Building Curiosity’s nuclear power source at Idaho National Laboratory

I have been fascinated by radioisotope thermal generators (RTGs), aka nuclear batteries, ever since I saw a display at the Maryland Science Center in Baltimore’s Inner Harbor sometime in the early 1990s.

In that energy exhibit, there was a tiny RTG that was designed to power a cardiac pacemaker. What impressed me the most was the fact that the battery could produce continuous power for an incredible period of time – after 10 years it would still be producing 89% of the current that it produced when new. After 87 years of continuous output, the battery would still produce 50% of the current of a new battery.

These long-life nuclear batteries are not only energy dense, but they are also power dense – the current output per unit mass is comparable to that of a lithium ion battery, but they last far longer. I can just imagine what it would be like to have a laptop whose battery never needed to be recharged.

Similar technology is what enabled the Voyager spacecraft to head out into the solar system to take pictures and send them back to Earth. Even in a place where the sun is a distant dot that produces virtually no power and where journeys are measured in decades, engineers had a reliable power source available – the heat produced by radioactive decay of a relatively long lived isotope like plutonium 238.

Until very recently, the last major use of Pu-238 powered RTGs for a space exploration mission was the Cassini voyage. On Saturday, November 26, NASA launched a Mars exploration mission carrying a vehicle named Curiosity that is powered by a Pu-238 enabled battery. That device will provide power for reliable mobility and communications for as long as the vehicle can last.

The US does not currently have any capability left to manufacture the required material, largely thanks to the continuous pressure against anything nuclear by organized opposition. Pu-238 RTGs are anything but cheap sources of power, but I hope you can tell from the below video that a large portion of the cost is due to the extreme measures taken to ensure reliability in space AND to protect people from the largely imaginary risks.

There are other isotopes that are useful for nuclear batteries and other ways to turn the heat or radiation from the batteries into useful electricity. One of my favorites is strontium 90, an isotope that is abundantly produced in all fission power reactors, but which is currently treated as a troublesome waste product whose heat production adds to the complexity of used fuel storage.

For applications where there is a need for reliable, continuous power over long periods of time, radioactive decay should be on the list of options to consider. Unfortunately, opposition to the use of nuclear energy, the recycling of useful nuclear materials and irrational concerns about the effects of low level radiation has virtually eliminated this valuable technology.

It is way past time to reconsider that situation and take action to change it. Since effective action has not yet been taken, there is no time like the present to get started.

You can read more about Curiosity’s nuclear power source and the Idaho National Laboratory’s involvement in the manufacture and testing of the power source at Dan Yurman’s Idaho Samizdat. His article is titled NASA Mars vehicle will use nuclear power source. The INL web site is also a useful source of information about RTG production and is the place where I found the above video.

INL also has posted additional photos of Curiosity’s RTG power source on its flickr site. You might also want to visit the fact sheet on the Pu-238 battery and the reasons why it was the only available power source that could meet the mission requirements.

About Rod Adams

23 Responses to “Building Curiosity’s nuclear power source at Idaho National Laboratory”

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  1. DV82XL says:

    Strontium-90 has a shorter half-life, much lower power density then Plutonium-238 and produces gamma radiation. Plutonium-238 also has the lowest shielding requirements.

    Jim Adams, deputy director of planetary science at NASA, says that there’s enough of the fuel for NASA missions to around 2022. He says if NASA doesn’t get more after that, “then we won’t go beyond Mars anymore. We won’t be exploring the solar system beyond the asteroid belt.”

    • Rod Adams says:

      DV82XL – you are correct that Pu-238 has characteristics that make it the best choice for space missions that are constrained by weight and heat rejection capability.

      However, certain land based applications for RTGs have no such concerns.

      • Jagdish says:

        Sr-90 and the Cs isotopes produce the most heat in the used fuel. They must be utilized to publicize the fact that even fission products are useful. These isotopes are also available in large quantities during reprocessing. There are tens of thousands of TV/Internet towers away from habitation and power grids where they can be usefully employed. They can be buried underground for shielding.
        Radiation is less of a problem in the unmanned space missions. Even the shorter half-life of 30 years would be useful.

  2. James Greenidge says:

    To this day it amazes me how little NASA hawks nuclear power in its spacecraft as opposed using solar cells! Nuclear is hardly EVER mentioned for Cassini, Galileo, Viking or the Pluto probe, but NASA’s falling all over themselves clucking that the twin rovers and Juno are solar powered! It’s like nukes were some kind of necessary evil! On NASA TV they had a MSL Twitter gathering with mission staff speakers and the RTG guy sounded almost APOLOGETIC that they had to use nukes! I gleaned from SpaceWorld that engineers with the Juno Jupiter probe balked at having solar cells used that far out because NASA at that conception period was in a massive PR binge to save its funding by appearing as “green” and PC friendly to a low science-amplitude public. We’re going to need nuclear reactors in space eventually for reliable deep space manned missions anyway so I wish NASA we’re so skittish about the atom tainting their squeaky-clean (at least to school kids) image.

    We’re coming up on the anniversary of the first nuclear reactor activation in Chicago back on Dec 2, 1942. The TRUE birthday of nuclear fission, NOT as a terrible explosion in a desert as the media and anti-nukers portray it. Any retrospectives possible?

    James Greenidge
    Queens NY

  3. Cal Abel says:

    Not only is 2 December the anniversary for CP-1 it is also the anniversary for Shippingport Criticality on 2 December 1957 at 04:30.

    It is my favorite day of the year. My kids look forward to Christmas. I look forward to Critmas.

    Merry Critmas!! And happy neutrons to all!

    The dawn of the nuclear era is as it should be in a time that was a historical celebration of the winter solstice. Where the beginning of the cold and hard winter brought a promise of hardship and privation, even in this modern age. I think it is appropriate to celebrate the birth of nuclear power as an end of suffering. This is why I look forward to December 2 more than any other day of the year.

  4. Daniel says:

    All those wastes like they say that propel us into space and cure us on this planet. Go figure.

  5. donb says:

    On one of the TV network news programs, they call the radioisotope thermal generator a nuclear reactor. Sigh…..

  6. Jerry says:

    NASA has purchases the entire amount Plutonium-238 for this spacecraft from Russia. So long as there is a demand for it, someone will be there to supply it.

    • DV82XL says:

      Pu-238 is no longer being produced in Russia (or anywhere else), and there is not a substantial amount of 238Pu left in Russia (or anywhere else) available to meet NASA’s needs, beyond that which Russia has already agreed to sell to the United States. Purchasing Pu-238 was intended as a stopgap measure until U.S. production was reestablished, and continued procurement from Russia cannot serve as a long-term solution to U.S. needs unless Russia itself reestablishes a 238Pu production capability. Such a move would require a major investment in Russian production facilities—an investment that Russia seems unlikely to make unless the United States pays for it.

      Restarting production of Pu-238 in Russia would take longer than restarting domestic production because of the long time it would take to negotiate an agreement with Russia and to complete the National Environmental Policy Act (NEPA, 1970) process, which would apply to Russian production of 238Pu if it were funded by the U.S. government. Based on prior experience, it would probably take 2 or 3 years just to negotiate and finalize an agreement with Russia before work could begin. In addition, Pu-238 obtained from Russia can be used only for civil applications and cannot be used to satisfy U.S. national security applications, should they arise. Russia has agreed to sell 238Pu to the United States with the limitation that it be used only for peaceful space missions, and that same stipulation would presumably apply to future purchases.


      • Daniel says:


        There are however a whole bunch of Plutonium based war heads in Russia.

        The megaton to megawatt program only target Uranium based bombs.

        So there is plenty of supplies still.

        • DV82XL says:

          Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, RTG’s use Plutonium-238. They are not interchangeable, nor are they made by the same process.

          Pu-239 is normally created in nuclear reactors by transmutation of individual atoms of one of the isotopes of uranium present in the fuel rods. Occasionally, when an atom of U-238 is exposed to neutron radiation, its nucleus will capture a neutron, changing it to U-239. This happens more easily with lower Kinetic Energy (as U-238 fission activation is 6.6MeV). The U-239 then rapidly undergoes two beta decays. After the 238U absorbs a neutron to become U-239 it then emits an electron and an anti-neutrino by β− decay to become Neptunium-239 and then emits another electron and anti-neutrino by a second β− decay to become Pu-239.

          Pure Pu-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-238.

        • Daniel says:

          I thought you needed some 238 to stabilize the 239 in bombs.

        • DV82XL says:

          Gallium, aluminium, americium, scandium and cerium are used to stabilize the δ phase of Pu-239. Pu-238 is considered to be a problematic isotope in nuclear weapon pits and is excluded along with Pu-240 from Supergrade plutonium

  7. Barry Sheridan says:

    A very interesting look at the engineering of an RTG. Thanks for this. Barry (UK reader)

  8. Jason Kobos says:

    According to the NASA plan if they had to go the alternative and use a solar array instead of the bigger RTG they would still have to put radioisotope heaters on the lander.

  9. Moebius says:

    It might be of some interest to readers here to note that Polaris Books have released a three book series this year by David Buden on radioisotope, nuclear thermal and fission electric space nuclear power systems. Available at Amazon. Might be worth checking out.

  10. Joel Riddle says:

    This comment string would be incomplete without some mention of the plutonium-powered pacemakers that had RTGs.

  11. Rod Adams says:

    @Joel – thanks for the comment, but I did mention the Pu-238 pacemakers in the original post. My initial interest in nuclear batteries was inspired by learning about the pacemaker batteries that were powered by 1/200th of an ounce of Pu-238.

  12. Sebastien says:

    Hello Rod,

    I didn’t know Atomic Energy was such an issue in America, I guess that’s why I was so surprised to learn RTGs have been used for years (NASA documents said they were used in Apollo missions too).
    In France, we are all in nuclear energy (until recently), so I think the reactions would have been different.
    I wrote a page on curiosity’s nuclear battery:
    Please tell me if there are any mistakes. I tried to explain how it works in simple terms.

  13. thammel says:

    Rod, since you saw the cardiac pacemakers at the MD science center, you might be interested to know that the RTG for the MSL mission, MMRTG, was made in Maryland but only fueled at INL. Teledyne Energy Systems actually made the generator and all the components that convert the heat directly to electricity. I’ve had the good fortune to work on this project and be involved with this technology for many years. As a result, I travelled to FL to watch the launch of MSL last November. The safety effort and analyses that are required make these generators extremely safe.