|
|
| Fig. 1: RTG on the Cassini spacecraft. (Courtesy of NASA) |
Radioisotope thermoelectric generators (RTGs) are a type of nuclear battery that utilize heat released from the radioactive decay of certain isotopes and generate electrical energy from differences in temperature. Within the RTGs there are hundreds of small arrays of semiconductors called thermocouples which can transfer heat into electricity due to the Seebeck Effect. [1] RTGs are commonly used for powering deep space missions and remote outposts like wheatear stations and lighthouses. RTGs offer several benefits, including long lifespan, high dependability, and lowˇenvironmental impact. [2]
Materials that emit charged particles and have half-lives of about 80-100 years make good candidates to be used in RTGs. Other characteristics include low radiation emissions, high power density, and a stable fuel form with a high melting point. With a half-live of 87.7 years and emitting α particles that have very low penetration power, Pu-238 has been the most popular isotope used for RTGs. [3] Pu-238 is proven to be a very dependable supplying energy for over 20 NASA space missions. There has not been a problem observed during a space mission in the past 50 years since the RTG's introduction. [4] Nevertheless, due to Pu-238 scarcity, a variety of radioisotopes has been evaluated to be used in RTGs. Am-241 with its half-life of almost 5 times that of Pu-238 had been found as one of the promising fuels for radioisotope heat sources. [5] Sr-90, which emits β particles, has also been used as a radioisotope, and Cm-243 has been investigated to replace Pu-238. [6,7] Yet, the National Research Council's analysis in 2009 stated that no other radioisotope was found available that exceeded the safety and performance characteristics of Pu-238. [7]
 |
| Fig. 2: A glowing Plutonium-238 fuel pellet. (Courtesy of NASA) |
The United States stopped producing bulk Pu-238 with the closure of its facilities in 1988. [8] With the aim of sustaining future space missions, a US project is underway to restart Pu-238 manufacturing and produce at a constant rate of up to 1.5 kg annually by 2025. [9] Before this, the US imported Pu-238 from Russia, and by this agreement, DOE took delivery of 20 kg of Pu-238 from 1992 to 2009, before Russia stopped exporting. [7]
As mentioned previously, radioisotopes have a high energy density compared to some other power sources. This allows Pu-238 to provide a significant amount of power in a relatively lightweight package. To examine what 20 kg of Pu-238 is capable of let's determine how long 20 kg of Pu-238 can supply energy before the power drops to 1 kW. We need to start by calculating the initial total radioactivity of Pu-238.
| A0 | = | λ × N |
| λ | = | ln(2) 87.7 yrs × 3.1536 × 107 s/yr |
= | 2.5091 × 10-10 s-1 |
| N | = | 20 kg × 6.023 ×
1023 atoms/mol 0.238 kg/mol |
= | 5.06 × 1025 atoms |
| A0 | = | 1.2694 × 1016 s-1 |
The total power P0 generated from these radioactive decays, each of which releases energy E0 = 5.593 MeV, is
| P0 | = | A0 × E0 |
| = | 1.2694 × 1016 × 5.593 MeV × 1.6 × 10-13 J MeV-1 | |
| = | 11.309 kW |
Knowing the power of RTG when it is first turned on, we can calculate the time T when Pu-238 would decay to only produce P = 1 kW:
| P | = | P0 e-λT |
| T | = | 1 λ |
ln( | P0 P |
) | = | 1 2.5091 × 10-10 s-1 × 3.1536 × 107 s yr-1 |
× ln( | 11.309 kW 1.0 kW |
) = 306.5 years |
We have reviewed the role of radioisotopes and RTGs in deep space exploration, emphasizing their practical applicability. Pu-238 was found to be the most ideal isotope for this job, but its scarcity is a limiting factor for the development of RTG-powered probes. Yet, a probe can leave the earth with a little Pu-238 pellet and be powered for decades. To demonstrate how even comparably small kgs of Pu-238 deposits are important in supplying energy, calculations of power generation from radioactive decay processes are performed. In conclusion, with their high reliability and ability to produce power in extreme environments, RTGs are found to be crucial in fueling space missions.
© Pelin Dedeler. 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] K. Uchidaet al., "Thermoelectric Generation Based on Spin Seebeck Effects," Proc. IEEE 104, 1946 (2016).
[2] K. Liuet al., "Experimental Prototype and Simulation Optimization of Micro-Radial Milliwatt-Power Radioisotope Thermoelectric Generator," Appl. Therm. Eng. 125, 425 (2017).
[3] R. G. Lange and W. P. Carroll, "Review of Recent Advances of Radioisotope Power Systems," Energy Convers. Manag. 49, 393 (2008).
[4] A. Crerend, "Radioisotope Thermoelectric Generators (RTGs)," Physics 241, Stanford University, Winter 2015.
[5] S. Spaugh, "Beyond Plutonium-238: Alternate Fuel for Radioisotope Thermal Generators," Physics 241, Stanford University, Winter 2022.
[6] H. E. Adkins, "Proposed Strontium Radiosotope Thermoelectric Generator Fuel Encapsulation Facility," AIP Conf. Proc. 271, 153 (1993).
[7] Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration (National Academies Press, 2009), pp. 14-45.
[8] T. Tinsley, M. Sarsfield, K. Stephenson and R. Ambrosi, "Progress and Future Roadmap on Am-241 Production For Use in Radioisotope Power Systems," 2019 IEEE Aerospace Conference, 8741817, 2 Mar 19.
[9] R. M. Ambrosi et al., "European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration," Space Sci. Rev. 215, 55 (2019).