TAGS-85: Power Conversion System for the Outer Planets and Beyond!

Hello, and welcome back to Beyond NERVA! Today, we take a break from the Topaz International Program to cover a subject that we haven’t touched on at Beyond NERVA yet, and sadly the inspiration is due to the death of one of the true luminaries of astronuclear engineering, Dr. Emanuel “Emil” Skrabek, who passed on March 14th of progressive supranuclear palsy and Parkinson’s disease.

Dr. Skrabek

His obituary can be found here: http://www.ruckfuneral.com/obituary/emanuel-andrew-skrabek-phd . May his family have comfort in his passing, and may his memory be eternal. His legacy within astronuclear engineering, and the discoveries that his inventions enabled within (and outside) our solar system, certainly make him one of the true unsung engineering heroes in our race’s ability to reach out beyond our planet and learn about our own solar neighborhood.

Today, we are going to talk about the bread and butter of astronuclear engineering: the radioisotope thermoelectric generator, or RTG. From the dawn of spaceflight, these systems have provided simple, solid state power for missions of all types, from orbiters to landers to rovers, and have enabled incredible science to be done in the far-flung reaches of our solar system – and just recently, beyond.

MMRTG cutaway diagram with materials used in various components, image DOE/NASA

Simply put, RTGs use the natural radioactive decay of a radioisotope, or radioactive isotope of a material, to produce electricity through the use of a pair (or more, but mostly just two) of materials that, at the place that they meet, produce electricity – assuming that there’s a hot side (where you stick the radioisotope) and a cold side (a radiator). They’re insanely attractive to mission planners for quite a few reasons. First, they’re completely solid state, which means that there are no moving parts to break. Second, they’re well-characterized, meaning that the problems that they face, their effects on a spacecraft, their behavior during launch, and a slew of other factors are well-known. Third, they can provide a heat source for components in a spacecraft, meaning that the freezing cold of space isn’t going to cause mechanical seizing or electronics failure thanks to the waste heat from the generator. Finally, they’re a legacy design that remains incredibly relevant, meaning that we’ve been doing them for a long time but they’re still useful.

This is an incredibly well-documented and widely-used application for in-space nuclear power, so I’m working on a page with more details on this technology, but when it will be complete is still up in the air. Follow me either on FB or Twitter to get notified when it is available!

Radioisotope Thermoelectric Generators: The Fuel

238PuO2 fuel pellet, image DOE

Everything is radioactive, but some things are more radioactive than others, and all fall into a broad set of categories of “radioactivity.” For the purposes of a radioisotope thermoelectric generator, the best option for fuel is an isotope that only undergoes alpha decay (and preferably only decay once) so the fuel needs only minimal radiation shielding to protect the spacecraft from any radiation that could damage the spacecraft’s on-board electronics and other payload. Because of this, and because deep space missions have very long timelines. This led RTG designers to decide to use an isotope of Plutonium, 238Pu, for many of its’ deep space mission.

Fuel pellet after cladding, image DOE

238Pu is rare in that it only emits alpha radiation, meaning that the fuel pretty much shields itself. It also emits a useful amount of heat for either producing electricity or heat, and over a useful timeframe. There are many other options for radioisotope fuel for RTGs, but of the flown missions the vast majority have used 238Pu for fuel. We wont go into the other fuel types here, but I’m currently working on a page on RTGs which will go into the different options.

Unfortunately, due to a number of organizational and planning difficulties within NASA and the DOE, who supply the 238Pu oxide fuel, the radioisotope itself is becoming scarce. With less fuel available, and more powerful sensors needing support from more powerful transmitters to get all the information back to Earth, the increased use of electric propulsion, and other power requirements, the transition is almost being forced on NASA’s mission planners.

Fortunately, recent advances in the precursor “target” for fuel production (https://www.scientificamerican.com/article/new-supply-could-prevent-deep-space-plutonium-shortage/ ), as well as a widely-cited Government Accountability Office report (https://www.gao.gov/products/GAO-18-161T ), seem to offer cautious promise that the fuel supply will begin to flow again soon.

Radioisotope Thermoelectric Generators: The Heat Sink

As with pretty much all other known forms of thermal-to-electric heating, there needs to be a hot side of the system and a cold side. In smaller power units, this heat rejection system is simple: a set of simple metal fins secured to the metal exterior of the power unit. This casing shields the very minor amounts of radiation coming from the fuel during the decay process (if you’re interested about the radiation environment for the payload of an MMRTG, it can be found here: https://trs.jpl.nasa.gov/handle/2014/45778 ), and also provides micrometeorite protection for the power supply. Due to the low power involved, these simple systems are more than sufficient to cool the converter.

Another use for the waste heat is to provide heat to temperature-sensitive sensors and electronics. In fact, the RTG is only one application for a broader category of systems” Radioisotope Heating Units.” which can provide not only heat for components, and electric power for on-board systems, but even propulsion as well, in the form of a radioisotope thermal rocket. This last option isn’t available in current systems, but New Horizons successfully used its RTGs to power maneuvering thrusters in the outer solar system. If it’s able to be used somehow, waste heat isn’t waste yet.

Radioisotope Thermoelectric Generators: The Thermocouple

MMRTG thermocouple exploded diagram, image NASA

RTGs use the thermoelectric effect to convert heat to electricity. The thermoelectric effect occurs when there’s a difference in temperature across the junction of two different metals. This is more properly known as the “Seebeck effect,” after its second discoverer (it was first described by Volta in 1794): Thomas Johann Seebeck in 1821 after his independent rediscovery. The temperature range that the system would be exposed to determines the materials that are best for the particular converter. that is being used. The efficiency depends on a specialized material property known as the “Seebeck coefficient;” a higher Seebeck coefficient means a more efficient power conversion process, and more electricity is generated for a given temperature gradient. This coefficient depends on a number of things, including the microstructure and crystalline structure of the materials being used for each element of the converter, meaning that changing the alloy of the base materials used can have a significant effect on the performance of the converter.

This application was actually widely in use already as a type of sensor called a “thermocouple,” which sends a voltage based on its temperature within a certain range of environments and is one of the most common forms of temperature monitoring in many fields. Designers of early astronuclear systems, such as the designers of the SNAP-3 RTG (which flew for the first time in 1961) scaled this concept, turned it inside out, and placed it on a spacecraft. Many different types of material combinations have been experimented with over the years, mostly based around lead and tellurium, PbTe. Thin strips were alternated around the radius of the fuel canister, with heat being provided by conduction and radiation using wide radiator fins. Another common option is silicon-germanium, a better option at higher temperatures.

Today most designs use lead telluride (PbTe) doped with a special material that Dr. Skrabek invented with Donald Trimmer while working for Teledyne Energy Systems: TAGS-85. In TAGS, the tellurium used in the converter. is carefully doped with silver (Ag) and antimony (Sb), hence TAGS. The most commonly used version of TAGS, TAGS-85, uses (PbTe)85(AgSbTe2)15. By using this combination of metals, the crystalline microstructure of the tellurides by adding materials that differ in one of two properties: the atomic size being either significantly larger or smaller, or the inclusion of something that either does or does not have a localized magnetic moment. The reasons for this are incredibly complex, and are still the focus of ongoing research in the field, with two big goals. The first is decreasing thermal conductivity, thereby enabling better heat retention in the fuel until it’s able to be converted into electricity, and something that can be affected by imperfections in the crystalline structure of a material. The other is to increase the power factor, which is the relationship between the Seebeck coefficient and electrical resistivity, which is where the localized magnetic moments come into play. A paper from 2012 looks at more recent proposed changes to TAGS-85, involving the use of dysprosium as an additional doping agent: https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1333&context=usdoepub .

SNAP-19 cutaway view, image AEC/DOE

This type of material was first used in the SNAP-19B RTG, for the Nimbus-B satellite which failed to launch. The fuel was recovered, and placed in the Nimbus-3 satellite, which launched April 3, 1969.

Pioneer 10 Jupiter flypast, painting by Rick Guidice, image via NASA

This was the first in an impressive list of missions powered by these power supplies: Pioneer 10, the first flyby of Jupiter (launched March 2, 1962, perijove on December 4, 1973); and Pioneer 11, which launched on April 6, 1973, flew by Jupiter on December 2, 1974, and then went on to Saturn on September 1, 1979 (just months before Voyager 1 and 2). The Martian Viking Landers 1 and 2 used a pair of modified SNAP-19’s each as well in their missions.

Back deck of Viking lander. SNAP-19 just visible on right rear. Image NASA
MMRTG cutaway, image NASA

The next major use of TAG-85 was in the Multi-Mission RTG (MMRTG), which is a true workhorse of American astronuclear engineering, currently powering the Curiosity rover. His legacy will fly again on the Mars 2020 rover, currently under construction at the Jet Propulsion Laboratory.

MMRTG on Mars Science Laboratory (Curiosity), image NASA

Current RTG design work is shifting away from the solid-state conversion systems so long favored by NASA, due to its low power conversion efficiency. This is nothing new, but the inherent simplicity of the solid-state systems have dominated the available supply of radioisotope power systems, and the mission needs of NASA, the USAF, and other major customers that using a heat engine to produce electricity has only been explored so far. The upcoming power conversion series will deal with these options in detail.

Thank You, Dr. Skrabek

While RTGs may not be the big, exciting power supplies that we often discuss here on Beyond NERVA, they have literally opened the outer solar system to our understanding, powering the missions that have amazed us all, no matter our level of education, and the knowledge and beauty we’ve all gained is due in part to Dr. Skrabek’s discovery and subsequent work on these systems. The bread and butter power supply for the outer solar system, and one that is powering the most advanced rover ever built by humanity on Mars, is possible thanks to his ideas, and his hard work.

While there is a transition happening to heat engine based RPUs (radioisotope power units, the broader category that a Stirling or Rankine powered RTG would fall under), this does not mean that the traditional RTG is going anywhere any time soon. Their inherent stability, durability, and ruggedness, combined with their ability to power a rover the size of a small truck around Mars, vaporizing rock at a distance with a laser beam to analyze its composition, is not something to be cast aside lightly.

Even if TAGS-85 never flies after Mars 2020 (something I very much doubt), his work will continue to inform us every day about the environment, both past and present, of Mars for years to come. Five of his power supplies (two each on Viking 1 and 2, one on MSL) will, all things being equal, end their days on the Red Planet, with a sixth on its way in another year. His work made us able to open our eyes on the beauty of the outer solar system, showed us Pluto in fascinating detail for the first time, and literally pioneered the path of the Voyager probes at Saturn (Pioneer 11 inserted in the same orbit to verify that particle density was within safe limits), and is now flying out of the solar system in two different directions. One small, but crucial, piece of materials engineering allowed these spacecraft and rovers to do more than they would be able to with other materials, and open our eyes that much more.

Thank you, Dr. Skrabek, for your life’s work. Your memory will live on in all the missions you have, are, and will make possible, and the knowledge that you’ve helped bring humanity as a whole.

Here are some of the pictures that Dr Skrabek helped enable:


Radiation Characteristics of Plutonium 238, Matlock and Metz, LANL 1967


SNAP-19 Nimbus B Integration Experience, Fihelly et al 1968

SNAP-19 Pioneer F &G Final Report, Teledyne Isotopes 1973

MMRTG and Advanced RTGs

MMRTG Fact Sheet, NASA 2013

NASA’s Radioisotope Power Systems Program (presentation slides), Dudzinski 2014

Multi-Mission RTG (MMRTG) (Presentation slides), Bechtel DOE

Nuclear Power Assessment Final Report, NASA/JHAPL 2015

Active Short Circuit – Chassis Short Characterization and Mitigation for the MMRTG, Bolotin and Keyawa 2015 (presentation slides)

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2 Responses

  1. The RTG was invented at Mound Laboratory in 1954 over lunch in the cafeteria by Ken Jordan and John Birden. Plutonium-238 came to Mound in 1959 from SRP for use in a weapons program. Its properties were unknown. https://www.dropbox.com/s/7h989o0uk3ie2x6/20085004Ofte.pdf?dl=0
    Mound was the ‘go to’ site for Pu-238 heat sources thru 1975, developing fuel forms, hardware and fabrication techniques. https://www.dropbox.com/s/qp5nfrl9elng5va/Mound%20Heat%20Sources.pdf?dl=0
    From 1976 thru 2002 Mound assembled all the Pu-238 RTGs, including the largest ones ever made, the 9.6 Kg. GPHSs. Mound analyzed Pu-238 for its characteristics, including using a Mound speciality, calorimetry, to determine its half life. Mound developed fuel forms in conjunction with the converter contractors, including oxygen-16 exchange to reduce the neutron emissions by a factor of 5 and substoichiometric oxide fuel to reduce corrosion of converter components.
    While Mound no longer exists, the legacy of its employees lives on thru the Mound Museum.

    1. I’ve got a few dozen papers out of Mound Labs on my computer, and every time I read them I’m stunned at all the ideas that they had developed – and then were not pursued.

      My favorite of their programs is Project Poodle, which IMO has the most frustrating ratio of cool developed idea to available documentation of any astronuclear program I’ve run across.

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