager 2 in 2018) and continue to transmit from interstellar space in 2021 with their RTGs operating at a reduced power level of about 331 watts after 44 years of Pu-238 decay during the mission. NASA plans to continue the Voyager missions until at least 2025.
