We’re going to be talking a lot about radioisotopes, which make up the fuel for an RTG, so let’s go ahead and define the term off the bat. First, the term “isotope,” which just means a particular atomic mass of a particular element. Elements are defined by the number of protons in the nucleus, but the number of neutrons varies. If the number of neutrons is too high, or too low, then the nucleus is unstable, or simply can’t form. At the lighter end, this is generally about one proton per neutron, but as you go up in atomic number you need more neutrons to hold things together, since they carry the strong nuclear force (the binding force of atomic nuclei). We REALLY don’t want to get into the vagarities of atomic stability here, but there’s an incredibly well-made video on this subject, available here:
If a nucleus has too many or too few nucleons (protons and neutrons), then it will undergo radioactive decay. There are four main types of decay: alpha decay, beta decay, gamma emission, and neutron emission. We’ll start from the back, and work our way forward.
Certain materials end up emitting a neutron only, but this is usually only something that occurs (to any statistically significant degree) in a nuclear reactor (artificial or natural, like a star). If you’re familiar with reactor dynamics, then you’re familiar with the fact that there are two neutron types: prompt neutrons and delayed neutrons. Prompt neutrons are released immediately upon fission (or some forms of fusion) occurring, and tend to be your more energetic neutrons. Delayed neutrons, on the other hand, are emitted by your fission products at some point afterward. This gets incredibly complex mathematically, so we won’t get into the details, but this is a large part of why most terrestrial power plants have a primary coolant loop and a secondary coolant loop: you don’t want neutrons messing up your turbine, or causing it to be radioactive, in most cases – it makes maintenance a headache. There ARE some single-loop systems, but they’re the exception rather than the rule. As a general rule, by the end of a few minutes, neutron emission is (generally) not a significant form of radiation.
The second type of radiation is gamma emission, which in many ways is the simplest. Picture the classic high-school solar system model of an atom (also known as the Bohr or solar system model): a number of rings surround the nucleus of the atom, and each one can hold a specific number of electrons. If the orbit isn’t completely filler, then that means that one of the electrons has extra energy. When it drops down to the lowest energy state it emits a photon; the details of how many electrons are in which orbital determine a number of things, including what color flame you get when you burn a sample of it in a fire (the classic high school chemistry example). The same mental image works (to a certain extent) in the nucleus, with each orbital having a number of protons and neutrons that can occupy each. When a nucleon is in too high an energy state, it also emits a photon, but the wavelength of this photon is on the high end of the electromagnetic spectrum (shorter wavelength), a gamma ray. These are incredibly hard to slow, mostly requiring direct impact with the nucleus of an atom. This is why gamma shielding is massive, dense materials like lead, tungsten, uranium and the like. Capturing this energy and turning it into work, outside gamma heating, is called gammavoltaics, and is in the realm of science fiction for now. I hope to cover the options for this in the future, though, and when that is done I’ll post a link here.
The third type, beta emission, comes in two forms: beta-plus and beta-minus, which mostly occurs when an isotope is CLOSE to stable, but not there yet. Basically, by quantum mechanical interactions that are insanely weird and esoteric without the math, a neutron becomes a proton and spits out usually an electron. This is called “beta-minus” decay, and is the most common form of beta decay. This electron is moving at very high velocities, for an electron, so it’s not useful for electricity, but can be slowed to the point that it is useful (this is called betavoltaics, another type of radioisotope generator that we won’t cover today, but is the idea behind the “diamond battery” that is being researched at the University of Bristol). The other version, beta-plus decay, still involves a neutron becoming a proton in the nucleus, but rather than producing an electron at high velocities it produces a positron, the antimatter version of an electron. This happens for… reasons (there’s a mathematical explanation, and there are some isotopes of certain elements that preferrentially do this, but I can’t even begin to follow the math that explains it). While it’s energetic, it doesn’t take much to stop a beta particle: a sheet of aluminum foil, like you probably have in your kitchen, will do it.
Finally, we come to alpha decay. Here, a nucleus is too massive, and so spits out a chunk of itself. This chunk is always (outside fission) a helium-4 nucleus: two protons, two neutrons. This is moving at a very high velocity, but it’s also prone to hitting materials around it, which means that it can be stopped by a sheet of paper – or your skin. However, the kinetic energy in that particle is significant, meaning that it will heat the surrounding material to a significant degree. Other than capturing the heat from elastic collisions in the material, the motion of the alpha particle itself can be used to generate electricity, which is called alphavoltaics. Again, this is the subject for a future page or blog post, which will be linked here when complete.
Every isotope has an incredibly predictable decay pattern as a mass. Predicting when a particular nucleus will undergo radioactive decay is impossible to predict, but en masse the process is insanely predictable. Knowing the element and isotope, the statistical likelihood of a particular outcome is almost completely certain, down to the wavelengths and energies of the decay products (what’s ejected from the nucleus) to a degree that is the envy of most other scientific disciplines. This level of confidence in the behavior of the materials involved sets a high bar, but given the resources, one that makes astronuclear engineers incredibly confident in their determinations in the behavior of their systems.
The net result of these complex processes is that there is a predictable pattern to the decay process involved. Due to the statistical nature of the particular physical processes, the best way to describe it is by what is called the half-life of a material. This means that half of the atoms in a particular sample have decayed within a particular time, and because of this the process is logarithmic in nature. Half decays, then half of that half, and so on, until the system reaches an asymptote: a point that can never be reached, but is constantly being approached over longer and longer time frames. The shorter the time period that this half-life occurs in, the more radiation is put out during a given period of time, but also the shorter the time period that the fuel produces enough radiation to be useful to convert into work (i.e. how long it’s useful, and how useful it is in that timeframe). Because of this, the half-life of a material is critically important to the choice of fuel, based on the length of the mission required.
Decay chains, as the process of nuclear decay is known, are readily available for any particular isotope, and form the basis for their use as a radiopower source. For spaceflight, another key component is the need for a large amount of energy for a given volume and mass. This varies between different materials, but in general an alpha decay emits a lot more energy than a beta or gamma decay, so this tilts the tables in favor of heavy (or purely) alpha emitters.
This is why it’s common to give the total radioisotope inventory of the fuel in terms of Becquerels (or curies… *sigh, I’ll convert as much as I can remember to). This is the total number of decays that will occur if every atom were to decay (functionally, this is about five half-lives, but the RTG will fail before that due to a lack of useful power output), and is the equivalent of the total change in velocity possible with a rocket: there’s only this much, and it’s a hard limit. It also allows engineers and materials scientists to start characterizing the system from their side, as well.
The other components in radioisotope selection are the chemical reactivity of the fuel, its stable forms, and the thermodynamic and physical properties of that fuel.
Given the incredible diversity of radioisotopes, though, covering all of them is a challenge to say the least. Instead, we’ll focus on the ones that have been most used historically, and have also been proposed.
In the Beginning
In 1957 Mound Laboratory was approached by the US Army Signal Corps Research and Development Laboratories, Ft. Monmouth, New Jersey to begin an evaluation of radioactive material for use as a heat source in a thermoelectric generator. Half-life, shielding required, efficiency, health hazards, availability and costs were to be considered. Once the radioisotope was selected, a heat source would be designed and prototypes would be constructed. The thermoelectric generator design would use conventional materials, or material and information furnished by the Signal Corps Laboratory. Initial elimination of isotopes as heat sources was made on the basis of half-life. Any isotope which had a half-life of less than 100 days or greater than 100 years was discarded. A few exceptions were made to insure not overlooking a likely isotope. Further eliminations were made of isotopes which (1) were gamma emitters only, (2) had radioactive transitions yielding gamma with energies greater than one million electron volts, (3) had transitions having an occurrence of gamma emission greater than ten per cent with energies over 0.1 million electron volts, (4) had decay schemes which involved daughter elements having any of the preceeding gamma characteristics, or (5) had particle energies so slight that it would require more than one per cent conversion efficiency to give a 0.01 watt output. A literature search of the nuclides was conducted to eliminate isotopes which did not have desired nuclear properties. Calculations were based primarily on values obtained from the Trillinear Chart of Nuclides by William H. Sullivan.NUCLEAR BATTERY-THERMOCOUPLE TYPE SUMMARY REPORT, Blanke et al 1962
These nuclear property restrictions eliminated consideration of all but the following isotopes: sulfur~35, argon-42, calcium-45, nickel-63, krypton-85, strontium-89, strontium-90, yttrium-91, cadmium-113m, tin-121m, tin-123, cerium-144, promethium-146, promethium-147, samarium-151, europium-149, europium-152, europium154, europium-155, gadolinium-148, thulium-170, thulium-171, lutetiura-174, tungsten-185, tungsten-188, osmium-194, thallium-204, lead-210, polonium-208, polonium-209, poionium-210, radium-228, actinium-227, thorium-228, uranium-232, Plutonium-236, plutonium-238, americium-241, americium-242, curium-242, curium243, curium-244, californium-248, californium-250, californium-252, einsteinium-252 and einsteinium-254. All of the above isotopes, with the exception of naturally occuring lead-210, polonium-210 and several of the fission products, must be manufactured, and almost all of them must be separated to obtain a sufficiently high specific activity
Perhaps the best known radioisotope for RTG use is plutonium 238, or 238Pu. This particular isotope of Pu will not undergo fission, and is produced by irradiating specially prepared targets of Neptunium 237. This can either be extracted from spent nuclear fuel (or removed during batch reprocessing in a fluid fueled reactor), or be created by first bombarding a target of Americium to produce the Np target. This material was the first isotope of Pu to be isolated. Its subsequent practical use was pioneered by the Mound Laboratories in the 1950s, who were the providers of many different radioisotopes, as well as radioisotope powered systems of many sorts.
238Pu has a half-life of 87.7 years, and when it decays it emits a single 5.593 MeV alpha particle to become the geologically stable uranium 234. Its emissions of gamma and beta radiation are incredibly small as a pure metal. However, other properties of plutonium dictate that it be formed into an oxide.
PuO2 is a well-understood, stable chemical structure, but complicates the radiation situation due to a reaction with two isotopes of oxygen in the chemical structure of the fuel: 17O and 18O both produce neutron radiation when bombarded with alpha particles, and as such they increase the neutron radiation amount by ten times! However, this flux is still extremely low, and isotopic enrichment brings the neutron production well within an acceptable dosage.
238Pu has fueled many of the RTGs that have been flown over the years, as well as those that have been used in naval and air defense capacities. The first satellite to be powered by an RTG was fueled with 238Pu oxide, as were most of the missions to the outer solar system (Juno being the single exception). It is also currently powering Voyagers 1 and 2, New Horizons, the Curiosity rover, and will power the Mars 2020 rover as well.
241Am is produced during irradiation of uranium, which goes through neutron capture during irradiation to 241Pu, which is one of the isotopes that degrades the usefulness of weapons-grade 239Pu. It decays in a very short time (14 day half-life) into 241Am, which is not useful as weapons-grade material, but is still useful in nuclear reactors. According to Ed Pheil, the CEO of Elysium Industries, the Monju sodium cooled fast reactor in Japan faced a restart problem because the 241Pu in the reactor’s fuel decayed before restarting the reactor, causing a reactivity deficit and delaying startup.
241Am is used as RHU fuel in the form of Am2O3. This allows for very good chemical stability, as well as reasonable thermal transfer properties (for an oxide). This fuel is encased in a “multilayer containment structure similar to that of the general-purpose heat source (GPHS) system,” with thermal and structural trade-offs made to account for the different thermal profile and power level of the ESA RTG (which, as far as I can tell, doesn’t have a catchy name like “GPHS RTG” or “MMRTG” yet). Neptunium oxide most often takes the form of NpO2, meaning that a deficit of oxygen will occur over time in the fuel pellet. The implications of this are something that I am completely unable to answer, and something that is definitely distinct from the use of 238Pu, which then becomes 234U, both of which have two oxygen atoms in their most common oxide state. However, considering there’s a stoichiometric mismatch between the initial material, the partially-decayed material, and the final, fully-degenerate state of the fuel element. I know just enough to know that this is far from ideal, and will change a whole host of properties, from thermal conductivity to chemical reactivity with the clad, so there will be (potentially insignificant) other factors that have impacts on fuel element life from the chemical point of view rather than the nuclear one.
The US also looked into 241Am RHU fuel. A recent report for the DOE’s Nuclear Energy University Programs by Sudarshan Loyalka at University of Missouri – Columbia, looked into the possibility of an American supply from the US weapons Pu stockpile. It is available here:
Perhaps the shortest-lived of the isotopes in this list, 210Po is best known as a poison, not a source of power. Alexander Litvinenko’s poisoning after his defection from Russia to the UK was carried out using this isotope, and in the process of grossly mishandling the material (other than the gross mishandling of deliberately causing radiation poisoning in another human being) exposed dozens to a highly chemically and radiologically (if ingested) toxic substance.
At the same time, this isotope offers incredible potential benefit for short-lived radioisotope power systems. Its high power density (641.3 W/g), low half-life (138 days), and low shielding requirements offer many advantages in certain applications. The best known of its applications is Project Poodle, which combined the very high specific power of 210Po with the low mass of H2 to produce 0.25 lbf of thrust at 710 s isp at 3600 F, from a 45 lb thruster.
For more information on 210Po, check out the Polonium-210 page, available here!
Far more popular in Russia than the US, 90Sr has a half-life of 28.8 years, making it a shorter-mission power option. Additionally, it is the only radioisotope on this list with goes through beta decay, rather than alpha decay. It has a lower power density of 0.46 w/gram, and hence a lower temperature, therefore it is less attractive for space missions.
This is just fine for industrial terrestrial applications, however, where mass isn’t the driving factor, instead fuel cost and availability are key. Since 90Sr is one of the most common fission products, the sheer availability of it for any country that does extensive reprocessing makes it a potentially economical and plentiful source for RPS.
The US used 90Sr as a fuel or numerous SNAP-era RTGs and other RPS of the time. Additional information and documentation is available on the SNAP RTG page, here.
More information on 90Sr, and its use in RTGs, will be available soon. I’ll post a link once it’s ready.
Another early option for radioisotope power supplies was 147Pm, another in the list of beta emitters considered for thermal radioisotope applications. With a half-life of 2.6 years, a beta decay energy of 224.1 keV, and a power density of 0.36 W/g, this was very much a mid-range option for an RPS. Both gamma and x ray emissions from 147Pm are minimal, with effectively no neutron emissions, meaning that shielding mass is negligible in the overall design requirements.
General Electric worked with Monsanto to modify a resistojet design to use a radioisotope instead of an electric heater using 147Pm as the fuel, but I’m still working on tracking down the details of the thruster.
147Pm has also been suggested as a betavoltaic power source, which I’ll discuss as I’m able to find more information and effectively summarize that type of power system. However, if you’re interested there’s a paper by Colozza and Cataldo on low-power betavoltaics using 147Pm available here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180007388.pdf
A page on 147Pm may be produced in the future if there is interest. If you want me to dig deeper into this fuel, let me know either in a comment here or on social media!
Perhaps one of the most esoteric of radioisotopes on this list, 144Cm was investigated at Mound Laboratory during the 1950s-70s as a potential power source. Primarily a beta emitter, it too is a fission product, which means it could potentially be an economical byproduct of using fission power plants.
It has a half-life of only 284 days, decaying into another beta emitter, 144Pr (with a half-life of 17.5 min and a specific activity of 2.78×10^18 Bq/g), before decayig into 144Nd, a low level gamma emitter with a very long half-life.
This short half-life makes it useful for only short mission durations, but its use in radioisotope thrusters is a possibility. It was also investigated as the fuel for the Task-2 RTG meant for space use (see the SNAP RTG page for more information). The only other design I found that used it was the SNAP-13 thermionic radioisotope generator, which also never flew.
However, while the designs using 144Cm never seem to have gotten off the drawing boards, the isotope found use elsewhere: as a beta decay antineutrino generator for particle physics experiments.
Calculation and measurement of 144 Ce- 144 Pr β -spectrum, Atroschenko et al 2017 https://www.researchgate.net/publication/321940309_Calculation_and_measurement_of_144_Ce-_144_Pr_b_-spectrum
The 144Ce source for SOX, Duero et al 2016 https://iopscience.iop.org/article/10.1088/1742-6596/675/1/012032/pdf
106Ru is another fission product beta emitter, with a half-life of 373.59 days and an average beta emission energy of 39.4 keV. Due to this low energy emission, it is not an ideal radioisotope, but is another potentially economic option, which was briefly investigated at Mound Laboratories. Unfortunately, information on its use is very sparse, and I have yet to find a design fueled by this radioisotope. If you know of one, let me know!
Other Options Abound!
There are a whole host of radioisotopes that are useful for radioisotope power sources in specialized applications. C14 has been used in betavoltaics, for instance, and a host of other options are available as well.
While they aren’t as high powered as a fission power plant, RTGs have a key role to play in astronuclear engineering, and their use will likely only expand as our interest – and investment – in space exploration increases. Their mission lifetime, power requirements, and a host of other considerations will define which fuels are used, and this list will grow as the designs and missions for these isotopes increases!
Further Reading and References
NUCLEAR BATTERY-THERMOCOUPLE TYPE SUMMARY REPORT, Blanke et al 1962 http://large.stanford.edu/courses/2013/ph241/jiang1/docs/4807049.pdf