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| Fig. 1: Comparison of volumes of Am-241 (blue) and Pu-238 (orange) required to produce 1 Watt of heat energy. (Source: S. Spaugh) |
Radioisotope power systems (RPS), including radioisotope thermoelectric generators (RTG), are a class of devices which produce electricity via nuclear radiation; unlike in fusion or fission reactors, the electricity is produced directly from the energy given off by a radioactive source. RTGs are particularly well suited for, and have found extensive use in, providing power to satellites and probes sent on long-haul space exploration missions. Other use cases that have been explored or demonstrated include remote unmanned weather stations and the like, implanted medical devices, and anti-tamper devices included in weaponry design as mandated by the Department of Defense. [1]
RTGs make use of the Seebeck effect: a thermal gradient across a junction between two different metals or semiconductors will produce an electric current. [2] At a high level, the design of a radioisotope power system can be split into the design of the heat source, which contains the radioactive fuel as well as necessary shielding; and the converter, which produces the electrical output from the radiated heat. [3]
The design and architecture of RTGs as they are most commonly found in US-affiliated projects has been covered fairly extensively in other entries to the PH241 archives. [1,4-6] These flight-proven RTGs have generally used plutonium, specifically Pu-238, as the fuel source. [4] The use of Pu-238 has met several supply challenges in the last 40 years, including a temporary cessation of production in the US; production has been reinstated in recent years at Oak Ridge National Labs, but in relatively small amounts. [3,4] Although the trajectory of US development of RTGs still focuses on Pu-238 as the primary fuel variant, there has been a body of recent work looking into alternative fuels. [2,7]
Fuel selection may be approached from a system design perspective as well as from a feasibility perspective. Some common criteria or considerations include:
Ability of the fuel isotope to reliably and consistently produce heat over a long period of time; this is quantifiable via radiation characteristics and the half-life of the material. [6,7]
Energy produced per unit mass (particularly for weight-critical systems such as extraterrestrial applications). [6,7]
Safety considerations, such as level of gamma radiation, rate of spontaneous fission, or chemical safety considerations such as toxicity or solubility in water. [7]
Technological feasibility of fuel production, as well as availability and affordability of the raw materials and processes involved in doing so. [2,7]
Initial fuel selection research during the early development of RTGs evaluated upwards of 1300 isotopes before narrowing down to 47 potential candidates. [6] As already stated, Pu- 238 emerged early on as the fuel of choice for long space missions, due to its favorable radiation characteristics and half life of 88 years. [6] However, there is a history of Strontium-90 (Sr-90) being successfully used in remote and unmanned terrestrial contexts, particularly Russian lighthouses, beacons, and weather stations. [2] RTG designs using Curium-244 (Cm-244) have also been demonstrated. [6]
In their 2021 paper, Dustin and Borrelli make a theoretical case study of 9 potential non-plutonium fuel isotopes, scoring them qualitatively on several metrics by how they compare to Pu-238. [7] Their comparison included a review of the suitability of Sr-90 and Cm-244, noting that the half-lives of both are too short to be well-suited for long term spaceflight, as the fuel would cease to produce adequate heat before the end of the mission. They conclude that the most promising alternative fuel is Americium-241 (Am-241). This isotope can be produced as a byproduct of nuclear waste, and has a half-life of 432 years, making it suitable for long-term use. [7] The most glaring trade-off is that Am-241 produces less energy per mass than Pu-238: Am-241 has a specific heat density of 0.115 W/g, while Pu-238's is 0.56 W/g. [7,8] Fig. 1 shows the difference in volume of material required to produce 1 Watt of heat-energy using Pu-238 vs Am-241.
OBrien et al. in 2008 made a similar study of 4 potential alternatives to Pu-238, also including Am-241. [2] They note that two Polonium isotopes, namely Po-208 and Po-210, may have potential for short-duration missions in which a decay to stable lead at the end of the mission is a desired characteristic. This is due to the short half-lives of these isotopes. Finally, they also conclude that Am-241 is the best potential alternative to Pu-238 for long-term power supply. [2] In addition, they note that Am-241 has lower neutron radiation levels than Pu-238, making it a good fit for engineering systems that are sensitive to neutron radiation damage. [2]
The promise of Am-241 as a future RTG fuel is supported by a project funded by the European Space Agency (ESA), ongoing since 2009, to develop an RTG designed specifically around Americium oxide Am2O3 as a fuel source. [8,9] The work has included development of production methods for Am-241 carried out by the National Nuclear Laboratory in the UK, which demonstrate the availability and feasibility of using this fuel for ESA projects. [8,10] On the engineering side, the ESA team has made use of a dummy fuel supply which uses electric heat to model the output of Am-241 to develop prototype designs of a modular and scalable RTG. [8] As of 2019, a prototype powered by the dummy fuel was lab-bench functional in both ambient and simulated space environments, capable of producing 10 W and having approximately 5% total system efficiency. [8] This is comparable to the ~6% system efficiency demonstrated by the Pu-238 RTGs which have been launched by NASA. [1]
Plutonium remains the de-facto fuel selection for US-based development of RTGs, due to its status as the legacy system employed by NASA and the US's recent return to production of this isotope. However, its relative scarcity and the emergence of space exploration programs outside the US and Russia provoke new developments in alternate fuel selections. Am-241 emerges as a promising fuel for radioisotope heat sources, and with a half-life of almost 5 times that of Pu-238, may even open the doors to more remote space exploration than was possible before.
© Sarah Spaugh. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Park, Review and Preview of Nuclear Battery Technology, Physics 241, Stanford University, Winter 2017.
[2] R.C. O'Brien et al., "Safe Radioisotope Thermoelectric Generators and Heat Sources for Space Applications," J. Nucl. Mater. 377, 506 (2008).
[3] R. Lange and W. Carroll, "Review of Recent Advances of Radioisotope Power Wystems," Energy Convers. Manage. 49, 393 (2008).
[4] A. Crerend, "Radioisotope Thermoelectric Generators (RTGs)," Physics 241, Stanford University, Winter 2015.
[5] J. Ruffio, "What Future for Radioisotope Thermoelectric Generators (RTG)?" Physics 241, Stanford University, Winter 2017.
[6] M. Jiang, "An Overview of Radioisotope Thermoelectric Generators," Physics 241, Stanford University, Winter 2013.
[7] J. S. Dustin and R. A. Borrelli, "Assessment of Alternative Radionuclides for Use in a Radioisotope Thermoelectric Generator," Nucl. Eng. Des. 385,111475 (2021).
[8] R. Ambrosi et al., "European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration," Space Sci. Rev. 215, 55, (2019).
[9] R. Ambrosi et al., "Radioisotope Power Systems for the European Space Nuclear Power Program," 2019 IEEE Aerospace Conference, IEEE 8742245, 1 Mar 19.
[10] J. Brown et al., "Americium and Plutonium Purification by Extraction (the AMPPEX process): Development of a New Method to Separate Am-241 From Aged Plutonium Dioxide For Use in Space Power Systems," Prog. Nucl. Energy, 106, 396 (2018).