Tag Archives: Los Alamos National Lab

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:

  • Idaho National Engineering Lab (INEL):
    • Store the Neptunium dioxide (NpO2) feed stock
    • Deliver feed stock as needed to ORNL
    • Irradiate targets provided by ORNL in the Advanced Test Reactor (ATR)
    • Return irradiated targets to ORNL for processing
  • Oak Ridge National Lab (ORNL):
    • Manufacture targets
    • Ship some targets to INEL for irradiation
    • Irradiate the remaining targets in the High Flux Isotope Reactor (HFIR)
    • Process all irradiated targets to recover and purify Pu-238
    • Convert Pu-238 to oxide and deliver as needed to LANL
  • Los Alamos National Lab (LANL):
    • Manufacture the Pu-238 fuel pellets for use in RTGs

Pu-238 production process

Source: Ralph L McNutt, Jr, Johns Hopkins University APL, 2014

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.

Since 2010, NASA’s RTG for spacecraft missions has been the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which It is based on the SNAP-19 RTG flown on the two Viking Mars landers (circa 1975) and the Pioneer 10 and 11 deep space probes (circa 1972). At beginning of life, the current MMRTG can provide about 2,000 watts of thermal power and 110 watts of electrical power from eight General Purpose Heat Source (GPHS) modules that contain a total of 10.6 pounds (4.8 kilograms) of plutonium dioxide fuel. Electric conversion efficiency is about 6%.

Assembled MMRTG on a transport dolly.  Source: NASA

MMRTG cut-away diagram.  Source: NASA

You’ll find a NASA MMRTG Fact Sheet here:  https://rps.nasa.gov/resources/86/multi-mission-radioisotope-thermoelectric-generator-mmrtg/?category=fact_sheets

NASA had a program to develop an Advanced Stirling Radioisotope Generator (ASRG), which was designed to produce about four times the power of the MMRTG per unit of Pu-238. Electric conversion efficiency was about 26%. The ASRG required a total of 2.7 pounds (1.2 kilograms) of plutonium dioxide in two GPHS modules. However, the ASRG would produce less waste heat, which can be used productively to warm electronics in the interior of a spacecraft, such as the Mars rover Curiosity.  In November 2013, NASA announced that ASRG development had been discontinued because of budget cuts. You’ll find a NASA ASRG Fact Sheet at the following link:  https://rps.nasa.gov/resources/65/advanced-stirling-radioisotope-generator-asrg/

You can read a history of RTGs and more information on current U.S. Pu-238 production status in a 2014 presentation by Ralph L McNutt, Jr, at the following link: https://www.lpi.usra.edu/sbag/meetings/jan2014/presentations/08_1545_McNutt_Pu238_SBAG.pdf

9 February 2016 Update:

On 22 December 2015, DOE reported the first U.S. production in nearly 30 years of Pu-238.   This production demonstration, which was partially funded by the NASA, was performed at ORNL and yielded 50 grams of Pu-238.  The last U.S. production of Pu-238 occurred in the late 1980s at the Savannah River Plant in South Carolina.

DOE reported that it plans to set an initial production target of 300 – 400 grams (about 12 ounces) of Pu-238 per year. After implementing greater automation and scaling up the process, ORNL expects to reach the the production target of 1.5 kg (3.3 lb) Pu-238 per year.

The next NASA mission that will use an RTG is the Mars 2020 rover, which will use an MMRTG,  as used on NASA’s Mars rover Curiosity. 

You can read the ORNL announcement of initial Pu-238 production at the following link: https://www.ornl.gov/news/ornl-achieves-milestone-plutonium-238-sample

3 January 2019 Update:

In the past three years, ORNL has made scant progress in producing Pu-238.  In a 13 December 2018 article, “NASA Doesn’t Have Enough Nuclear Fuel For Its Deep Space Missions,”author Ethan Siegel reports:  “Although current production (at ORNL) yields only a few hundred grams per year (less than a pound), the laboratory has the eventual goal of ramping up to 1.5 kilograms (3.3 pounds) per year by 2023, at the earliest.  Ontario Power Generation in Canada has also begun producing Pu-238, with the goal of using it as a supplemental source for NASA.”  You can read the complete article on the Forbes website at the following link: https://www.forbes.com/sites/startswithabang/2018/12/13/nasa-doesnt-have-enough-nuclear-fuel-for-its-deep-space-missions/#1a73d47e1c18

The Canadian plans for becoming a source of Pu-238 was announced on 1 March 2017:  “Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, are participating in a project to produce isotopes in support of deep space exploration. Under the agreement, OPG would help create isotopes at the Darlington nuclear station east of Toronto that will help power space probes.” You can read the complete OPG press release here: https://www.opg.com/news-and-media/news-releases/Documents/20170301_DeepSpace.pdf

Also see the OPG public relations piece, “OPG looks to the stars,”  here: https://www.opg.com/news-and-media/our-stories/Documents/20170802_OPG_Deep_Space.pdf

4 August 2020 Update:

The NASA Mars rover, Perseverance, was launched from Cape Canaveral on 30 July 2020, with an expected landing date of 18 February 2021 in the Jezero crater on Mars. Once on the surface, Perseverance will be powered by an MMRTG.

The Pu-238 and some other special materials for the Perseverence MMRTG were produced in the U.S. at ORNL, as described in the following short (2:03 minutes) video, “ORNL-produced tech fuels NASA’s Perseverance mission to Mars”:

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-mars

At 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.

For more information:

Scientists Used Natural Cosmic Radiation to Peer Inside Fukushima’s Mangled Reactor

Peter Lobner, updated 4 March 2023


A muon is an unstable elementary subatomic particle in the same class as an electron (they’re both leptons), but with a much greater mass (207 times greater).  A useful property of muons is that they can penetrate matter much further than X-rays with the added benefit of causing essentially zero damage to the matter it passes through. Muons scatter and lose energy as they pass through matter, slowing down and eventually decaying, typically into three particles: an electron and two types of neutrinos. The higher the average density of the matter encountered along the muon’s flight path, the more quickly the muon slows down.

Muons are created by the interaction of high-energy cosmic rays with the upper regions of Earth’s atmosphere and they account for much of the cosmic radiation that reaches the Earth’s surface. This means that the existing flux of muon radiation at the Earth’s surface (about 10,000 muons/square meter/sec) is a free resource for clever researchers.

Muon tomography uses this free muon flux and the muon’s characteristic of slowing down more quickly in denser matter to create a density map of the field-of-view available to a muon detector.  There are two types of muon imaging, transmission and scattering. The differences are addressed in LA-UR-15-24802, listed below. In both types of muon imaging, denser objects and structures in the detectors field of view appear as shadows (muon shadows) that are darker (fewer muons getting thru to the detectors) than less dense areas.

Muon tomography at the Fukushima Nuclear Power Plant

Tokyo Electric Power Company (TEPCO) supported the use of muon tomography at the Fukushima Nuclear Power Plant to help determine what damage was done to the reactor cores at Units 1, 2 and 3 during the 11 March 2011 accident, which was precipitated by a 9.0 magnitude earthquake followed by a 15-meter (49.2-ft) tsunami.

A 2013 muon tomography feasibility study (Hauro Miyadera, et al.) reported: “Muon scattering imaging has high sensitivity for detecting uranium fuel and debris even through thick concrete walls and a reactor pressure vessel. Technical demonstrations using a reactor mockup, a detector radiation test at Fukushima Daiichi, and simulation studies have been carried out. These studies establish feasibility for the reactor imaging. A few months of measurement will reveal the spatial distribution of the reactor fuel.”

At Reactor #1, two 22 ton (20 metric ton), 21-foot by 21-foot (6.4 m by 6.4 m) muon detectors were installed and used to collect data over periods of months to develop high-resolution images of the damaged reactor core and surrounding areas. Placement of the muon detectors and the general scan geometry is shown in the following diagram.

Fukushima muon tomography setup 

Source: LA-UR-12-20494

Reactor #1 muon scan results

In March 2015, TEPCO announced that its muon tomography scanning efforts at Fukushima were successful, and confirmed that the nuclear plant’s Reactor #1 suffered a complete meltdown. The muon scans showed no corium (i.e., the lava-like product of a reactor core meltdown containing the melted nuclear fuel, fission products, control rods, and structural materials) remained in the reactor pressure vessel (RPV). The muon scans did not show the distribution of the corium that flowed out of the bottom of the reactor vessel into the primary containment vessel (PCV).

Muon tomography scan of Reactor #1. The corium, if present in the 
RPV, should have been visible as a dark shadow inside the RPV. 
Source: TEPCO via ExtremeTech (2015)
Muon tomography scan of Reactor #1, focusing on the RPV. 
Source: TEPCO via ExtremeTech (2015)

Reactor #2 muon scan results

World Nuclear News (WNN) reported (2016 & 2017), “TEPCO said analysis of muon examinations of the fuel debris shows that most of the fuel has melted and dropped from its original position within the core (and resolidified)…..Measurements taken between March and July 2016 at unit 2 showed high-density materials, considered to be fuel debris, in the lower area of the RPV.”

A muon tomography image of Reactor #2. 
Source: TEPCO via WNN (2016)

Reactor #3 muon scan results

In 2017, WNN reported, “Some of the fuel in the damaged unit 3 of the Fukushima Daiichi plant has melted and dropped into the primary containment vessel, initial results from using a muon detection system indicate. Part of the fuel, however, is believed to remain in the reactor pressure vessel.”

Muon tomography image of Reactor #3. 
Source: TEPCO via WNN (2017)

Summary of fuel debris status at Fukushima 

Based on the results of the muon tomography program and other means of investigation, TEPCO created the following graphic summary showing the estimated distribution of core and containment vessel fuel debris in Fukushima Units 1, 2 & 3.

Source: TEPCO

For more information