Converting heat into electricity without moving parts

by Rod Adams

I’ve been fascinated by nuclear batteries—also known as radioisotope thermal generators or RTGs—since I first saw a pacemaker battery in an exhibit at the Maryland Science Museum. As my wife and children will testify, I am one of those slow moving people at museums who insists on reading nearly every placard under every exhibit. I’ll never forget my feeling of wonder as I found out that 1/200th of an ounce of plutonium-238 could provide sufficient heat to that tiny battery to make it produce a continuous electrical current sufficient to run a heart pacemaker.

Since an isotope with an 87 year half life will still be producing about 90 percent of its initial heat after decaying for 14 years, those batteries were designed to be able to supply a heart patient for as long as they would be expected to survive. They did not need for an external power supply—with its attendant infection risks and they did not require additional surgeries to replace chemical batteries.

That museum experience fired my curiosity and encouraged me to find out more about the technology that allows the direct conversion of heat into direct current power. I found out that plutonium-238 was not the only possible isotope. There is a whole list of possibilities depending on the specific applications and measures of effectiveness.

I learned that pacemaker batteries were only one of several applications where a small amount of long-lived, reliable power was valuable enough to provide a positive cost benefit. Other applications include remote lighthouse power, navigational buoys, satellites, deep space probes, and communications facilities in remote locations.

I also learned that the batteries had a substantial body of experience that showed that they were dependable and needed little maintenance.

For a variety of reasons, including the sustained campaign against all things nuclear, RTG technology is virtually unknown and nearly all efforts to use nuclear batteries have been abandoned. About the only time that the topic gains much attention is when the leaders of a high-profile space program like Cassini or the Curiosity tell the public that their mission has been enabled by one or more RTGs.

The below video offers a detailed look at the construction and testing performed for the 110 watt RTG that is continuously charging the chemical batteries that supply more concentrated surges of power to the Curiosity rover.

There is one potential application of RTG technology that continues to intrigue me, especially as I learn more about the challenges that faced the operators at the Fukushima Daiichi nuclear power station as they gradually depleted all of their available power supplies.

Though RTGs are not a huge source of concentrated power, they provide a steady stream of current that can recharge chemical storage batteries. A 100 W RTG, for example, provides 2.4 kilowatt-hours per day, but there would be no need to limit the inventory to just a single unit. There is also no need to compete with space applications for plutonium-238; shielding weight is not a major consideration in a large nuclear power facility. The most abundantly available isotope is strontium-90, an isotope that commercial nuclear reactors produce in large quantities.

Of course, that isotope is not readily available in the United States since we are not recycling our used nuclear fuel, but I would imagine that it could be purchased from the French, the Japanese, or the Russians.

RTGs, or their Stirling engine heat to power cousins, operate on a completely different principle than a diesel engine and do not need any external support to keep doing their job.

Though they would need only infrequent attention, there are plenty of nuclear-trained people available to ensure that emergency power RTGs would not run into some of the same difficulties that have given the technology a bad name in remote areas of the former Soviet Union.

I am sure that there are obstacles to overcome, and some of them may even be show stoppers, but just imagine how comforting it would be to know that a sustained, complete loss of electricity is virtually impossible at a facility with a few RTGs installed in the power system.



Rod Adams is a nuclear advocate with extensive small nuclear plant operating experience. Adams is a former engineer officer, USS Von Steuben. He is the host and producer of The Atomic Show Podcast. Adams has been an ANS member since 2005. He writes about nuclear technology at his own blog, Atomic Insights.

17 thoughts on “Converting heat into electricity without moving parts

  1. DV82XL

    This is so true, and your observation also applies to lose and misplaced medical sources as well. Why these have gotten a free pass beyond a bit of media outrage at each incident (which quickly passes) is a mystery to me.

    Having said that the depraved indifference that the Russians have shown to their RTGs in the Artic is criminal so I suppose we should be grateful that these are not getting too much notice.

  2. Reese

    Yet another is the improvement of direct conversion beta cells that includes self-healing (from radiation damage) icosahedral boride semiconductors. The idea is to greatly improve efficiency over RTGs or your phosphor sandwich by skipping the intermediate step. Search for United States Patent 6479919. (Darn– I was set to work on that in September 2011 when I had to go back to a job where I was “needed” more.)

  3. Rod Adams

    Joel – you are sort of correct. The Chernobyl accident happened as a result of a very poorly planned and executed attempt to see if the heat available in the reactor after shutdown could be drawn off for a short period of time to supply steam to a turbine to allow time for the grid to transfer to another power source.

    There are a huge number of details about the experiment and the added complexities imposed by the actual grid conditions, the lack of proper briefing and the incredibly stupid decision to disable all safety systems that tell me there are lessons to be learned, but the take away is not “never do anything with any similarity ever again.”

    I happen to know that it is possible to safely use the thermal energy stored in a sub-critical reactor and the associated coolant system to provide some emergency power for a little while.

    With regard to plugging in to supply power – that is simple if there is a place to connect. It is darned near impossible if there is not or if the loads are designed for 50 cycle instead of the 60 cycle that US designed electrical systems use.

    One of the complicating factors at Fukushima was that about 50% of Japan is 60 cycle and about 50% is 50 cycle. Apparently, during the confusion after the event, generators producing the wrong frequency arrived on site.

  4. Mark

    Good article.

    May I just add that whilst many anti -nuclear peeps worry about nuclear power plants etc ( and recent events in Japan etc), it is the misuse (careless control) of Sr-90 based RTGs that have probably actually done more physical damage to people. I am not suggesting they should not be used – far from it. However, the focus on the control of High Activity Sealed Sources (HASS) should be higher up the anti’s agenda rather than the tiny risks posed by nuclear energy.


  5. CK

    You have to watch for shielding of gamma radiations coming from other numerous radioisotpes if you want to harness power directly from spent fuel rods.

  6. James Greenidge

    The Michael Crichton book/movie “The Terminal Man” featured a surgically implanted plutonium powered brain-pacemaker computer. I also recall seeing in our family’s old World Book encyclopedia (1964 c.) pictures of a shirt button-sized nuclear battery made in the form of a “sandwich” with alpha emitting layers emitting current. I THINK Bulova was also interested in this for its cool new Accutron watches.

    James Greenidge
    Queens NY

  7. Joel Riddle

    Jeff S.,
    I may be mistaken on this, but if not, I think the accident at Chernobyl actually occurred during a safety test to see if it was possible to utilize power from the main generator to supply the reactor’s safety systems.

    Can someone that was older than 1.8 years old in April of 1986 correct/clarify me on that?

    Somewhat on the subject of using RTGs for backup power, while the Fukushima incident was unfolding and it was clear that not having AC power was their primary problem, I had the thought that it would be a great story if a U.S. Naval submarine could pull up just off-shore and plug into Fukushima’s emergency (Class 1E) power supplies. That would have made for a heroic story.

  8. donb

    The reason that micro-RTGs aren’t used for pacemakers more often is that the patient might develop cancer after a few thousand years ;-).

  9. Jeff S

    Why not just use the decay heat from the reactor core itself, to power the Thermal Generators? It’s ridiculous to me that a nuclear power plant can destroy itself because it has *too much heat* but at the same time, can’t cool itself, can’t run sensors and controls systems – because it doesn’t have “enough energy”.

    Use the decay heat in the fuel itself to keep the plant’s emergency operations running during an emergency.

  10. Joffan

    Reprocessing… might not even be necessary. For a LWR nuclear power plant, why not just have a RT generator that works off a spent fuel rod or several? It could even work from inside the spent fuel pool somewhere, changing out the power-generating spent fuel rods after each refueling cycle. Perhaps the freshest rods wouldn’t be suitable, but older rods could give a fairly steady heat for a couple of years.

    The regulators would require a lot of convincing, no doubt, but provided the rods were contained and assessed to stay below (say) 300C, I don’t see any great risk. The heat quality would be lower than the space RTGs but volume is not really a consideration.

    If anyone knows why this wouldn’t work – I’m all ears.

  11. John Englert

    Another example of a nuclear battery is to make a promethium-147, phosphor, and photoelectric sandwich. The beta particles strike the phosphor which emit light that then generate electric power. My World Book encyclopedia (ca. 1972) has a picture under atomic power.

  12. Suzy Hobbs

    Thanks Rod! My husband and I were wondering about this technology over dinner last night- it does seem to be an underutilized tool, with many more potential applications.

  13. DV82XL

    The failure to develop this technology due to antinuclear interference is one of the greatest tragedies of the nuclear age. Few understand that the plutonium isotope being used is not suitable for nuclear weapons nor is it produced by the same process.

  14. Mark Miller

    …and the ONLY reason that the far-superior micro-RTG isn’t still used in human heart pacemakers is that the physician who INSTALLED the pacemaker in the first place is forever liable to recover the radioactive source when the patient finally dies (rather than letting it be buried with him/her). After all, that 1/200th of an ounce of plutonium-238 could expose the public to FAR MORE DEADLY RADIATION (NOT!) than nuclear medicine patients that are routinely sent home with signifiant (though “acceptable”) quantities of short-lived radioactivity in their bodies. Oh well….

  15. Purgatus

    Fuel cells are also being deployed as emergency backup power solutions. I would be interested in thinking about the pros and cons of each of these options. One of the main drawbacks I can think of for the RTG is that it would be a significant waste of its capacity to have it sitting unused, generating power, 99.9% of the time.

  16. Ruwan Ratnayake

    Enjoyed reading (with my morning cup of coffee) – Wonderful article. Thanks Rod.